OtoVFHSJTY of ILLINOIS LMm - e igi? SOME REACTIONS INVOLVED IN SECONDARY COPPER SULPHIDE ENRICHMENT. E. G. Zies, E. T. Allen (Chemical Study), and H. E. Merwin (Microscopic Study) . Economic Geology Publishing Company CONTRIBUTION No. 7 CHEMISTRY univusity or niwois lwwry FEfi - £ idi7 5ner tube was used. The mixture of sulphide and quartz was distributed around this tube, which projected above the mixture about i cm. The solution was introduced through the inner tube, care being taken not to let it overflow until the solu- tion came through to the top of the quartz and sulphide. SECONDARY COPPER SULPHIDE ENRICHMENT. 439 TABLE X. Pyrite and Cupric Sulphate. Analyses of the Enrichment Products. Exp. Cu in KCN, g. S in KCN, g. Total, g. Per Cent, of Cu. Per Cent, of Cu 2 S. Per Cent, of CuS by Difference. 13 0.6352 s3 0.1800 0.8152 77.92 86 ± I % 14 14 0.5770 33 O.1645 0.7415 77.82 85 “ 15 17 0.1478 O.O420 0.1898 77.87 85 “ 15 18 O.1995 O.0577 O.2572 77.60 83 “ 17 The residues in 13, 14, and 17 were compressed and examined microscopically. This examination showed that a large percent- age of the pyrite was still unattacked and also that both cuprous and cupric sulphides were present as alteration products on the pyrite. The film of alteration products formed on the pyrite in 18 was too thin to permit a similar microscopic examination. The analyses of the enrichment products show that in 13, 14, and 17 the percentage of cupric and cuprous sulphides remained fairly constant but changed several per cent, in 18, where the amount and surface of the pyrite were much greater than in the other experiments. Analyses of the Solutions . — The filtrates from the residues were analyzed in the usual manner. These analyses, together with the molecular ratios based on them, are tabulated below. TABLE XI. Pyrite and Cupric Sulphate. Temperature , 200°. Exp. Dura- tion, Days. Analyses of Solutions. Molecular Ratios on the Basis of the Analyses. Copper, g. Total Iron as Ferrous, g ■ Acid, g. h 2 so*. Cu Deposited h 2 so 4 Cu HjSO*' Initial. Final. Depos- ited. Fe in Solution' Fe ' 13 9 O.642O None 0.6420 0.2205 Not det. 2.56 34 34 14 8 O.6424 None O.6424 0.2209 “ 2.56 34 34 15 20 0.5m None O.511I O.I728 0.6600 2.60 2.18 1.20 16 8 0.5084 0.0255 O.4829 0.1630 0.6225 I 2.66 2.17 1.23 17 8 0.5084 0.0113 O.4971 O.1696 0.6392 2-57 2.15 1.20 18 2 0.5405 | None l 0.5405 O.1838 Not det. i 2.58 34 34 33 Both copper and sulphur were determined ill aliquot portions of the solu- tion of copper sulphides in potassium cyanide. 34 H 2 S0 4 not determined since Jena glass reaction tubes were used. 440 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. Discussion . — Experiments 13 to 17 were carried out under similar conditions, namely, the same amount of pyrite was ex- posed to the action of a 5 per cent, solution of cupric sulphate; likewise a large percentage of pyrite was unattacked. The mo- lecular ratios obtained in these experiments show that the condi- tions at the end of the experiments must have been nearly the same. Now the analyses of the alteration products and their microscopic examination proved the presence of both cuprous and cupric sulphides, and the uniformity of the conditions at the end of the experiment was again shown by the fact that the per- centages of the two sulphides of copper were about the same. The following hypothetical equation has been suggested to repre- sent the alteration of pyrite into cupric sulphide. 4FeS 2 + 7 CuS 0 4 + 4H 2 0 = 7CuS + 4FeS0 4 + 4H 2 S0 4 . (6) The molecular ratios demanded by this equation and by equation (5), the correctness of which we have proven, are tabulated below : Equation. Cu J h 2 so 4 Cu fT Fe H 2 SO 4 (5) 2.80 2.40 I,I 7 (6) i-75 1. 00 i-75 The ratios obtained in experiments 13 and 18 all lie between these values. This evidence, together with that furnished by the anal- yses of the alteration products and by their microscopical exami- nation, serves at least to indicate that the reaction represented by equation (6) is involved when cupric sulphide is obtained as one of the enrichment products replacing pyrite. We have proven that covellite reacts with cupric sulphate to form cuprous sul- phide, and we have shown that this reaction takes place readily at 200°. This being the case, the evidence seems good that when pyrite unde'r the conditions of our experiments alters into cuprous sulphide, both reactions just indicated are involved, thus : 4FeS 2 + 7 CuS 0 4 + 4H 2 0 = 7CuS + 4FeS0 4 + 4H 2 S0 4 . (6) 5Q1S + 3CuS 0 4 + 4H 2 0 = 4Cu 2 S + 4H 2 S0 4 . ( 1 ) SECONDARY COPPER SULPHIDE ENRICHMENT. 441 According to these equations, all of the iron in solution would be derived through (6), but the sulphuric acid would be derived through both reactions. Now, if we know the total amount of copper which took part in the reaction, and also the total iron in solution, we should, on the basis of these equations, be able to calculate the amount of cuprous and cupric sulphides present in the alteration products on the pyrite. We should also be able to determine both sulphides, knowing the amount of total copper which reacted and the amount of sulphuric acid formed. These calculations were made for experiment 17, Table X, and the values thus found compared with those obtained by analysis of the alteration product on the pyrite: Per Cent. C112S. Based on analysis 85 Based on Cu and Fe 83 Based on Cu and H 2 S 0 4 87 Per Cent. CuS. 15 1 7 13 The agreement is as close as could be expected when all of the experimental errors involved are considered. The same calcula- tions were made in the case of experiments 12, 13, and 17, and similar agreement was found between the values thus obtained and those found by analysis of the alteration product. The percentages of CuS and Cu 2 S, shown in Table XII., indicate that reaction (1) proceeds faster than (6). The relative rates of the two reactions under the conditions of our experi- ments are in all probability influenced by the extent of the altera- tion of the pyrite. Thus as the thickness of the enveloping coat- ing of the sulphide of copper which replaces the pyrite increases, there will be greater opportunity for reaction (1) to take place than for (6), due to the fact that the enrichment products replac- ing the pyrite prevent ready access of the cupric sulphate solu- tion. Consequently, as the reacting surface of the pyrite is in- creased, for a given concentration of copper, more cupric sul- phide should be formed. This is indicated at least in experi- ment 17, where the amount of cupric sulphide is several per cent, greater than in the other experiments, and probably approaches 442 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. more closely the relative rates of the two reactions involved if the copper enrichment products derived from the pyrite were to remain detached from the pyrite. We do not wish to leave the impression that all of the cupric sulphide is formed at one time, and that this is then further at- tacked by the cupric sulphate; it is entirely possible that the two reactions go on simultaneously, but when the pyrite is covered with its alteration products, the opportunity for reaction between cupric sulphide and cupric sulphate is more favorable than before. (c) The Influence of Sulphuric Acid on the Reaction between Pyrite and Cupric Sulphate at 200°. The following experiments were carried out in order to de- termine in what manner sulphuric acid influenced the reaction between pyrite and cupric sulphate. TABLE XII. Influence of Sulphuric Acid. Initial Conditions of the Experiments ( Temperature , 200°). Exp. Material. Weight, g . Solution. Group I . . ! 19 Elba pyrite, ioo mesh and finer. I. OOOO 50 c.c. Cupric sulphate, 5% and acid, 5%. 20 Elba pyrite, ioo mesh and finer. 1. 0000 50 c.c. Cupric sulphate, 5% no acid. Group II . 21 Leadville pyrite, 125-200 mesh. 1. 0000 40 c.c. Cupric sulphate, 5% and acid, 2 %. 22 Leadville pyrite, 125-200 mesh. 1. 0000 40 c.c. Cupric sulphate, 5% no acid. 23 Leadville pyrite, 125-200 mesh. 1. 0000 40 c.c. Cupric sulphate, 5% no acid. Group III 24 Leadville pyrite, 125-200 mesh. 1. 0000 50 c.c. Cupric sulphate, 5% and acid, 1!%. 25 Leadville pyrite, 125-200 mesh. 1. 0000 ! 50 c.c. Cupric sulphate, 5% no acid. The solutions and residues obtained in these experiments were analyzed in the same manner as in the previous experiments on pyrite. Each group of experiments is to be considered sepa- SECONDARY COPPER SULPHIDE ENRICHMENT. 443 rately, since the experiments in each group were carried out under comparable conditions, though it should be remembered that in the second and third groups the tubes in which acid was used were exposed to the temperature of 200 ° for a longer period than those in which no acid was used. The solutions obtained in these experiments* analyzed as follows : TABLE XIII. Influence of Sulphuric Acid. Group. Exp. Duration. Analyses of the Solutions. Initial. Copper, g. Final. Deposited. Acid, g. Initial. I 19 5 days O.6325 O.5568 0.0757 2.4500, 5% 20 5 “ O.6325 O.0963 O.5362 None II 21 15 “ O.5111 O.IO65 O.4046 0.8000, 2% 22 8 “ 0.5084 0.0255 O.4829 None 23 8 “ 0.5084 O.OI33 O.4951 None III. . . 24 7 " O.6366 O.1846 0.4521 0.7500, \\% 25 4 “ O.6366 O.0273 0.5093 None On comparing 19 and 20 in Group I., we find a marked differ- ence in the amount of copper deposited, the duration of the ex- periment being the same in both cases, a fact which indicates a distinct retarding influence on the part of the acid. In the second and third group of experiments, a similar retarding influence is noted, even though the tubes containing the acid were heated for a longer period of time than those in which no acid was used initially. In experiment 21, Group II., the tube was removed from the furnace at the end of eight days but the appearance of the pyrite and that of the solution indicated that very little action had taken place. Thereafter the tube was removed from the furnace daily for examination until the end of the experiment, and it was noted that the solution of cupric sulphate gradually lost its characteristic color, indicating that reaction was taking place and that the acid exerted a retarding influence but did not inhibit the reaction, as would have been the case if it were a 444 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. question of increased solubility of one of the sulphides due to the presence of a great excess of hydrogen ions. This inhibiting effect of the acid is probably due, in part at least, to another cause, to which we shall refer subsequently. Analyses of Enrichment Products. — The enrichment products which replace the pyrite, both when acid was used initially and when no acid was used initially, were analyzed in the usual manner. TABLE XIV. Influence of Sulphuric Acid. Group. Exp. Analyses of the Enrichment Products. Cuin KCN, S • S in KCN, g • Total, g • Per Cent, of Cu. Per Cent, of CU2S. Per Cent, of CuS by Difference. I 19 acid 0.0763 0.0225 O.O988 77-23 80 ± 3% 20 20 none 0.1442 0.0414 O.1856 77.69 84 ± I % 16 II 21 acid 0.1093 0.0311 O.1404 77-85 85 ± 1 % 15 23 none 0.1478 0.0420 O.1898 77.87 85 ± 1 % 15 III. . . 24 acid 0.1330 0.0384 O.1714 77-59 83 ± 1 % 17 25 none 0.2010 0.0570 6.2580 77-90 85 ± 1 % 15 On comparing the analyses in each group, we find that, within the limits of the experimental error, the acid has not changed the relative amounts of cupric and cuprous sulphides; therefore the acid has exerted only a retarding influence and has not changed the nature of the reaction between pyrite and cupric sulphate. Experiment 26. Action of Sulphuric Acid on Pyrite. — We find that pyrite is only very slightly attacked at 200° by a 2 per cent, solution of sulphuric acid, as the following will show : 1 g. of Leadville pyrite, sized as usual between 125 and 200 mesh, 35 and mixed with three times its volume of quartz, was exposed at 200° for a period of three days to the action of a 2 per cent, solution of sulphuric acid, and the amount of hydrogen sulphide in the tube determined. The tubes were opened without expos- ing their contents to the air, in the manner described on page 418. 35 The pyrite was carefully sized in order to compare the reactivity of pyrite and chalcopyrite toward sulphuric acid. SECONDARY COPPER SULPHIDE ENRICHMENT. 445 The hydrogen sulphide was driven out by means of hydrogen gas and absorbed in cadmium acetate. Under these conditions 0.6 milligram of hydrogen sulphide and 4 milligrams of iron as sulphate were found. These small amounts certainly show that pyrite is not readily attacked by sulphuric acid. The tube in which this experiment was carried out was very carefully evacu- ated before sealing, therefore no oxidation due to the presence of air in the tube could have taken place. As a matter of fact, we have just shown that sulphuric acid exerts a markedly retarding influence on the reaction between pyrite and cupric sulphate. This certainly could not be the case if pyrite were readily attacked by sulphuric acid with the formation of hydrogen sulphide. (1) Secondary Reactions . It will be remembered that in our preliminary experiments on the reaction between pyrite and cupric sulphate, hematite, cuprite, and metallic copper were formed together with the sulphides of copper, thus confirming a similar observation by Stokes. 36 It will also be remembered that we succeeded in establishing such experimental conditions that at the end of an experiment only the sulphides of copper were found as alteration products on the pyrite. While this was the case at the end of any one experiment, in its earlier stages hematite, cuprite, and metallic copper were also formed. The question now arises, how were these three sub- stances formed and why did they disappear. When pyrite alters into the sulphides of copper, ferrous sul- phate is formed as one of the soluble products. Stokes 36 showed that ferrous sulphate and cupric sulphate react at 200° to form hematite, cuprite, and metallic copper, and showed that their presence could be explained by the following equilibrium : 2FeS0 4 -f- 2CuS0 4 ?=* Cu 2 S 0 4 + Fe 2 (S 0 4 ) 3 . When no acid is used as an initial constituent of the solution, the ferric sulphate so formed will be, at 200°, largely hydrolyzed, 36 H. N. Stokes, Bull. U. S. Geol. Survey, No. 186, p. 44. 446 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. . and the iron will be precipitated as hematite, thus permitting the equilibrium to shift more and more to the right until the acid formed as the result of the hydrolysis of the ferric sulphate is in equilibrium with the precipitated hematite. This shift of the equilibrium brings the concentration of the cuprous sulphate to a point where it also hydrolyzes, yielding cuprite and sulphuric acid. Just as with ferric sulphate, this hydrolysis will go on until the cuprite and acid are in equilibrium. The formation of the metallic copper is easily explained by another well-known reaction of cuprous sulphate : Cu + CuS0 4 ^ Cu 2 S0 4 . This reaction is readily reversible, and the amount of copper pre- cipitated at any temperature will depend upon the concentrations of cuprous and cupric copper. It is therefore obvious that the addition of sufficient sulphuric acid to a reaction mixture con- sisting of copper and iron sulphates will, by preventing the hydrolysis of one of the products, ferric sulphate for instance, prevent the formation of hematite, cuprite, and metallic copper. Now let us consider what happens when pyrite and cupric sul- phate react. We have shown that in this reaction ferrous sul- phate and sulphuric acid are formed, and that, depending on the conditions of the experiment, pyrite alters into cuprous sulphide, or into cuprous and cupric sulphides. In the first stage of the experiment, we have present a small amount of ferrous sulphate and sulphuric acid and a relatively large amount of cupric sul- phate; in consequence of the reaction between ferrous sulphate and cupric sulphate explained above, hematite, cuprite, and metal- lic copper will form until all three are in equilibrium with the sul- phuric acid present. In the later stages of the reaction between cupric sulphate and pyrite, enough acid is formed to redissolve both hematite and cuprite, and it will be evident from what has just been said that the ferric sulphate formed by the solution of the hematite must be reduced both by metallic copper and by the cuprous sulphate formed by the solution of the cuprite. One can readily see, therefore, that in the reaction between SECONDARY COPPER SULPHIDE ENRICHMENT. 44 7 pyrite and cupric sulphate, the more rapidly the copper in solu- tion is used up, the more rapidly sufficient sulphuric acid will be formed to prevent the hydrolysis of the ferric sulphate resulting from the reaction between ferrous and cupric sulphates. The copper can be used up rapidly by presenting a large surface of the pyrite to the action of the solution; thus, after carrying out a large number of preliminary experiments, it was found that at 200 ° one gram of ioo-mesh pyrite mixed with three times its volume of quartz will use up in eight days all of the copper in 50 c.c. of a 5 per cent. CuS0 4 .5H 2 0 solution, and no hematite, cuprite, or metallic copper will be present at the end of the experi- ment. When no quartz was mixed with the pyrite, several weeks were required to accomplish the same result. An examination of Tables XI. and XIII. will show that, as the reacting surface of the pyrite was increased and made as available as possible by admixing with quartz, the time required for the pyrite to use up all or nearly all of the copper in solution and leave no hema- tite, cuprite, or metallic copper at the end of the experiment is greatly decreased. The formation of these secondary products can also be avoided if sulphuric acid of the proper concentration is present as one of the initial constituents of the solution. We have shown, how- ever, that sulphuric acid retards the reaction between pyrite and cupric sulphate but does not change the nature of the reaction. We believe that the retarding influence of the acid is due to its maintaining in solution the ferric iron formed in the reaction between cupric and ferrous sulphates. When no sulphuric acid is used initially, we have the oxidizing action of the cupric sul- phate augmented by the action of the cuprous sulphate formed in the reaction between cupric and ferrous sulphates . 37 As the acid increases, the ferric iron is maintained in solution and the augmenting action of this cuprous sulphate is rendered negligible. When sulphuric acid is used as one of the initial constituents of the solution, the ferric iron is maintained in solution but the amount at the end of the experiment is very small. (See page 437)- 37 See page 446. 448 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. ( d ) The Influence of Ferrous Sulphate on the Reaction between Pyrite and Cupric Sulphate at 200° . It has been thought by several investigators that ferrous sul- phate plays a very important part in the reaction between pyrite and cupric sulphate. We tried to obtain information on this point at 200°, but were compelled to discontinue the experimental work for the following reason: When ferrous sulphate is used as an initial constituent of the solution, the troublesome secondary reactions discussed on page 446 become more troublesome than ever; if sulphuric acid is added to overcome the hydrolysis of the ferric sulphate and cuprous sulphates, the retarding influence of the acid which we have previously described comes into play, consequently nothing could be gained by carrying out extensive experiments along these lines. B. The Reaction between Pyrite and Cupric Sulphate at ioo°. The experiments at this temperature were carried out as fol- lows : One gram of pyrite was mixed with three times its volume of pure quartz and exposed to the action of a 1 per cent, cupric sulphate solution. Under these conditions no hematite was found at the end of the experiment. At the same time experiments were carried out in order to study the influence of sulphuric acid, and also that of ferrous sulphate, on the reaction between pyrite and cupric sulphate. TABLE XV. Pyrite and Cupric Sulphate. Initial Conditions: Weight of Leadville pyrite (200-220 mesh), 1.0000 g. ; temperature, ioo°. Exp. Dura- tion, Months. Initial Concentrations of Solutions. Analyses of Soli Copper, g. itions. Per Cent. Acid, Initial. Per Cent. Ferrous Iron, Initial. Initial. Final. 1 Deposited. 27 2 50 C.C. 1% CUSO4.5H2O 0.1286 0.II50 O.OI36 None None 28 2 “ “ “ 0.1286 O.II60 0.0126 “ 29 2 50 c.c. 1% CUSO4.5H2O, and 1% H2SO4 0.1286 O.1230 O.OO56 1% il 30 2 50 C.C. 1% CUSO4.5H2O, 1% FeSC>4.7H20, and 1 % H2SO4 0.1286 Q-I 233 0.0053 1% 1% SECONDARY COPPER SULPHIDE ENRICHMENT , 449 It was necessary to determine the ferrous iron and sulphuric acid formed during the experiment in aliquot portions of the solution, and the amounts found were too small and the experi- mental errors too large to permit their being determined with accuracy. The copper, however, could be accurately determined and the determinations are believed to be correct to within 0.3 milligram of copper. The pyrite in 27 and 28 was completely covered with a grayish blue alteration product; the pyrite in 29 and 30 was barely tarnished, indicating that very slight action had taken place. On comparing the amount of copper deposited in 29 with the amounts deposited in 27 and 28, one can readily see that the acid has also at ioo° exerted a retarding influence on the reaction between pyrite and cupric sulphate; the agreement be- tween the amounts of copper deposited in 27 and 28 shows that the difference observed between the amounts of copper deposited in 29 and the amounts deposited in 27 and 28 is too great to be attributed to experimental error ; the difference must be attributed to the retarding influence exerted by the acid. On comparing 29 and 30 we find that within the limits of the experimental error the same amounts of copper were deposited. It will be noted that sulphuric acid was present along with the ferrous sulphate; this was added to prevent the hydrolysis of the ferric sulphate formed in the reaction between ferrous sulphate and cupric sul- phate; it is therefore evident that when sulphuric acid is present, ferrous sulphate exerts no influence on the reaction between pyrite and cupric sulphate. The enrichment product replacing the pyrite in experiment 28 was removed with a 1 per cent, solution of potassium cyanide, and analyzed : Analysis of the Enrichment Product. Exp. Copper in KCN, g. S in KCN, g. Total, g. Per Cent, of Cu. Per Cent, of C112S. Per Cent, of CuS. 28 O.OO82 O.OO25 0.0107 77 76 ± 8 24 In view of the small amounts of copper and sulphur found in the KCN solution, the percentages of Cu 2 S and CuS shown 450 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. above can be considered only as an approximation. We made a mixture of pure chalcocite and covellite containing the above per- centage of Cu 2 S and CuS, and dissolved it in KCN, using a por- tion containing about the same amount of copper and sulphur as indicated in the analysis, and found on the basis of the result that the percentages of Cu 2 S and CuS found above are correct only to within about 8 per cent, of Cu 2 S or CuS. This accuracy is suffi- cient, however, to prove that both Cu 2 S and CuS were present. Discussion . — Thus pyrite and cupric sulphate will react also at ioo° and yield the same end products as those obtained at 200°, namely, cupric and cuprous sulphides; and just as at 200 °, sul- phuric acid exerts a retarding influence on the reaction. Ferrous sulphate is apparently without effect when sulphuric acid is present. C. Pyrite and Cupric Sulphate at 40°-^o°. Experiments carried out by Sullivan indicated that pyrite and cupric sulphate react also at ordinary temperatures. 38 He found that 20 grams of finely divided pyrite caused a solution of cupric sulphate which originally contained 0.097 g. Cu in 40 c.c. to lose in three days 0.040 g. Cu ; thus indicating that pyrite reacts com- paratively easily with cupric sulphate at ordinary temperatures. We desired, if possible, to obtain additional evidence on this point. Experiment 31 . — At first 15 grams of Elba 39 pyrite sized between 125-200 mesh to the linear inch was exposed to the action of a solution of cupric sulphate which contained 0.3195 g. Cu in 100 c.c. of solution. The experiment was carried out at 40° and extended over a period of two months. The containers in this and in the subsequent experiment were, as usual, care- fully evacuated and placed in the shaking machine described on page 415. Only 2 milligrams of copper were lost by the solution. Experiment 32 . — Another experiment was carried out this time at 50° in the following manner: 15 grams of pyrite finer than 200 mesh were exposed to the action of 200 c.c. of a copper sul- 38 E. C. Sullivan, Trans. Am. Inst. Min. Eng., 37, 894 (1907). Unfortu- nately no analysis of the pyrite used in this experiment is mentioned. 39 See page 438 for analysis. SECONDARY COPPER SULPHIDE ENRICHMENT. 451 phate solution of the same strength as used in the experiment above. In a period of six months, 0.0523 g. Cu reacted with the pyrite. The- resulting solution was decidedly acid and con- tained about 0.052 g. Fe as sulphate. The acid and iron were not accurately determined since in a previous experiment it was found that on washing the very finely divided pyrite, oxidation of the pyrite by the air caused iron, as sulphate, to pass into solution. 40 Therefore on washing the cupric sulphate out of the pyrite, iron, as sulphate, passed into solution in addition to the iron that was derived from the reaction between pyrite and cupric sulphate. The general appearance of the pyrite had not changed in any marked way, but on washing the pyrite with 1 per cent. KCN and determining the copper in solution, 0.0520 g. Cu were found. On adding a small amount of nitric acid to the KCN solution the usual precipitation of brownish copper sulphide was noted. The sulphur in the copper sulphide dissolved by the KCN solution could not be accurately determined since the presence of consider- able iron as sulphate was noted along with copper. The iron was derived by oxidation of the finely divided pyrite in the same manner as above indicated. The determination of the sulphur could not be satisfactorily corrected by the running of a blank for the correction was uncertain and too large to permit its being properly applied. The results show that pyrite and cupric sulphate react, slowly 41 to be sure, at a temperature much below ioo° and that the prod- ucts of the reaction are the same as at the more elevated tempera- tures, namely, sulphides of copper, sulphuric acid and iron sulphate. 5. THE ACTION OF CUPROUS SULPHATE ON PYRITE. It is evident that the reaction between pure pyrite and cupric sulphate is, at lower temperatures, an exceedingly slow one. On 40 Our apparatus for filtering out of contact with air was so badly clogged up by this large amount of finely divided pyrite that we were -compelled to carry out the filtration in the usual manner. 41 The more rapid rate indicated by Sullivan’s experiments is possibly due to his having used a pyrite ground more finely than that used by us. 452 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. the other hand, the evidence is good that cuprous sulphate and pyrite react far more readily. Thus Winchell 42 has shown that pyrite exposed to the action of cupric sulphate and sulphur diox- ide alters rapidly into copper sulphide at ordinary temperatures, even in the presence of sulphuric acid. Spencer 43 has shown that when pyrite in contact 44 with metallic iron is treated at ordinary temperatures with cupric sulphate, it is coated over in a comparatively short time with copper sulphide enrichment prod- ucts. In both experiments cuprous copper is present in the solu- tions; in the first experiment the reduction is caused by the sul- phur dioxide and in the second by the metallic iron. Inasmuch as cuprous copper can play such an important part in altering pyrite into copper sulphides, we should, on the basis of the following equilibrium, expect a similar alteration when pyrite is exposed to the action of cupric and ferrous sulphate, be- cause ferrous iron partially reduces cupric copper to cuprous copper, as explained on page 446. 2Q1SO4 -f 2FeS0 4 ^Cu 2 S04 + Fe 2 (S 0 4 ) 3 . 45 (4) This equilibrium has been by no means fully investigated, but we feel certain that the quantities of ferric iron and cuprous copper that can coexist in solution are very small as compared with the quantities of ferrous iron and cupric copper. 46 Of course, if the cuprous copper were partially removed by pre- cipitation, more would form by reaction from left to right (see equation (4)), but in this reaction ferric iron must also form. Thus, as cuprous copper is precipitated, the ferric iron continually increases, which means that the concentration of cuprous copper must continually decrease. If the removal of the ferric iron could proceed pari passu with the precipitation of cuprous copper, 42 H. V. Winchell, Bull. Geol. Soc. Am., 14, 269-276, 1903. 43 A. C. Spencer, Econ. Geol., 8, 629. 44 It is not necessary for the metallic iron to be in contact with the pyrite, since cuprous copper must be present, even if the iron is suspended in the solution out of contact with the pyrite. 45 See footnote, page 446. 46 Based on calculations made by Dr. E. Posnjak of this laboratory; work unpublished. SECONDARY COPPER SULPHIDE ENRICHMENT. 453 the reduction of cupric copper by ferrous iron would go on un- hindered. If pyrite is added to a solution containing all these four sub- stances, the system is considerably complicated by the fact that three out of four of the substances — all except the ferrous iron — react with pyrite. The action of cuprous copper on pyrite is similar to that of cupric copper, in so far as the product is con- cerned, but the rate of reaction of the latter is so much slower that we may neglect it for our present purposes. Ferric iron, on the other hand, has quite a different action; it not only oxidizes the pyrite to ferrous salt and sulphuric acid, but it oxidizes the enrichment products, dissolving them also. The ferric iron acts so much more readily on the sulphides of copper than on pyrite that in the case of a partially enriched pyrite the action would be entirely confined to the former. Thus the chemical action of the ferric iron is to dissolve what the cuprous copper precipitates. If, now, the ferric iron could be removed from solution, the process of alteration of the pyrite would undoubtedly be greatly accelerated. In this connection, we repeated an experiment carried out by Spencer , 47 who added calcite to a system consisting of pyrite, cupric and ferrous sulphates. He found that the pyrite became coated with a sulphide of copper in a comparatively short time. Our results confirm Spencer’s in so far as the film on the pyrite had all the appearance of copper sulphide , 48 though it was too thin for microscopic examination. Too little is yet known of the detailed chemical processes of natural enrichment for us to make any complete application of the foregoing facts to the phenomena of nature; we can only point out what will happen in certain contingencies. In the first place, there can be no question that pyrite and other ferriferous sulphides, when subjected to the action of a copper sulphate solu- tion, will give a complex system containing, besides an enriched 47 Spencer, loc. cit., p. 649. 48 Spencer, loc. cit. Metallic copper also was observed. Likewise, it was found that calcium carbonate does precipitate metallic copper from a solution of cupric and ferrous sulphates. 454 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. sulphide, a solution of cupric, ferrous, cuprous, and ferric sul- phates. In the second place, if the ferric iron is removed contem- poraneously with the enrichment process, the latter should be greatly accelerated. 6. ALTERATION OF PYRITE TO THE COPPER-IRON SULPHIDES. The geological evidence seems clear that pyrite can alter to chalcopyrite and subsequently to bornite. Up to the present, how- ever, we have not been able to induce pyrite to alter to these sul- phides when cupric sulphate is used as the enriching solution. As a mattter of fact, we do not see how they can form under our conditions, for in the work on chalcopyrite and bornite it will be shown that these sulphides are attacked by cupric sulphate much more rapidly than pyrite at all temperatures. It is rather difficult to see, therefore, how these sulphides can form from pyrite when cupric sulphate is the enriching solution. This must especially be the case with bornite, which, as will appear later in this paper, is very sensitive to cupric sulphate' and also to sulphuric acid, the latter being formed when pyrite is. attacked by cupric sulphate. 7. THE REACTION BETWEEN PYRRHOTITE (FeSx) AND CUPRIC SULPHATE. A. At 200 0 Inasmuch as pyrrhotite forms one of the primary constituents of a number of ore bodies which give evidence of having under- gone secondary enrichment, it was thought desirable to study the reaction between pyrrhotite and solutions of cupric sulphate. For this purpose both natural and synthetic pyrrhotites were used. A number of preliminary experiments were first carried out in order to obtain a clue as to the best method for studying the problem. In these preliminary experiments it was found that when one gram of pyrrhotite ground to pass a‘ 100-mesh sieve is exposed to the action of 50 c.c. of a 5 per cent, copper sulphate solution for 3 days at 200°, a very energetic reaction takes place in which all of the copper is deposited. In addition to the sulphide en- SECONDARY COPPER SULPHIDE ENRICHMENT. 455 richment product, hematite, metallic copper, cuprite, and pyrite (or marcasite) were formed. Ferrous sulphate, hydrogen sul- phide, and sulphuric acid were found in solution. Pyrrhotite therefore resembles pyrite in behavior, but its reaction is even more complicated. The hematite, metallic copper, and cuprite, in both reactions are derived in the same way. At the very base of the residue in the reaction tube, a few grains of material were noticed, the yellowish color of which bore a striking resemblance to chalcopyrite. Another experiment gave a similar result. The pyrrhotite and its alteration products were so caked in the bottom of the tube that the circulation of the solution through the mass must have been greatly impeded. It seemed to us that the yel- low substance might have been the first product to form ; that in its formation the copper in the immediate vicinity might have been entirely removed from solution; and that thereafter the product might have been saved from further change by the fact that additional copper could reach it only with great difficulty, owing to the impeded circulation of the solution. Acting in line with this suggestion, in the next experiment we increased the surface of the reacting pyrrhotite, and to make the action more uniform the particles were separated by quartz. Three grams of pyrrhotite were mixed with three times their volume of pure quartz and exposed at 200° to the action of a 5 per cent, solution of C11SO4 . 5 H 2 0 . 49 Under these conditions, apparently only the yellow product was obtained. The conditions of the experiment were thus greatly simplified, and it now remained to be proved that the resulting sulphide was chalcopyrite, for our previous experience with pyrite had taught us not to trust too much to surface color evidence. 50 Experiments were now carried out on synthetic and natural pyrrhotite of the following compositions : Pyrrhotite Used. Fe. S. Cu. Synthetic (1) 38.12 “ (2) 61 17 Auburn, Maine 38.92 O.03 49 See footnote 12. 50 See page 439. 456 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. The synthetic pyrrhotites were made according to a method de- veloped in this laboratory. 51 Microscopical examination showed that the natural pyrrhotite contained a very minute amount of chalcopyrite. Below are tabulated the initial conditions of the experiments at 200°. TABLE XVI. Pyrrhotite and Cupric Sulphate. Initial Conditions at 200°. Exp. Material. Weight, g. Solution. Quantity. Copper Initial, g. 33 Synthetic (i) 3.0000 50 c.c. 5% CUSO4.5H2O O.6363 34 “ “ “ “ 35 ii << it 4 4 36 (2) “ €t 44 44 44 37 Auburn, Maine << ll n 44 In all of the experiments, the pyrrhotite was mixed with three times its volume of pure quartz sized between 60 and 80 mesh. The tubes containing the pyrrhotite and solution were heated at 200 0 for a period of three days. At the end of this time, all the copper originally present in solution was used up in altering the pyrrhotite. Analysis of Enrichment Products Replacing the Pyrrhotite . — The residues obtained in these experiments were washed, dried over sulphuric acid in a vacuum desiccator, and analyzed in the following manner : The pyrrhotite was separated from the quartz by the use of a 100-mesh sieve, the pyrrhotite passing through and most of the quartz remaining on the sieve. The sulphides were then digested with 1-10 sulphuric acid at ioo° in an atmos- phere of H 2 S in order to remove the unattacked pyrrhotite. The wet sulphides were then ground in a mortar, taking care to simply break up the grains rather than to grind them finely by any rub- bing motion of the pestle, as very finely divided chalcopyrite was found to be attacked by the acid. After regrinding, the sulphides were again digested with the dilute sulphuric acid in an atmos- 51 Allen, Crenshaw and Johnston, Am. J. Sci. (4), 33, 194 (1912). SECONDARY COPPER SULPHIDE ENRICHMENT. 457 phere of H 2 S. The residue was then dried and analzed. Below are tabulated the results of these analyses ; the values given have been corrected for silica, which was always present in the purified residues : TABLE XVII. Pyrrhotite and Cupric Sulphate. Analyses of Residues. Exp. Per Cent, of Cu. Per Cent, of Fe. Per Cent, of S. Summation. 33 32.39 35-20 33-50 30.55 29.50 30.88 28.OO 36.44 99.71 34 34a 30.21 33-20 32.69 36.07 99.78 36 37 30.12 32.95 Calculated for CuFeS2 34.63 30.42 34-94 The chemical analysis was supplemented by a microscopic ex- amination. The latter revealed, in addition to chalcopyrite, the presence of pyrite and pyrrhotite in all of the purified residues, thus accounting in a large measure (except in experiment 34) for the discrepancy between the calculated values for chalcopyrite and those obtained by analysis of the residues. The residues from experiments 33 and 35 were examined by Professor Graton. A portion of each was imbedded in sealing wax, polished, and ex- amined under the metallographic microscope. The presence of shells of chalcopyrite was noted in the residue. In some cases these shells contained a core of pyrrhotite but in most cases were hollow, thus indicating that the pyrrhotite had been removed by the treatment with acid. We found that the shells varied in thickness from 0.002 to 0.005 mm - The residue obtained in experiment 34 was examined before treatment with acid, and, in addition to chalcopyrite and pyrrhotite, bornite also was found. This residue was again examined after purification with acid, and it was found that the bornite had altered to sulphides of copper. Later on, we shall show that bornite is easily attacked by sulphuric acid and is altered by it to the sulphides of copper, thus accounting for the absence of bornite when the residue was 458 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. examined after treatment with acid. The presence of these sul- phides of copper also accounts for the high percentage of copper found in 34. The copper sulphides were later on removed with dilute potassium cyanide and the residue again analyzed. This analysis is shown in 34a. Discussion . — The microscopic evidence, together with the chemical analysis, show that chalcopyrite is the end product when pyrrhotite reacts with cupric sulphate at 200°, if the surface of the pyrrhotite and the amount of copper in solution are properly proportioned. If they are not properly proportioned, the chal- copyrite which is first formed envelops the pyrrhotite to such an extent as to prevent ready access of the solution to the pyrrhotite. When this happens, the cupric sulphate attacks the chalcopyrite, forming sulphides of copper. If a large excess of copper is pres- ent, hematite, cuprite, and metallic copper are formed. 52 In view of the fact that pyrrhotite varies in chemical compo- sition, it is obviously impossible to establish an equation which will adequately represent the reaction involved when pyrrhotite and cupric sulphate react to form chalcopyrite, consequently no effort was made to analyze quantitatively the solutions obtained in these experiments. Ferrous sulphate and sulphuric acid were present in solution, but when one remembers that sulphuric acid attacks pyrrhotite with great readiness, it becomes obvious that the acid found at the end of an experiment is not the total acid which is formed during the experiment ; the acid which is present owes its existence to the fact that pyrrhotite is protected from attack by the chalcopyrite which envelops it. It is also obvious that the ferrous iron in solution is derived partly from the reaction be- tween pyrrhotite and cupric sulphate and partly from the action of the acid on the pyrrhotite ; the hydrogen sulphide formed in this latter action can not of course persist in the presence of cupric sulphate, but after all of the copper has been precipitated considerable of the gas remains. Subsequent experiments have led us to believe that this hydrogen sulphide plays an important 52 See page 446. SECONDARY COPPER SULPHIDE ENRICHMENT. 459 part in the altering of pyrrhotite into chalcopyrite under these conditions. 53 It is evident that under the proper conditions pyrrhotite and cupric sulphate react very readily at 200° to form chalcopyrite. In our experimental work on the reaction between pyrite and cupric sulphate, we did not succeed in obtaining chalcopyrite as an alteration product on pyrite, and we doubted very much that such an alteration could take place in the presence of cupric sul- phate, owing to the fact that chalcopyrite reacts more readily with cupric sulphate than pyrite does. In a subsequent paper on the role of hydrogen sulphide we hope to suggest an explanation for the difference in behavior of pyrite and pyrrhotite towards cupric sulphate. 54 B. Pyrrhotite and Cupric Sulphate at ioo°. These experiments were carried out under the same conditions as those at 200 °, namely, the pyrrhotite was mixed with three times its volume of pure quartz and exposed, in evacuated Jena glass tubes, to the action of the cupric sulphate solution. Below are tabulated the initial conditions of the experiments and the analyses of the resulting solutions. TABLE XVIII. Pyrrhotite and Cupric Sulphate. Temperature, ioo°. Initial Conditions. Analyses of Solution. Exp. Dura- tion Weight, g. Copper, g Days. Material. Solution. Initial. 9 Final. De- posited. 38 45 86 2.000 Orange Co., Vt. f 125-200 mesh. 25 c.c. of a 2 % CUS04.5H20. 0.1272 None O.I272 39 45 1. 000 Orange Co., Vt., 125-200 mesh. 25 c.c. of a 2% CuSO-j.stLO. 0.1272 O.I272 53 Work in hand. 54 Work in hand. 65 It is quite possible that complete deposition of the copper did not require this long period of time. 460 E. G. Z1ES, E. T. ALLEN AND H. E. M ERWIN. This natural pyrrhotite analyzed as follows: Fe = 6i.62 per cent.; 8 = 38.24 per cent.; Cu = o.04 per cent. The micro- scope revealed a minute amount of chalcopyrite. In these experiments the concentration and amount of the solu- tion were the same, but the quantity of sized pyrrhotite was varied. In experiment 38, 2 grams of pyrrhotite were used, and in this case the color of the chalcopyrite was very pronounced; a very small amount of some oxide of iron was present; no hydro- gen sulphide could be detected. This gas was tested for by pass- ing hydrogen through the solution in the reaction tube, which in turn was connected with an absorption apparatus containing cad- mium acetate. The solution in the tube contained only ferrous sulphate and a very little sulphuric acid; the oxide of iron re- ferred to above was as usual derived from the hydrolysis of the ferric iron formed in the reaction between ferrous sulphate and cupric sulphate, which also takes place at ioo°, especially when little or no acid is present. A few grains of the deposit were examined microscopically. It was shown that the copper was deposited as chalcopyrite, which replaced the outer portion of the grains of pyrrhotite. In experiment 39, where only one gram of pyrrhotite was used, all other conditions being the same, the residue in the tube pre- sented a somewhat different appearance; instead of the usual chalcopyrite color, the bluish color often seen on a badly tarnished chalcopyrite was noted. All of the copper originally present in solution had been deposited. Ferrous sulphate, hydrogen sul- phide, and 9 milligrams of sulphuric acid were present in solu- tion. The solid product was treated with a 1 per cent, solution of KCN in the cold, which completely removed the tarnish and exposed the characteristic color of chalcopyrite, the presence of which was further proved by a mineralographic examination. At ioo° we were thus able to show that pyrrhotite reacts with a solution of cupric sulphate to form chalcopyrite, and that this chalcopyrite is further attacked, forming sulphides of copper, if the reacting surface of the pyrrhotite is not large enough to rapidly use up all the copper in solution. SECONDARY COPPER SULPHIDE ENRICHMENT. 4 61 C. Experiments at 40 ° . Finally, experiments with pyrrhotite and cupric sulphate were carried out at 40°. Two natural pyrrhotites differing in chem- ical composition were used; these were carefully sized in the usual manner between 125 and 200 mesh. 56 These pyrrhotites analyzed as follows : T Per Cent. Locality. t of Fe _ Per Cent, of S. Per Cent, of Cu. Per Cent, of Residue. Copper Mt., Alaska 61.10 Orange Co., Vermont 61.62 38.68 38.24 00.06 OO.O4 00.06 These experiments were carried out in the shaking machine described on page 415. If it is desired to compare the relative reactivity of two pyrrhotites, differing in chemical composition, towards cupric sulphate, the bottles and their contents should be shaken rather gently because pyrrhotite is a soft mineral and easily broken by attrition. The necessity of doing this will be especially evident when we consider that the duration of these experiments was two months. Below are tabulated the initial conditions under which the experiments were carried out and also the analyses of the resulting solutions : TABLE XIX. Pyrrhotite and Cupric Sulphate at Ordinary Temperatures. Initial Conditions. Analyses of Solutions. Exp. Dura- tion, Months. Weight, Material. Solution. Copper, g Initial. Final. Depos- ited. 40 2 4-759 Orange Co., 125-200 mesh. l£% C11SO4.5H2O in 400 c.c. I.2765 I.2285 0.0480 41 LT 4-759 Copper Mx, 125-200 mesh. i\% CUSO4.5H2O in 400 c.c. I.2765 1.2003 0.0762 In both experiments, the pyrrhotite changed color after the. first 12 hours at 40°, assuming a yellowish cast which became a pro- 56 See page 410. 462 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. nounced chalcopyrite yellow in about seven days. The yellow color persisted in the case of the pyrrhotite from Orange County for about seven days more, but in both experiments the yellow color slowly gave way to a bronze-like color which persisted in 40 until the end of the experiment. Subsequent examination with the microscope showed that this bronze-like color was due to iridescence caused by a very thin film of sulphide which had formed on the chalcopyrite. The color in the case of the pyr- rhotite from Copper Mt. was not bronze-like, but decidedly blu- ish, and also showed iridescence when examined under the microscope. The conditions of the experiments were made as nearly com- parable as possible, so that this rather striking difference in amount of copper deposited is significant and can be attributed either to the difference in chemical composition or to some differ- ence in physical structure, meaning by the latter that the more reactive specimen may have been more fissured and thus pre- sented a greater surface to the solution. Additional work must be done, however, in order to prove this point, and it is especially essential to secure a large number of pure natural pyrrhotites dif- fering in chemical composition and in physical structure, and likewise to secure a number of synthetic pyrrhotites differing in chemical composition. The securing of the natural pyrrhotites is in itself a rather difficult problem. The residues obtained in these experiments were compressed into tablets in the manner previously described, polished, and ex- amined under the microscope. The films formed on the pyrrho- tite were, however, too thin to be identified by this method. Both residues were then treated with a dilute solution of KCN '( J 4 per cent.). In a few minutes the tarnish disappeared, giving way to the characteristic color of chalcopyrite, a uniform yellow, and not due to iridescence. The color evidence seems strong enough to warrant our making the statement that at ordinary temperatures,, as well as at higher temperatures, chalcopyrite is the first enrich- ment sulphide formed when pyrrhotite reacts with a solution of cupric sulphate. SECONDARY COPPER SULPHIDE ENRICHMENT. 463 8. THE REACTION BETWEEN CHALCOPYRITE (CuFeS 2 ) AND CUPRIC SULPHATE. It is known that chalcopyrite in nature alters into cuprous sul- phide and also into cupric sulphide. Efforts were made to bring about these alterations and to study the reactions involved. The experimental work on chalcopyrite was done in much the same manner as that indicated under pyrite. This sulphide, how- ever, is reactive enough towards cupric sulphate solutions at ordi- nary temperatures to make experimental work worth while. In all of the experiments above 40 °, the sulphide was mixed with three times its volume of pure quartz and exposed to the action of the solutions in the usual manner. When the experiment extended over a longer period of time than three days, silica tubes were used instead of Jena glass tubes. A. Chalcopyrite and Cupric Sulphate at 200 ° . The behavior of chalcopyrite with cupric sulphate resembles that of pyrite in that the secondary products, hematite, cuprite and metallic copper are likely to form; in fact, chalcopyrite can not be converted entirely into chalcocite in this way at 200 0 with- out obtaining these secondary products. In such a case the mix- ture is of course too complicated for analysis. We have shown that these secondary products, when obtained in the alteration of pyrite, will eventually redissolve owing to the action of the sul- phuric acid which the oxidation of the sulphur in pyrite yields. Chalcopyrite yields much less acid, hence it is necessary to have acid present as one of the initial constituents of the solution when it is desired to convert chalcopyrite at the higher temperatures completely into chalcocite. Before carrying out quantitative ex- periments with chalcopyrite and cupric sulphate together with sul- phuric acid, it was first necessary to learn if sulphuric acid readily attacks chalcopyrite at 200°. (a). Action of Sulphuric Acid on Chalcopyrite . The experiments with chalcopyrite which we are about to de- 464 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. scribe are of a qualitative nature only. The chalcopyrite used was obtained from Evora, Portugal, and analyzed as follows : 57 Evora, Portugal Cu = 34.36; Fe = 30.61; S = 35.01 Calculated for CuFeS 2 Cu = 34.63; Fe = 30.42; S = 34.94 Experiment 42. Action of Acid at 200°. — One gram of this chalcopyrite was sized between 125 and 200 mesh and treated with 50 c.c. of 2 per cent, sulphuric acid at 200° for a period of two days. Ferrous sulphate, and hydrogen sulphide were found in solution. While it is true that these experiments were of a qualitative nature only, yet we are justified in making the state- ment that the amounts of ferrous sulphate and hydrogen sulphide found were far greater than those found when pyrite was simi- larly treated. 58 In addition to noting the presence of these sol- uble products, the presence of some sulphide of copper was ob- served as a coating on the chalcopyrite. In another experiment, carried out under the same conditions and extending over a period of ten days, the presence of crystal aggregates, together with the other products just mentioned, was observed. These were ex- amined microscopically and shown to be marcasite. 59 Action i of Acid at ioo°. — Chalcopyrite sized between 125 and 200 mesh was exposed to the action of sulphuric acid also at ioo°. The duration of the experiment was 2 weeks. If hydrogen sul- phide was formed in this case, the amount was less than could be detected by the method of analysis detailed under pyrite. This method we know to be very accurate. ( b ) Alteration of Chalcopyrite t\o Chalcocite. As a result of our experiments on chalcopyrite and sulphuric acid at 200°, we could see no reason for believing that the acid could in any way seriously affect the formation of cuprous sul- 57 Analysis by Dr. J. L. Crenshaw. 58 Pyrite when treated under similar conditions yielded 0.6 milligram of hydrogen sulphide, whereas chalcopyrite yielded about 39 milligrams. 59 This is the form of FeS 2 obtained at ordinary temperatures when the acid is of the concentration indicated. See Allen, Crenshaw and Merwin, Am. J. Sci. (4), 38, 393, 1914. SECONDARY COPPER SULPHIDE ENRICHMENT. 4^5 phide, therefore in the experiments which immediately follow we used sulphuric acid in order to prevent the formation of the sec- ondary products mentioned on page 446. TABLE XX. Chalcopyrite and Cupric Sulphate. Initial Conditions: 0.4000 g. chalcopyrite from Evora (125-200 mesh) ; solu- tion, 25 c.c. 5 per cent. CuS 0 4 - 5 H 2 0 and about 2 ^ 4 % H 2 S 0 4 ; temperature, 200 0 . 60 Analyses of the Solutions. Exp. Duration, Days. Copper, g. Acid, g. Total Iron (Ferrous), gr- Initial. Final. Deposited. Initial. Final. Formed. 43 8 0.3197 O.0248 O.2949 0.6063 O.9410 0-3347 O.I2I7 44 1 8 0.3197 0.0205 O.2992 0.6063 O.9502 0-3439 O.I222 All of the copper lost by the solution was precipitated on the chalcopyrite as sulphide, and exhibited the characteristic prop- erties of an enrichment product in that it adhered firmly and re- placed the chalcopyrite. Molecular Ratios . — On the basis of the above analyses, the following molecular ratios were determined : Exp. Cu H2SO4 Cu FV Fe h 2 so 4 * 43 2.13 i-57 1.36 44 2.15 1.60 1.34 The residues were compressed, polished, and examined micro- scopically. In experiment 43 this examination proved the pres- ence of cuprous -sulphide together with a few per cent, of cupric sulphide; likewise it was shown that a few grains of unaltered chalcopyrite were still present. In 44, it was shown that in so far as the microscope could determine, all of the chalcopyrite had altered to cuprous sulphide. This evidence taken together with 60 Similar experiments were made with no acid present initially, but the complications mentioned on page 464 forced us to discontinue experimentation along this line. 466 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. the ratios points to the following equation as representing the reaction between chalcopyrite and cupric sulphate under such conditions : 5 CuFeS 2 +nCuS 0 4 + 8 H 2 0 = 8 Cu 2 S+ 5 FeS 0 4 + 8 H 2 S 0 4 (7) The ratios demanded by this equation are the following : Cu H2SO4 Cu Fe ‘ Fe ’ H 2 SO/ 2.2 1.6 1-375 The agreement between these ratios and those obtained in experi- ment 44 are as close as the experimental errors involved will permit. The microscopical examination of the residue in 43 showed that cupric sulphide was present along with the cuprous sulphide. The enrichment products obtained in both experiments were dissolved in a 2 per cent, potassium cyanide solution and ana- lyzed in the usual manner. In view of the very small amount of chalcopyrite left in the residue the error introduced into the anal- ysis, due to the slight solubility of the chalcopyrite in the 2 per cent, solution of potassium cyanide, is negligible. TABLE XXI. Chalcopyrite and Cupric Sulphate. Analyses of the Enrichment Products. Temperature, 200 . _ Cu in KCN, S in KCN, Total, Per Cent. Per Cent. Per Cent. Exp. j g . g- g. of Cu. of CU 2 S. of CuS. 43 0.1460 O.O39O O.1850 78.93 93 ± 1% 7 44 1 0.1471 0.0375 O.1846 79.71 99 “ I These analyses confirm the observations made microscopically, namely, that in experiment 44 the chalcopyrite had altered to chalcocite, and that in 43 both cupric and cuprous sulphides were present. SECONDARY COPPER SULPHIDE ENRICHMENT. 4 6/ ( C ) ALTERATIONS OF CHALCOPYRITE TO CUPRIC AND CUPROUS SULPHIDES AT 200°. Efforts were made to find conditions under which chalcopyrite and cupric sulphate will react to form cupric sulphide as the only enrichment product. In this we were not successful, but we have been able to show that chalcopyrite does alter into cupric sulphide with subsequent alteration into cuprous sulphide; the amount of cupric sulphide which can form depending on the conditions of the experiment. TABLE XXII. Chalcopyrite and Cupric Sulphate. Initial Conditions : Evora chalcopyrite (ioo mesh and finer) ; solution, 50 c.c. 5 per cent. CuS 0 4 - 5 H 2 0 ; temperature, 200°. Dura- tion, Days. Analyses of the Solutions. Exp. Weight, g ■ Copper, g. j Acid h 2 so 4 , g ■ Total Iron (Ferrous), g • Initiai. Final. Deposited. 45 25 1. 000 O.6363 O.OO08 O.6335 0.5085 O.3026 46 3 5.000 O.6363 O.OO44 O.6319 1 0.2782 0.4304 In the experiments on the alteration of chalcopyrite into cu- prous sulphide, it was found necessary to have sulphuric acid present as one of the initial constituents of the solution. This was not found necessary when the conditions shown in Table XXII. obtain. In neither experiment was hematite present at the end of the experiment. The tubes were left in the furnace at 200° until the solutions had become colorless, indicating that most of the copper had been deposited. Increasing the amount of this finely divided chalcopyrite from one to five grams has evidently caused marked change in the length of time required for the solution to become colorless. The analyses bring out two other interesting points. It will be noted that in experiment 46, five times as much chalcopyrite was used as in 45 and that in both cases quartz was added to make available, as nearly as possible, the total surface of chalcopyrite, yet in 46 much less acid and much more iron were present in solu- 468 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. tion at the end of the experiment. The increase in the amount of iron shows that a greater amount of chalcopyrite reacted in 46 than in 45, and the decrease in the amount of acid shows that less oxidation of the sulphur by the cupric sulphate took place in 46 than in 45. This fact is shown somewhat more clearly by the molecular ratios derived from the above analyses : Exp. Cu H2SO4 Cu Fe' Fe ' H^SCV 45 1.85 O.96 1-93 46 I.29 0-37 3-50 These ratios differ greatly not only from one another but also from the ratios found when chalcopyrite alters into cuprous sul- phide. The ratio Cu/Fe is approaching unity, the ratio H 2 S 0 4 /Fe is approaching o, and the ratio Cu/H 2 S 0 4 is increasing rapidly. The residues obtained in these experiments were examined microscopically. 61 In addition to the unaltered chalcopyrite, the presence of cupric and cuprous sulphides as alteration products on the chalcopyrite was established. All the evidence brought forward leads us to believe that chal- copyrite when exposed at 200° to the action of cupric sulphate will alter to cupric and cuprous sulphides, and that the greater the percentage of cupric sulphide present in the enrichment product the greater will be the amount of iron obtained when a given amount of copper as sulphate reacts with chalcopyrite and the smaller will be the amount of sulphuric acid; also, the greater the surface of chalcopyrite exposed to a given amount of cupric sulphate, the greater is the tendency to form cupric sulphide. 62 B. The Reaction between Chalcopyrite and Cupric Sulphate at 40°. In order to obtain additional evidence along this line, the fol- lowing experiments were carried out in the shaking machine at 61 Examined by Graton and Murdoch. 62 See page 442. The argument used in the case of pyrite applies here also. SECONDARY COPPER SULPHIDE ENRICHMENT. 469 40°, using chalcopyrite sized between 125 and 200 mesh. 63 The object of this sizing will be referred to later. 64 The chalcopyrite known as Japan “y,” analyzed as follows: Per Cent, of Cu. Per Cent, of Fe. Per Cent, of S. Japan “y” 65 34-63 34-63 30.70 30.42 Calculated for CuFeS2 34-94 TABLE XXIII. Chalcopyrite and Cupric Sulphate. Temperature, 40°. Initial Conditions. Analyses of Solutions. Exp. Dura- tion, Months. Weight, g- Copper, g Total Iron, g. Acid, Material. Solution. Initial. Final. Depos- ited. H 2 SO 4 , g- 47 2 12.000 Ugo, Japan, 66 125-200 mesh 200 C.C. l \ % CUS04.5H20 O.6359 O.5740 O.0619 O.0437 0.020 48 2 4-253 Japan “y” 125-200 mesh 400 C.C. Ij% CUS04.5H26 I.2744 1.2538 0.0206 O.OI36 49 2 5.000 Evora, 125-200 mesh 400 C.C. l\% CUS04.5H20 I.2718 1. 2410 0.0308 O.OI92 The chalcopyrite gradually became coated with a thin iridescent film whose color resembled bornite 67 but which the analysis showed to be sulphides of copper. 68 The copper and iron were determined in aliquot portions of the solution, and both are correct within 0.5 milligram of substance. The copper was determined electrolytically as usual ; the iron was determined gravimetrically after removing the copper with hy- drogen sulphide under the proper conditions. The gravimetric determination was checked by dissolving the oxide of iron in potassium bisulphate, reducing and titrating with potassium per- manganate. Sulphuric acid was always present in the solutions, but in such small amounts that its determination in the presence of copper could not be accurately made except in experiment 47, 63 See page 410. 64 See page 471. 65 Analysis by Dr. J. L. Crenshaw. 66 Examined microscopically and found to be very pure. 67 See also Welsh and Stewart, Econ. Geol., 7, 785-7, 1912. 68 See page 439. 4 ;o E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. in which 20 milligrams of H 2 S 0 4 were found; this determina- tion is correct to within 1 milligram. 69 The following molecular ratios were calculated on the basis of these analyses : Exp. Cu Fe' Fe H 2 S 0 4 ‘ Cu h 2 so 4 ' 47 I.24 ± O.05 0-3 4.8 48 1.3 ±0.1 49 1.4 ± 0.06 Analysis of Enrichment Products . — The enrichment products formed on the chalcopyrite in experiment 47 were brought into solution with 1 per cent. KCN. If the operation is done rapidly and the chalcopyrite sized as indicated, the error introduced into the analysis by the solubility of the chalcopyrite in the KCN is small and can easily be corrected for by exposing pure chalco- pyrite to the action of the 1 per cent, solution for the same length of time as that which elapsed between the disappearance of the coating and completion of the filtration; the same amount of sized chalcopyrite must of course be used in the blank as was used in the experiment. The correction amounted in the follow- ing analysis to 0.4 milligram of copper, and the same amount of sulphur, quantities obviously negligible. Analysis of Enrichment Products. 1 Exp. Cu in KCN. g. S in KCN. g. Total, g. Per Cent, of Cu. Per Cent, of CU2S. Per Cent, of CuS by Difference. 47 0.0871 0.0386 O.1258 69.30 21 ± 3 79 (a) Influence of Sulphuric Acid. In the case of pyrite, we found that sulphuric acid retarded the reaction between pyrite and cupric sulphate ; efforts were made to learn if this acid would also retard the reaction between chalco- pyrite and cupric sulphate. This work was carried out at 40° and is tabulated below. 69 The total amount of the solution was used in making this determination. SECONDARY COPPER SULPHIDE ENRICHMENT. 4 71 The amount of copper deposited can be determined with great accuracy, consequently the conclusion is justified that the rela- tively large difference in the amounts shown in the table is due to the retarding influence of the 2 per cent, sulphuric acid. TABLE XXIV. Influence of Sulphuric Acid, Chalcopyrite and Cupric Sulphate. Initial Conditions. Analyses of Solutions. Exp. Dura- tion, I Weight, ** Copper, g. Acid, Initial. Total Months. Material. Solution. Initial. Final. Depos- ited. Iron, g. 47 2 12.000 Ugo, Japan, 125-200 mesh 200 C.C. l\% C11SO4 . 5H2O No acid. O.6359 0.5740 0.0619 None 0.0437 50 2 12.000 Ugo, Japan, 125-200 mesh 200 C.C. i|% C11SO4.5H2O land 2 %H 2 S 0 4 0.6359 0.5915 O.0444 2 % O.O376 ( b ) Influence of Ferrous Sulphate at 40 0 . 70 TABLE XXV. Initial Conditions. Exp. Material. Weight, g- Solution. Duration. 47 Ugo, Japan, 125-200 mesh 12.000 200 C.C. Ij% C11SO4.5H2O. . . . 2 months 5 i n it << “ 200 C.C. Ij% C11SO4.5H2O and 1% FeSC>4.7H20 “ “ The solution in both experiments became cloudy after three days owing to the hydrolysis of the ferric iron formed by the action of cupric sulphate on the ferrous sulphate. This cloudi- ness gradually increased, reached a maximum, and then slowly disappeared. During this time a marked difference in the ap- pearance of the coatings formed on the chalcopyrite was noted. The color of the coatings in both experiments resembled that of a tarnished bornite; previous experiments had shown that this appearance is a kind of composite effect, inasmuch as the films were so very thin that iridescence was noted when the substances 70 See also A. C. Spencer, Econ. Geol., 8, 625 (1913). 472 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. were examined under the microscope. In experiment 51, in which ferrous sulphate was used, the coating at the end of five days became perceptibly darker than the coating formed in 47. The difference became more marked and reached its maximum about the same time that the maximum cloudiness of the solution was noted. When the cloudiness of the solution decreased, the difference in appearance of the coatings decreased, until at the end of the experiment, which extended over a period of two months, no difference in appearance was noted, an observation borne out by the analysis of the solutions. TABLE XXVI. Analyses of the Solutions. Exp. Copper Initial, g. Copper Final. Copper Deposited. 47 O.6359 0.5740 O.0619 51 O.6359 O.5790 O.0569 As a matter of fact, at the end of two months less copper was deposited when ferrous sulphate was used. The coatings were dissolved in the usual manner with a 1 per cent, solution of KCN and analyzed. TABLE XXVII. Analysis of the Enrichment Products. Exp. Cu in KCN, g. Cu KCN, g. Total, g. Per Cent. . of Cu. Per Cent, of Cu 2 S. Per Cent, of CuS by Dif- ference. 47 Si N 1 > r- ^ 00 t'- 0 0 d d 0.0386 O.0324 O.1258 O.IO71 69.30 69.74 21 d= 3 24 ± 3 79 76 The cupric and cuprous sulphides agree within the limits of the experimental error, consequently the ferrous sulphate has ex- erted no appreciable influence in changing the relative amounts of these sulphides. The changes observed in the appearance of the coating of copper sulphides on the chalcopyrite when ferrous sulphate is present as an initial constituent of the solution can be explained on the basis of the reaction represented by equation SECONDARY COPPER SULPHIDE ENRICHMENT. 473 (4). The cloudiness of the solution was caused by the precipita- tion of the hydrolyzed ferric iron. The more extensive altera- tion of the chalcopyrite at first was caused by the reaction be- tween chalcopyrite and cuprous sulphate. The reaction between chalcopyrite and cuprous sulphate continued until the hydrolyzed ferric iron and the resulting sulphuric acid were in equilibrium. We have seen from experiment 47 that in the reaction between chalco- pyrite and cupric sulphate, sulphuric acid is also formed, and this latter acid will disturb the equilibrium just mentioned and, in the course of time, all of the precipitated ferric iron will be redis- solved. While this is going on the ferric iron will carry a corre- sponding amount of copper back into solution. Thus at the end of the experiment, the influence of the ferrous sulphate must be reduced to a negligible quantity. The experiment showed that such was the case. C. Discussion of Experimental Work with Chalcopyrite. When we compare the molecular ratios obtained in our experi- mental work on the reaction between cupric sulphate and chalco- pyrite, we note the following interesting features : TABLE XXVIII. Exp. Temp. Molecular Ratios. Analyses of Enrichment Products. Cu IT H 2 S 04 Cu Per Cent, of Cu 2 S. Per Cent, of Cu 3 . Fe H 2 S 0 4 44 200 2.13 I.60 1-34 99 I 43 * ‘ 2.15 1-57 1.36 93 7 45 * ‘ 1.85 0.96 1-93 49 40 1.4 48 * * 1-3 46 200 1.29 0-37 3-50 47 40 1.24 0.3 __ 4 l 8 21 79 The analysis of the enrichment products formed in 44 and 43 at 200° shows that mostly cuprous sulphide was present, whereas in 49 at 40° mostly cupric sulphide was present. Judging from these results, the formation of the maximum amount of cupric sulphide is accompanied with a minimum of sulphuric acid. The 474 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. limits which the ratios Cu/Fe and H 2 S0 4 /Fe are approaching indicate that no acid whatsoever will be formed when chalcopyrite alters only into cupric sulphide. The simplest equation which expresses these facts is the following : CuFeS 2 + CuS0 4 = 2-CuS + FeS0 4 . (8) The cupric sulphide formed in this equation is attacked by the cupric sulphate still present and altered into cuprous sulphide; this action increasing as the chalcopyrite becomes coated with its alteration products which render it less accessible to the solution. If we assume that all of the iron found at the end of an ex- periment was due to the reaction represented by (8), and that the acid found was due to the reaction between covellite and cupric sulphate which we have shown to take place not only at elevated temperatures but also at ordinary temperatures, 71 namely, 5CuS -f- 3 CuS 0 4 4H 2 0 = 4Cu 2 S -f- 4H 2 S0 4 , ( i ) we should be able to calculate on the basis of these equations the amount of cupric and cuprous sulphide present at the end of an experiment. This was done in the case of experiment 47, page 4 73, and the values found compared with those obtained by anal- ysis of the enrichment product. CuS Cu 2 S By analysis 79 + 3 21+3 Calculated 75 25 The agreement is as close as can be expected in view of the ex- perimental errors and difficulties involved. 9. THE REACTION BETWEEN CUPRIC SULPHATE AND BORNITE (Cu 5 FeS 4 ). Bornite proved to be the most difficult and at the same time 71 See pages 491 and 496. SECONDARY COPPER SULPHIDE ENRICHMENT. 475 the most interesting in its behavior of any natural sulphide which we studied. Bornite was found to react even at ordinary tem- peratures rather easily with sulphuric acid and also with cupric sulphate. It is likewise attacked by dilute solutions of potassium cyanide. The enrichment products formed by replacement of the other natural sulphides studied, could, by observing the proper precautions, ! be removed with potassium cyanide without affecting the unchanged sulphide appreciably and the resulting solution analyzed; but it was found that even a i per cent, solution of KCN will attack bornite to a degree which is sufficient to render this method of analysis useless; thus preventing the use of an extremely helpful analytical method . 72 It was also found diffi- cult to obtain a sufficient amount of pure bornite to carry out our work. The material studied was the purest obtainable. The analyses of the various samples of bornite used in our experi- ments are tabulated below : 73 TABLE XXIX. Analyses of Bornite. Locality. Cu. Fe. •s. Pb. A g. Superior, Ariz Costa Rica 62.99 62.99 II.23 11.20 25.58 25-54 O.IO Unknown, (Z) Calculated for (Cu 5 FeS 4 ) 74 63.19 63.33 II.31 II . 12 25-44 25-55 .02 A. Bornite and Sulphuric Acid. While trying to purify some bornite we found that this sul- phide is rather sensitive to sulphuric acid, especially so at elevated temperatures. It was therefore advisable to obtain an idea as to the products formed. The initial conditions and duration of these experiments are tabulated below : 72 The correction for solubility of the bornite is large and at present can not be applied with certainty. 73 E. T. Allen, Am. I. Sci., XLI., 409-413 (1916). 74 Idem, p. 410. 476 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. TABLE XXX. Initial Conditions and Duration of Experiments. Exp. Weight, g- Temp. Solution. Duration. 52 Bornite from Superior Ariz., small lump and 100 mesh and finer 1.0 200° 12.5 C.C. 2 \% H2SO4. . 2 days 53 Bornite from Costa Rica, lumps and finer 200° 12.5 C.C. 2% H2SO4. . . 8 days 54 Bornite from Costa Rica, 125—200 mesh 76 I. OOO 100° 50 C.C. 2% H2SO4 2 days 55 Bornite from Superior Ariz., 125-200 mesh 76 . . 4.000 O 0 400 C.C. I % H2SO4 2 months Products Formed at 200 ° . — The residue in 52 presented quite a different appearance from the original bornite and on being examined microscopically was shown to be made up as follows : Twinned cubes of cuprous sulphide were found growing on the lump, the fine powder had a grayish blue appearance suggesting the presence of cuprous together with cupric sulphide ; the lump was broken in two, polished, and examined microscopically. This examination revealed the presence of chalcopyrite in thin lines seemingly following fracture and cleavage directions. This bor- nite had been examined in a similar manner before treatment with acid and no chalcopyrite found. In addition to the copper sul- phides and chalcopyrite, the presence of very fine crystals of one or both disulphides of iron was noted. The solution contained ferrous sulphate and hydrogen sulphide. We have here a very complicated reaction, the detailed study of which would lead us beyond the scope of this paper. The formation of the chalco- pyrite within the lump of bornite was studied in a qualitative manner and will be dealt with in a subsequent paper on which we hope to report shortly. The evidence brought out by this study led us to the following explanation for these interesting facts : The acid first attacked the surface of the bornite altering it to cupric and cuprous sulphides ; ferrous sulphate and hydrogen sul- phide being formed at the same time. The interior thus became protected from the direct action of the relatively strong acid and 75 See page 410. SECONDARY COPPER SULPHIDE ENRICHMENT. 4 77 the small amount of acid which did penetrate was greatly weak- ened by reacting with the bornite. Under these conditions we have found that ferrous sulphate, cupric or cuprous sulphide, and hydrogen sulphide will react and form chalcopyrite. The most likely place for the chalcopyrite to develop will be of course along the lines of fracture and cleavage since they afford to the acid the readiest means of ingress. The appearance of the residue in Experiment 53 was much the same as that observed in 52 and the same formation of chalco- pyrite within the lump was noted. In experiment 54, carried out at ioo°, the appearance of the residue was again markedly different from that of the original bornite. The surface of the grains was coated over with what the microscope proved to be covellite and chalcocite, giving the residue a bluish appearance. Ferrous sulphate and hydrogen sulphide were found in solution in approximately the following amounts: Fe = 0.039 g. and H 2 S = 0.023 g. The exact values are of course not significant, but the figures show that bornite is appreciably attacked by sulphuric acid at temperatures where pyrite and chalcopyrite, similarly sized, show no action. 76 In order to compare the action of acid on the sulphides, it is obvious that the sulphides should be carefully sized so as to present com- parable surfaces. 77 At 40°, bornite is also markedly attacked by sulphuric acid; not as easily, to be sure, as at ioo°, yet at the end of 2 months, during which the bornite, sized as indicated, had been exposed to the action of a 1 per cent, solution of sulphuric acid, 0.0236 g. of iron in the ferrous condition was found in solution, a quantity which must have been derived from the alteration of 0.2120 g. of bornite. Hydrogen sulphide was very easily recognized by its 76 See page 465. 77 For work along similar lines,, see J. D. Clark, Bull. Univ. of Mex., 75, 11 6; also referred to in Tolman and Clark, Econ. Geol., IX., 570. No close sizing was attempted in the experiments carried out by Clark, hence it is somewhat difficult to understand how the results obtained in his experiments are comparable with one another. Thus the 200-mesh pyrite may have con- tained more very fine flour than the 200-mesh bornite. 478 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. odor. 78 The residue at 40° had the same bluish appearance noted in the case of bornite treated in the same manner at ioo°. B. Bornite and Cupric Sulphate at 200° C. (a) The Alteration of Bornite to the Sulphides of Copper. In the preliminary experiments carried out in studying this reaction, it was found that so little acid is formed when one gram of bornite is treated with 25 c.c. of a 5 per cent, cupric sulphate solution that the hematite resulting from the reaction between fer- rous sulphate and cupric sulphate persists for a long time — as a matter of fact, in experiment 57, tabulated below, which was car- ried out with the idea of converting the bornite completely into chalcocite, some hematite persisted even at the end of one month’s time. In experiment 55, the surface of the bornite was increased by using a larger amount of bornite ground to pass through a 1 oo- mesh bolting cloth. When this large surface was made as avail- able as possible by mixing the bornite with three times its volume TABLE XXXI. Bornite and Cupric Sulphate. Temperature, 200° . Initial Conditions. Analyses of Solutions. Exp. Duration. Weight, g- Copper, g Acid, Total Iron, g. Material. Solution. Initial. Final. Depos- ited. H2SO4, S ■ 56 i day 5.000 Bornite, Superior, Ariz., 100 mesh and finer. 50 c.c. 5% CuSC>4.- 5H2O O.6363 0.0040 O.6323 O.0963 O.5015 79 57 i month I. OOO Bornite, Superior, Ariz., 100 mesh and finer. 50 c.c. abt. 2% CuS0 4 .- 5H2O 0.2657 0.0135 0.2522 O.3040 O.IO44 80 1 78 Of course, in the presence of cupric sulphate together with the acid, no odor of H 2 S would be noticed. 79 All ferrous. 80 0.0122 as hematite. SECONDARY COPPER SULPHIDE ENRICHMENT. 479 of pure quartz, no hematite was formed, and the copper in solu- tion was used up very quickly. In experiment 57 hematite persisted even after the lapse of one month. Its presence indicates the hydrolysis of some ferric salt the formation of which has been several times pointed out in similar cases. The molecular ratios obtained in experiment 57 are affected, though only to a slight degree, by this secondary action. Molecular Ratios Based on Analyses of the Solutions. Exp. Cu H2SO4 Cu Fe‘ Fe ‘ h 2 so 4 ‘ 56 I. II O.II 10. 1 57 2.12 HI 1.3 These ratios bring out some rather striking differences in the two experiments and will be discussed together with the microscopic evidence derived from the examination of the residues. Microscopic Examination . — These residues were compressed as usual, polished, and examined. The color of each was com- pared with that of a pure specimen of chalcocite. In experiment 56, both cupric and cuprous sulphide were observed as distinct constituents. In addition, the color of the cuprous sulphide was darker than that of the pure mineral indicating the presence of cupric sulphide in solid solution. 81 This residue was also ex- amined after imbedding a few of the grains in sealing wax, and polishing deeply into the grains. 82 In addition to the sulphides of copper, small blades of chalcopyrite were found, seemingly following the cleavage cracks in the original bornite. This bor- nite had been examined before the experiment, and no chalco- pyrite found. It will be remembered that sulphuric acid, also, caused an indirect formation of chalcopyrite within the bornite, 83 and since we know that acid was present at the end of the experi- ment, the presence of this chalcopyrite is accounted for. The 81 Posnjak, Allen and Merwin, Econ. Geol., X., 506 (1915). 82 This examination was made by Professor Graton. 83 See page 477. 480 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. amount of the chalcopyrite, however, could not have been more than a fraction of a per cent. In experiment 5 7, the color of the residue matched that of the standard chalcocite very closely, and it was therefore concluded that the bornite in this case had been altered to chalcocite. ( b ) Discussion of Results. The results of the microscopic examination obtained in experi- ment 57, taken together with the molecular ratios, indicate that the reaction progresses according to the following equation : 5Cu 5 FeS 4 -f - 1 iCuS 0 4 + 8 H 2 0 = i8Cu 2 S + 5 FeS 0 4 + 8 H 2 S 0 4 . (9) The molecular ratios demanded by this equation and those ob- tained by experiment are tabulated below : Cu Deposited h 2 so 4 Cu Deposited ' Fe ' Fe H 2 S 0 4 By equation (9) 2.2 I.60 1-375 By experiment 57 2.12 1-7 1-3 The agreement is as close as can be expected, considering the experimental difficulties involved. In experiment 56, both cupric and cuprous sulphide were found in rather large amounts together with a little chalcopyrite. As well as could be determined all of the bornite had altered to these substances. No hematite was observed, and the amounts of copper, iron, and acid were such that they could be determined with great accuracy. There is an error introduced by the forma- tion of a little chalcopyrite, but its magnitude must be very small since the amount of this substance was very slight. Even though the acid does attack the bornite, forming, as we have seen, ferrous sulphate and hydrogen sulphide, yet in the presence of cupric sul- phate all of the acid used up in this manner is regenerated owing to the action of the hydrogen sulphide on the cupric sulphate. SECONDARY COPPER SULPHIDE ENRICHMENT. 481 We infer, therefore, that the ratios calculated on the basis of the analysis of the solution are accurate. The molecular ratios obtained in these experiments show that for a given amount of copper deposited, less acid was formed in experiment 56 than in 57,; the microscopic examination showed in 57 that the bomite had altered into chalcocite, but that in 56 both cupric and cuprous sulphide were present. It is evident then that when cupric sulphide is present less acid is formed. The molecular ratios differ so greatly in the two experiments that we are led to the conclusion that if the surface of the bornite were large enough to react at once with all of the copper present in solution, no acid whatsoever would have been formed and that the acid which was formed was derived from the reaction be- tween cupric sulphide and cupric sulphate. If we assume that the acid was derived as indicated, then, knowing the amount of acid formed, we should be able to calculate on the basis of equa- tion ( 1 ) the amount of copper which reacted with cupric sulphide to form this acid. Acid to the amount of 0.0963 g. was found in experiment 56. The formation of this amount must have been accompanied by the deposition of 0.0468 g. of copper which re- acted with the cupric sulphide to form cuprous sulphide. De- ducting this from the total copper deposited, we obtain the amount of copper which reacted directly with the bornite, namely, 0.5855 g. The 0.5015 g. of iron found in solution was formed during this reaction. The molecular ratio Cu/Fe calculated from these data is 1.03 g. This ratio agrees well with that de- manded by equation (10), namely, unity. Cu 5 FeS 4 + CuS 0 4 = 2Cu 2 S + 2CuS + FeS 0 4 . ( 10) Furthermore, 1 g. of the residue obtained in experiment 56 was analyzed and yielded' the following results, corrected for the silica present in the residue : Cu — 0.7414, Fe= : o.oioi, S = undetermined. 482 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. The iron found in the residue is attributed in part to the chalco- pyrite, whose presence in the residue was revealed by the micro- scopic examination, and in part to unattacked bornite. Assum- ing that all the iron was derived from chalcopyrite (CuFeS 2 ), we find by calculation that this amount of iron is equivalent to 0.0332 g. CuFeS 2 . This amount of CuFeS 2 contains 0.0115 g. Cu. Deducting the weight of the CuFeS 2 from 1.0000 g., the total weight of the residue, and deducting the amount of Cu in the CuFeS 2 from the total Cu found by analysis, we obtain by calculation 75.5 per cent. Cu. This is the percentage of Cu pres- ent in the residue as sulphides of copper. We find, by using this percentage as the basis of our calculation, that the residue con- tained 60 per cent, of Cu 2 S and 40 per cent, of CuS. Knowing the amount of acid formed in experiment 56 and the total amount of copper which reacted, we can calculate on the basis of equa- tions (10) and (1), assuming for the present the correctness of (10), the amount of Cu 2 S and CuS which should be present in the residue. Thus we obtain 65 per cent. Cu 2 S and 35 per cent. CuS. In view of the uncertainty involved in making the proper correction for the iron found by actual analysis of the residue, and in view of the experimental errors involved, the agreement between the amounts of Cu 2 S and CuS just calculated with those calculated from the analysis is as good as can be expected. Thus the analysis of the residue furnishes additional evidence in favor of equation (10) as representing one of the stages of the reaction when bornite reacts with cupric sulphate. It would have been desirable to carry out other similar experi- ments with still larger quantities of finely ground bornite, but we were prevented from doing so by the scarcity of the pure mineral. C. Bornite and Cupric Sulphate at ioo°. The one experiment carried out at this temperature is tabulated below ; the bornite was mixed, as usual, with three times its vol- ume of pure quartz. SECONDARY COPPER SULPHIDE ENRICHMENT. 4^3 TABLE XXXII. Bornite and Cupric Sulphate. Initial Conditions: 1.000 g. bornite from Superior, Ariz. (ioo mesh and finer) ; solution, 50 c.c. 1 per cent. CuS 0 4 - 5 H 2 0 ; temperature, ioo 0 . Analysis of Solution. Exp. Duration, Months. Copper, g. Total Iron ; Acid, Initial. Final. Deposited. ( Ferrous), g. H 2 SO 4 , g. 58 I O.I265 None 0.1265 O.O974 84 O.0357 Molecular ratios based on this analysis : Cu H 2 S 0 4 Cu Fe - I - I4: Fe ' ~ 0-21 ’ H 2 S 0 4 “ 5 ' 47 ' The residue obtained in this experiment was compressed and ex- amined microscopically. The presence of both cupric and cu- prous sulphides was noted. When we compare the ratios obtained in this experiment with those obtained when bornite alters to cuprous sulphide only, we see that here again for a given amount of copper deposited as sulphide on the bornite less acid is formed when both cupric and cuprous sulphides are present than when cuprous sulphide alone is present : J Cu Deposited H2SO4 Cu Deposited Fe Fe H2SO4 Exp. 57 at 200 (all chalcocite) 2.12 1-7 1-3 Exp. 58 at 100 1. 14 0.21 5-47 The ratios obtained in the experiment at ioo° also approach the values demanded by equation (xo), but not quite so closely as in the case of 56 at 200°. If we deduct the small amount of copper involved in the formation of the acid, namely, through equation (1), and then calculate the molecular ratio of the re- maining copper to iron in the same way as before, the ratio be- comes 1 : 1 as demanded by equation ( 10). 84 A very little hematite remained attached to the wall of the tube and amounted to about 1 milligram. 484 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. D. Bornite and Cupric Sulphate at Ordinary Temperatures. These experiments were carried out in the manner previously described. 85 Inasmuch as the bottles were shaken throughout the experiment, no quartz was required to facilitate the circulation of liquid through the mass of the mineral — an absolute necessity when the experiments were done in sealed tubes. TABLE XXXIII. Bornite and Cupric Sulphate. Initial Conditions. Exp. Material. Weight, g- Temp. Solution. 59 Bornite from Superior, Ariz., 100 mesh and finer 3.000 30 ±5 200 C.C. l\% CUSO4.5H2O 60 Bornite from Superior, Ariz., 125-200 mesh 6.000 30 ± 5 400 C.C. Ij% CUSO4.5H2O TABLE XXXIV. Analyses of the Solutions. Exp. Copper, G. Total Iron. Acid. Duration. Initial. Final. Deposited. 59 60 0.6407 I.2718 0.5405 I.2495 0.1002 0.0223 0.0875 O.OI97 Trace 86 “ 88 1 month 2 months The copper and the iron were determined in the total filtrate from the bornite in the same manner as indicated in the experi- ments with chalcopyrite. It will be noted that more copper was deposited in 59 than in 60 even though the weight of bornite was greater in 60 than 59. Thus in 59, 3 grams of bornite were ground to pass through a 100-mesh bolting cloth; of this amount, 48 per cent, passed through a 200-mesh sieve. Thus all of the fine flour was included. In 60, the 6 grams of bornite were care- fully sized between 125 and 200 mesh and the adhering fine flour removed by elutriation. 87 It is therefore believed that the dif- 85 See page 415. 86 Very faintly acid to methyl orange. The color of the solution makes it impossible to determine very small amounts of acid. 87 See page 410. SECONDARY COPPER SULPHIDE ENRICHMENT . 4^5 ference in the amounts of copper deposited in the two experi- ments is due in part at least to the difference in the amount of surface of bornite exposed to the action of cupric sulphate. The molecular ratio Cu/Fe was determined for each experi- ment: Cu/Fe in experiment 59=1.00 and in 60=1.0. The residue in 59 was examined microscopically and the presence of cupric and cuprous sulphide determined. 88 ’ 89 This evidence taken together with the evidence of the ratios at temperatures ranging from 30° to 200°, but especially at 30° again point to the probability of the reaction represented by fhe following equation : Cu 5 FeS 4 -f CuS 0 4 = 2Cu 2 S + 2CuS -f- FeS 0 4 (10) , . , , . Cu deposited in which the ratio _ = 1. Fe (a) The Influence of Sulphuric Acid. The following experiments were carried out at the same time and under the same conditions in order to determine the influence of sulphuric acid on the reaction between bornite and sulphuric acid. TABLE XXXV. Bornite and Cupric Sulphate. Influence of Sulphuric Acid. Initial Conditions: 4.000 g. bornite No. 2 from Superior, Ariz. 90 (125-200 mesh) ; temperature, 30 + 5 0 . Exp. Dura- tion, Months. Initial Concentration of So ution. Analyses of Sol Copper, g. utions. Acid , Initial. Total Iron, g ■ Initial. Final. Depos- ited. 6 l 2 400 C.C. 1% CUSO4.5H2O 1.0227 1.0182 O.OO45 None O.OO43 62 2 400 c.c. 1% CUSO4.5H2O and 1% H2SO4 1.0227 1.0078 O.OI49 1% O.OI7I 63 2 400 C.C. I % H2SO4 None None None 1% O.O236 and h 2 s 88 Examined by Professor Graton. 89 This was not done in the case of the second experiment as not enough bornite had altered to make this examination worth while. 90 This bornite contained about 8 per cent. CtuS and was the purest available when these experiments were carried out. 486 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. We do not wish to lay any stress on the absolute values shown above, but rather on the relative values; they vary sufficiently to make a comparison worth while. It is evident from the amount of copper deposited that the acid exerted an accelerating influ- ence. We see from the amount of iron in solution that more bornite was altered when I per cent, acid alone was used than when i per cent, acid together with cupric sulphate was used. This is presumably due to the protective action exerted by the firmly adhering alteration products formed on the bornite. ,IO. THE REACTION BETWEEN SPHALERITE (ZnS) AND CUPRIC SULPHATE. A. The Reaction at 200° . The sphalerite used in our experimental work was carefully selected from material obtained from two sources, namely, from Sonora, Mexico, and from Joplin, Missouri. These samples of sphalerite were analyzed for impurities. Zn. Fe. S. Summation. Sonora, Mexico 66.98 O.I 5 •23 32.78 99.91 Joplin, Missouri Both the microscopic examination and the chemical analysis showed that both samples of sphalerite were very pure. Experi- ments were carried out at 200° and at ordinary temperatures. TABLE XXXVI. Sphalerite and Cupric Sulphate. Initial Conditions: Material, sphalerite from Sonora, Mexico (100 mesh and finer) ; solution, 20 c.c. 2 per cent. CuS 0 4 - 5 H 2 0 ; temperature, 200°. Exp. Duration, Days. Weight, g ■ Analyses of Solutions. Copper, g. Zinc in Solution, g • Acid, H2SO4, g. Initial. Final. Deposited. 64 65 2 2 2.000 1.300 0.1030 0.1032 None None 0.1030 O.IO32 O.089O O.0858 0.0361 Not det. SECONDARY COPPER SULPHIDE ENRICHMENT. 4% 7 It is evident from these analyses that copper sulphate and sphalerite react, copper being deposited and zinc dissolved, while sulphuric acid is also formed. Microscopic Examination . — On examining the solid products microscopically, it was found that copper had been deposited as sulphides partly replacing the sphalerite. In all probability both cupric and cuprous sulphides were present, as indicated by the color of the grains of sphalerite, which were coated with firmly adhering blue and bluish gray films. Furthermore, it was found that some of the grains were covered with films so thin as to transmit the green light characteristic of covellite. The precipitated copper sulphides were then analyzed chem- ically in the usual manner. If a i per cent, solution of potas- sium cyanide is employed to dissolve the copper sulphides, and the solution and washing of the residues are rapidly done, the solvent action of the reagent on the sphalerite is negligible. Sphalerite, sized between 125 and 200 mesh, thus behaves quite differently from precipitated zinc sulphide, for the latter is readily attacked by even a 1 per cent, solution of potassium cya- nide. The following analyses of the enrichment products replac- ing the sphalerite were carried out in the manner just indicated: TABLE XXXVII. Sphalerite and Cupric Sulphate. Analyses of the Alteration Products. Exp. Cu in KCN, g. S in KCN, g. Total, g. Per Cent, of Cu. Per Cent, of CU2S. Per Cent, of CuS. 64 0.0663 0.0253 0.0916 73.38 44 ± 3 56 65 0.0675 0.0246 O.0931 73-29 51 ± 3 49 These analyses confirm the observation made with the micro- scope, namely, that cupric and cuprous sulphide are present in the enrichment products. Furthermore, the analysis of the solu- tion in experiment 64 showed the presence of sulphuric acid, which was no doubt derived from the conversion of some of the cupric sulphide into cuprous sulphide, a reaction which is com- paratively rapid at 200°. On this assumption, we may, as usual, 488 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. calculate the amount of cupric and cuprous sulphide which should be present from the amount of copper deposited and the amount of acid formed. In experiment 64, 0.0361 gr. of acid was found. This, on the basis of equation (1), is equivalent to 0.0586 grams of Cu 2 S, which in turn contains 0.0468 grams of copper. This amount of copper, when deducted from the total copper deposited, namely, 0.1030 grams, gives the amount of copper present as cupric sulphide, or 0.0562. This amount of copper is contained in 0.0846 grams of CuS. Such a mixture contains 41 per cent. Cu 2 S and 59 per cent. CuS, while we found by analysis 44 per cent, of Cu 2 S and 56 per cent, of CuS. On the basis of equation ( 1 ), we can also determine the amount of copper which reacted with the cupric sulphide to form cuprous sulphide, knowing the amount of sulphuric acid formed. Mak- ing this calculation for experiment 64, we find that 0.0176 grams of copper reacted in this manner. This amount deducted from the total copper deposited gives the amount of copper which reacted with the sphalerite to form cupric sulphide. We find this amount to be 0.0854 grams. When we calculate the molecular ratio of this copper to that of the zinc found in solution, we obtain Cu/Zn = o.99. This value points to the following equa- tion as representing the first reaction which taken place between sphalerite and cupric sulphate: ZnS + CuS 0 4 = CuS + ZnS 0 4 . (11) The question will no doubt be asked how can the acid formed in the reaction persist in the presence of sphalerite. The solution in experiment 64 was carefully examined for hydrogen sulphide, according to the method previously described, 91 but none was found. So long as copper is present in solution, any hydrogen sulphide that may be formed by the action of the acid on the sphalerite is used up in precipitating sulphide of copper and at the same time setting free the same amount of acid as was used up in attacking the sphalerite. Therefore, so long as copper is present in solution, so long will the amount of acid formed accord- ing to experiment 64 persist. Later on when all of the copper 91 See page 445. SECONDARY COPPER SULPHIDE ENRICHMENT. 4% 9 has been deposited, as was the case in 64, the coating of sulphides of copper formed on the sphalerite protects the latter, and if the tube is removed from the furnace as soon as all of the copper is deposited, little if any of the acid is used up. In the second experiment on cupric sulphate and sphalerite, acid was likewise formed, though it was not determined. Never- theless, here also we may calculate the amounts of cuprous and cupric sulphides formed, using as data the quantity of zinc dis- solved and the quantity of copper precipitated, if we assume equa- tions (1) and (11), which we are now perfectly justified in doing. In this way, we obtain 46 per cent, of Cu 2 S and 54 per cent of CuS ; the agreement with the amounts found by analysis of the coating on the sphalerite is again as close as the experi- mental error involved will permit, especially in view of the fact that a very faint test for hydrogen sulphide was obtained in this experiment, showing that the acid which formed during the ex- periment attacked the sphalerite after all copper had been de- posited. This will make the amount of cupric sulphide calculated on the basis of the zinc in solution higher than when no hydrogen sulphide is present finally. Comparing the calculated amount of cupric sulphide with that found by analysis, we note that such is the case. B. Sphalerite and Cupric Sulphate at Ordinary Temperatures. The experiments at ordinary temperatures were carried out under the conditions previously described. 92 TABLE XXXVIII. Sphalerite and Cupric Sulphate. V Initial Conditions. A nalyses of Solutions. d J-S « a g Weight, S ■ Copper, g. Zinc, g- W - 0 a a V H Material. Solution. Initial. Final. Depos- ited. 66 2 30 ± 5 4.881 Sonora, Mexico, 125-200 mesh 400 C.C. l \% C11SO4.- 5H2O I.2718 1. 2112 O.0618 0.0515 67 2 40 ± 1 12.000 Joplin, Mo., 125-200 mesh 200 C.C. l\% C11SO4 .- 5H2O 0.6359 O.54OI 0.0958 O.0855 92 See page 415. 490 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. Sulphuric acid was present in both solutions, but the amount was too small to determine with any accuracy . 93 The residues were examined microscopically but the films of the enrichment products which partly replaced the sphalerite were too thin for identification. These films were removed with a I per cent, solu- tion of potassium cyanide and analyzed in the usual manner. TABLE XXXIX. Sphalerite and Cupric Sulphate. Analyses of the Enrichment Products. Exp. Copper in KCN, g. S in KCN. g. Total, g. Per Cent, of Cu. Per Cent, of Cu 2 S. Per Cent, of CuS. 66 O.0616 0.0228 0.0844 73-0 49 ± 5 51 67 0.0572 0.0234 0.0806 71.0 34 ± 5 66 The experimental evidence at 200° showed that sphalerite and cupric sulphate reacted to form first cupric sulphide, and that this sulphide in turn reacted with the cupric sulphate according to equation (i) to form cuprous sulphide. The presence of sul- phuric acid in the solution and the presence of cuprous sulphide in the enrichment product on the sphalerite indicate that the same reactions take place at the lower temperatures. If this is the case, we should be able to calculate, on the basis of equations (n) and (i), the amount of cuprous and cupric sulphides present at the end of the experiment, if we know the amount of copper lost by the solution during the experiment and the amount of zinc found in solution. The calculation was made, and the results are tabulated below: Exp. Calculated. Analyzed. Per Cent, of Cu 2 S. Per Cent, of CuS. Per* Cent, of Cu 2 S. Per Cent. 1 of CuS. 66 46 54 49 ± 5 51 67 31 69 34 ± 5 66 The agreements are within the limits of experimental error. (a) The Influence of Sulphuric Acid. The sphalerite used in these comparison experiments was carefully sized. 93 See page 437. SECONDARY COPPER SULPHIDE ENRICHMENT. 49 1 TABLE XL. Initial Conditions: 12.000 g. sphalerite from Joplin, Mo. (125-200 mesh); temperature, 40° ; duration of experiments, two months. Analyses of the Solutions. Exp. Solution. Copper, g. Zinc in Solu- tion. Acid, g. Initial. Initial. Final. Depos- ited. 67 200 C.C. l\% CUSO4.5H2O O.6359 O.5401 0.0958 O.0855 None 68 200 c.c. i|% CUSO4.5H2O and 2 % sulphuric acid O.6359 O.5282 O.IO77 0.1033 2% 69 200 c.c. 2 % sulphuric acid None None None 0.0532 2% The enrichment products partly replacing the sphalerite were removed with a 1 per cent, solution of potassium cyanide and analyzed : TABLE XLI. Analyses of the Enrichment Products. Cu in KCN, S in KCN, Total, Per Cent. Per Cent. Per Cent. Exp. g. g- g- of Cu. of CU2S. of CuS. 67 (no acid) 0.0572 O.0234 O.0806 71.0 34 66 68 (acid) 0.0695 O.0324 0.I0I9 68.2 13 87 These analyses show that when sulphuric acid is present as an initial constituent of the solution, more cupric sulphide is formed than when no acid is present initially. The analysis of the solu- tion in experiment 69 showed that the acid had attacked the sphalerite, carrying zinc into solution and liberating hydrogen sulphide. No doubt this reaction also takes place in experiment 68, but the hydrogen sulphide is used up in precipitating cupric sulphide. 94 Thus the acid should accelerate the reaction repre- sented by the equation ZnS + CuS 0 4 = CuS -f- ZnS 0 4 . Inasmuch as the reaction represented by equation ( 1 1 ) is ac- celerated by sulphuric acid, we should find more zinc in solution in experiment 68 than in experiment 67. An examination of the analyses of the solutions will show that such was the case, but it is rather interesting to note that the difference between the amounts of zinc thus found is not as great as the amount of zinc found when only acid was used. This is probably due to the fact 94 See Posnjak, Allen and Merwin, this journal, X., 528, 1915. 492 E. G. Z1ES, E. T. ALLEN AND H. E. MERWIN. that the sphalerite on becoming coated with the enrichment prod- ucts is largely protected from attack. If the enrichment prod- ucts when formed had remained distinct from the sphalerite, and thus permitted a fresh surface of the sphalerite to be exposed to the action of the solution, the difference between the amounts of zinc found in experiments 67 and 68 should be at least as large as the amount of zinc found when only acid is used. The en- richment products, however, adhere firmly, thus exhibiting the characteristic feature of natural enrichment products. Discussion . — The evidence brought out by our experimental work on the reaction between sphalerite and cupric sulphate shows that sphalerite at all temperatures first reacts with cupric sulphate to form cupric sulphide, and that this in turn is further attacked by the cupric sulphate, yielding cuprous sulphide and sulphuric acid. Finally, sulphuric acid accelerates the reaction in which cupric sulphide is formed. II. THE REACTION BETWEEN GALENA (PbS) AND CUPRIC SUL- PHATE AT ORDINARY TEMPERATURES. The preliminary experiments carried out on this sulphide showed that accurate deductions could be made from work done at ordinary temperatures, inasmuch as galena exhibited a sur- prising activity towards cupric sulphate solutions under these conditions. As a matter of fact the work at elevated tempera- tures in the bombs was not as satisfactory as that done at the ordinary temperatures where the shaking machine could be used ; for the insoluble lead sulphate which was formed clogged up the voids between the grains of the galena and practically stopped the reaction. This difficulty was avoided at ordinary tempera- tures where the contents of the flasks were agitated. The galena used in the experiments came from the Mississippi valley region in Wisconsin. It was carefully selected from several pounds of material and analyzed as follows : Material. Pb. S. Cu. Fe. Zn. SiO*. i PbS 0 4 . Mn. Galena from Miss. Valley 86.53 1 86.58 I 3 - 3 I 13.41 Trace 95 Trace 95 Trace Trace 95 | 00.06 None Calculated for PbS . . . 95 Less than 0.01 per cent. SECONDARY COPPER SULPHIDE 'ENRICHMENT. 493 The analysis shows that no manganese was contained in the galena, hence the surprising activity mentioned above can not be due to the presence of alabandite (MnS). 96 In the first experiment tabulated below, the galena was ground to pass through a ioo-mesh bolting cloth; no closer sizing was resorted to. In the second experiment, the material was sized between 125 and 200 mesh. In these experiments the copper was deposited on the galena as a firmly adhering, bluish coating of copper sulphide. TABLE XLII. Galena and Cupric Sulphate. Initial Conditions. Weight, g- Solution. | Exp. Material. Quantity. Copper, Initial, g. Temp. 70 Galena, Miss. Valley, 100 mesh and finer. 3.000 ij% CUSO4.5H2O in 200 C.C. O.639O 35 ± 5 71 Galena, Miss. Valley, 125-200 mesh. 9.048 i\% CUSO4.5H2O in 400 C.C. I.2718 35 ± 5 The analyses of the resulting solutions and of the lead sulphate are shown below : TABLE XLIII. Analyses of Solutions and of Lead Sulphate. Exp. Duration, * Months. Copper, g. Acid, H 2 S0 4 , g- PbS0 4 . Equiv. Pb. Initial. Final. Deposited. 70 5 97 0.6390 O.2540 0.3850 0.065 1.6936 1. 1570 71 2 I.2718 1.1652 O.IO66 0.020 — — The surface of galena exposed in 70 was determined approxi- mately and found to be about the same as that exposed in 71. 98 This comparison is, however, only approximate. The difference 06 See G. S. Nishihara, Econ. Geol., IX., 743, 1914. 97 Remained in shaking machine for a period of one month, but contents of bottle were not analyzed until four months later. 98 Determined on the basis of microscopic examination. 494 E. G. ZIES, Et T. ALLEN AND H. E. MERWIN. in amount of copper deposited in the two experiments is prob- ably largely due to the difference in duration of the experiments. Methods of Analysis . — The results shown in Table XLIII were secured as follows : After filtering off the solution and drying the residue in a vacuum desiccator over sulphuric acid, the copper in solution was determined as usual by electrolysis after deter- mining the sulphuric acid by titration ; when such small quantities of acid as represented above are to be determined by titration in the presence of copper sulphate, the difficulty involved in obtain- ing a sharp end point renders an exact determination impossible ; the figures are therefore only approximate. The lead sulphate formed during the experiments was separated from the galena and its adherent coating of copper sulphide by taking advantage of the solubility of lead sulphate in ammonium acetate; galena is insoluble in this reagent; the lead sulphate is reprecipitated with dilute sulphuric acid and determined in the usual manner. The amount of lead sulphate which remains in solution with the cupric sulphate is negligible." The firmly adhering sulphides of copper were removed from the unchanged galena by a 2 per cent, solution of KCN and analyzed as usual. A blank test proved that galena is unaffected by this solution. It is impossible to determine both lead sulphate and the cop- per sulphides in the same sample, for the ammonium acetate used to dissolve the former attacks the sulphides of copper; and fur- thermore the potassium cyanide which is used to dissolve the copper sulphides changes some of the lead into sulphide. Each of the substances may however be determined separately, for pure lead sulphate may be precipitated from the ammonium acetate solution, and though a part of the sulphur to be determined in the potassium cyanide solution is precipitated as lead sulphide, an equivalent of sulphate passes into solution at the same time thus: K 2 S + PbS0 4 = PbS + K 2 S0 4 . Besides, lead sulphate itself is not appreciably soluble in potassium cyanide. The deposit on the galena in the second experiment therefore was analyzed as usual. r '° That is, as compared with the total lead sulphate deposited. SECONDARY COPPER SULPHIDE ENRICHMENT . 495 Analysis of Enrichment Products. Exp. Cu in KCN, g. S in KCN, g. Total, g. Per Cent, of Cu. Per Cent of CU2S. Per Cent, of CuS. 71 O.O428 O.O197 0.0625 68.5 15 ± 5 85 It is evident that the reaction between galena and cupric sul- phate can not be expressed by the following simple equation alone : PbS + CuS0 4 = CuS + PbS0 4 . ( 12) The presence of the cuprous sulphide in the enrichment products is further shown by the presence of sulphuric acid in the solu- tion, this product being readily accounted for by the further change of covellite with copper sulphate (see equation (i)). If this view is correct we should be able to calculate the quantities of cupric and cuprous sulphide from the quantity of acid found in solution and the total copper precipitated. The calculation is not very exact because the acid can be determined only approxi- mately. The determination tends to be too high on account of the disturbing influence of the cupric sulphate on the end point. The calculation gives 79 per cent, of cupric sulphide and 21 per cent, of cuprous sulphide which agree with the direct determina- tions as closely as can be expected. In experiment 70, on page 494, the presence of sulphuric acid and the molecular ratio of copper lost by the solution to the lead deposited, again indicate the presence of both cupric and cuprous sulphide : Exp. I Copper Lost by Solution, g. Lead Deposited, g. T? • CU Rauo Pb' 70 0.3850 I.I 57 I.O84 Here the amounts of copper and lead are large and can be deter- mined very accurately. If all the copper were present as cupric sulphide, the above ratio should be unity as demanded by equa- tion (12). 49 6 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. Assuming that the acid formed during the experiment was due to the formation of cuprous sulphide according to equation (i), and knowing the amount of the acid, we can determine the amount of copper which reacted with the cupric sulphide to form cuprous sulphide. Making this calculation we find that 0.0316 g. Cu reacted in this manner. We have just seen that 0.3850 g- of copper took part in the total reaction. Deducting the former from the latter we have 0.3534 g. of copper which took part in the direct reaction between cupric sulphate and galena. The molecular ratio of this copper to the lead deposited as lead sulphate is Cu/Bb=i.oo. This is the ratio demanded by equation (1 2). In consequence of all this evidence on the action of cupric sul- phate on galena, we feel confident in stating that when these sub- stances react, cupric sulphide is first formed and that this cupric sulphide, even at ordinary temperatures, is further attacked by the cupric sulphate, yielding cuprous sulphide. V. RELATIVE REACTIVITIES OF THE SULPHIDES TOWARDS CUPRIC SULPHATE, AT 40°. It is obviously very desirable to know the relative rate of copper enrichment and also the relative amounts of mineral changed when the sulphides react with cupric sulphate. After carrying out several preliminary experiments, we became con- vinced, however, that it is impossible to obtain accurate data on this problem. First: In making such a comparison it is, of course, necessary to compare equal surfaces of the sulphides. It is impossible to do this since, owing to the presence of fissures and cleavage cracks, the apparent surface is not the true surface exposed to the action of the solution, and each of the minerals is likely to differ in amount of Assuring and cleavage. Second: We have shown in the experimental work just dis- cussed that the copper sulphide enrichment products envelop and adhere very firmly to the grain of the sulphide which precipitated them. It is evident, therefore, that we can not determine the rate SECONDARY COPPER SULPHIDE ENRICHMENT. 497 of the reaction since the coating will prevent ready access of the solution and retard the reaction more and more as its thickness increases. Third: Then, too, the determination of the rates of enrichment is further complicated by the fact that at ordinary temperatures, within the limit of the time which is feasible for carrying out these experiments, the sulphides do not alter to an enrichment product common to all of them. Thus when chalcopyrite, bor- nite, sphalerite and galena are enriched, both covellite and chal- cocite are found in the enrichment product, and the relative amounts of the two copper sulphides differ for each sulphide used. Pyrrhotite alters first to chalcopyrite but the enrichment products of chalcopyrite are also present. Fourth: The determination of the rate of alteration of the sulphides to chalcocite is without doubt also very desirable and would form an excellent basis for comparing the relative reac- tivities of the sulphides. In order to approach natural enrich- ment conditions, experiments directed along this line must be carried out at ordinary temperatures but at such temperatures the alteration of the sulphides to chalcocite is exceedingly slow and at present beyond the possibilities of laboratory study. At elevated temperatures, 200° for instance, the experimental work has shown that alteration to chalcocite takes place far more readily but we can by no means be certain that relative rates determined at such a temperature are applicable at lower temperatures. This being the case, a comparison of the rates of enrichment must be confined at present to a comparison of the amounts of copper (as sulphide) precipitated by the sulphides and the extent of the alteration of these sulphides. All of the foregoing statements are of course only applicable when cupric sulphate is the enriching agent. Experiments car- ried out by Winchell and Spencer on pyrite and our own experi- ments on covellite strongly indicate that enrichment will proceed faster when cuprous sulphate is the enriching agent . 100 It must be remembered, however, that these same principles 100 See page 428 . 498 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. are applicable to natural conditions. The relative values shown in the table below furnis'h at least an indication of what to expect as to the rapidity with which the various sulphides are altered and enriched in nature. Experimental Work . — In order to obtain approximately equal surfaces, first, the sulphides were carefully sized between 125 and 200 mesh, 101 and second, equal volumes, based on the densities of the sulphides, were used. The sulphides were exposed, at 40°, to the action of a 1% per cent, solution of CuS 0 4 . 5 H 2 0 , for a period of two months. The contents of the flask were gently agitated 102 throughout this period. TABLE XLIV. I. II. III. IV. V. VI. ‘ VII. VIII. IX. H3 . II s 73 c3 § Wt. of Sulphide Use Based on Equal Vol umes (g.). Copper Deposited (&■). Metal Derived from Sulphide (g.)- 104 Wt. of Mineral Altered (g.)jCalcu- lated from IV. Mols. ot Mineral Altered Based on IV. Relative Mols of Mineral Altered CusFeSi = 1 . Relative Wt. of Min- eral Altered, CuFeS 2 = 1 Based on V. oS "2 0 5 11 >73 tn ‘•2 h 3 Galena, Miss. Val., Wise. . . Pyrrhotite, 9.048 0.1070 0.3258 Pb 0.3762 PbS 0.00157 4-5 5-5 3-0 Copper Mt., A In qItq 5-593 O.0896 106 1 Sphalerite, Sonora, Mex. Pyrrhotite, 4.881 O.0616 0.0528 Zn 0.0787 ZnS 0.00081 2.3 1.1 1. 1 Orange Co., V^r 5-593 O.0564 106 Chalcopyrite, Evora, Portugal .... 5.000 0.0308 0.0210 Fe 0.0691 CuFeS2 O.OOO38 1. 1 1.0 1.0 Bornite, Superior, Ariz 5-952 0.0223 ! 0.0197 Fe 0.1771 Cu5FeS4 O.OOO35 1 2-5 2.1 101 See page 410. Also, up to the present we have not been able to obtain bolting cloth which would permit even approximately accurate sizing of finer material. 102 Violent agitation, of course, would have broken up the grains by attri- tion, thus materially changing the surface. 103 The analyses of these sulphides are given under the description of the previous experimental work on the reactions between them and copper sulphate. 104 This column shows the amount of lead deposited as sulphate and also SECONDARY COPPER SULPHIDE ENRICHMENT. 499 The action at 40 ° of cupric sulphate on both Elba pyrite and Butte covellite, sized in the manner indicated, is so slight, and the experimental errors are so large that the values obtained were not included in the table. When a method is developed for accu- rately sizing material ground finer than that used in the experi- ments, it will be possible to assign values to these substances. The table brings out the following interesting features : Galena is very much more reactive towards cupric sulphate than any of the other sulphides considered in this paper, no matter on what basis the comparison is made; the pyrrhotites differ mark- edly in their precipitating power; bornite follows chalcopyrite in precipitating power, but follows galena in volume of mineral altered. VI. SUMMARY. 1. The reactions of a number of natural sulphides with copper sulphate solutions have been quantitatively investigated. Atten- tion has been confined to the following: chalcocite (Cu 2 S), covellite (CuS), bornite (Cu 5 FeS 4 ), chalcopyrite (CuFeS 2 ), pyrrhotite, pyrite (FeS 2 ), sphalerite (ZnS) and galena (PbS). In all cases a copper enrichment product is formed, either a sulphide which varies with the conditions, or as a special case, metallic copper and cuprite. In all cases also the sulphate of the metal contained in the original sulphide is formed and usually sulphuric acid as well. This acid is derived from the oxidation of the sulphur in the sulphide with cupric sulphate. In these reactions cupric sulphate plays the role of an oxidizing agent; not only at elevated temperatures but at lower temperatures as well. 2. The sulphide enrichment products are crystalline and all adhere firmly to the altered sulphide, as in nature. When cupric sulphate is the enriching agent, pyrite alters to covellite and chal- the amounts of zinc and iron found in solution as sulphates. They are the total amounts which took part in the reactions between the sulphides from which they were derived and cupric sulphate. 105 Based on V and the densities of the sulphide. 108 The amount of iron in solution is not the total amount of iron which took part in the reaction between pyrrhotite and cupric sulphate. See page 459 for further explanation. 500 E. G. ZIES, E. T. ALLEN AND H. E. MERWIN. I cocite. It has been shown that the alteration to chalcocite is rep- resented by the following equation: 5 FeS 2 + 14 G 1 SO 4 + i 2 H 2 0 =^ 7 Cu 2 S +; 5 FeS 0 4 + I2H 2 S0 4 , and that in the alteration to covellite the following equation in all probability represents the reaction: 4 FeS 2 -\- yCuS0 4 — (- 4 FT 2 0 7 ^uS d - 4FcS0 4 -f- 4 H 2 S 0 4 . The evidence is good that this reaction is involved when pyrite alters to chalcocite. Pyrrhotite alters to chalcopyrite and very probably to bornite. The reaction can not at present be satis- factorily worked out on a quantitative basis owing to the fact that pyrrhotite varies in composition and is attacked by one of the reaction products, namely sulphuric acid. Chalcopyrite alters to covellite and chalcocite. The reaction between chalcopyrite and cupric sulphate to form chalcocite has been shown to be repre- sented by the equation: 5 CuFeS 2 + nCuS0 4 -H,8H 2 0 = 8 Cu 2 S + 5FeS0 4 + 8H 2 S0 4 . When chalcopyrite alters to covellite the experiments point strongly to the reaction represented by the following equation : CuFeS 2 + CuS0 4 = 2CuS + FeS0 4 , and also indicate that this reaction is involved in the alteration to chalcocite. Bornite alters to chalcocite as follows : 5Cu 5 FeS 4 +iiCuS0 4 +8H 2 0=i8Cu 2 S+5FeS0 4 +8H 2 S0 4 . It has also been shown that bornite may alter to covellite and chal- cocite thus : Cu 5 FeS 4 + CuS0 4 = 2 Cu 2 S + 2 Q 1 S + FeS0 4 . Covellite alters to chalcocite as follows : SECONDARY COPPER SULPHIDE ENRICHMENT. 50 5C11S + 3C11SO4 + 4 H 2 0 = 4Cu 2 S + 4H2SO4. The experiments also furnish evidence that this reaction proceeds in two stages thus : CuS — f- 7CUSO4 -f- 4H2O = 4CU2SO4 — |- 4^2^04, CuS -f- Cu 2 S 0 4 = Cu 2 S -f- CuS 0 4 . It is very probable that these two reactions are involved when the sulphides discussed in this paper react with cupric sulphate to form chalcocite. Sphalerite and galena alter first to covellite and subsequently to chalcocite : ZnS + CuS 0 4 — CuS + ZnS 0 4 , 5CuS -f- 3CUSO4 + 4H2O == 4 Cu 2 S -f- 4H2SO4. PbS + CuS 0 4 == CuS + PbS 0 4 , 5CuS -f- 3CUSO4 -f- 4H 2 0 == 4 Cu 2 S + 4H 2 S0 4 . 3. The order of stability of the sulphide enrichment products toward cupric sulphate solutions is : chalcopyrite, covellite, chap cocite ; each of them changing into the succeeding sulphide by the further action of cupric sulphate. Chalcocite is by far the most stable sulphide of all, under these conditions, but it may finally be converted into metallic copper and sulphuric acid, though very slowly indeed even at 200°. The most favorable conditions which have been observed for the formation of the intermediate products, chalcopyrite and covellite, are the exposure of a large surface of the reacting sulphide to the action of a comparatively dilute solution of copper sulphate. 4. All these reaction's have been studied at several tempera- tures ranging from 200° down to 30°. In the main the rate rather than the nature of the reaction is changed by raising the temperature, but there are a number of secondary reactions, slight or negligible at low temperatures, which become pronounced at 5 02 E. G. ZIES, E. T. ALLEN AND H. E. M ERWIN. higher temperatures. Thus ferrous sulphate is partly changed into ferric sulphate by cupric sulphate : 2Cife0 4 + 2FeS0 4 ^ Cu 2 S 0 4 4 " Fe 2 (S 0 4 ) 3 . At elevated temperatures, hydrolysis generally conditions the for- mation of considerable hematite, cuprite and metallic copper from the two primary products of the above reaction. To what extent the metallic copper and cuprite sometimes found in the natural enrichment zone are derived from the hydrolysis of cuprous sul- phate is not clear from these experiments. 5. The results of qualitative experiments indicate that enrich- ment proceeds faster in the presence of cuprous sulphate than in the presence of cupric sulphate. 6. The influence of sulphuric acid on the enrichment reactions has been studied. The enrichment of chalcopyrite and pyrite in our experiments has been retarded by an increase in the con- centration of sulphuric acid. The explanation for this is found in the fact that hydrolysis of the ferric sulphate, formed as we have stated above (paragraph 4 of summary), is either hindered or prevented, and thus the influence of the cuprous sulphate formed from cupric sulphate by the reducing action of ferrous sulphate is held back. The result is that the formation of cuprous sulphate is limited, and since the rate of reaction of cuprous sul- phate on the sulphides is much faster than that of cupric sul- phate, enrichment itself is retarded. The enrichment of galena, sphalerite, pyrrhotite, and bornite is accelerated by sulphuric acid, for the “solubility” of these sulphides is thus materially in- creased. Chalcopyrite is one of the products at higher tempera- tures between bornite and 2 per cent, sulphuric acid alone. 7. The influence of ferrous sulphate on the enrichment reac- tions has also been studied to some extent. The first effect is to increase the rate by increasing the quantity of cuprous sulphate in solution, and cuprous sulphate is more rapid than cupric in its action on the sulphides. However, the effect is soon lost unless the ferric iron formed is removed from solution. SECONDARY COPPER SULPHIDE ENRICHMENT . 503 It may be stated here that a reversal of the principal enrich- ment reactions, such for example as f 5FeS2+i4CuS0 4 +^H 2 0=7Cu2S+5FeS0 4 +\H2S0 4 , has not been realized experimentally. The attempt to intro- duce iron into bornite by allowing the sulphide to react with fer- rous sulphate alone has not met with success. Geophysical Laboratory, Carnegie Institution of Washington, Washington, D. C.