SCREEN ANAEYSIS AS AN AID IN PEJEP EVALUATION Revised November 19^9 Ml Of FDRESTUY \MM OF U*S< vhll p w UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison, Wisconsin In Cooperation with the University of Wisconsin SCHc^N ANALYSIS AS AN AID III PULP EVALUATION .A resume" of work at the Forest Products Laboratory ' By :.. R. SCHAFER, Engineer This article supercedes a publication of the same title (l) and brings together the results of several independent investigations, some of which have not been previously reported. The Laboratory first became in- terested in the fractionation of fiber masses into length classes as a means of palp evaluation in .a study of insulating board from sawmill waste* The fractionation was effected with an experimental flat-plate diaphragm screen provided with a series of interchangeable slotted plates of the type ordinarily used in pulp mill screening operations. The apparatus was later modified fcr application to evaluating grcundwood and chemical pulps by substituting wire screens for slotted plate: . Further study brought improved efficiency through the d- v-lopment of a -tiple plate screen in which the stock flowed through four compartments each fitted with a screen of different size openings. - Experiments will be described of .the screen analysis of insulatii- board pulps, groundwccd pulps, and chemical pulps with both the si:.. - and multiple diaphragn? plate screens as well as a limited amount of work with the Bauer-:,:c:e-tt Classifier. - '.: ANALYSIS OF INSULATING BOARD 3T ' Single Flat Screen The plates used in the single-plat-; diaphragm scj : - . l) were 10 inches wide by 12 inches long and contained to slots, :h k inches long. The widths of th<= slots in the series of plal • ; 0,015; 0,012, and 0.GC3 inch, respectively. The material irst screened on the scr n having the smallest slots. raction of t.. material passing through the :;.•:■ n was caught in a drain box with -Acknowledgement is made to the Appleton Machi] ] ■ ;■ :.y, Apple ten, Wis., for cooperation in working out details of design of this sc; :.d for building models in the various stages of developm is commercially kn< .. as the Appleton :tiv :r R88U Fourdrinier wire bottom. The- .sc-ree-n- plate- was then replaced by the next larger in the series and the screening repeated. The material during each screening was vigorously 'agitated on the screen with water Vprays. One hundred grams of the original fiber required about 10 minutes screening en each plate to effect a practical separation. Continued screening caused the various sized fractions to blend into each other. The varioas frac- tions obtained in this way were then dried and weighed. The screen analysis and other properties of three well-known com- mercial insulating boards and one of an experimental Douglas fir board are given in table 1. These boards are composed of various materials that have been processed in different ways. It" is, therefore, not surprising to find no general relationship between the strength properties and the screen analysis* Nevertheless, a change in the fibrous composition of any one of them would undoubtedly influence not only the strength properties but its density, formation, insulating value, and appearance. Having developed a product of satisfactory quality, the screen analysis, therefore, presents a means of control in manufacture. Plate 1 shows the fiber aggregates obtained from insulating boards. T he Bauer-McNett Classifier The Bauer-McNett Classifier is a four compartment apparatus in which the stock flows by gravity through a mesh wire screen from one compartment to the next. The screen plate is set vertical in the compartment and a propeller agitates the stock suspension in a manner claimed to orient the fibers parallel to the screen surface. Ten grams of material are used in each test. The fiber distribution of seven commercial insulating board stocks as obtained on this classifier are given in table 2. SCREEN ANALYSIS OF GROUNDWOJD PULP Since groundwood pulp will practically all pass through a plate with the usual 0.008-inch width slots, the separation of this type of pulp must be made with screens having openings smaller than those obtain- able in the form cf slots. For this purpose a series of screens con- sisting of various mesh wires of the Tyler standard screen scale were soldered to perforated brass plates. The screens were 24, }2, 42, and bO mesh of the Tyler series which, respectively, have screen openings of 0.701, O.U35, 0.351, and 0.24b mix., in width. The ratio of the width of screen opening in one screen to that of the next one in this series, is approximately equal to the square root of 2. The procedure for the single-plate screen consisted in using these plates in the apparatus in the order of their size, starting with the smallest. The. fraction of the material passing through the screen was caught in a catch screen the bottom of which was made up of a piece of R8S4 -2- c ID CD u o CO a M -: ft a! ■H C a> S ■H fn D ft X D (D aS rH ft C -o I I to - ■H I Digitized by the Internet Archive in 2013 http://archive.org/details/screenaOOfore © fl o d cd w I f ' j ttfl a • H cd d CD W P ■ : CO ,H cd • -H nJ rH • •H r-H ■H fH D 1 cd CO -J fH •rl CD . 1 1 d o o Q v, • rl B 1 1 o .-, fH - 1 «H ." q © .-' tJ cd fl B ID Tl ■■< fn O cd [fl o rQ <: M cd fl ^ • H -t-3 i cd - rH ■ i B T) CO *H -< ■H • .' ! rH cd r : -l-> J ■ : • • ,- i fH • H a) rH ■3 Eh cd U cd ■H Td CD P cd rH ft 0) r-H fl • H 00 CD ^ -P B* d m .: «H i . 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Reduced to 0.9 natural size. zm r£SS4F u CD •H O ^D OJ o o rci • CD S LT\ r-i d d o ^ to . -p to r<~\ OHH o o o rH rH rH MD VO V£> co i — r— V£> C rH CT\ LT> U5 ^O VD rH d d r^- LOUD to a CM CJ OJ to to to r— to r— r^to o w ocn i-i.cn o CM CM CM rH rH rH LOJ" LPi CM CM CM CM CM CM VD I — r— J" J" .zj- rH rH rH CM CM OJ CT> zl- nj- ltmo 'lTn cm ro cm CMCMCM CM CM CM CMCMCM ^t <£> IT\ CMJ-rO nMO 'PiOJ CO LCMTiin CM CM CM CM CM CM OJ r-cn HOlTl r-^o o en o-\ J" H OJ ITiJ- -=f r — to to ud r— ^o ro r-o r<-\ cnoo LTYzl- J- OJU) _zt- CMLC^I^v CT»LPiCM CMtOO .-1 rQ CD '-. CD > '■ rQ CM CD rH 0) 5 CT5 rQ r^ CD rH > 03 rQ J" CD cti (h CD 5 Cd rQ Lf> flj rQ Ci rQ Q) V_- gj tU) AID cd cti Ss rH CD UD CD > t> <1 Table 3» — Fiber size distribution of the various screen fractions of groundwood pulp Fiber length range Passing 60 mesh screen Retained between 42 and 60 mesh screens Retained between 32 ar.d 42 mesh screens Retained between 24 and 32 mesh scr< ns From mm« To Fibers measured Fibers measured FVbers mear-vred Fibers measured mm ( : Number: Percent: Number: Percent; Numb e.c . Pe -cent: Number: Percent : 0.073 . . 485 : 65.70 : 2 : 0.40 '> 3 0,82 1 1 i , 0.88 0.073 i .147 • 135 : 18,70 : 15 : 3.76 : 8 2,15 7 2.05 .147 .220 . 78 : 10.50 : 41 : 8.12 s 9 2,47 . 7 2.05 .220 , , .294 : 15 : 1.92 1 37 : 7.33 : 8 2.15 . 9 : 2.63 .294 . ! .%& . 8 : 1.07 : 54 : 10.70 : 11 . 3 - 2 3 : .88 .36S , , .440 i 4 : .54 : 45 : 5.70 : 11 : j.C2 9 ! 2.63 .440 : c5i5 : 7 : .54 : 53 : 10.50 : 22 6cC2 . 15 , 4.38 .515 ; .538 , 1 : .15 : 72 : 14.20 : 22 6.02 13 3. 80 .588 ! .062 : 1 : .15 : 28 : 5.52 : 4b 5.85 , 10 ! 2.92 .6o2 : •736 : : 35 : 6.52 : 21 5.75 ! 5 : 1.46 •736 : .810 : : 22 : 4.36 : 34 . 5.32 ! 17 - 4.97 .810 i .884 ; : 13 ! 2.58 . 42 , 11.50 : 11 ! , 3.22 .884 : .956 ! : 15 I 3.76 S 2b . 7.13 ! 16 ! 4.68 •956 i 1.030 ; : 15 : 3.97 1 17 4.65 . 17 . 4.97 1.030 : 1. 102 ; : 7 : 1.38 ! 10 ; 2.74 U , 3. 80 1.102 : 1.175 ■ : 9 : 1.78 : 11 ; 3-02 : 27 i 7.90 1.175 i 1.250 : 11 : 2.18 : 23 : 6.03 20 - 5-85 1.250 : 1.322 : : 3 : .59 : 12 3.25 15 , 4.38 1.322 1 1.398 : 3 : .59 : 10 ; 2.74 , 24 ! ! 7.02 1.398 : 1.470 : 3 : .59 : 8 2.15 13 , 3. 80 I.470 ! I.5U5 : 1 : .20 : 3 .82 2o . 7.60 1.545 : l.bl8 : : 3 : .59 : 5 1.37 , 11 ! 3.22 1.618 ! I.65I : : : 2 ■ .55 G : 2.03 1.691 - 1.765 : 1 : .20 : 3 .82 b : 1.75 1.765 : 1.839 : 3 : .55 : 2 ■ -55 . 4 : 1.17 1.839 , 1.912 : 2 : .40 : 1 : .27 , 10 : 2.92 1.512 , 1.986 : 1 : .20 : 1 : .27 : 4 : 1.17 1.986 . 2.060 ; 4 ! 1.17 2.060 . 2.115 ; 1 : .29 2.115 . 2.205 : : 2 : .59 2.205 ! 2.382 : 1 ...... . : 2 : .59 2.382 : 2.375 : ; . 3 : .88 2-375 : 2.430 : : : 3 : .88 : Above C ).Qte" : Above I.580 mm. : Above < B.OoOmm. ... . -'•"O • ♦ : 3 : 0.45 ; ] : 0.20 : 4 : 1.10 : 5 : l.4o Tota : 7^5 : 100 : 505 : 100 : 365 : 100 : 342 : 100 R884 SO mesh wire. Ordinarily no attempt was made to catch the mat through the 6' -mesh screen. One hundred grams on the basis of of ove] pulp was used for the test. Af r scr i In . all oth fraction- dried and i hed and the portion oassing throug] 60-mesh screen was obtained by subtraction. When it was desired to ob- tain a ouantity of the fine fraction to determine its properties, a piece of lir. c -eling was laid over the wire screen in the catch box. this method more than 75 percent of the fiber passing the I -n sh screen was reclaimed. tie appearance of the different fractions obtained with this series of screens is characteristic and indicates a fairly definite separation of the fibers into groups according to their size (3 ) • Table 3 shows the results of fiber measurements made to determi the fiber-size distribution in the various fractions. The data represent the average of five commercial groundwood pulps. The five pulps were fractionated and the fiber measurements made on each fraction. The five sets of fiber measurements for a given fraction ■ r t'J n combined. From 350 to 500 measurements were made on each of the fractions retained on the 2^+, 32, U2, and 60-mesh screens, and because of the inaccuracy in- volved in measuring very snort fibers, about 75- measurements were made in the material passing the 60-mesn screen. The manner in which the fiber lengths tend to .-re . .••Ives about the average, or the freauency distribution, was determined by + number of fibers falling within a given range of length instead of tl number of fibers of a given single length. This was due * the :'- ; that even with the apparently large number of measurerr.> tits relation of the rercentage of fibers of a given length to that of the r.ftxt higher or lower length was very irregular and therefore, a drawn through such data would not present a true picture of the normal distribution of the fiber lengths. A much larger number of measur 1 I would have smoothed out these irregularities and permitted t) of smaller ranges than rerorted here. Tne average fiber length, the stand- deviation, and the coefficient of variability of each fraction are she in table h. A high degree of variability is indicated in all fractions, that for the material passing the 60-mesh screen b< almost twic as great as the other fractions. Table k. — Average fiber length, standard deviation, and coefficient of variability of groundwood s n fractions Fraction Average fib lengt Standard ■viation Coefficient of var i i Retained between: : 1+2 and 60-mesh : 32 and 1+2-mesh : mm. 797 .5675 .8120 l.llUO ±0.0796 •33^ •365 ■513 re •9 • • ■ j.l The abscissas of the joints on the curves shown in figure 3 are plotted as the midpoints of the range of length chosen. In spite of the efforts to insure a complete separation of the fibers there is a consider- able overlapping of sizes in every fraction. This does not, however, invalidate the usefulness of the method. In commercial work the series of screens needed will depend on the characteristics of the pulp and the degree of separation desired. In some cases a sharp separation into fiber classes may not be necessary. If a sharp separation is desired, a study such as described above, although requiring a large amount of careful work, will be needed to determine the number and mesh of the screens required. Detailed studies of the physical and chemical properties of the screen fractions of groundwood pulp have been presented in other publica- tions (J+,5.)' These investigations have indicated some very definite re- lationships between fiber size distribution and pulp properties. SC3LEN ANALYSIS OF CH3MICAL PULP The screen analysis has been found useful in studies of the effect of beating. When beating a mixture of spruce and birch sulfite pulps for glassine stock, samples were taken from the beater at intervals and development of hydration and strength of the pulp followed by means of the freeness and strengtn tests. The cutting action of the beater roll was followed by the screen analysis, in this case with the single-plate screen. The results of the analysis are shown in figure 4. It may be noted that the cutting action of the roll, which was of stone was rapid during the first 120 minutes. Cutting was less rapid dur- ing the next 80 minutes but increased again during the last period of 40 minutes. It may be noted further that an increase in the amount of material that would ^ass the 60-mesh screen was accompanied with an approximately equal decrease in the amount retained on the 24-mesh screen. Tne intermediate fractions remained practically constant in amount throughout. This example is cited to illustrate a use the screen analysis may serve in determining what is happening to pulp in processing. Another example of the aid of the screen analysis in the process- ing of pulp is shown by the data in table 5- This was a study of the production of bond paper from bleached sulfite pulp. The screening was done on the four-compartment screen shown in figure 5- The analysis indicates the trend of the processing effect on fiber distribution. The percentage of fiber retained on the 24-mesh screen was reduced proportionately as much by beating as by jordaning although the beating treatment was very mild. The effect of beating on increase of the amount of material passing ll^-mesh screen was nearly twice as great as the effect of jordaning the beaten pulp. Generally speaking more than two-thirds of the fiber size reduction of the "on 24-mesh" material by beating appeared as increase in the amount of "through 115-mesh" material whereas only a little more than one-third of the size reduction of "24- mesh material" by jordaning the beaten stock appeared as fine material. R884 -4- -P •H i— i a o o o n - C H S-i O O M CO fl '— O CJ • r-H Q) o ^ ctf a) m w f! (D 0) ^H ^ EH O -— ' CO C) o ■H | S % OJ £ - O OJ •h r^N -p ■ ■H -H Jh W a CD ,0 £> O •H r> ( i i o O Q O O VD LPi J? r^i OJ (^uaojr-j — sj3qt : jo .xa q el o (D i-i u W • : CO OJ e o tuD o u Eh a CU u o CO S i OJ : CO CO CO O ft) 0) s e s OJ .VI o t^ t) r d a a a _3" CO OJ CD ' CD pq CD : CD m CD a> -p m ; -yd J «■ ,-c > -j \ / / / 11 1 1 \. / / 1 1 l\ 1 I 1 / \ i i 1 1 \ \ \ 1 \ / \ j / ft ^ ■* A 1 / / > «- / l V j > } -\. ; \ . 1 1 i 1 1 \ \ \ j . i L •^ 1 LM O OJP rHCTJ 0) OP ) a •H o vx> O LOv O o O OJ O •p ■ :, O P -P •H CD - i— I -H M •H ?H I CD ft -P (D •H •H D P a (OfUaoaaj — t .783jos no pauiT?i9' ^irroaiv) Table 5» — Screen analysis of unbeaten and proce ss ed bleached sulfite pulp ■ ; - • * ■ • Processing: . : Retained • . • Retained betw n : Passing:' 'treatment : on • • , ii 1___ : H5-mesli :. 24-mesh • 2k and r 42 and : SO and : • • • 42-mesh : -80-raesH :ll'5-mesh : • .. : Percent • Percent : • Percent: Percent : Percent. . 6.4 : 4.3 : J.^ ; _ 10. 4 10.1 : 6.5 : '3.2' : 22. 0;. After beating. . . .: " 58.4 I After jordaning. .: U5.0 • l4.0 : 10.0 : 4.0 " : 27. .. It was also' observed in this investigation that the percentage of fine material in the furnish was associated with' the porosity of th finished paper. The aata in table o show that a higher porosity value (more dense sheet) was_obtained with furnishes containing the high r proportions of fines. Table b. — Influence of amount of fines in the furnish on the porosity ( Df sulfite bond paper • •Machine run ', Proportion of passing 115* furni sh : -mesh 'Pc rosity of sheet by Gur ley densometer : After jordaning : At head box Number 1257 1272 1261 : Percent : Percent 33-2 35.6 30.0 9.3 p • > ■ > Seconds 360 230 5o 50 1 12o7 1255 : 19.2 :. R884 -5- STUDIES ON THE SCREENING. PROCEDURE In the original screen" analysis test employing a single-plate diaphragm screen it was necessary to interchange :screen -plates for each f-ract-icri. This' apparatus was improved upon by building four diaphragm screens* into one machine. I,n the improved machine each compartment con- tains a : screen plate with different size of openings than the other three. The screen is shown in figure 5» I n operation, from 50 to 100 grams of the pulp (depending on the fiber size distribution of the sample), in a dilute suspension, flows by gravity from one compartment to the next through the series of screen plates, the openings i-n -t'he plate in each successive compartment being smaller than in the one preceding. As with the single-plate screen; mesh wires of the Tyler series have been used, but with the introduction of one or two screens of finer mesh to the series. The fibers under the action of the -vibrating diaphragms and agitating water sprays in each compartment, pass through the screens until they arrive on one, the openings of which are too small to allow their passage. The fibers retained in each compartment at the end of the screening are washed out, filtered, dried, and weighed. Analyses of several commercial groundwood pulps and one experimental groundwood pulp were made on both screens in order to compare the results of the multiple-plate and the single-plate screens. The results shown in table 7 indicate that the two screens are not comparable except perhaps in the amount passing the 60-mesh wire. The effect of the amount of pulp used in the test is shown in the analysis of pulp G-21 (table 7) • The analyses obtained by the two screens are more comparable with ^0 grams of pulp than when greater amounts are used. In general varying the amount of pulp causes greater variations in the results from the single-plate screen than from the four plate screen. This is shown in table 8. The variability is greater in the four-plate screen for the "on SU-mesh" and passing n b0-mesh n but is less for the intermediate fractions. R88U -6- Tab 1 e 7 • — Comparison of screen analyses made with the single-plate and the four-] □late fractionating screens • Sample^: Screen • Amount :Petained: retained between Passing # t taken fo] analysis p-> • ^ vt mesh • t • ■ < . • on :24 mesh • :24 and :}2 and :42 and • • • :32 mesh: 42 mesh: 60 mesh. Number : • Grams :Percent : Percent: Percent ;:Percent Percent P 888 t • Single plate • 100 : 15.0 : 4.5 : 11.7 : 4.0 , DC": • Pour plate '• 100 : 24.5 : .4 : 2.4 : 10.4 02.3 P 895 I Single plate ■ 100 : 14.0 : 4.0 : 5*1 : 7.0 ► b5-5 • Four plate • 100 : 28.1 : .5 : 2.3 : 0.5 o2.2 P 856 \ Single plate ■ 100 : 12.2 : 8.2 : 7-5 : 5.8 : 6o.2 • Pour plate • ICO : 15.0 : .5 : 5.3 : 8.8 ■.4 p 857 • Single plate j 100 : 27. 4 : 5.4 : 7.4 : 6.6 53.1 '• Pour plate • 100 : 28.7 : .5 : 4.3 : 8.2 . 52.3 p 501 • Single plate * 100 : 5.0 : 5.3 : 6.5 : 5.2 ,-3 • Four plate ' 100 : 18. 5 : .5 : 5.0 : 11.1 . o4.5 P 502 J Single plate * 100 : 10.3 : 8.7 : 10.3 : 10.2 b( .' : Pour plate • 100 : 24. 3 : 3.7 : b.O : 10.6 , 5.4 G 21 * Single plate • 100 : 3o.O : 3-5 : 5.S : 0.7 -.2 : Four plate > 100 : 43.5 : 7.8 : 4.3 : 11.3 . 33«l G 21 ; Single plate • 72 : 37.6 : 3.2 : 7-4 : 8.5 , 42.7 • Four plate • 75 : 47.7 : 11.4 : 5.0 : 14.4 , 21.5 G 21 • Single plate • 50 : 33.0 : 6.0 : 5»0 : 12.5 . 34.5 • Four plate • 50 : 35.6 : 10.5 : 4.0 : 13o 34 . c -Commercial groundwood pulp indicated by "P", experimental by "G". R884 Figure 5 zfi in 65 r -Laboratory size four-plate diaphragm fractionating pulp screen. Table 8. — Comparison of single-elate and four-plate fractionating screens in r'v~":r-i to vri 1 ill-;.- in results caused by varying the amount of sample Fraction Variability- Single plate screen : Four plate sc/ Retained on 24 mesh. Percent 3-6 Percent 11.2 Retained between: 24 and J>2 mesh. 32 and 4-2 mesh. 42 and 60 mesh. Passing 60 mesh. . 41.2 28.8 42.0 21.8 23.9 15.0 15.8 28.2 -Calculated by dividing the • ge of all differences between values by the average of all values and multiplying by 100. The amounts of sample were 50. 75. an d 100 grams of pulp from r run No. 21. (See table 7- ) Table 9 sho-'s a number of duplicate analyses made on the four- plate screen picked at random from a large number of tests on ^round'-'ood pulp. The results show duplicability to be ffood for this type of pulp. It is emphasized, however, that although the procedure to be followed is not difficult, it is not possible to attain good duplication out attention to manipulative details. Experience has further shO'- r n thai analytical precision cannot be expected from a fractionating screen • it is for this reason that an apparatus which may employ a fairly lar - sample is to be preferred. The sample taken should be sufficiently 1- for the smallest fraction obtained to be weighed on a balance reading to a minimum of one-hundredth of a gram. All tests should be run in duplicate. The degree of duplicability'-' that may be expected on the four- compartment screen of the Forest Products Laboratory is a maximum dif- ference of 2 in fractions amounting to 10 percent or more of I ilp, a maximum difference of 1 in fractions amounting to between 1 and 9 p< r- cent', inclusive, and a maximum difference of 0.3 in fractions amoui I to less than 1 percent of the whole pul] . R884 ■7- METHODS OF USING SCREEN ANALYSIS DATA A screen analysis as ordinarily made gives from 2 to 5 values depending on the number of screens used. These may be used for comparison with a similar analysis on a pulp chosen as a standard. The use of several numbers for a test result is somewhat inconvenient and several methods have been devised to make the data more useful. Discussion of the relative merits of these methods follow. Screen Analysis Curves Screen analyses may be conveniently shown by curves. The plot of the percentages of the various fractions against the widths of the openings of the screens upon which they were retained is usually so irregular that a smooth curve cannot be drawn through the points. How- ever, if the cumulative percentages are plotted against the screen open- ing a smooth curve may generally be drawn which rises continuously from the amount retained on the coarsest screen opening to the amount retained on a hypothetical screen having openings of zero widths. Since the cumulative percentage is defined as the amount of material that would be retained on a given screen if it were the only one used in the analyses, the amount retained on the screen with openings of zero width is, of course, 100 percent. This type of curve is, therefore, useful in estimating the amounts of material that would be retained on screens not used in the testing series. Either Cartesian or semilogarithm coordinates may be used for plotting the widths of the screen openings. Both have certain advantages* Owing to a constant ratio between the widths of openings in the Tyler screen series the ordinates representing them on the uniform or arithmetical scale may become very close together as the widths decrease, whereas on a logarithmic scale these ordinates are not so compressed and thus offer a more convenient method of plotting. Since the logarithm of the screen opening equal to zero is infinity, it is impossible to draw the curve on semilogarithm coordinates between the smallest screen opening used and screen with zero openings. Thus it is impossible to estimate the amounts that might be retained on screens smaller than that used in the testing series when the plot is made on semilogarithm coordinates. Figure 6 shows several typical cumulative curves for groundwood pulp drawn on uniform or Cartesian coordinates* Screen analysis data shown in the form of curves may only be compared visually. However, certain properties of the curves may be expressed numerically and used as a means of comparison. A discussion of these factors follows. E88U Table 9» — Degree of du. | ;li?a"oi lity oi' scr- 1 Commercial pulp indicated by "P" , experimental by "G" . four-plate f; rac itionating s screen Sample-= • « • * Retained on 24 mesh • • Retained bet 1 , 2k and : 42 and 42 mesh : 80 mesh veen : 80 and : 150 mesh • : Passing -: 150 rash ■ ■ Numb< 3r '• Percent • Percent ' Percent ; Percent : Percent G 57 a b • • 4.0 3.7 • 1.6 1.4 • • lb. 4 14.5 : 19.8 : 21.2 : 58.2 : 59.2 G 80 b • * ±4.0 12. 4 • • 6.8 7.2 • • • '20.0 19.6 : 23.4 : 23.6 : 35.2 : 37.2 G l48 a b • • 2.1 2.9 • 7.9 10.0 • • 21.5 20.2 : 18.4 : 19.5 : 50.1 ; 47.4 G 217 a b • • • 4.0 4.0 s 15.0 19.4 • 11.6 11.6 : 15.4 : 15.8 : 50.0 : 49.2 G 312 a b • • 15.2 14.2 t 7.3 8.4 • 12.9 13.1 : 15.4 : 15.4 : 4^.2 : 48.9 p 999 a b • • • 17.2 18.8 ff 4.8 4.0 I 13 .4 13.6 : 20.0 : 20.8 : 44.0 ; 42.8 P 1101 a b • 18. 9 17.2 • 12.1 12.7 ■ 12.1 12.8 : lo.b : 16.3 : 40.3 : 40.9 p 1196 a b • • 11.9 12.5 • 17.3 18.1 • 15.9 lo.l : 14.8 : 15.0 : • .1 : 33.4 R884 — - - — - — u> Screen Analyses Expressed in Single Numbers A number termed the coarseness modulus has been discussed in pre- vious oublications (1,2,3)- This factor is the sum of the cumul- I centages of the screen analysis (omitting the fraction that passes t'i finest screen) divided by 100. The number is approximately proportio to the area beneath the screen analysis- cumulative percent curve or. semi- logarithm coordinates, t I n the limits of the widths of the openin of the coarsest and finest scree, s sed. The area under the curve dravn on uniform coordinates Iso be calculated. This is more complete than the area under the semi- logarithm plot because the fraction passing the finest sc be included in the calculation. Although several ways may sed to determine this are* ' . following approximation based on the summation of the areas of rectangles is sufficiently accurate and does not involve an undue amount of work: Cumulative percentages are read from the curve at intervals 0.05 mm. ,:T idth of screen openir: : Let: a r ■ ■ nt the amount r I or. the coarsest scr : b_ represent the cumulative amount for a screen openin.-": 0.05 1 .less in Lth than that of the coarsest screen; n represent the cum amount for a screen opening of 0.05 mm. in width; and s_ r< ■ r .' nt I sum of all cumulative percentage readings taken from t :urvi at 0.05 mm. intervals betveen b_ and n: The approximate area under the curve is; A = (19a + 56b + 50s + 56n + 1875) 7 1000 The area under the senilogarithmic curve, as coarseness modulus, bo calculated directly from the screen analysis data v/hereas the curve must first be drawn before calculi under tfte uniform coordiiv ; irv . Under normal circumstances I values of these areas increase as the proportion of long fibers incr^ and the proportion of the short fibers decrease. T] .in such cases they may be used as a measure of • iber 1 ;1 re- lationship is not rigid, however. It is evident thai an; cl shape of the screen analysis curve alvays indicates a ch; Lbi r- size distribution, but a change in s] \ oes no* . be a c in the area under the curve or a change in i fiber length of pul . A value found to be more closely associated to I 'age J length of the pulp has been called th< ... ... pulp"(J4). It is proposed that this rather to the "fiber length index," which is a morp descriptive term. This index may be defined as the average of the widths of * a pair of screens of which one will just permit fibers of the av< 1 ; H8SU -9- length to r>ass and the other just -permit fibers of average length to be retained. It is calculated from screen analysis data by assuming that the summation of fiber lengths per unit weight of pulp is a con- stant and that the number of fibers in a Unit weight of a .^iven frac- tion relative to the number in another fraction is in inverse ratio to the widths of the screen openings. Table 10 illustrates one method of calculation. T^ble 10. — I/lethod of calculating the fiber-length index of a. ~oulp from the screen analysis Range of screen Average of widths of •openings Factor for relative munber of fibers Relative : Relative v: e ight of: numb er pulp : of . retained: fibers between : screens : Relative total length of fibers i. esh— Widths of opening A B T : BE A3L mm. Percent : 16- 24: 24- 42: 42- SO: 80-150: 150- : 991-0. 701: 0.846 701- .351: .526 351- -175: -263 175- -104: .139 104- .0 0^2 1.00 12.3 : 12.3 : 10.4 1.61 : 2.7 : 4.3 : 2.3 3.22 : 20.0 : 64.4 : 16.9 6.09 ■ 63.O : 384. : 53-3 S.30 : 2.0 : 32.6 : 1.7 Total : 100 . n+97 • ° "" : ^84.6 Fiber-length index of the pulp. .84. 0/497. 6 = 0.170 mm. -Tyler standard. The l6-mesh screen represents, in this case, the smallest mesh through which all of the puln will pass. 2 —It will be noted that for a given set of screens the summation of ABD will be a constant eaual to 100 times the average of widths of openings of the two coarsest screens. R384 _]/_ A.TURE CI . (1) Gchafer, E.R. and Carpenter; L.A. "Screen Analysis as an Aid in Pulp Evaluation." Paper Trade J. _^C, No. l\j, 57 (May 8, l^j . (2) Schafer, E.R. and Carpenter, L.A. "Groundwood Pulp Evaluation by .'- ans of Static Bending Screen Analysis and Kate of EIovy Tests." Tech. Assoc. Papers 13,(1): 267 (May 1330) . Paper Trade J. No. 3; bl (July 17, 1930). (3) Schafer, E.R. and Heinig, Melburn. "Further Studies on Groundv/ocd Pulp Evaluation." Tech. Assoc. Papers 1J4, 1, (May i L j}l) • Pa] Trade J. £3, No. 10, 55 (Sept. 3; 1331) • (H) Schafer, E.R. and Santaholma, Matti. "Effect of Diiferent Sized Fibers on the Physical Properties of Groundwood Pulp." Tech. Assoc Papers JJ (l):U^2 (June l^) . Paper Trade J. %_, No. ly UO (Nov. ^, 1J,33). (5) Schafer, E.R. and Santaholma, katti. "Chemical Properties of Screen Fractions of Bla.ck Gum and Slash Pine Groundwood Pulps." Tech. Assoc. Papers 1]_ (l):43l (June 133*+). Paper Trade J. %]_, No. J. U6 (Nov. 5, 1^33). RggU -U- UNIVERSITY OF FLORIDA 3 1262 08928 5869