U. S. DEPARTMENT OF COMMERCE

DANIEL C. ROPER, Secretary NATIONAL BUREAU OF STANDARDS

LYMAN J. BRIGGS, Director

NATIONAL BUREAU OF STANDARDS MISCELLANEOUS PUBLICATION M124

PAPER PULP FROM CEREAL STRAWS

BY A MODIFIED SULFATE PROCESS

By

BAKER WINGFIELD, E. R. WHITTEMORE, C. B. OVERMAN

O. R. SWEENEY, and S. F. AGREE

Issued August 4, 1936

UNITED STATES

GOVERNMENT PRINTING OFFICE WASHINGTON : 1936

For sale by the Superintendent of Documents, Washington, D. C. Price 5 cents

M124

PAPER PULP FROM CEREAL STRAWS BY A MODIFIED

SULFATE PROCESS

By Baker Wingfield, E. R. Whittemore, C. B. Overman, 0. R. Sweeney, and

S. F. Acree

ABSTRACT

A modified sulfate process is outlined for the conversion of oat, rye, and wheat

straws to pulps which would be suitable for the production of typewriter, book,

and other grades of paper. The variables in digesting, beating, screening,

and bleaching were investigated separately for the optimum conditions. Pulps

of the above grades were produced in large laboratory equipment. Test sheets from these pulps showed a yield of 38 to 43 percent of dry weight of raw material

for wheat straw; 42 to 46 percent for rye straw; and 40 to 45 percent for oat

straw. CONTENTS

Pago

I. Introduction 1 II. Equipment used 2

III. Experimental procedure 3

1. Mechanical pretreatment 3

2. Presteep 3

3. Chemical cooks 4

(a) Pressure and time studies 4

(b) Chemical concentration and interchemical ratio 5

4. Washing 6

5. Beating 7

(a) Mechanical fiber treatment 7

(b) Mechanical fiber treatment combined with partial

bleaching 7

6. Screening and screen operation 7

7. Concentrating 8

8. Bleaching 8

9. Formation 9

10. Testing 9 IV. Discussion and comparison of the process with wood processes 10

V. Summary and conclusions 11 VI. References 12

I. INTRODUCTION

The total production of wheat, oat, and rye straw in the United

States during 1931, assuming a low yield value of three-quarters of a ton per acre and using the figures for acreage in production given

in the 1932 Yearbook of Agriculture, was 84,621,000 tons, of which

60,039,000 tons was produced in the North Central area and Oklahoma and Texas. The distribution of tonnage in the latter figure was 48.5 percent of wheat, 46.6 percent of oat, and 4.9 percent of rye. Approximately 25,000,000 tons of paper could be produced from the

69444—36 1
2 Miscellaneous Publications, National Bureau of Standards

straws grown in this central area of the United States. This is more

than twice the quantity of paper now consumed annually in this

country.

Straw is composed of cellulose, lignin, pentosans, and small amounts

of other materials. The presence of a large amount of cellulose

fibers in straws first suggested their use as a raw material for the

manufacture of paper pulp. They have been employed from time

to time in this country for the production of paper, but recent eco- nomic causes have limited such use. Approximately 600,000 tons

is used annually for [1]

1 the making of strawboard and egg-case

paper. In other countries such as Argentina and Chile, the Pomilio

[2] process is being successfully used for converting straw into highquality papers. In certain European countries other processes have

been adopted to some extent. Chlorination processes, such as Pomilio's [3], Traquair's [4] (Mead Co.) and others, are patented. These

all require chlorine and caustic, which are not available at sufficiently

low prices in the. Straw Belt of the United States at present; conse- quently, these processes were eliminated as possibilities in this study.

Of the other methods in general use in the paper industry—the soda,

sulfate, and sulfite—the sulfate process was chosen because a good

yield of strong pulp can be obtained by its use. The initial experimental work made it apparent that in most respects commercial

pulping conditions for wood are too drastic for straw. The factors

in each process step were therefore investigated sj^stematically. The

plan was to develop on a large laboratory scale a process specifically

suited to the production of a high yield of strong pulp from straw,

and to define this process within fairly narrow limits.

II. EQUIPMENT USED

Digester A, figure 1, was a 15-gallon insulated steel shell, arranged

for indirect heating by means of a steam jacket surrounding the sides

and bottom, with a thermo-siphon, B, connecting at the top and bottom of the shell. It was fitted with a charging hole at the top and a

sieved discharge at the bottom. Expansion chamber C, figure 1, fitted with a combination bleeder and blow-off line, was connected

with the cover of the charging hole. Beating engine A, shown in figure 2, was a 12-lb hollander type.

The tub and roll were iron and the adjustable roll turned above a

fixed V-bar bedplate. Both roll and bedplate were wood filled. Screening apparatus B, in figure 2, was of the hinged, jig type,

built around a typical 1- by 4-ft commercial bronze screen plate,

machined with 0.004 in. slots. The stock compartment extended

18 in. above the plate and the enclosed space was fitted with gear- driven, bent brass stirring rods.

Purifier C, figure 2, was a rotating basket with solid walls, open at the top and fitted with several concentric retaining baffles on its inside periphery. It was 8 in. in diameter and 6 in. deep and was mounted in a steel shell. The basket revolved at 350 rpm and had a capacity of 2 to 3 gallons of liquid per minute.

Pulp concentrator D, in figure 2, was similar to a trommel type of screen. It consisted of a double-wire cylinder, no. 80 screen inside,

reinforced with no. 30 screen on the outside, 4 ft long, and 11 in. in

i Figures in brackets throughout this paper refer to the numbered references on page 12.

NBS Miscellaneous Publication Ml 24

Figure 2.Pulping and screening equipment.

A, Beater; B, screen; C, purifier; D, concentrator; E, jordan.
Paper Pulp from Cereal Straws 3

diameter. Two baffles were mounted on the inside surface in the

form of screws. The cylinder was pitched 0.5 in. /ft downward toward

the discharge end and rotated at 15 rpm.

The jordan E, in figure 2, was of the standard type, 18 in. long, 8

in. in diameter at the inlet, and 11 in. in diameter at the discharge

end. It operated at 400 rpm.

The sheet forming apparatus consisted of four parts; the mold, the

sheet frame, the grid, and the suction box. The mold and the

suction box were each 9 by 12 by 4% in. cypress boxes. The grid

forming the top surface of the suction box was a perforated brass

plate faced with no. 30 bronze screen. The sheet frame consisted of

two hollow frames with 9 by 12 in. openings between which were clamped a no. 80 screen and a no. 30 reinforcing screen. The hydraulic press was a diaphragm type with 9 by 12 in. bronzefaced platens. The press was equipped with a hand-operated hydraulic pump capable of producing a pressure of 200 lb/in.2 on the

plates.

The drying roll was a hand-operated, steam-heated dryer with a

surface of polished, nickel-plated steel, which was 12.5 in. in diameter and 16 in. in length. The drier felt was heavy canvas wrapping

around three-quarters of the roll. The testing equipment consisted of a standard MIT folding

tester, the usual tensile machine, and a Mullen burst tester. All

physical testing was done at Iowa State College in a room maintained at 70° F and 70-percent relative humidity.

III. EXPERIMENTAL PROCEDURE

Many factors influence the preparation of paper pulp from any raw material. In this study, the effects of varying each factor

discussed below were determined with the other conditions constant. These results made it possible to establish a fairly specific range of

conditions for the process.

1. MECHANICAL PRETREATMENT

In the manufacture of paper from straw abroad it is common

practice to shred the straw before chemical treatment to facilitate the

loading and blowing of the digesters and, in general, the handling of

the material. However, since the design of the digester used in this

work was such that the cooked pulp was more easily washed in the

digester, and more easily removed if whole straw was used, the

stalks were used directly from air dry bales.

2. PRESTEEP

It was found that a preliminary treatment of the raw straw with

hot water greatly improved the color of the cooked straw and decreased the amount of chemical needed in digesting and bleaching.

Steeping at atmospheric pressure was found to be practically as

effective as pressure steeping in its results, though it removed slightly

less material than could be dissolved at higher pressures. This pre- treatment lowered the amount of chlorine necessary for bleaching

by 2 percent.
4 Miscellaneous Publications, National Bureau of Standards

3. CHEMICAL COOKS

The cooking of cellulosic materials for the preparation of paper

pulp is one of the most important phases of the process. In this

step, varying any of a number of factors will affect the product.

These factors are: pressure and temperature, the duration of cooking, the ratio of material to water, the total amount and concentration of chemicals, and, in the sulfate cook, the proportion of sodium

sulfide to sodium hydroxide. The best conditions for these factors are given in the following sections, with the exception of the straw- to-water ratio, which is the maximum for the digester used—1 part

of straw to 9 parts of water, by weight. Twelve pounds of air-dried

straw was about the maximum amount which could be conveniently

charged into the digester, and this weight was used throughout the

work. 108 lb of water was required to cover the material and

prevent undercooking at the surface.

(a) PRESSURE AND TIME STUDIES

Experiments were conducted with wheat straw at gage pressures

of 35, 50, 65, and 85 lb/in.2 with variations in cooking period of 1,2,

3, 5, and 7 hours, using 10 percent of total chemical based on the

air-dried straw, in the ratio of 3 parts of sodium sulfide to 7 parts of caustic soda, by weight, in each case. Each run was followed by

90 min. of beating, circulated screening, and two-stage bleaching.

The results are given in table 1. Table 1.

Effect of pressure and duration of cooking on the yield, strength, and

bleaching quality of pulp made from wheal straw digested with 3 percent of sodium

sulfide and 7 percent of caustic soda

Run

Digestion Properties of band sheets Yield; weight

of paper

XlOO-h weight

of straw

Bleaching

quality Gage

pressure Time Weight i Bursting 2 strength

Folding 3 endurance

Tensile 4 breaking

load

Double

lb hr lb Points folds kg %

120—. ._ 35 3 48.1 43 1,200 6.7 45.2 Fair.

121 35 5 44.5 42 1,200 6.6 45.3 Good.

122 35 7 47.0 33 1,000 6.9 44.0 Do.

20.. 50 1 47.3 39 360 6.3 31.0 Fair. 123 50 44.1 42 930 6.3 41.7 Good.

92. 50 5 49.6 40 780 6.4 44.4 Do.

107.. 65 1 47.6 41 790 6.0 41.8 Good.

106 65 2 50.6 47 990 6.5 42.1 Do.

101.... 65 3 52.5 46 1,300 6.6 44.4 Fair. 110 85 1 51.1 47 1,300 6.7 44.6 Fair. 104 85 2 44.0 40 920 6.4 44.2 Very poor.

102. 85 3 53.6 50 1,600 6.6 36.8 Do.

1 The figures given are typical and represent the weight of 500 sheets, 25- by 40-in.

2 Bursting pressure, in pounds per square inch, through a circular orifice of 1.2 in. diameter.

3 For test specimens 19 mm wide and 90 mm between jaws.

4 For test specimens 15 mm wide and 90 mm between jaws. It may be seen that all the high-pressure cooks yielded pulps with

poor bleaching quality, and that excessive cooking time was required

for good bleaching quality in low-pressure cooking. From a consid-
Paper Pulp from Cereal Straws 5

eration of the strength and bleaching quality of the pulps, and the

time and steam requirements for different cooks, it appeared that

50-lb pressure for 3 hr was optimum.

(b) CHEMICAL CONCENTRATION AND INTERCHEMICAL RATIO

The three straws—wheat, oat, and rye—were studied throughout a range of chemical concentrations and interchemical ratios, cooking 3

hours under 50 lb pressure and using uniform practice in steeping,

beating, screening, and bleaching. The results are tabulated in

tables 2, 3, and 4. Bearing in mind the fact that pulps cooked with 3 percent of sodium

sulfide had poor bleaching quality, unless at least 1 1 percent of caustic was also used, the tables indicate that the best results were obtained

by the use of 2 percent of sulfide with 9 to 10 percent of caustic on

oat straw; 1 to 2 percent of sulfide with 9 to 10 percent of caustic on raw straw; and 1 to 2 percent of sulfide with 8 to 10 percent of caustic on wheat straw. Of the pulps from the three straws, that from oat straw was the

strongest, but most difficult to bleach; that from rye straw was

easiest to bleach, but brittle; while wheat straw was about intermediate between these two with respect to bleaching and strength.

Table 2.

Physical properties and yields of pulp from oat straw cooked with

different percentages of chemicals

Run

34

25

3536

21 to 23. 22

24

26

9

10

1112

1314

i The figures given are typical and represent the weight of 500 sheets 25- by 40-ineh.

! Bursting pressure, in pounds per square inch, through a circular orifice of 1.2 in. diameter.

8 For test specimens 19 mm wide and 90 mm between jaws.

4 For test specimens 15 mm wide and 90 mm between jaws.

Properties of hand sheets Yield: weight

of paper X100+

weight of straw

Chemical sodium

sulfide

Chemical Weight i Bursting

strength » Folding

endurance 3

Tensile breaking

load *

sodium

hydroxide

lb57.1 58.8

54.6

54.8

Points

7170

62

69

Double

folds970

1,200

1,000

1, 300

kg

8.9 8.9

8.7 8.4

%

41.0

42.2

40.

1

45. 1

%

1

1

1

1

%

»

9

10

11 56.2

57.0

59.6

57.7

787080

78

1,400

1, 700

1, 100 1,600

9.6

8.5

11.2 9.5

46.3

45.4 41.4

43.7

22

22

89

10

11

57. 1 59.2

55.9 58.0

54.3 56.4

7888

67

7764

65

2, 100 2,000

1,500 1,800 980

870

8.8

10.2

9.1 10.2

7.6 8.0

50.2

47.1 44.5

47.1

42.6 44.2

3

33

33

3

6

789

10

11
6 Miscellaneous Publications, National Bureau of Standards

Table 3.

Physical properties and yields of pulp from rye straw cooked with

different percentages of chemicals

Run

Properties of hand sheets Yield; weight

of paper

xioo-^ weight

of straw

Chemical

Weight 1 Bursting strength 2 Folding

endurance * Tensile breaking

load 4 SodiuTi

sulfide Sodium

hydroxide

Double

IKId Points folds kg % % %

39 57.9 66 340 9.8 44.0 1 9 40 54.

4

62 360 8.

4

41.8 1 10

41 53^5 59 440 8i0 35.4 11 28 56.6 66 1,000

260 8.6 44.

1

2 8 30 59.7 61 9.8 46.

1

2 9

37 54.0 60 290 8.1 42.3 2 10

38 55.5 58 2.50 9.3 42.3 2 11 15 57.8 56 1, 100 8.7 47.2 3 6 16 58.3 56 780 7.6 48.0 3 7 17 62.4 69 850 9.9 48.9 3 8

18 56.3 45 230 7.8 43.4 3 9

19 59.7 53 840 8.1 45.8 3 10 20 58.0 55 550 8.9 44.7 3 11

1 The figures given are typical and represent the weight of 500 sheets 2.5- by 40-in.

* Bursting pressure, in pounds per square inch, through a circular orifice of 1.2 in. diameter.

* For test specimens 19 mm wide and 90 mm between jaws.

4 For test specimens 15 mm wide and 90 mm between jaws. Table 4.

Physical properties and yields of pulp from wheat straw cooked with

different percentages of chemicals

Run

f_i.b.

a.h.

6-d.

Ci.k.

1.m

Properties of hand sheets

Weight

50.8 57.9 61.8

51.6

58.8 65.0

59.1 47.4

45.4 49.7

47.3

Bursting strength 2

Points

58

46

51

59

47

566264

42

40

4137

Folding

endurance 3

Don hie folds 1,200 910

780

410

670

790

740 660

640 580

610

510

Tensile breaking

load 1

kg

6.9 6.7

6.8

6.4

6.7

7.4

8.6

7.1 6.4

6.2

Yield; weight

of paper

XlOO-^ weight

of straw

Chemical

Sodium

sulfide

1 The figures given are typical and represent the weight of 500 sheets 25- by 40-in.

i Bursting pressure, in pounds per square inch, through a circular orifice of 1.2 in. diameter.

* For test specimens 19 mm wide and 90 mm between jaws.

* For test specimens 15 mm wide and 90 mm between jaws.

4. WASHING

It was found that poor yields wrere obtained w

rhen the pulp was washed in the beater through the medium of a rotary washer covered

with a no. 80 screen. Recovery of fiber from the wash water by

filtering indicated that the poor yields were caused by loss of fines through the washer wire. A method of washing the pulp in the

cooker was adopted. This was accomplished by using four to five batches of hot water to take advantage of the leaching action, or a
Paper Pulp from Cereal Straws 7

continuous washing with hot water for 10 to 15 min. When this was

combined with three-stage bleaching and circulated screening, the

yields were increased to over 40 percent.

5. BEATING

(a) MECHANICAL FIBER TREATMENT

A series of beating trials was made of pulps cooked at 50- or 65-lb

pressure with 3 percent of sodium sulfide and 7 percent of caustic

soda. The method was to feed the pulp into the beater with the roll raised to a 0.07-in. clearance until circulation was established at the

desired consistency. The roll was then lowered to 0.0285-in. clear- ance. At definite intervals this clearance was reduced in steps to a minimum of 0.00285 in. The results indicated that after 90 min

the strength remained practically constant and that further beating was unnecessary.

(b) MECHANICAL FIBER TREATMENT COMBINED WITH PARTIAL BLEACHING

To investigate multistage bleaching and recovery of fines, a series of experiments was made in which the cooked and washed pulps were

beaten and partially bleached simultaneously. Two-fifths of the

total bleach required was added at the start of the 90-min. beating

period. The pulp was then allowed to stand for 2)i to 3 hr, before

screening, to utilize the chlorine completely. The high dilution

necessary for screening, together with the subsequent concentration

for further bleaching, were attained by washing and draining between

the bleaching stages. Further advantages of this method were a

reduction in the amount of tailings and a consequent increase in the

rate of screening, increased recovery of fines owing to their precipitation from a suspended state by the action of the bleach, and a

resultant shortening of the processing time.

6. SCREENING AND SCREEN OPERATION

The usual commercial papermaking screens for the removal of undisintegrated fiber bundles and other objectionable material have

slots that vary from 0.008 to 0.015 in., according to the grade of pulp

desired, and have a production rate of from 21 to 30 pounds of bone

dry pulp per hour per plate [5]. Straw pulp requires small slots to remove the objectionable material. Screens having 0.004-in. slots were investigated and found to be satisfactory. The use of these narrow screen slots required modifications in the usual methods of

screening to obtain satisfactory production. Neil [6] investigated the

modified method and determined the following optimum conditions

for factors which affect the rate of screening: (1) Kate of agitation

above the plate, 325 rpm; (2) amplitude, 1 to 1.5 in., and rate of

vibration of the jig, 800 vpm; (3) consistency of the stock, 0.3 to 0.5 percent; (4) difference in head on the screen, 28 to 30 in.; and (5) the

concentration of tailings in the stock above the plate, 0.3 to 0.5 lb

of tailings allowed to accumulate and 15 min taken for cleaning.

Observing these conditions, screening rates of 26 to 33 lb hr can be

attained, which is comparable to wood-pulp screening and shows the

advantage of improved screen control.
8 Miscellaneous Publications, National Bureau oj Standards

The dilute pulp suspension from the screen was passed through a

centrifugal separator, described previously. This operation aided

greatly in removing the denser impurities from the pulp. The separ- ator removed material which amounted to 1.0 to 2.0 percent of the

bone-dry straw and contained 30 to 60 percent of ash.

7. CONCENTRATING

The dilute pulp suspension from the screen and separator had to be concentrated in order to bleach the pulp efficiently. The continu- ous concentrator described previously proved to be very satisfactory

for this purpose. The machine, with moderately free pulp, will concentrate from 0.5 percent consistency to 5 to 7 percent consistency

at the rate of 5 to 6 pounds of bone-dry material per hour. It has the

advantage of rapid and efficient concentration of pulp with low initial cost and low power consumption.

8. BLEACHING

Recent trends in pulp bleaching have been toward multistage or countercurrent methods, for the reason that the action of concentrated

chlorine solutions has a decided weakening effect on the fibers, and

also because the use of successive stages of bleaching reduces the time

of treatment and the amount of chlorine necessary to produce a white

fiber. The effect of single-stage and multistage bleaching was determined [7J for wheat-straw pulp made by a 3-hr, 50-lb pressure cook,

with 7 percent of caustic soda and 3 percent of sodium sulfide. The

bleaching was accomplished in quart jars, using 20- to 25-gram samples

at approximately 3-percent consistency. The samples were mixed

with chlorine as sodium hypochlorite and allowed to stand for varying

lengths of time, after which the liquor was drained off. The active

chlorine content of the liquor was determined by titrating an aliquot

portion with sodium thiosulfate in the presence of potassium iodide

and acetic acid. The treated pulps were then washed clean with

cold water, and made into sheets of approximately 42 lb per ream. White pulp could be obtained in single-stage bleaching only when

the bleaching agent was present in decided excess: 17 to 23 percent

active chlorine based on the weight of the pulp. Poor yields were

obtained by this method and the paper had poor strength, particularly

folding endurance.

Two-stage bleaching decreased the amount of bleach necessary to produce a white pulp without the large losses in yield and in strength.

Table 5 shows results typical of those obtained in this study.

While applying two-stage bleaching to larger scale experiments, it was observed that a loss of fine material could be minimized by

addition of the bleach solution before washing or screening the

beaten pulp. To take advantage of this, a three-stage bleaching

process was evolved. Four percent of chlorine, based on the weight

of the crude pulp (which was taken as one-half of the original raw

material), was used for the first bleaching stage and was added

during the beating process. This low percentage was used in order

to reduce further the deleterious effect of the strong chlorine and

not to waste the bleach on the coarse material which would later be
Paper Pulp from Cereal Straws 9

screened out as tailings. After screening, centrifugal cleaning and

concentrating, the pulp, diluted to 2- to 3-percent consistency, was

again returned to the beater, where an additional 4 percent of chlorine was added and thoroughly mixed into the pulp. Sixteen hours was

allowed for the action of this second stage of bleaching. Two per- cent of chlorine was then added and mixed with the pulp and allowed

to act for 24 hr. The bleached pulp was washed on flat, no. 80

screens until the wash water was clean and gave no color with

phenolphthalein.

Table 5.

Properties of wheat-straw pulp bleached in two stages

Ratio of CI added

in 1st to that added

in the 2d stage

Time of treat- ment, in stages Total

CI added

X100-=- bone-dry pulp

CI re- maining

xioo-s- bone-dry pulp

Yield of bleached pulp

Properties of hand sheets

Color

1st 2d Weight i Bursting 2 strength Tensile breaking load 3

Folding endurance 4

Double

hr hr % % % !b Points kg folds 1:1 8 32 11. 36 0. 44 90.5 41.7 32 6.4 720 Good.

7:5 8 32 11. 26 .46 92.7 41.9 32 6.3 470 Do.

8:5 8 32 11.36 .65 89.7 41.8 31 6.0 420 Do.

9:5 8 32 11.38 .67 88.7 37.3 26 5.5 430 Do.

2:1 8 32 7.71 .04 96.7 40.6 28 6.2 560 Poor

2:1 8 32 8. 52 .12 86.3 38.3 29 5.7 800 Fair.

2:1 8

. 8

32 9.96 .39 38.8 29 5.9 600 Good.

2:1 32 11.37 .43 84.3 38.2 26 4.2 320 Do.

2:1 8 32 12.81 .60 86.3 38.3 25 5.6 530 Do.

i The figures given are typical and represent the weight of 500 sheets. 25- by 40-in.

' Bursting pressure in pounds per square inch through a circular orifice of 1.2 in. diameter.

3 For test specimens 19 mm wide and 90 mm between jaws.

4 For test specimens 15 mm wide and 90 mm between jaws.

9. FORMATION

The bleached and washed pulp was passed through the jordan at approximately 0.7-percent consistency, with the jordan plug set to give a light brushing action. This treatment was sufficient to

float the fibers and insure good felting properties.

The jordaned pulp was diluted with water to a consistency of 0.6 percent. The suction box and mold were filled with water. Twopound samples of the diluted pulp were poured into the mold and

evenly distributed by hand. The water was removed by suction and

the fibers allowed to mat on the 80 screen of the sheet frame. Suction was maintained for about 1 min after the visible water was removed.

The sheet frame containing the felted fibers was removed and a piece

of 12-oz canvas was stretched over the screen-covered grid. The

sheet frame was then inverted over the grid, suction applied, and the

wet sheet transferred to the canvas. The canvas backings and sheets were placed in a hydraulic press between felts and pressed for about

3 min at 200 lb /in2. The pressed wet sheets were then removed from

the press, stripped from the canvas, and dried on the roll drier at

slightly over 100° C.

10. TESTING

The physical tests were made on samples from hand sheets. At

least 10 samples, which had been conditioned for a minimum of 2 hr

at 70° F and a relative humidity of 70 percent, were used for each
10 Miscellaneous Publications, National Bureau of Standards

test. Samples for test were taken from several band sheets as follows: Mullen, 10 to 12 squares, 10 by 10 cm; tensile, 10 to 12 strips, 19 by

175 mm; folding, 10 to 12 strips, 15 by 135 mm. The weight per ream

was determined from the weight of the conditioned Mullen test samples.

IV. DISCUSSION AND COMPARISON OF THE PROCESS

WITH WOOD PROCESSES

Any attempt to give practical cost data on small-scale work would

be, at best, only a rough approximation. The process in general, as outlined in this paper, made use of no new types of equipment, and

the treatment was in accordance with established practice. Hence,

amounts, yields, and time comparisons are of more significance than

cost approximations derived from small-scale work. Such comparisons are given in table 6. Table 6. -Comparison of the sulfate process for straw pulp with standard wood

processes

Treatment

Amount (in tons) of bone-dry cel- lulosic material required per ton

of pulp. Total chemical (%) for cooking based on the bone-dry raw material . Time (hours) of cooking Maximum temperature (°C) of cook. Yield (%) of the unbleached pulp.. Bleach (%), as bleaching powder,

required. Yield (%) of bleached pulp

Sulfate process on

straws

2.2 to 2.6..

11.8 to 12.9.

4147 45 to 50. 27

36 to 40.

Sulfate process on wood

2.1 to 2.5 »; 2.8 to 3.4 30 to 33 » «; 10 to 17 »». 4 to 5« e

; 1.5 to Rh. 170 to lSlk—

.

Soda process on wood

2.5 to 2.6»; 2.3 to 3.0^

4 to 5' 1214 to 6«>.

38 to 40 »; 28 to 44 b. 8 to 30+»; 8 to 13« «; 8.4b.

26.6 to 41.8*

Sulfite process on wood

2.0 to 2.7 b.

9.4 to 15« (as SOj).

8 or less »; 20to30» «; 10.5 to 14.5 b. 125 to 160»; 120b. 37 to 50 b. 16 to 24"; 10 to 14 b«.

35.2 to 47.5 <».

» Manufacture of Pulp and Paper, TAPPI. 3 (McGraw-Hill Book Co., 1927).

b Sutermeister, Chemistry of Pulp and Paper Making, 2d ed. (John Wiley and Sons, 1929). Easy bleaching.

d Assuming a 5 percent loss during bleaching. The comparative data, as tabulated from authoritative sources, shows that straw pulp in many cases has advantages over wood; and some apparent disadvantages, the most noticeable of these being the

relative amount of bleach required for straw in comparison with wood.

It should be mentioned here that no attempts were made to bleach

at a high consistency or at the increased temperatures which are in

general practice in commercial plants, neither was there a comparison

made with small-scale wood-pulp bleaching under identical conditions.

The application of commercial bleaching methods and equipment to straw pulp would probably materially decrease the time of bleaching

and bleach consumption and may affect yields as tabulated.

The capacity of a digester unit is low for straw in comparison with

wood. Sutermeister gives the capacity of digesters at 4.445 [8] lb of wood pulp per cubic foot of digester space. The digester used in

this experimental work produced 3.05 lb of straw pulp per cubic foot

of digester space. The duration of the digestion must also be considered. Assuming a charging and discharging time of 1.5 hr, the
Paper Pulp from Cereal Straws 11

capacity per cubic foot per hour for sulfate wood pulp is 1.48 to 0.59 lb, for soda pulp is 1.27 to 0.59 lb, for sulfite pulp is 0.56 to 0.17

lb, and for sulphate straw pulp is 0.55 lb. Modifications in digester

design and methods of packing straw into the digester would materially increase straw-pulp capacity.

The amount of dirt in straws varies considerably with the method

of gathering, baling, and storing, and with the weather conditions

when the straw is gathered and baled. Some experimental work has

been done on washing the straw with cold water previous to the water

steeping. Prewashing removes the dirt almost entirely and greatly

increases the effectiveness of the cooking and bleaching and improves

the quality of the pulp produced. This effect is apparently due to the removal of water-soluble materials which, when present, hinder

the chemical processes. Prewashing could very well be combined

with shredding, thereby lowering the cost. The wet shredding process would eliminate the fines produced in the dry shredding operation.

V. SUMMARY AND CONCLUSIONS

1. Experimental work has shown that a strong bleached pulp can be produced from straws by using a modified sulfate process by the

following procedure:

Water presteep at 100° C for 1 hr. Chemical cook at 147° C for 3 hr with 1 to 2 percent of Na2S and

9 to 10 percent of NaOH.

Wash.

Beating 90 min with 4 percent of chlorine added and allowing the

stock to stand for 4 hr. Screening through 0.004-in. slots and centrifugal cleaning.

Concentrating for further bleaching.

Final bleaching with 4 percent of chlorine for 16 hr and 2 percent

additional for 24 hr. Washing to remove excess chlorine.

Jordaning.

Paper formation.

2. Yields of 38 to 43 percent of strong, bleached pulp from wheat

straw; 42 to 46 percent from rye straw; and 40 to 45 percent from

oat straw, have been obtained.

3. Various grades of pulp can be produced by changing the amounts

of the chemicals in the digestion process. Yields of 45 to 55 percent

of unbleached pulps can be obtained.

The authors express their appreciation to the staff members of the Chemical Engineering Department and the Engineering Experiment Station of Iowa State College for their aid and cooperation in carrying out this work at Ames, Iowa; to Dr. Edward Wolesensky for advice and criticism; and to V. H. Peterson, Wesley Walker, J. R.

Northrup, P. D. Hurley, L. Sweeney, W. J. Neil, and William Harrison for their aid in the experimental work.
12 Miscellaneous Publications, National Bureau oj Standards

VI. REFERENCES

[1] Skinner. Ind. Eng. Chem. 24, 695 (1932).

[2] U. Pomilio. Ind. Eng. Chem. 24, 1006 (1932).

[3] U. Pomilio and G. Consiglio. Fr. 646, 154, Dec. 24, 1927.

[4] J. Traquair and F. G. Rawlings. Can. 314, 416, Aug. 18, 1931.

[5] J. O. Mason. Manufacture of Pulp and Paper, III, section 7, p. 20 to 50. (McGraw-Hill Book Co., 1927.)

[6] Wm. J. Neil. Unpublished B. S. thesis, Iowa State College (1932).

[7] V. H. Peterson. Unpublished B. S. thesis, Iowa State College (1931).

[8] Sutermeister, Chemistry of Pulp and Paper Making, 2d. ed. (John Wiley

and Sons, 1929.) o