AIRICUL1URE ROOM

SCREEN ANALYSIS AS AN AID I N

PULP EVALUATIONRevised November 1939

M

SCHOOL OF FORESTR YOREGON STATE COLLEG E

CORVALLIS, OREGO N

UNITED STATES DEPARTMENT OF AGRICULTUR EFOREST SERVIC E

FOREST PRODUCTS LABORATOR YMadison, Wisconsi n

In Cooperation with the University of Wisconsin

"0

SCREEN AlgLYSI.S AS AN .AID IN PULP EVALUATION

A, resume of work at the :Forest Product s - Laboratory

By

E . R . SCHAFER,Engineer

This article supercedes a publication of the same title, (1) ; :-.and ..brings together the results of several'indepe'identinvestigations, some o fwhich have not been previously reported . The ' Laboratory first became iin-terested in the fractionation .offiber masses into length classes as,ameans of pulp , evaluation in a study of insulating board from sawmil lwaste. The fractionation was effected with an experimental flat-plat ediaphragm screen provided with a series of interchangeable slotted plate sof the type ordinarily used in pulp mill screening operations . Theapparatus was later modified for application to evaluating groundwoo dahd chemical pulps by substituting . wire screens for slotted plates .Further study brought improved ef`iciency• through the development o fmaltiple"plate screeriin which the stock flowed through four compartment seach fitted`with a:screen of different site openings .-

Experiments will be described of the screen analysis of insulating board pulps, groundwood pulps, and chemical pulps with both the single an d

multiple diaphragm plate sereea-as well as a limited amount of work with . :.the Bauer-McNett Classifier .

SCREEN ANALYSIS . OF INSULATING BOARD STOCK S

Single Plate Screen

The plates used in the single-plate diaphragm screen (see fig . 1)were 10 inches wide .by l2 inches long .and contained 6o slots, eack 4 -inches long. The widths of the slots in the series of plates . were 0 .020 ,0 .015, 0.012, and o.OCg inch, respectively . The material was firs tscreened on the screen having the smallest slots . The fraction of thematerial passing through the screen was caught in a drain box with a

Acknowledgement is made to the Appleton Machine Company, Appleton, Wis . ,for cooperation in working out detail s , of, design, ,of; this screen an dfor building models in the various stages of development . . The machin eis commercially known as the Appleton Selective Screen ,

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Fourdrinier wire bottom.. Thescreen ,plate was then replaced by the nex tlarger in the series and the screening rep'a tads _ ...' :T e

ter ,lduring eachscreening was vigoroi.sly .,agitated on the screen with water sprays . Onehundred grams of the original fi'b e 'r ' required about- lO r minirtes screening oneach plate to effect a practical separation . Continued screening cause dthe various sized fractions to blend into each other . The various frac-tions obtained in this way were then dried and weighed .

The screen analysis and other 'propertie's-of three well-known com-mercial insulating boards and one of an 'experimental Douglas fir board aregiven in table 1 . These boards are composed of various materials that hav ebeen processed in different ways . It is, therefore, not surprising t ofind no general relationship between the strength properties and th escreen analysis . Nevertheless, a change in the fibrous composition o fany one of them would undoubtedly influence not only the strengt hproperties but its density, formation, insulating value, and appearance .Having developed a, product of, satisfactory quality, the screen analysis ,therefore, presents, a means of Oontrol'in manufacture . Plate 1 shows thefiber aggregates obtained from insulating boards .

The Bauer=McNett -Classifie r

The .Bauer-McNett Classifier is a ` four compartment apparatus in whic hthe stock flews . by gravity through a'-mes h ' wire screen from one compartmen tto the next . The screen plate"is set vertical in the . compartment and apropeller agitates the stock suspension in a=manner claimed•to orient th e

fibers parallel to the screen surface . Ten grams of material are used i neach test . The fiber .distribution of seven commercial insulating boar dstocks as obtained :on this classifier are given in table 2 . .

SCREEN ANALYSIS OF GROL'NDWOOD PULP

Since groundwood pulp will practically all pass through a plat ewith the usual 0 .00 g-inch width slots, the separation of this type o f

pulp must be made with screens having openings smaller than those obtain -

able in the form of slots . For this purpose a series of screens con-sisting.of various mesh wires of the Tyler standard screen scale wer e

soldered to perforated brass plates . The screens were 24, 32,-42, and 60mesh of. the Tyler series which, respectively, have screen openings o f0 .701, 0.495, 0 .351, and 0.246 mm. in width. The ratio of the width of

screen opening in one screen to that 'of the next one in .-this series. i sapproximately equal to the square root of 2 .-

The procedure for the single-plate screen consisted in using thes e

plates in the apparatus in the order of their size, starting with th esmallest, The fraction of the material passing through the screen wa scaught in a catch screen the bottom of which was made up of a piece o f

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on 0.008 inch width slot s

Passing through 0 .008 inchwidth slot s

PLATE 1

Fibers obtained in the fractionation of a pulp used for insulat-ing board. Reduced to 0 .9 natural size .

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Table 3 .--Fiber sizedistributionof the various screen fractions o fgroundwood pulp

Fiber length : Passing 60 :

Retained :

Retained

Retainedrange

: mesh screen :

between

:

between

:

between: 142 and 60 mesh:32 and 42 mesh; 24 and 32 mes h

:

screens

:

screens

:

screens

From : To

Fibers

Fibers

?.ers

Fiber smeasured

measured

:

mei. ;_17-.ed

measured------------------ -

mm. : mnn. :Number : Percent : Number : Percent: Numbe :c• J. °cent : Number: Percent

0

0 .073 : 489

: 65 .70 : 2 :

0.40

: 3

: 0 .82 :0 .073 :

.147 :

139

:

18 .70 : 19 :

3 .76

. 8

: 2019 :.147 .

.220 :

78

:

10 .50 : 41 8 .12

. 9

t, 2147 :37 •

7 .33

. 8

: 2

9.220 :

.294 :

15

:

1.92 ..294 :

.368

.

8

:

1.07 : 54 :10.70

: 11

: 3 .02.36s

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4

: .54 : 49 :

9.70

: 11

: 3 .52.44o

.515 :

7

: . 94 : 53 :1o.50

: 22

: 6 . 0.2-41 :-.515

:

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1

: .15

: 72 :14.20

: 22

: 6 .02 :.588 :

.662 .

1

: .15

: 28 :

5 .52

. 46

: 9 .85

:.6o2

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6 .52

: 21

: 5 .75 :.736

:

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:

4 .36

: 34

: 9 .32

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2.58

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: 11 .50 ..884

:

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3 .76

: 26

: 7 .13

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3 .97

: 17

: 4.65 :1 .030

:

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1 .38

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: 2 .74 :1 .102 .

1 .175

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1 .78

: 11

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: 6 .03

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1 .322 : 3

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: 121 .322 :

1 .398

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1 .470 : 3

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: 83

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: 2 .430

: :

3

: 0 .45 ; 1

: 40 .20

:

: 1410

51 .46

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745

: 100

: 505

: 100

:

365

: 100

: 342

: 100

10

14 :

1 .171 :

.292 :

.5 92 :

.5 93 :

.883 :

.88Above 0 .662 :Above 1 .986 irrn .:Above 2 .060 mm.:,Above 2 .430 mm.

80 mesh wire . Ordinarily no attempt was made to catch the material passin g

through the 60-mesh screen . One hundred grams on the basis of weigh t

of oven-dry pulp was used for the test . After screening, all othe r

fractions were dried and weighed and the portion cassing through th e

60-mesh screen was obtained by subtraction . When it was desired to ob-tain a auantity of the fine fraction to determine its properties, a piec eof linen toweling was laid over the wire screen in the catch box . Bythis method more than 75 percent of the fiber passing the 60-mesh scree n

was reclaimed .

The appearance of the different fractions obtained with this serie sof screens is characteristic and indicates a fairly definite separatio nof the fibers into groups according to their size (3) .

Table 3 shows the results of fiber measurements made to determin e

the fiber-size distribution in the various fractions . The data represen tthe average of five commercial groundwood pulps . The five pulps wer efractionated and the fiber measurements made on each fraction . The fivesets of fiber measurements for a given fraction were then combined . From350 to 500 measurements were made on each of the fractions retained o nthe 24, 32, 42, and 60-mesh screens, and because of the inaccuracy in-volved in measuring very short fibers, about 750 measurements were mad ein the material passing the 60-mesh screen .

The manner in which the fiber lengths tend to group themselve sabout the average, or the freauency distribution, was determined by th enumber of fibers falling within a given range of length instead of th e

__number of fibers of a given single length . This was due to the fac tthat even with the apparently large number of measurements taken, th erelation of the percentage of fibers of a given length to that of th enext higher or lower length was very irregular and therefore, a curv edrawn through such data would not present a true picture of the normaldistribution of the fiber lengths . A much larger number of measurement swould have smoothed out these irregularities and permitted the use o fsmaller ranges than reported here . The average fiber length, the standar ddeviation, and the coefficient of variability of each fraction are show nin table 4 . A high degree of variability is indicated in all fractions ,that for the material passing the 60-mesh screen being almost twice a sgreat as the other fractions .

Table L+ . ;--Average fiber length, standard deviation, and coefficient o fvariability of groundwood screen fraction s

Fraction

: Average fiber : Standard

: Coefficient o f•

length

: deviation

variability : . - ..

Passing 60-mesh 0 .079 7Retained between:

42 and 60-mesh .5675

32 and 42-mesh .8120

24 and 32-mesh :

1 .11-1.0

4

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mm . Percen t

+0 . 0796

99 . 9

. 334

58 . 9

.365

45 . 0

.513

56 . 1

-3-

The abscissas of the pofints on the curves show:n in. figure '3 are:

plotted as the midpoints of the range of length chosen . In spite •o-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'o f

screens needed will depend on the characteristics of the pulp and th e

degree of separation desired . In some cases a sharp separation into fibe r

classes may not be necessary . If•a sharp separation is desired, a-studysuch 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 (4,) . These investigations have indicated some very definite re-lationships between ,fiber size distribution and pulp properties . .

11 '

SCREEN ANALYSIS OF CHEMICAL PUL P

The screen analysis has been found useful in studies of the effec tof beating . Then beating a mixture of spruce and birch sulfite pulps forglassine stock, samples were taken from the beater at intervals an d

development of hydration and strength of-the pulp followed by means ofthe'freenessand strength tests . The cutting action of the beater ril lwas followei .b'y-the screen analysis, in this case with the single-plat escreen . The results of the analysis are shown in figure :4 .

-

It may be noted that the cutting action of the roll, which was o f

stone Was rapid 4uring the first .120 minutes . Cutting was less rapid, dur-ing the . next80 minutes but increased again during the last period of 4 0minutes : It may be noted further that an increase in the amount o fmaterial that would pass the 60-mesh screen was accompanied with anapproximately equal decreas.e in the amount retained on the 24-mes hscreen . The intermediate fractions remained practically constant i namount throughout . This example is cited to illustrate a use the scree nanalysis may serve in determining That 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 th eproduction of bond paper from bleached sulfite pulpi Th4 4creening wa sdone on the four-compartment screen .showh in figure 5- . The analysi sindicates the trend of the processing effect on fiber distribution .The percentage of fiber retained on the ' 24-mesh screen was redu.ced - -proportionately as much by beating as by jordaning although the beatin gtreatment was very mild . . The effect of beating on increase of th eamount of material passing 115-mesh screen was nearly twice as great a sthe effect of jordaning the beaten pulp . Generally speaking more thantwo-thirds of the fiber size reduction ofthe "on 24-mesh" material b ybeating appeared as increase in the amount of "through 115-mesh" materia lwhereas only a little mbre than one-third of the size reduction of "24 -mesh material ft by jordaning the beaten stock appeared as fine material .

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Table 5 .--Screen apalysis .of_unbeaten and processed bleached sulfitepulp

Processing: ,_ := Retained.

'Retained between

: Passingtreatment

on

: -= :,115-mesh24 and

42 and : 80 andU2=mesh 80-mesh :115-mesh

Percent

Pexrcent' : Percent . Percent . ,percent

Unbeaten c

75 .0

6 .)4.

4+ .3

'319

10.4 ,

After beating . . . . :

58 .4

10 .1

:

6 .5

3 .2 .

22. 0

After jordaning . . : .

45 .0

:

14.o

:

10.0

4 .0

, 27 . 0.5 r

It . .was also observed in this investigation that the percentage o ffine material in the furnish was associated with the porosity of thefinished paper . The data in table 6 show that a higher porosity valu e(more dense sheet) was obtained with furnishes containing the highe rproportions of fines .

Table 6 .--Influence of amount of fines in the furnish on the porosityof sulfite bond pape r

Machine

:

Proportion of furnish

: Porosity of shee trun

passing 115-mesh

by Gurley: :

densometer: After jordaning : At head box :

---------------------------------------------------------------------Number Percent

Percent Seconds

1257 : :

33 .2 3601272 35 .6 2301261 30.0 1431267 : :

9 .3 561259 1y.2 50

R884

STUDIES ON THE SCREENING PROCEDURE '

In the ori-Omal screen analysis test employing' asingle-plat ediaphragm screen it was necessary to interchange screen plates for eac hfraction. This appaus was improved upon by building_ four diaphragmscreens into- olie . machine . In the improved machine - each compartment con-tains a,: $cregn plat e: ;with different size- of. openings than the other three .The screeni - shown -in figure 5 . I-n operation, from 50 to 100 grams o fthe pulp (depeh$ .ng on the-,fiber size distribution Of the sample), in adilute suspension., flows by gravity from one compartment to the nex tthrough the series . of screen plates, the openings in the plate ineac hsuccessive compartment being . smaller than in the one preceding . As withthe 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 th eseries : The fibers under the action of . the vibrating diaphragms andagitating water sprays in .eaeh compartment, pass through . the- screensuntil they arrive on one, the openings of which are too small to allo wtheir-passage . The fibers retained in•each compartment'at the end of th escreening are washed out, filtered, dried, and-weighed ,

Analyses of several commercial groundwood pulps and one experimenta lgrow:ndwoodpulp were made on both screens in order to compare the result softhe multiple-plate and'the single-plate screens . The results shown-intable 7 ' indicate that-the two screens are not comparable except perhap sin the amount passing the 60-mesh wire .

The effect of the amount of pulp used in the test is shown in theanalysis of'pulp Gi21 (table 7) . The analyses obtained by the two screen sare'more comparable with 50 grams of pulp than when greater amounts ar eused. In general varying the amount of pulp causes greater variations i nthe -results"from the .,single-=plate screen than from the ■ four dilate ' so ' I

This isshown in table -. -The variability is greater in the four-pla tscreen for the non 24-mesh" and passing "60-meshn but is less for the

intgrtnpdiate fractions . ' :

-

-,

Table 7 .--Comparison of screen analyses made with the single-plate andthe four-plate fractionating screen s

Samplel :

Screen Amount

:Retained :

Retained between

:Passingtaken for :

on

: :o0-mesh:analysis :24 mesh :24 and :32 and :42 and :

:32 me sh : 42 me sh: 60 mesh:

Number Grams

:Percent Percent :Percent :Percent :Percent

P 888 : Single plate : 100 19 .0

: 4.9

: 11 .7

: 4 .0

: 60 . 4Four .plate 100 24 .5

: .4

: 2 .4

: 10 .4

; 62 .3

P 895 : Single dilate 100 l4.o 4 .o 9.1

: 7,0

: 65 . 9Four plate

;:; 100

: 28 .1

: .5

: 2 .3

: 6 .9

: 62 . 2

P 896 Single plate 100 12 .2

: 8 .2

. 7 .5

: 5 .8

; 66 . 2s Four plate

. 100 19 .6

: .9

: 5 .3

: 8 .8

: 65 . 4

P 097 :

Single plate 100 27 .4

: 5 .4

: 7 .4

: 6 .6

: 53 . 1Four plate

. 100 28 .7

. .5

: 4 .3

: 8 .2

: 58 . 3

P 901 : Single plate 100 9 .0

: 5 .3

: 6 .9

: 9 .2

: 69 . 3Four plate

. 100 18 .5

: .9

: 5 .0 11 .1

: 64 . 5

P 902 :

Single plate 100 10.3

: 8 .7 10 .3

: 10.2

: 60 . 5Four plate

. 100 24 .3

: 3 .7

: 6 .0

: 10 .6

; 65 .4

G

21 : Single plate 100 36 .0 3 .3 5 .8

: 6 .7

: 4 g . 2Four plate

. 100 43 .5

: 7 . g

. 4 .3

: 11 .3

: 33 . 1

G

21 : Single plate 72 37 .6

: 3 .8

: 7 .4

: 8 .5

: 42 . 7Four plate

. 75 4707 11 .4

. 5 .0

: 14 .4

: 21 . 5

G

21 : Single plate 50 38.0

: 6 .0

: 9 .0

: 12 .5

; 34 . 5Four plate

. 50 35 .6

: 10 .9 4 .o

: 13 0 5

; 34 .0

-Commercial groundwood pulp indicated by "P", experimental by "G" .

R884

Figure 5 .--Laboratory size four-plate diaphragmfractionating pulp screen .

zM 16105 F

O

Table g .--Comparison of single-plate and four-plate fractionating screen s

in regardtovariability in results caused by varying th e

amountofsample

Fraction

Variability-

Single plate screen : Four plate scree n

Percent

Percent

Retained on 24 mesh :

3 .6

11 . 2

Retained between :24 and 32 mesh 41 .2

23 . 932 and 42 mesh 2g . g

15 . 0

42 and 60 mesh 42 .0

15 . g

Passing 60 mesh 21 .g

2g . 2

-Calculated by dividing the average of all differences between value sby the average of all values and multiplying by 100 . The amount sof sample were 50, 75, and 100 grams of pulp from grinder run No . 21 .

(See table 7 . )

Table 9 shows a number of duplicate analyses made on the four -plate screen picked at random from a large number of tests on groundwoo dpulp . The results show duplicability to be good for this type of pulp .It is emphasized, however, that althougn the procedure to be followed i snot difficult, it is not possible to attain good duplication withou tattention to manipulative details . Experience has further shown thatanalytical precision cannot be, expected from a fractionating screen an dit is for this - ,reason that an apparatus which may employ a fairly larg esample is to be preferred . The sample taken should be sufficiently larg efor the smallest fraction obtained to be weighed on a balance reading t oa minimum of one-hundredth of a gram . . A11 tests should be run i nduplicate .

The degree of dupli,cabil6ety 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 the pulp ,a maximum difference of 1 in fractions amounting to between 1 and 9 per-cent, inclusive, and a maximum difference of 0 .3 in fractions amountin gto less than 1 percent, of the whole pulp .

Rgg4

- 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•ctmparisonwith a similar analysis on a pulp chosen as a standard . The use o fseveral numbers for a test result is somewhat inconvenient and severalmethods have been devised to make the data more useful . Discussion o fthe relative merits of these methods follow .

• Screen Analysis Curve s

Screen analyses may be conveniently shown by curves . The plo tof the percentages of the various fractions against the widths of th eopenings of the screens upon which they were retained is usually s oirregular that a smooth curve cannot be . drawn. through the points . Ho yv-

ever, if the cumulative percentage 's are. plotted against the screen open-ing a smooth curve may generally be drawn which rises continuously fro mthe amount retained on the coarsest screen opening to the amount retaine d

on a,hypothptical screen having openings of zero widths, : Since thecumulative percentage is defined as the amount of material that would b eretained 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, o fcourse, 100 percent . This type of curve is ., therefore, useful inestimating the amounts of material that, would. be retained on screen snot used in the testing series ,

Either Cartesian or semilogarithm coordinates may be used fo rplotting the widths of the screen openings . Both have certainadvantages. Owing to a constant ratio between the widths of openings

in the Tyler screen series the ordinates representing them on th euniform or arithmetical scale may become very close together as th e

widths decrease, whereas on a logarithmic scale these ordinates are no tso compressed and thus offer a more : convenient method of plotting . •Since the•logarithm of the screen open-lag equal to zero is infinity ,it is impossible to draw the curve on- stiemilogarithm coordinates betwee nthe smallest screen opening used and screen with zero openings . Thus

it is impossible to estimate the amounts that might be retained o nscreens smaller than that used in the testing series when the plo tis made on semilogarithm coordinates . Figure 6 shows several typicalcumulative curves for groundwood pulp drawn on uniform or Cartesiancoordinates* .

Screen analysis data shown in the form of curVes may o y be •compared visually . However, certain properties of the curves )

b eexpressed numerically and used as a means of comparison . A di®u's+ioAof these factors follows .- .

R884

Table 9.--De ree of du)licabilit of screen analyses made with thefour-plate fractionating scree n

Sampl el : Retained :on

Retained between

: Passing----_- --------------------- 150 mesh

Number

:

24 mesh

: 24 and42 mesh

:

42 and

:

g0 and

.:

80 mesh :

150 mesh

:

Percent: Percent

: Percent

: Percent

: Percent

:

G 57 a 4 .o 1 .6 16 .4 19 .8 58 . 2b 3 .7 1 .4 14.5 21 .2 59 . 2

G 80 a 14.0 6 . g 20 .0 23 .4 35 . 8b 12.4 7 .2 19.6 23 .6 374 2

G 148 a 2 .1 7 .9 21 .5

: 18 .4 50 . 1b 2 .9 10 .0 20 .2 19 .5

: 47 . 4

G 217 a 4 .0 19 .0 11 .6 15 .4 50. 0b 4.0 19.4 11 .6 15 .8 49 . 2

G

312 a 15 .2 7 .3

: 12 .9

: 15 .4 49 . 2b 14 .2 g .4 13 .1 15 .4

: 48 . 9

999 a 17 .2 4 .8 13 .4 20 .0 44 . 6b 18 .8 4 .0 13 .6 20 .8

: 42 . 8

P 1101 a 18 .9 12 .1 12 .1 16 .6 40 . 3b 17 .2 12 .7 12 .8 16 .3 40 . 9

P 1196 a 11 .9

. 17 .3 15 .9 14 .8 40 . 1b 12 .4 18 .1 16 .1 15 .0 38 .4

1Commercial pulp indicated by 'P') experimental by "G" .

Rgg4

no =•vr_gla. ,.

i

•

1'

--p -1---t''.

1

E

i

1

L

oo

-a--I .

1-

'D

0 o Oto

o

o 0 0

(41190.10a

peLIT"eq.G .1 4T.12TGn2 OAT1.13TTLUMD)

•

Screen Analyses Expressed in Single Number s

A number termed the coarseness modulus has been discussed in pre-vious publications (1,2,1) . This factor is the sum of the cumulative per-centages of the screen analysis (omitting the fraction that passes th efinest screen) divided by 100 . The number is approximately proportionalto the area beneath the screen analysis-cumulative percent curve on semi -

logarithm coordinates, between the limits of the widths of the opening sof the coarsest and finest screens used .

The area under the curve drawn on uniform coordinates may als obe calculated . This is more complete than the area under the semi -logarithm plot because the fraction passing the finest screen may b eincluded in the calculation . Although several ways may be used t odetermine this area the following approximation based on the summatio nof the areas of rectangles is sufficiently accurate and does not involv ean undue amount of work : Cumulative percentages are read from the curv eat intervals 0 .05 mm . width of screen opening :

Let :a represent the amount retained on the coarsest screen used ;

b_ represent the cumulative amount for a screen opening 0 .05 mm . less i nwidth than that of the coarsest screen ; n represent the cumulative amoun tfor a screen opening of 0 .05 mm . in width ; and s represent the sum of al lcumulative percentage readings taken from the curve at 0 .05 mm . interval sbetween b and n :

The approximate area under the curve is :

A = (19a + 56b + 50s + 56n + 1875)

1000

The area under the semilogarithmic curve, as represented by th ecoarseness modulus, may be calculated directly from the screen analysi sdata whereas the curve must first be drawn before calculating the are aunder the uniform coordinate curve . Under normal circumstances th evalues of these areas increase as the proportion of long fibers increas eand the proportion of the short fibers decrease . Therefore, in suchcases they may be used as a measure of average fiber length . The re-lationship is not rigid, however . It is evident that any change in theshape of the screen analysis curve always indicates a change . in fiber -size distribution, but a change in shape does not always denote a chang ein the area under the curve or a change in average fiber length of th epulp .

A value found to be more closely associated to the average fibe rlength of the pulp has been called the "average-screen opening for th epulp"(4) . It is proposed that this rather awkward phrase now be change dto the "fiber length index," which is a more descriptive term . Thi sindex may be defined as the average of the widths of the openings in a

1111

pair of screens of which one will just permit fibers of the averag e

Rg 8 1+

-9-

length to pass and the other just permit fibers of average length t o

be retained . It is calculated from screen analysis data by assumin g

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 given frac-tion relative 'to the number in'another fraction is in inverse rati oto the widths of the screen openings . Table 10 illustrates one method

of calculation .

Table ' 10 .--1%:ethod of- calculatin• the fiber-leri th index of a pul • fro mthe screen analysi s

Range of screen :Average : Factor :Relative :Relative : Relative : of

for

:weight of : number

total

i.-esh- : Widths of : widths :relative : pulp

of

length. opening

of

:mumber of : retained : fibers

o f:openings : fibers : between :

fibersf

screens

A

B

D

BD .

:

ABD

mm .

: Percent .

16- 2 )4 : 0 .991-0 .701 : 0 .846 : :

1 .00 : 12 .3

: 12 .3

: 10 . 4

214- 42 :

.701- .351 :

.526 :

1 .61 :

2 .7

4 .3

2 : 3

42- 80 :

.351- .175 :

.263 :

3 .22 : 20 .0

: 64 :4

. 16 : 9

80-150 :

.175- .10 14 :

.139 .

6 .09

.

63 .0

: 384 .•o

53 . 3

150- 0 :

.10 1+- .o :

.052 :

16 .30 : 2 .0 : 32 .6 1 . 7Total : 100 .0

: 1+97 .

. L8 1+ . Z

Fiber-length index of the pulp . .8 14 .6/497 .6 = 0 .170 mm .

-Tyler standard . The 16-mesh screen represents, in this case, th esmallest mesh through which all of the pulp will pass .

?It will be noted that for a given set of screens the summation o fABD will be a constant equal to 100 times the average of width sof openings of the two coarsest screens .

R88 1 +

-lC-

.. -: •v

LITERATURE CITED

yam., .

(1) Schafer, E.R. and Carpenter, L.A. "Screen Analysis as an Aid i nPulp Evaluation ." Paper Trade J .

, No . 19, 57 (May g , 1930) .

(2) Schafer, E .R. and Carpenter, L .A. "Groundwood Pulp Evaluation byMeans of Static Bending Screen Analysis and Rate of Flow Tests ."

{Tech . Assoc . Papers li,(l) :267 (May 15530) . Paper Trade J . 91 ,No . 3, 61 (July 17, 1930) .

(3) Schafer, E .R. and Heinig, Melburn . "Further Studies on Groundwoo dPulp Evaluation ." Tech. Assoc . Papers 14, 1, (May 1531) . PaperTrade J .

, No . 10, 55 (Sept . 3, 1931) .

(4) Schafer, E .R . and Santaholma, Matti . "Effect of Different SizedFibers on the Physical Properties of Groundwood Pulp." Tech .Assoc, Papers

(1) :442 (June 1934) . Paper Trade J.

, No . 194o (Nov . 9, 1933) .

(5) Schafer, E .R. and Santaholma, Matti . "Chemical Properties of Scree nFractions of. Black Gum and Slash Pine Groundwood Pulps ." Tech.Assoc . Papers 13 (1) :#31 (June 1934) . Paper Trade J. 0j, No . .1946 (Nov . 9, 1933)