'3-2.*) 14 • ^ cJsV.^SK- C- ?-£ U. S. Department of Agriculture, Forest Service FOREST PRODUCTS LABORATORY In cooperation with the University of Wisconsin MADISON, WISCONSIN EFFECT OF HIGH PIT TEMPERATURE AND OF PREHEATING OF THE WOOD ON THE GRINDING OF LOBLOLLY PINE By EARL R. SCHAFER Associate Engineer J. C. PEW Assistant Engineer and R. G. KNECHTGES Assistant Scientific Aid I )EPO HUME LIBRARY OCT ul 1972 £AS. - Univ. of Florida Published in PAPER TRADE JOURNAL July 9, 1936 Digitized by the Internet Archive in 2013 http://archive.org/details/fhighpittOOfore EFFECT OF HI O H FIT TEMPERATURE AND ?T PREHEATING CF THE WOOD ON THE GRINDING CF LOBLOLLY PINE- B3 E. R. SCHAFER, Associate Engineer J. C. PEW, Assistant Engineer and R. G. KNECHTGES, Assistant Scientific Aid The results reported here of grinding loblolly pine at high temperatures are "believed to "be applicable to any coniferous species. Elevation of grinder pit temperatures, with other variables constant, re- sults in longer fibered and stronger pulps in relation to the power con- sumed. This trend previously reported to 190 o F. , continues on up to 210 c F. Preheating of the wood in water has relatively small effect as com- pared with the pit temperature. The small beneficial effect noted seems to be due not merely to the elevation in temperature of the wood, but to a mild chemical or cooking action taking place during the preheating period. The increased plasticity of the wooc when charged at higher tempera- tures appears to be more of a detriment to quality than an aid. With a stone carrying sufficient water in its surface, the temperature at the wood- stone interface is not likely to exceed the boiling point of water at atmospheric pressure. Control of temperature by controlling the pulp carry- ing capacity of the stone surface is possible, but is not so practicable or so easily done as is controlling the pit temperature. Since the results noted cannot be attributed alone to the effect of heat on the wood it is suggested that the viscosity of the water may be largely responsible. Introduction A previous publication on the effect of temperature in pulp wood grinding^ pointed out that high temperatures in the grinder pit were -Presented at the Annual Meeting of the Technical Association of the Pulp £ Paper Industry, Waldorf-Astoria Hotel, New York City, Feb. 17-20, 193 : . 2 -Effect of Temperature and Consistency in Mechanical pulping, by E. R. Schafer and J. C. Pew. Paper Trade J., 101, No. 13; 71 , Sept. 26, 1935- RlllU advantageous in producing longer fibered, stronger pulps, generally with lower power consumption and higher grinding rate. The results reported here are an extension of the prior work. In general three types of exper- iments were made using (1) higher pit temperatures than previously employed, (2) wood preheated external of the grinder, and (3) steam projected on the stone just before it passed in rotation under the wood. The loblolly pine used was from the same shipment as that used in part of the previous work. The selection of this species for the pres- ent experiments was prompted by the current studies now being made on the effect of growth variables in the pulping of the southern pines. The re- sults of the temperature studies on this species are believed to be applicable to any coniferous species. Procedure The equipment and the general methods of operation were the same as previously used with loblolly pine.?. In the present experiments, how- ever, the wood was in all cases brought to a definite temperature before charging into the grinder. This was accomplished by placing the 9-i ncn - long wood bolts in a tank in which water was circulated at the required temperature for approximately 5 hours. A control bolt with a thermometer inserted to the center was used to determine when the wood was heated to the desired temperature. The wood was left in the tank until required for charging the grinder. Ho material change in moisture content of the wood or the color of the pulp was caused by this heat treatment. The unit pressure of wood against the stone surface was constant at 1M- pounds per square inch and the temperature of the shower water was adjusted so as to give a consistency of pulp in the pit of about 5 percent in all experi- ments. The surface of the artificial stone, from past experience, was known to have remained constant throughout the relatively short time covered by the experiments. Discussion of Results The data obtained are summarized in Table 1. Series 1 shows the results of charging wood of a temperature of 60° and grinding it with pit temperatures of l6o°, I90 ', and 210° 7. This series is similar in part to series 7 of the prior tests. 2 "but because of the slightly different grinding conditions the actual values of pulp strength and power are not comparable with the previous work. Comparison can, how- ever, be made in the general trends. It is noted in series 1 that increas- ing the pit temperatures caused the pulp to become longer fibered and to have greater strength, this tendency continuing above the 190° maximum temperature previously employed. Rill 1 ! _2- As previously observed th* increase in pit temperature (with other variables constant) caused a consistent decrease in the power input, "but contrary to the former work the production did not consistently in- crease. This resulted in irregularity in the power consumed per unit of production which, within relatively narrow limits appeared to increase at 19 )° and decrease at 210 c . From the standpoint of economy of production it is of irterest to consider the quality per unit of power consumption, obtained by dividing the strength values by the horsepower days per ton.-^ The strength values thus factored are shown in Table 1. The most economi- cal use of power seems to be definitely indicated at high pit temperatures. As previously pointed out, however, high pit temperatures must be accompanied with a sufficiently low consistency that the pulp flows freely over the dam. It is evident that the relation between power consumption and quality is empirical. The energy absorbed in the grinding process represents the resistance of the wood to the various forces made to act upon it and therefore has an influence on the quality only as these forces affect quality. The conditions that bring about improvement in quality often require the consumption of more energy, but one does not neces- sarily follow the other. Therefore, since it is conceivable that pulps of equal quality may be produced with the consumption of different amounts of energy, the evaluation of quality by means of the factoring of strength values by the power consumed has no theoretical significance and is justified only when used for economic considerations. The effect of varying the temperature when the wood was preheated to and charged at the pit temperature is shown in series 2. The trend of increased strength with increased temperature is much as shown in series 1. In series 2, however, the bursting-power factor values are slightly higher and the tearing and tensile strengths, except in two instances, slightly lower than in series 1. At 190° F. and 210 c F. the fiber lengths are con- siderably greater in series 2 than in series 1. The power consumption was markedly higher at the two higher temperatures when the wood was preheated to the grinding temperature. The only advantages of preheating the wood are a slight improvement in bursting strength and a fairly marked increase in fiber length, but to accomplish this the preheating and grinding must be done at high temperatures. This is borne out further in series 3 i n which the pit temperature was maintained at l6o c F. and the temperature of wood varied from 60° to 21 r F. in 50° steps. Considering this wide range of temperature variation the results showed surprisingly small differences in pulp quality. The fiber lengths remained unchanged except for a slight increase at 210° 7. The strength values factored by the power showed little or no change in bursting strength and a slight decrease in tearing and tensile strengths -^This factor bears a reciprocal relationship to the "quality-price" number used oy Brecht in determining the cost of maintaining a siven quality. Papier Fabrikant 33, 13; 113, Mar. 31 , 1935; 33, l 1 ^; 121, Apr. 7, 1^35; 21, 15; 129, Apr. lU, 1935. RlllU -3- with increase in the temperature of the wood charged, prior heating of the wood alone, even to a higher degree than the grinding temperature, apparently had no helpful influence on the pulp produced, hut tended for the moot part in a direction opposite to that generally ohserved on the effect of increased temperatures. This would indicate that certain advantages claimed in practice as due to the mild warming and steaming that the wood receives in the grinder pocket, either do not exist or must he attributed to other causes in addition to heat and moisture. If "benefits accrue it would seem that something more than softening of the wood must he the cause. The foregoing indicates definitely that (l) improvement in quality results as the temperature at the wood-stone contact increases, (2) a slight improvement in certain pulp properties with an accompanying detriment to other properties results if in addition to raising pit temperatures the entire mass of wood is "brought slowly to the grinding temperature in water external to the grinder, and (3) no appreciable "benefit is derived by raising the temperature of the wood only. In connection with the last point it "became of interest to determine whether wood so treated is permanently affected or whether the softening action is reversible upon cooling. For run No. !%• the wood was heated to 210° F. according to the procedure adopted and then rapidly cooled to oC c F. and ground at a pit temperature of l60°. The pulp from this run (see series h) was not only "better than that of a similar run (No. 1S5) i- n which the wood was not heated, hut also "better than the run (No. 183) in which the wood was heated to 210° and not cooled before grind- ing. The strength-power factors indicate further that the wood in this run was ground with the most economical use of power as compared with the other t: - o. The experiment indicates that the wood is permanently affected hy slowly heating in water to approximately the boiling point. This permanent alteration is undoubtedly due to a mild chemical reaction or extraction of the wood constituents. An additional fact is that the higher plasticity of the wood charged at high temperature is not so "beneficial as hitherto supposed. In series 5 "the effect of heating the stone surface just "before it traveled under the wood was noted when the wood was charged at room temperrture and the grinder pit maintained at the normal operating tempera- ture of l6C c . The heating was accomplished hy projecting steam on the stone at that point. Measurement of the amount of steam was not made, therefore, the results are only qualitative. That the temperature at the interface was higher than normal is shown hy the lowered consistency. A distinct increase in fiher length, "bursting power and tensile-power factors is evident in run No. 189 i n which steam was applied as compared with run No. I85 in which no steam was used. In attempting to account for the effects noted it seems that the controlling causes are not attributable alone to the effect of temperature on the physical properties of the wood. Brecht- has also reported that k " Loc. cit. ElllU _U- the heating of pulp, produced "by the "cold" grinding process (European practice) for 30 minutes in water at various temperatures has but little effect on any of the physical properties. Similar experiments at the Forest Products Laboratory with commercial pulp, produced "by the "hot" grinding process (American practice), heated at various intervals in boiling water confirm this statement except that it was noted that boiling for only 1 minute caused an appreciable increase in the freeness. Cer- tainly contact with hot water cannot have any effect on the fiber length of the pulp. Hence, as an alternative, for which, at present, there are no data to su.bmit as supporting evidence, it is suggested that the princi- pal factor producing the effects may be the viscosity of the water used as the grinding medium. Certain experiments are in mind that may give information on this point. T heoretical Discussion of the Heat Developed in Grinding In the work carried out so far consideration has been given only to the temperature of the pulp in the pit and prior thermal treatment of the wood. The actual temperatures attained between the wood and stone are probablj'- of most importance, but unfortunately these temperatures are very difficult if not impossible to measure. However, the amount of heat developed may be estimated by analysis from several angles of approach. The following example serves to give an idea of the magnitude of these temperatures . Assume a grinder stone operating with a pressure of 3- pounds per square inch of wood-stone contact. The grinding or friction coefficient as calculated from operating data may be shown to vary from 0.15 to 0.35 or even more. Assume an average value of 0.25. Allow one square inch of stone surface to advance one foot under the wood at the above pressure. Then the energy expended will be 30 x 0.25 x 1 = 7»5 foot pounds per square inch of stone surface. This energy is almost entirely transformed into heat, yielding 7*5 x 0.324 = 2.U3 calories. The heat is absorbed by the stone, by the pulp suspension carried under the wood in the grooves in the stone, and by the new wood ground off. twing to its mobilitjr and high specific heat the major portion of the heat is probably absorbed by the pulp sus- pension in the grooves. It is likely that the grooves in the stone sur- face are full of pulp as they advance under the wood, since pulp from the grinder pits is seen to be doctored off at the first block of wood in the first pocket. It may be estimated by rough calculation that the volume of the grooves on a moderately rough stone will be from 0.2 to 0.3 cc. per square inch of stone surface. Assuming 0.25 cc « as a^ 1 average and that the heat is absorbed entirely by this pulp in the stone surface, the temperature rise attained in one foot of travel would be 2.U3 -«- 0.25 = 9«7 C 0. or 9.7 x 1.3 = 17. 5° ?• Actually some of the heat is absorbed by the stone and by new fibers ground off the wood so that the increase in temperature attained would be less than this amount. Thus from the standpoint of energy input it is seen that with a fairly deeply grooved stone running at ordinary pit temperatures the temperature between the wood and stone BlllU _5_ could never reach the extreme values sometimes postulated. In the fore- going example, if the pit temperature is l6c°, about 3 feet of continuous wood-stone contact would he necessary for the pulp suspension to reach the boiling point. The explosion theory suggested by Schoengut2 would require steam pressures in excess of atmospheric. A temperature suffi- cient to cause such pressures is probably seldom if ever attained in the grinding process. Higher temperatures are, of course, attained when the stone surface does not carry sufficient water to prevent burning of the wood, but this is not normal operation. It is further evident from these considerations that the depth and character of the stone grooves has a decided effect on the tempera- ture produced between the wood and stone. It is possible, by varying the depth and type of burring, that is, by varying the free space under the periphery of the stone, to control the average temperature between the wood and stone without changing the pit temperature. If the amount of pulp-carrying space is decreased the average temperature between wood and stone is increased. However, when this is done the range of temperature from the point where the wood and stone first come into contact to the point where the stone emerges is also extended. The relative advantages of varying the stone surface or the pit temperature may be illustrated by the following example. Assume the pre- ceding grinder stone (which carried 0.25 cc « °f P^lp per square inch) to be running in a pit at 150° F. and that all of the heat developed is absorbed bv the pulp in the stone surface. When a given area of the sur- face first makes contact with the wood the temperature of the pulp in the grooves is 150° F. When the area has traversed one foot the temperature would be I5O + 17o = 167. 5°« The average temperature for the one foot travel would be 15S.8 C F. Now to raise the temperature suppose the pulp- carrying space in the stone is decreased by one-half. The pulp in the grooves on entering the wood area would still be 150°, but (since to absorb the same quantity of heat the temperature would be doubled) after travers- ing a distance of one foot the temperature would be I5O + 35° = 185° F. and the average I67.5 F« On the other hand, if instead of changing the space in the stone surface the pit temperature had been raised to 153° then the temperature after one foot of travel would be 155° + 17.5° = 176. 5° with an average of 167.8°. Thus by raising the pit temperature 9° the average temperature under the wood has been increased an amount equal to that caused by reducing the pulp -carrying space in the stone surface one-half, Furthermore, by raising the pit temperature the range of temperature under the wood is only half of that caused by changing the stone surface. It appears that uniform temperatures within narrow limits are more easily ob- tained by controlling pit temperature than by varying the pulp -carrying capacity of the stone surface. 2-Schoengi.it, J. Fapier-fabr. ]£, No. 3; 25, Jan. 20, 1935, ' - BlllU _6_ Conclusions Raising the temperature of the grinder pit, other variables remaining constant, has "been found to have marked effects in mechanical pulping. Preheating of the wood in water external to the grinder caused slight effects that may he attributed to a mild cooking or extraction. The increased plasticity of the wood caused by increased temperature had but little influence on the resulting pulp. The experiments as well as the theoretical considerations presented indicate that preheating the wood has onljr an insignificant effect on the temperature of the wood-stone interface, since the amount of new fiber added during the passage of an area of stone surface through the grinding zone is small in comparison with amount of pulp already in the surface. Consideration of the energy absorbed in the production of groundwood fiber leads to the conclusion that the temperature of the interface is not likely to exceed the boiling point of water at atmospheric pressure. Since the marked effects observed in controlling the temperature of the grinder pit or of the wood cannot be attributed alone to changes in either the physical properties of the wood or of the pulp it is sug- gested that the viscosity of the water may be the controlling factor. This may be difficult to prove experimentally, but it is believed further work along this line will yield valuable information. 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