The pulping of straw and hardwood with magnesium monosulphite

•nnnmmfmifi^r^m^iijiv will

THE PULPING OF STRAW AND

HARDWOOD WITH MAGNESIUM

MONOSULPHITE

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN

DOCTOR IN DE TECHNISCHE WETENSCHAP

AAN DE TECHNISCHE HOGESCHOOL TE

DELFT KRACHTENS ARTIKEL 2 VAN HET

KONINKLIJK BESLUIT VAN 16 SEPTEMBER

1927, STAATSBLAD No 310. OP GEZAG VAN

DE RECTOR MAGNIFICUS DR O. BOTTEMA,

HOOGLERAAR IN DE AFDELING DER

AL-GEMENE WETENSCHAPPEN, VOOR EEN

COMMISSIE UIT DE SENAAT TE VERDEDIGEN

OP WOENSDAG 8 DECEMBER 1954 DES

NA-MIDDAGS TE 4 UUR.

PETRUS MARTINUS SMOLDERS

U/i^^^f

DIT P R O E F S C H R I F T IS G O E D G E K E U R D DOOR DE PROMOTOR: P R O F . I R T . T R O M P

CON-TENTS

C h ^ t e r I Introduction 1

Chapter IT Delignification of lignocellulosic raw materials by

sulfo-n a d o sulfo-n 7

Chapter III Review of the monosulphite p r o c e s s M

Chapter IV The possibility of recovery of residual liquor

compo-n e compo-n t s icompo-n the mocompo-nosulphite p r o c e s s 24 a. Burning of evaporated monosulphite w a s t e liquor with

a limited air supply 25 b . Burning of the evaporated waste liquor with a full air

supply 27

Chapter V The use of magnesia as a base in pulping with sulphur

dioxide, bisulphite or monosulphite 29

Chapter VI Small-scale pulping of rye straw with magnesium

mono-sulphite 34 1. Chemicals 38 2 . Temperature 39 3 . Time 39 4 . Straw/liquor ratio 39

Chapter VII Preparation of chemical pulp from rye straw and espar-t o g r a s s wiespar-th magnesium monosulphiespar-te on a

semi-technical s c a l e 42

Chapter Vni Preparation of semichemical pulp from rye straw with

magnesium monosulphite on a semi-technical s c a l e 50 Chapter IX Tentative experiments on the pulping of hardwoods

with magnesium monosulphite 54

Chapter X Reuse of magnesium monosulphite w a s t e liquor 60

Chapter XI Tentative combustion experiments on a laboratory s c a l e

with magnesium monosulphite waste liquor 64 Appendix P a r t I: Description of analytical procedures 69

Part II: T a b l e s containing beating c h a r a c t e r i s t i c s

of paper and board p u l p s 79

Slunmaty 90 Samenvatting

C h a p t e r I INTRODUCTION

Pulp for papermaking may be defined a s the fibers obtained when cellulose-containing materials are treated in such a way that non-fibrous matter i s removed to a certain e x t e n t . However, not pulp but rags were the chief source of paper until about the middle of the nine-teenth century.

The growing scarcity of rags stimulated the search for other raw materials. Britain successfully developed a method of making paper from esparto, a g r a s s imported from North Africa. Experiments with cereal straws resulted in the development of a practical process of producing straw paper and board in commercial q u a n t i t i e s . For a few d e c a d e s straw became an important raw material.

Due to inventions of papermaking machinery a s the hollanderbeater, the cylinder- and the Fourdrinier machine, together with the discovery of methods producing wood pulp, a very rapid progress in the manufac-ture of paper started in the second part of the last century.

F i g . 1 shows the progress made in the manufacture of paper from the beginning of the present century:

f .^ .^ > / ^ \< > jT* / ^ ^ ^ ^ A P E R AMD BOARD •* - CHEMICAL PULP MECHANICAL PULP

y^

y

y^

y^"^

Moreover, purified chemical pulp manufactured by the sulphite and kraft p r o c e s s has been used in an increasing rate a s dissolving pulp for the production of rayon:

J * '

J

y

k

A/

T

A

//

1

Fig. 2. World production of rayon ^

It i s striking that in the United States in 1948 more than 24 million tons of paper and board were consumed, corresponding to more than 160 kgs per c a p i t a . T h e s e figures acquire their full significance when compared with the 8 million tons consumed by more than 250 million Europeans whose per capita consumption averaged l e s s than 25 k g s . Even the highest figure reached by any European country, viz. 75 kgs per capita in Sweden, i s far below the American l e v e l . The United States have reached their present level of consumption at the end of a fairly rapid evolution which, though influenced by the ups and downs of the general economic situation, has shown a remarkably uniform up-ward trend (see fig. 3).

However, a detailed study of these figures shows that consumptions of the different grades of paper and board have developed at greatly different r a t e s . T h i s i s shown clearly in the following graph. E s p e -cially striking i s the extraordinary increase of paper-board consumption. ( F i g . 4 ) .

T h i s upward trend in the pulp and paper industry of the United States i s , of course, an exponent of the high standard of living in t h i s country. It s t a n d s to reason that the increasing demand of paper and board can only be satisfied by an adequate source of raw material. As indus-trialization programs proceed in underdeveloped areas in Latin America,

Kgs 229 1(0 139 90 19

•17 1920 1929 «lO 1919 I9M IMS 1990 ItSS I9M 1MI

F i g . 3 . Apparent per capita consumption of paper and board in the United States of America (5 to K SO IS a. 10 < bl " o. "Ax 25 20 IS 10 5 0 t - - ^ v . ^ J" "-^ .'--' paper

^J

lA

yf

A/

,v

t/

A/

' V

r

F i g . 4 . U.S. per capita consumption of the different grades of paper and board

A s i a , and Africa, and a s Europe strains to restore and then to rise above i t s pre-war living standard, the over-all demand for paper in t h e s e regions will a l s o increase steadily and insistently. In continen-tal Europe, though to a l e s s extent in the northern countries, forests were heavily overcut during the war and first post-war years and these accelerated r a t e s of cutting could not be sustained indefinitely without d i s a s t r o u s r e s u l t s .

It i s important that F . A . O . (Food and Agricultural Organisation of the United Nations) organized a "Preparatory Conference on World Pulp P r o b l e m s " in 1949 ^ to draw up a "world pulp balance" for 1948, together with a prognose of the situation in 1955. As 85-90% of the raw material used in the pulp industry c o n s i s t s of softwoods, it i s important, too that F . A . O . consulted the opinion of s p e c i a l i s t s and held a conference on "Raw Materials for More P a p e r " in 1952 * . T h e main purpose of this l a s t conference was to undertake extensive in-vestigations to determine what p o s s i b i l i t i e s there are for expanding facilities for the manufacture of pulp and paper in order to meet require-ments anticipated in the future. Special emphasis was laid upon other raw materials a s : hardwoods, agricultural residues and other non-wood materials, in order to estimate their possibilities for manufacturing pulp and paper.

Many s t e p s have been taken by the pulp and paper industry to obtain a better utilization of the present fiber r e s o u r c e s . Among t h e s e are the increased u s e s of fiber w a s t e s , the use of coated papers for more pur-p o s e s , increased u s e of groundwood in some pur-papur-per grades, and pur-progress in the development of p r o c e s s e s for the use of fibrous agricultural r e s i d u e s . Prominent among these developments are high yield s e s obtained by modifying the conventional kraft and sulphite proces- proces-s e proces-s and the adaptation of the full kraft proceproces-sproces-s to the utilization of hardwoods.

E s p e c i a l l y after World war II pulping techniques have been more and more modified in order to reach s t i l l higher yields and to utilize more hardwood. T h e s e pulping techniques are generally called "semichemi-c a l " pulping. A"semichemi-c"semichemi-cording to F I E L D ^ semi"semichemi-chemi"semichemi-cal pulping "semichemi-c o n s i s t s of

selectively softening and partially dissolving most of the cementing components in a cellulose-containing raw material by a mild treatment with a chemical agent, followed by a reduction by abrasion and attrition to a pulp-like material by which the individual fibers are liberated in a relatively undamaged condition. These conditions differ from those of chemical pulping in which the cementing components are dissolved to a large extent so that little or no physical effort i s required to fiberize the cooked c h i p s , and a l s o from those of mechanical pulping, where the entire fiber separation i s accomplished by mechanical action. In the former c l a s s of pulping p r o c e s s e s a certain chemical damage i s

ed to the cellulosic portion of the wood, while in the latter, severe mechanical damage is incurred.

Semichemical p r o c e s s e s , i . e . p r o c e s s e s employing a mild chemical treatment followed by mechanichemical action to complete the fiber s e p -aration, have undergone a very large expansion during post-war years and, a s pointed out by S A P p * this expansion has paralleled the ex-pansion in manufacturing facilities for corrugated b o x e s . Moreover, it has been influenced by a number of other factors, primarily:

1. Semichemical p r o c e s s e s are economical in that only relatively small quantities of chemicals are consumed and high yields are obtained. 2 . High quality corrugating medium can be made from semichemical

p u l p s .

3 . Hardwoods can be utilized satisfactorily in this manner, thereby decreasing the drain on the scarcer softwood supply and aiding in efficient forest management.

The growing popularity of semichemical pulps i s indicated by the fact that the consumption i s increasing rapidly ( s e e fig. 5).

SEMICHEMICAL PU ^ ^ / ^ / /

J

Fig. 5. World production of semichemical pulp

The most prominent process in this new field i s the neutral sulphite semichemical (NSSC) p r o c e s s . In the history of the NSSC process the Forest Product? Laboratory of the U.S. Department of Agriculture play-ed an important part. Much of the existing knowlplay-edge i s summarizplay-ed in a recent publication ' " .

However, the idea expressed by the word "semichemical" i s not brand-new. As a matter of fact "semichemical" pulping of straw has 5

^ e a d y been applied e . g . in the Netherlands for more than fifty y e a r s with an annual capacity of some hundredthousand tons of strawboard and a yield of 80—85% on straw.

In pulping any cellulose-containing raw material with a chemical agent, an extract i s obtained containing dissolved organic matter re-moved from the raw material, together with the chemicals used for pulping. Utilization of t h i s extract and recovery of the cooking chem-i c a l s was achchem-ieved chem-in the c a s e of alkalchem-ine pulpchem-ing and nowadays such utilization i s a l s o possible of the acid waste liquor remaining in the sulphite p r o c e s s . A successful solution of this problem has not yet been found for waste liquors of the neutral sulphite process and the lack of a chemical recovery i s the greatest obstacle to a more universal a c c e p t a n c e of this p r o c e s s .

B e c a u s e of the importance of this waste liquor problem much re-search i s already being done. Also at the Netherlands Experiment Station for the Utilization of Straw extensive investigations in this field have been carried out. T h e s e experiments resulted in the devel-opment of a modified neutral sulphite p r o c e s s . The present publication i s a report of t h i s r e s e a r c h . The author wants to express his gratitude towards the Station's Board of Management and Director for their per-mission to use the r e s u l t s of this work for h i s t h e s i s .

L i t e r a t u r e c i t e d :

1 . Geschaftsberichte 1948-1949-1950: Zellstofffabrik Waldhof, Wiesbaden, Germany.

2 . Geschaftsberichte 1948-1949-1950: Zellstofffabrik Waldhof, Wiesbaden, Germany.

3 . The Pulp and Paper Industry in the U.S.: O.E.E.C. report 1951, p . 27. 4 . The Pulp and Paper Industry in the U.S.: O.E.E.C. report 1951, p . 28. 5 . FAO Report of the Preparatory Conference on World Pulp Problems:

Mon-treal, June 1949.

6. FAO Forestry and Forest Products Study No 6, April 1953.

7 . F I E L D , H.L.: Semichemical Pulping of Hardwoods, Tappi, 36, Aug. 1953, 140A.

8. S A P P , J.E.: The Production and Refining of Semichemical Pulp for Cor-rugating Board,Pulp and Paper Mag.Can. 54, March 1951, p . 96.

9 . Sulfite de Soude; S A . J O H N C O C K E R I L L , Service "Chimie", Seraing, Belgium, estimated of U.S. Bureau of Census combined data.

10. C H I D E S T E R , GM.: Semichemical Pulping of Hardwoods, Paper Trade Jour nal 129, no 20, 84-89, (1949).

'IHfll'ilF«'^?l^"WflH ' • i " ^ " » — • « ^ • ^ ^ ^ p w ^ ^ T .

C h a p t e r II

DELIGNIFICATION OF LIGNOCELLULOSIC RAW MATERIALS BY SULFONATION

From the standpoint of the pulp and paper industry, the carbohydrates contained in wood, straw, e t c . , are of primary i n t e r e s t . Digestion of the raw material in chemical and semichemical pulping amounts to the more or l e s s extensive removal of incrusting s u b s t a n c e s like lignin, leaving the larger part of the cell-wall carbohydrates behind.

The term "delignification" constitutes an incomplete description of this p r o c e s s , s i n c e , b e s i d e s lignin, a l s o some ash constituents and r e s i n s , and part of the hemicelluloses are removed.

It i s unfortunate that the chemistry of lignin i s not yet fully eluci-dated and that the chemical reactions involved in delignification are not thoroughly understood.

The difficulties until recently encountered in the isolation of the larger part of the lignin in the cell wall and of the monomeric building units from t h e s e lignin preparations are chiefly responsible for our still inadequate knowledge of the structure of lignin a s it occurs in the raw material.

However, it i s firmly established that the building units of lignin, whether from hardwood or from softwood, are aromatic. It i s also certain that these building units include guajacylpropane units in the c a s e of softwoods, and guajacyl — a s well a s seringylpropane — units in the c a s e of hardwoods. F R E U D E N B E R G ' considers that the three-carbon side chain may be either:

H 1 H - C - O H 1 H - C - OH 1 H - C - OH H - C = 0 H - C - H or 1 H - C - OH H CH, 1 • ' 1 C = 0 1 - C - OH and that t h e s e are attached to the aromatic nuclei:

CH O T OCH, OH ^ serin

gyl-Lignin may be regarded a s a product resulting from the condensation of t h e s e building u n i t s , e . g . through a phenolic hydroxyl group and an alcoholic hydroxyl group of the side chain. The condensation products may undergo further etherification and condensation with other building units and complex products of high molecular weight might be formed.

T h e s e main r e s u l t s appear to be in general accordance with ^he hy-p o t h e s i s advanced a s early a s 1897 by K L A S O N ^, namely that lignin i s a condensation product of coniferylalcohol, hydroxyconiferylalcoholor coniferylaldehyde. K L A S O N ' S hypothesis was supported by the previous finding of T I E M A N N and H A A R M A N N ^ that coniferin, t h e glucoside of coniferyl alcohol, i s present in the cambial sap of certain conifers.

( C g H j j O j ) - O ^ ^ A - C H - C H - C H J O H

CH3O coniferin

As long a s the structure of lignin itself remains in doubt, it will be difficult to define the action of any cooking chemicals on lignin; how-ever, it i s considered that the following statements are essentially cor-r e c t .

In alkaline pulping p r o c e s s e s lignin goes into solution a s sodium s a l t s of phenolic and, p e r h a p s , enolic hydroxyls. This i s accompanied by an often l e s s desirable partial dissolution of the hemicellulose fraction. Moreover, in the c a s e of straw pulping a strong c a u s t i c liquor undoubtedly removes a s i z a b l e amount of the s i l i c a in the straw.

In the p r o c e s s of pulping with a digestion liquor containing sodium monosulphite, it i s assumed by K L A S O N * that the sulphite adds on

the lignin molecule at an ethylenic double bond to produce soluble sodium lignin s u l p h o n a t e s . While the hemicellulose i s a l s o attacked, this attack i s l e s s vigorous than in the c a s e of an alkaline cook.

In Sweden a considerable amount of work on the sulphonation of spruce lignin has been carried out during the last few y e a r s . Under the direction of H X G G L U N D *) various investigators studied the mechan-ism of this sulfonation r e a c t i o n . The r e s u l t s of these studies were summarized by AD L E R a n d L i N o c R E N .

T h e work of various investigators, H A G G L U N D and J O H N S O N ^, E R D T M A N ^, MiKAWA and co-workers * and L I N D G R E N ' about the effect of pH and cooking time on the degree of sulfonation and the

• ) E . H X G G L U N D , Head of the Department of Wood Chemistry of the Swedish F o r e s t P r o d u c t s R e s e a r c h Laboratory and of the Department of C e l l u l o s e Technology and Wood Chemistry of the R o y a l Institute of Technology in Stockholm.

r^vmr^^nmt^m

solubilization of spruce lignin lead to the concept that distinctly ferent sulfonation s t a g e s are involved because of the existence of dif-ferent sulphonatable groups. Some of these groups can react directly, some only after hydrolysis. T h i s i s clearly displayed in fig. 6 of L I N D

-G R E N which shows the rate of sulphonation of lignin in spruce wood by sulphite solutions of varying pH:

OCH, 0.5 0.4 0.3 0.2 0.1 pH

y;r^.

•oy

é-73fi

\ J

VM

F i g . 6 . Sulphur uptake (S/OCH.) by spruce wood meal on heating with sulphite solutions of different pH at 135° C

Within the pH interval 5—9, the lignin is sulphonated very rapidly up to approximately 0.15 sulphur atoms per methoxyl group. The groups involved in this rapid sulphonation are designated a s X-groups. Sub-sequently a considerably slower sulphonation of the so-called Z-groups takes place until the lignin contains approximately 0.30 sulphur atoms per methoxyl group. Within this pH interval the sulphur contenf^^ lignin cannot be further i n c r e a s e d . When more acidic sulphite solutions are employed the lignin t a k e s up more sulphur,the so-called B-groups being sulphonated. It i s assumed that by a hydrolytic action of the acid liq-uor the B-groups are set free and thus become sulphonatable. E R D T -MAN has designated these newly-formed groups a s B ' - g r o u p s . T h i s investigator and co-workers ^° studied the chemical nature of the B*-groups and their r e s u l t s indicate that they are hydroxyl B*-groups which are converted to sulphonic acid groups during the heating with sulphite solution.

It has not been possible so far to establish the nature of the other abovementioned groups in lignin, v i z . the X, Z and B-groups.

Practically all of the information about the chemical nature of these groups in lignin has been derived from comparisons of the sulphonation of lignin with that of model s u b s t a n c e s . The model s u b s t a n c e s that have proved to be of greatest interest in this connection are benzyl alcohols and their e t h e r s . The theory first proposed b y H o L M B E R c '^ that the sulphonatable groups are benzyl alcohols or benzyl ethers i s supported by the many similarities between the reactions of lignin and of model s u b s t a n c e s of this t y p e .

A D L E R and L I N D G R E N divide the model substances into three c l a s s e s on the b a s i s of their reactivity (see fig. 7):

I

I

—c —

1. p-hydroxybenzyl alcohols and ethers (1) are the only model sub-s t a n c e sub-s hitherto examined by L I N D G R E N and S A E D E N ^^ which are sulphonated a s rapidly as the X-groups. This i s in agreement with the finding that the number of X-groups i s of the same order of magnitude a s the number of free phenolic hydroxyl groups.

2 . p-alkoxybenzyl alcohols (II) which are sulphonated slowly by ap-proximately neutral sulphite solutions and more rapidly a s the hy-drogen ion concentration i n c r e a s e s . According to E R D T M A N and L E O P O L D ^ ' , L I N G R E N ''* and A D L E R and Y L L N E R **, these model s u b s t a n c e s behave like the Z-groups, They are about a s nu-merous as the X-groups.

3 . p-alkoxybenzyl ethers (III) which are not sulphonated by approxi-mately neutral sulphite solutions but, according to L I N D G R E N are sulphonated readily in more acidic solutions, resemble the B-groups of lignin. This i s in harmony with the assumption that these groups are converted to B^groups by hydrolysis (II). On this b a s i s the B ' -groups should be p-alkoxybenzyl alcohol -groups like the Z--groups.

However, the Z-groups are sulphonated by neutral sulphite solu-tions considerably more rapidly than the B'-groups (see L E O P O L D

—C — O R , OCH, ( D R i (II) Ri (III) Ri Fig. 7

'ms'v^^^ . n i l IL.I.' .. . I • • ' •'.. • .1 i^n^yjigBirpiiii ,111 ••.JU. ^ II Ü—IÜ.IL..H. ji II.. • iLiiiim I

j-' * and L I N D G R E N ^ ) . T h i s difference in reactivity may be due to differences in the side chain of the two types of groupings.

In particular the /S-guaiacyl ether of veratrylglycerol: H^COH CH3OV

„i_o_Q

HCOH

I OCH3 OCH 3

exhibits most of the e s s e n t i a l properties of spruce lignin.

Although on the b a s i s of the above reasoning it seems p o s s i b l e to build up a theoretical picture of the structure of lignin, it would be premature to do so before the conclusions that have been drawn from comparisons of the reactions of lignin and model s u b s t a n c e s have been checked by more direct methods.

A justification of this view l i e s in the fact that the sulphonation of lignin appears to be inhibited by p h e n o l s . The latter phenomenon i s , for i n s t a n c e , responsible for the well-known fact that pine heartwood cannot be delignified by the normal bisulphite p r o c e s s , i . e . by heating with acid sulphite s o l u t i o n s . According to E R D T M A N '', the lignin in

the pine heartwood under t h e s e conditions does not become sulphonated but undergoes condensation with the phenol pinosylvin which is present in this wood.On the other hand, if pine heartwood i s heated with neutral sulphite solutions sulphonation does take place i . e . of the two simul-taneous reactions, sulphonation i s the faster one.

C R E I G H T O N , G I B B S and H I B B E R T *' showed that the lignins from gymnosperms, dicotyledons and monocotyledons differfrom one another. Lignin from gymnosperms i s built up from phenylpropane monomers which have one methoxyl group (cf. B L A N D , H O and C O H E N ' * and L E O P O L D ^') lignin from dicotyledons from monomers with both one and two methoxyl groups, and lignin from monocotyledons from mono-mers with no, one or two methoxyl groups.

In order to see whether the lignins from these different types of plants differ also in their content of groups which can be sulphonated at pH 6 and 135° C ( e . g . X- and Z-groups) L I N D G R E N and S A E D E N ^^ studied the action of sulphite solutions of pH 4—7 at 135° on a typical dicotyl-edon: birch (Betula verrucosa Ehrk), and a typical monocotyledon, c a n a guadua (Guadua augustifolia Kunth) a grass from Ecuador, very closely related to bamboo.

In the c a s e of birch, only a small part of the lignin i s dissolved by neutral sulphite solutions; t o d i s s o l v e the bulk of the lignin, acid solu-tions must be u s e d . In this respect birch resembles spruce, and, there-fore, it i s p o s s i b l e that the bulk of birch lignin i s bound by the same linkages a s the major part of spruce lignin.

The lignin of cana guadua, on the other hand, i s dissolved to a large extent by neutral sulphite s o l u t i o n s . At lower pH the lignin i s not d i s -solved so rapidly, at l e a s t initially. At pH 9 the lignin i s dis-solved more rapidly than at pH 7 . As L I N D G R E N ^"«^^ determined the lignin content from the methoxyl content of the wood and the lignin of birch contains only 77.5 per cent of the total methoxyl groups in the wood (compared with 92 per cent for cana guadua and 89 per cent for spruce) the values for the lignin content of birch are relatively uncertain. In fig. 8 the amount of undissolved lignin (methoxyl) in spruce, birch and cana guadua i s plotted a s a function of the time of heating in the treat-ment with sulphite solutions of various pH.

SPRUCE IIIICH CANA GUAOUA

*/• mctheiyl 'h ntHheiyl V. mclhuyl

J I I 1 I I I ' ' ' 1 I L

S 10 15 20 25 5 10 15 20 2S S ID I I 20 2S

heurt hows houn

Fig. 8. The methoxyl content of the sulphite-treated spruce, birch and cana guadua (expressed as a percentage of the methoxyl content of the original wood) as a function of the (corrected) time of heating at 135 C, for sulphite solutions of various pH's.

The r e s u l t s obtained by L I N D G R E N and S A E D E N ^° with European

dicotyl-e d o n s a s y dicotyl-e l l o w b i r c h , p a p dicotyl-e r b i r c h , m a p l dicotyl-e p o p l a r a n d a s p dicotyl-e n . It s dicotyl-e dicotyl-e m s t h a t d i c o t y l e d o n o u s l i g n i n s c o n t a i n a s m a l l e r number of X a n d Z -g r o u p s t h a n t h e l i -g n i n of -g y m n o s p e r m s . T h a t a major p o r t i o n of t h e l i g n i n of c a n a g u a d u a i s d i s s o l v e d r a p i d l y by n e u t r a l s u l p h i t e s o l u -t i o n s , i s in a c c o r d a n c e wi-th -t h e f a c -t -t h a -t a g r i c u l -t u r a l r e s i d u e s l i k e s t r a w and b a g a s s e a r e e a s i l y d e l i g n i f i e d by n e u t r a l s u l p h i t e s o l u t i o n s . L i t e r a t u r e c i t e d : 1 . F R E U D E N B E R G , K . , M E I S T E R , M . and F L I C K I N G E R , E . : B e r . 7 0 , 500 (1937). 2 . K L A S O N , P . : Svensk Kem. T i d . 9 , 133 (1897). 3 . T I E M A N N , F . and H A A R M A N , W.: B e r . 7 , 606 (1874), 8, 5 0 9 ( 1 8 7 5 ) . 4 . K L A S O N , P . : B e r . 5 3 , 706, 1862, 1864 (1920). 5 . A D L E R , E . and L I N D G R E N , O . : Svensk P a p p e r s t i d n . 5 5 , 563 (1952). 6.HXGGLUND, E . and J O H N S O N , T . : P a p p e r s - T r a v a m t i d . , F i n l a n d 1 6 , 282 (1934). 7 . E R D T M A N , H.: Svensk P a p p e r s t i d n . 4 3 , 255 (1940), Cellulosechemie 18, 83 (1940). 8 . MiKAWA, H., S A T O , K . , T A K A S A K I , C , O K A D A , H . : u n p u b l i s h e d . 9 . L I N D G R E N , B.Q.: Svensk P a p p e r s t i d n . 5 5 , 78 (1952).

10. E R D T M A N , H . , L I N D G R E N , B . O . and P E T T E R S O N , T.: Acta Chem.

S c a n d . 4 j 228 (1950).

1 1 . HoLMBERG, B . : Svensk P a p p e r s t i d n . 3 9 , s p e c i a l number p . 113 (1936). 12. L I N D G R E N , B.O.: Acta Chem. Scand. 3 , 1011 (1949).

L I N D G R E N , B.O. and S A E D E N , U . : Acta Chem. Scand. 6, 91 (1952). 1 3 . E R D T M A N , H . and L E O P O L D , B . : Acta Chem. S c a n d . 3 , 1358 (1949). 14. L I N D G R E N , B.O.: Acta Chem. S c a n d . 3 , 1011 (1949), 4 , 1 3 6 5 ( 1 9 5 0 ) .

L I N D G R E N , B.O. and S A E D E N , U . : Svensk P a p p e r s t i d n . 5 5 , 245 (1952). 1 5 . A D L E R , E . and Y L L N E R , S . : Svensk P a p p e r s t i d n . 5 5 , 238 (1952). 16. L E O P O L D , B . : Acta Chem. Scand, 6, 57 (1952).

1 7 . C R E I G H T O N , R J i . J . , G I B B S , R J D . and H I B B E R T , H .: J J l m . C h e m . S o c .

66, 32 (1944).

C R E I G H T O N , R J i . J . and H I B B E R T , H.: J.Am.Chem.Soc. 66, 37 (1944).

1 8 . B L A N D , D . E . , H o , G. and C O H E N , W.E.: A u s t r . J . of Scientific R e s .

A 3, 642 (1950).

19. L E OP OL D, B.: Acta Chem.Scand. 6 , 38 (1952).

2 0 . L I N D G R E N , B.O. and S A E D E N , U . : Svensk P a p p e r s t i d n . 54, 795 (1951). 2 1 . L I N D G R E N , B.Q.: Acta Chem.Scand. 5 , 603 (1951).

C h a p t e r III

REVIEW OF THE MONOSULPHITE PROCESS

Knowledge of the pulping properties of sodium monosulphite d a t e s from the work of C R O S S ' in 1880,

In 1900 S C H A C H T ^ in Germany patented the use of a pulping liquor for the manufacture of pulp from straw, e s p a r t o , wood and the l i k e . As a new reagent, sodium monosulphite and some sodium thiosulphate had been added to the sodium hydroxide-containing liquor. By the use of this liquor a higher yield of pulp was obtained than by the soda or sulphate method; the fibers were stronger and had better felting proper-t i e s and bleach a b i l i proper-t y .

S C H W A L B E ^ in 1909 boiled vegetable fibers with a solution of sodium monosulphite to which had been added an acid in aqueous so-lution, a s a gas or a s an acid s a l t . The acid might be introduced grad-ually into the solution during the digestion process and the total quan-tity of acid should not be more than that corresponding to half the equivalent of the quantity of sulphite u s e d .

In the United States D R E W S E N * in 1917 took h i s first patent on

pulping of cornstalks and b a g a s s e in an aqueous solution of sodium monosulphite containing about 25% of chemical of the b a s i s of the raw material. The pulping action continued for five to six hours at a pres-sure of 4/^ to 9 kg/cm^ a b s .

Between 1918 and 1927 B R A U N * patented pulping p r o c e s s e s mainly suitable for agricultural r e s i d u e s ; the pulping liquor contained sodium monosulphite with an addition of sodium hydroxide and/or s a l t s like sodium carbonate J s i l i c a t e , and sulphide. To B R A U N * was a l s o grant-ed a patent on the production of pulp with wood a s a raw material; however, the amount of chemicals employed, v i z . 50% sodium mono-sulphite and 8% sodium sulphide on wood, was extremely high.

The use of a bisulphite liquor with a b a s e content three to four times a s high a s that in the usual bisulphite liquor, was proposed by S c H O L Z , P O S S A N N E R V O N E H R E N T A L and V O N H A L L E ^ in 1917 for the cooking of fibrous materials such a s hemp, jute, b a g a s s e and the like. T h i s treatment was followed by an alkaline cook.

T h e production of pulp from t h e s e materials by the usual bisulphite process i s not p o s s i b l e .

From I 9 2 6 - I 9 3 3 numerous patents were issued in the United S t a t e s to B R A D L E Y and M C K E E F E * concerning their "Keebra" p r o c e s s with ail i t s variations. T h i s process utilizes a digestion Hquor containing

u p a » II . l . n j i p iiaiinpiiif llll ii,ui|i«f .1 I .uM ui.j.u.in,MMii"i "!"•' - ' I - " — - >

sodium monosulphite a s i t s main constituent. The resemblance of t h e s e p a t e n t s to those of S C H A C H T and of B R A U N i s striking.

There was a great deal of controversy about the merits of the mono-sulphite p r o c e s s . Especially S C H A C H T and B R A U N disputed each other a s appears from the d i s c u s s i o n of the merits of B R A U N ' S process by O D R I C H ^. According to A R N O T ' " the so-called ''Keebra"process introduced in North America and B R A U N ' S monosulphite p r o c e s s in Germany are merely revivals or modifications of the process suggested by C R O S S in 1880 and first developed by S C H A C H T about twenty years later. They all used sodium monosulphite or mixtures of this salt with sodium hydroxide or other sodium s a l t s .

D R E W S E N ^^ in 1924 patented the treatment of straws and g r a s s e s with a solution of a monosulphite followed by treatment with an acid solution, preferably sulphurous acid or a bisulphite. K L E I N ^^ ques-tioned the validity of D R E W S E N ' S sulphite process in 1925 on the ground that the use of a monosulphite had been previously patented in Europe, and had been used successfully on a mill s c a l e . H e a l s o doubt-ed the necessity of the secondary acid treatment.

RAW LING ^^, however, obtained a patent in 1928 for a pulping meth-od consisting of a steaming of wometh-od (aspen), followed by a digestion with a solution containing sodium monosulphite and sodium carbonate at 125° C for about an hour. After the liquor had been withdrawn from the digester, sulphur dioxide was introduced until the pressure was increased by 1,3—2,0 kg/cm^ a b s .

No attempt i s made here to cover completely the literature on the sodium hionosulphite p r o c e s s . Excellent reviews were given by v.d. W E R T H ^*, MusMEci *^, H A N S E N ' * , S C H E L H O R N ^ ' , R U E ^*, H A F F N E R and K O B E ' ^ , W E L L S ^°, R I T M A N ^^ and M U R D O C K ^ ^ . Moreover, W E S T ^^ compiled references on the process in an extensive bibliography.

It was not until more than 40 years after C R O S S * invention that a definite commercial interest was shown in the monosulphite p r o c e s s . During the 1920's several mills started pulping with sodium monosul-p h i t e . However, in smonosul-pite of advantages from the viewmonosul-points of monosul-plant operation, strength and bleaching properties of the pulp, sodium mono-sulphite was not used to any great extent in commercial pulp produc-tion. Mainly because of the relatively high cost of sodium monosul-phite, it was necessary to recover the large amounts of chemical used in the p r o c e s s . Since at that time this recovery had not been s a t i s f a c -torily developed, the process was gradually abandoned. Further, the subsequent development of methods for bleaching sulphate pulps prac-tically made monosulphite pulps superfluous.

During the last fifteen years the interest in sodium monosulphite pulps has reappeared. The reasons for this renewed interest are the following:

I

1. Technical anhydrous sodium monosulphite i s now available at rea-sonably low c o s t , a s a byproduct of the chemical industry (phenol manufacture).

2 . T h e d e s i r e to obtain s u i t a b l e pulps from little-used s p e c i e s such a s the hardwoods, which seemingly cannot be pulped satisfactory by convential methods.

3 . T h e desire to produce high yield pulps; the semichemical pulping p r o c e s s employing sodium monosulphite has shown a remarkable expansion in recent y e a r s .

4 . T h e rising interest in p u l p s form agricultural r e s i d u e s like straw and b a g a s s e .

In the re-examination of t h e p o s s i b i l i t i e s of sodium monosulphite a s a pulping agent of wood s p e c i e s , the Forest Products Laboratory *) had played an important r o l e . E s p e c i a l l y the development of the neutral sulphite semichemical p r o c e s s which has been found particularly applicable to hardwood s p e c i e s i s to be credited to t h i s laboratory.

In the 1920*s a new type of pulp was put into commercial production a s a result of the work of R U E , W E L L S and RAW L I N G ^* at this labor-atory. T h i s "semichemical" pulp i s manufactured today in quantities in the neighbourhood of 800.000 tons a year. It was the opening wedge to the use of other types of corrugating medium, in addition to straw 9 point, in the manufacture of corrugated fiber containers. An e s s e n t i a l part of t h i s development was the introduction of the idea of "beating with r o d s " . The material produced by cooking in spherical rotary dig e s t e r s i s not pulped, but merely softened uniformly so a s to be s u s -ceptible to mechanical disintegration. The complete separation of the fibers, without shortening or formation of debris, was obtained at that time in a rod mill. Nowadays the higher strength of welded rotary di-g e s t e r s makes it p o s s i b l e to conduct monosulphite didi-gestion at hidi-gher temperatures, while the rod mills have been replaced by more efficient disintegrating apparatus like d i s c mills. This process for woodpulp i s known as the NjSSC-process (Neutral Sulphite Semi-Chemical p r o c e s s ) .

Conclusions of a conference on semichemical pulping ^ ' held in 1949, a s formulated by W E S T , are that the over-all action in pulping a hardwood to 75% yield at a temperature of 170° C can be described roughly a s follows:

1. The alkaline reagents will take out somewhat more extractives than the others.

2 . Decreasing removal of lignin and increasing removal of hemicellulose will be experienced in comparing acid sulphite, monosulphite, sul-phate, soda and soda ash p r o c e s s e s in the respective amounts of 50, 40, 30, 20 and 10% for lignin and 30, 45, 50, 55 and 65% for h e m i c e l l u l o s e s .

*) Forest Products Laboratory, United States Department of Agriculture; Hbdison, -Wisconsin; cooperating with the University of Wisconsin.

3 . Removal of pentosan does not follow the same rule, since the re-moval i s least in monosulphite (30%), intermediate with sulphate and caustic soda (40%), and greatest with acid sulphite and soda ash pulping (50%).

4 . Small amounts of a-cellulose will a l s o be removed or become soluble in c a u s t i c solutions of mercerising strength. In soda pulping this removal amounts to about 15% of total a-cellulose, whereas in mono-sulphite pulping only about 8% are removed.

T h e s e data are only approximate because of the complexity of the chemical composition of wood.

In making comparisons at approximately the same pulp yield, obvious groupings of wood s p e c i e s are:

I . W o o d s like oak, with high density and high lignin content, which have a low chemical requirement and yield pulps with ] .>w strength and high lignin content.

2 . Low-density low-lignin woods like those of the poplar and re-lated families, which have intermediate chemical requirements and yield pulps with good strength and relatively low lignin content. 3 . High-density, intermediate-lignin content woods like birch which

have intermediate chemical requirements and yield pulps with excel-lent strength and intermediate lignin content.

4 . Softwoods, which have a high chemical requirement and yield pulps with poor strength and high lignin content.

The question of impregnation and the factors which govern penetration of liquors was considered on this conference. Pulps made with the same yields were obtained in a considerably shorter time with a liquor of high chemical concentration and a high chemical-wood ratio and high residual chemical at the end than with a low ratio to give a mini-mum residual chemical. The temperature factor was found to be roughly two for a 10° C r i s e . Carbohydrate removal was slightly l e s s at low than at high temperatures for the same lignin removal. Sodium bicarbo-nate, sodium carbobicarbo-nate, sodium hydroxide and sodium sulfide were investigated as buffers over a certain range of alkalinity. When buffer-ing to neutrality similar r e s u l t s were obtained. Pulp brightness was decreased by increasing alkalinity, the sulfide giving kraft-like bright-n e s s . Semichemical pulp strebright-ngth ibright-ncreased with decreasibright-ng y i e l d . Ibright-n the case of jack pine sulphate and monosulphite pulps, those made at low yields (55%) would be very strong, but at 75% yield the sulphate pulp was much reduced in bursting, tensile and folding strength and somewhat reduced in tearing strength, whereas the monosulphite pulp had lost a relatively small portion of its strength.

Comparison of properties of semichemical monosulphite linerboards, obtained by the U.S. Forest Products Laboratory are shown in table 1 and a comparison of properties of bleached monosulphite chemical pulps obtained by t h i s laboratory at 45° S,.R.. are given in table 2.

Table 1

Semichemical monosulphite linerboards

Pulp Weight Caliper Bursting Tear Compres-yield strength *) resist- sion % of ance *) resists Species Aspen P a p e r birch Douglas fir Douglas fir (sulphate) o.-d. wood 78.2 78.6 76,6 63.9 g/m' 299 270 240 240 mm 0.37 0.39 0.49 0.40 kg/cm^ a b s . 0.37 10.5 8.0 62 g 260 250 560 670 a n c e kg 22,5 30.0 1 5 . 1 . 13.6 *) 300 s/m Table 2

Bleached monosulphite pulps

S p e c i e s Aspen P a p e r Birch Douglas fir Yield %of o.-d. wood 60 57 42 Bright-n e s s % 77 83 84 Bursting strength *) kg/cm^ a b s . 3.9 3.9 3.9 Tear r e s i s f ance *) g 48 42 122 Double F o l d s *) 500 850 1000 •) 66.7 g/m''

From 1947 onward investigations were carried out at the Northern Regional Research Laboratory *) dealing with the re-examination of the monosulphite p r o c e s s applied to agricultural r e s i d u e s . Also the com-parison of the effects of monosulphite pulping of straw with those of other pulping p r o c e s s e s , for fine specialty pulps a s well as for board p u l p s , was investigated at t h i s laboratory. The results indicate that sodium monosulphite pulping h a s a number of advantages over the other p r o c e s s e s for delignification of these raw materials.

A series of cooks of wheat straw on a laboratory s c a l e was made by the soda, kraft, monosulphite and acid sulphite p r o c e s s e s , under

uni-•) Pulp and Paper Section, Agricultural residues Division, Northern Regional search Laboratory, Peoria, 111. U.S.A. One of the laboratories foe Agricultural Re-search Service of the U.S. Department of Agriculture.

form pulping conditions. The resulting pulps were analysed chemically and tested for their strength c h a r a c t e r i s t i c s . The following conclusions may be drawn from the data obtained by A R O N O V S K Y , E R N S T , S U T

-C L i F F E a n d N E L S O N ^ * :

1. The acid pulping p r o c e s s produced relatively weak and brittle pulps from straw and i s apparently unsuitable for use with agricultural r e s i d u e s . T h e brittleness of the pulp and i t s r e s i s t a n c e to hydration, a s evidenced by the relatively long beating times required, may be due to reduced fiber strength and to its low pentosan content.

2 . The monosulphite process produced the highest pulp yields from wheat straw, and the kraft process gave somewhat higher yields than the soda p r o c e s s .

3 . Bleaching resulted in a lower decrease in yield with monosulphite pulp than with soda or kraft pulp.

4 . The strongest pulps were produced by the soda p r o c e s s , followed closely by the kraft and then by the monosulphite p r o c e s s .

5 . The bleached pulps had better strength properties, with the excep-tion of tensile strength, than the unbleached pulps, and the largest increase in strength was evidenced by the monosulphite pulp.

Some of the data obtained at the Northern Regional Research Labora-tory are given in t a b l e s 3, 4 and 5 shown in fig. 9. Beating, strength evaluation and chemical a n a l y s e s were carried out in accordance with the TAPPI standard methods.

Table 3

Cooking conditions for wheat straw

Chemical *) Ratio *) Temp. Time pH of liquor liquid/

% solid "C hrs. fresh spent

Process Soda NaOH Kraft MonosiJphite Acid sulphite 12.0 NaOH 5.3 N a j S O g 13.7 MgO 3.8 -Na^S 2.6 -S O , 30.0 7 : 1 7 : 1 7 : 1 6 : 1 170 170 170 104 9.8 8.6 7.0 140 2!^ 1.9 1.9

•) B a s i s original moisture-free straw

Studies on the monosulphite digestion of straw in relation to the manufacture of chemical pulp were also carried out at the Netherlands 19

T a b l e 4 U n b l e a c h e d p u l p y i e l d s *) o b t a i n e d with w h e a t straw P r o c e s s Soda Kraft Monosulphite Acid sulphite Crude % 52.0 57.8 57.4 49.4 Screened % 42.0 44.4 49.9 42.6 Screenings % 2.6 1.8 0.8 OJ6 F i n e s % 7A

n£

*) Basis original m o i s t u r e ^ e e straw

Table 5

Data on pulps obtained from wheat straw

Wheat straw Soda pulp

Soda pulp, bleached Kraft pulp

Kraft pulp, bleached Monosulphite pulp Monosulphite pulp,

b l e a c h e d

Acid sulphite pulp Acid sulphite pulp,

bleached Chemical a n a l y s e s "' Ash % 8.1 4 . 7 2.5 6.7 5.0 8.6 6.9 9.7 9.1 P e n t o -s a n -s % 27.6 23.2 27.1 26.5 26.7 28.1 24.0 — 1 2 £ Lignin % 20.1 6.8 3A 9.0 2.5 3.9 2.9 — 22 Bleaching Chlori-ne con- sumpt-ion''> % ^ — 7.8 — 18.0 — 4.4 — 15.6 Bright-n e s s *^^ % ^ 28 77 18 71 39 74 29 80 P u l p Yields '> % _ 42.0 38.2 44-4 40.2 49.8 46.6 42.6 38.7

a) Basis moisture-^ree original straw

b) Basis moisture-free screened unbleached pulp c) Hunter reflectometer

Experiment Station for the Utilization of Straw from 1942 onward by R I T M A N e.a. ^* and resulted in the following conclusions:

1. Monosulphite digestion only becomes effective at higher tempera-t u r e s , preferably above 150° C .

2 . Since a low dose (< 10% Na^SO^) at high temperature c a u s e s a sharp

.> nul I f • n»««ojp»pi- " W W n W W S * " ' ' '

ACIO SULFITE

Fig. 9. Yield and strength properties of bleached and unbleached wheat straw pulps produced by different pulping processes

fall in the pentosan yield a s a result of a pH drop during cooking, monosulphite digestion requires a high dose of c h e m i c a l s .

3 . As to removal of lignin, monosulphite digestion reacts very little to an increase of the concentration beyond 12% Na^SO^ ( b a s i s o.d. straw). This holds especially for temperatures around 150° C .

4 . With application of a sufficiently high concentration of sodium mono-sulphite, pulps may be obtained with an especially high pentosan content.

5. Also with a high concentration of sodium sulphite the digestion yields pulps with a high ash content.

6 . Monosulphite digestion of straw at high temperature and with a high dose of chemicals yields a very easily bleachable pulp. The yield l i e s above 50%.

7 . Monosulphite digestion i s unsuitable for the manufacture of d i s -solving pulp.

T h e differences in chemical composition of straw pulps obtained by c a u s t i c s o d a , sodium monosulphite, and the combination of these chem-i c a l s , are shown chem-in fchem-ig. 10 chem-in whchem-ich the most chem-important chemchem-ical con-s t i t u e n t con-s of the pulp are con-superimpocon-sed and a l con-s o the total a con-s well a con-s the screened pulp yields are given.

DU -• 0 1 to < iO t 20 1 ^ * . . . -\- , >"— > ' * - ^

H

'""1

Fig. 10. Chemical analyses of pulps obtained by cooking rye straw with NaOH, Na^SOj and combinations of these chemicals. The contents of a-cellulose, pentosan and lignin have been superimposed

L i t e r a t u r e c i t e d :

1 . C R O S S , CJ".: British Pat. 4,984 (1880).

2. S C H A C H T , W I L L I : German patent 122,171 (Oct. 10,1900). Wochenbl. Pa-pierfabr. 32:2351.

3 . S C H W A L B E , C.G.: German P a t e n t 231,078 (Aug. 1 5 , 1909). J.Soc.Chem. Ind. 29:1299.

30: 1 2 6 .

4 . D R E W S E N , ViGGo: U.S. P a t e n t 1,229,422 (Juni 12, 1917). C A . 11:2276. 5 . B R A U N , C.A.: German p a t e n t s : 638,011 (Juli 4 , 1927).

3 8 8 , 9 9 8 (Aug. 8 , 1920). h. 3 9 7 , 9 2 7 ( O c t . 3 1 , 1923). 4 0 0 , 8 5 8 ( D e c . 2 5 , 1923). 4 0 1 , 1 2 4 ( F e b . 2 2 , 1923). 6 . Ibid.: . 309,236 (May 3 0 , 1918). 364,992 ( D e c . 17, 1 9 U ) . 7 . S C H O L Z , K., P O S S A N E R V O N E H R E N T A L and H A L L E , M . V O N : German P a t e n t 325,918 (March 21,1917); P u l p and P a p e r Mag. of Can. 19:791. 8 . B R A D L E Y , L . and M C K E E F E , E . P . : P a p e r Trade J . 8 8 , no 8: 1 3 1 - 1 3 3

(1929); CJi.. 23:3086.

9 . O D R I C H , G . : P a p i e i - F a b r . 2 6 , no 9: 1 3 2 - 1 3 3 (1928); C . A . 22:3775. 10. A R N O T , ]M.: Worlds Paper Trade Review 9 5 , 961 (1931).

1 1 . D R E W S E N , V.: U . S . patent 1,511,644 (Oct. 14, 1924); C . A . 19:177. 12. K L E I N , A.St.: P a p e r 3 5 , 511 (1925); C . A . 19:1053.

13. R A W L I N G , F.G.: U . S . patent 1,679,682 (Aug. 7, 1928); C . A . 22:3778. 14. W E R T H , A.v.d.: Zellstoff und P a p i e r 17, 4 3 8 , 469, 531 (1937).

15. MusMECi, L.: L'industria della carta 6 , 11 (1939). 16. H A N S E N , J.: Paper Trade J . 8 4 , no 9: 5 1 - 6 0 (1927). 17. S C H E L H O R N , F . S . : ibid 119, no 2 3 : 3 9 - 4 6 (1945). 18. R U E , J.D.: ibid 8 1 , no 16: 5 4 - 5 6 (1925).

19. H A F F N E R , L . C . and K O B E , K.A.: ibid 1 1 1 , no 9: 9 3 - 9 8 (1940). 2 0 . W E L L S , S.D.: Ibid 119, no 21: 1 0 7 - 1 0 8 (1944).

2 1 . R I T M A N , E . L . : P u b l i c a t i e no 4 , Nederlands Proefstation voor Stroverwer-werking, Groningen, Netherlands, p . 59—84 (1942).

2 2 . M U R D O C K , H J l . : P a p e r Trade J . 124, no 4 : 5 1 - 5 2 (1947).

2 3 . W E S T , C . J . : Bibliographic Series No 175, P a r t . FV (1950). T h e Institute of Paper Chemistry, Appleton.

2 4 . R U E , J . D . , W E L L S , S . D . and R A W L I N G , E.G.: U . S . P a t e n t s 1,859,845,

- 6 , - 7 , -8 (May 24, 1932). P a p e r Trade J . 8 3 , no 13: 5 0 - 5 3 (1926).

25. Report on Conference on Semichemical Pulping; U . S . Forest Products Laboratory, Fiber Containers, 3 4 , no 11: 66—74 (1949).

2 6 . A R O N O V S K Y , S.I., E R N S T , A . J . , S U T C L I F F E , H . M . and N E L S O N , G J i . : Paper Trade J . (June 24, 1948).

2 7 . S U T C L I F F E , H.M., A R O N O V S K Y , S.I., W I L K I N S O N , R.M. B U R N H A M ,

WJ)., P H I L I P S , W.A. and C A R P E N T E R , E J R . : Tappi, 3 3 , 3 5 3 - 3 5 7 ( 1 9 5 0 ) .

2 8 . R I T M A N , E X . , c . s . : P u b l i c a t i e no 4 and 9, Nederlands Proefstation voor Stroverwerking, Groningen, Netherlands (1942 and 1943).

C h a p t e r IV

THE POSSIBILITY OF RECOVERY OF RESIDUAL LIQUOR COMPONENTS

IN THE MONOSULPHITE PROCESS

A great disadvantage of the pulping p r o c e s s e s applied to lignocellu-losic raw materials and based on the sulphonation reaction l i e s in the fact that from t h e s e p r o c e s s e s a waste extract remains containing the sulphur compounds and the organic matter extracted from the raw mate-r i a l . T h e s e waste liquomate-rs amate-re often disposed of by dischamate-rge into public water courses causing severe pollution. In addition to this aspect of the monosulphite waste liquor problem there i s , of course, the econom-ic a s p e c t , a s in this way the cooking chemeconom-icals as well as the organeconom-ic matter extracted from the raw material are l o s t . In the sulphate process it i s p o s s i b l e to recover the cooking chemicals, utilizing at the same time the fuel value of the organic matter. In this process, concentrating the liquor by evaporation r e s u l t s in a fuel which, on burning with a limited air supply, yields heat, soda ash and sodium sulphide.

By causticizing the soda ash is converted to caustic soda which, together with the sodium sulphide, i s reused a s a cooking a g e n t . Chem-ical l o s s e s in the system are made up by adding salt cake (Na^SO^) to the concentrated black liquor before burning. However, burning the eva-porated waste liquor of the monosulphite process will generally yield either sodium sulphide or sodium sulphate, depending on the amount of air supplied during burning. Conversion of t h e s e compounds to sodium monosulphite on a technical scale seems only possible in the c a s e of sodium s u l p h i d e . Sometimes a c r o s s recovery process is used in which the sodium monosulphite waste liquor i s substituted for the salt cake make-up in a kraft recovery system. However, the amount of waste liquor that can be recovered in this way i s limited by the salt cake requirement of the kraft p r o c e s s and this means that a production of monosulphite pulp must be combined with a six- to sevenfold production of kraft p u l p .

As long a s there has been an interest in pulping with monosulphite, efforts have been made to attack the problem of chemical recovery. In the recovery of chemicals from soda-base monosulphite cooking liquors by evaporation and burning, an ash is produced containing varying quan-t i quan-t i e s of sodium carbonaquan-te, sulphide, sulphaquan-te, and sulphiquan-te. The raquan-tio of these constituents depends largely on the burning conditions.

a. Burning of evaporated monosulphite waste liquor with a limited air supply

From this burning operation an ash or smelt remains consisting of about 40% Na^S and 60% N a ^ C O j . As pointed out by W E L L S ' direct sulphiting of the dissolved smelt (green liquor) i s i m p o s s i b l e . The Na^S will react according to the following equation:

2 Na^S + 3 SOj - 2 Na^S^Oj + S.

The sulphur thus formed combines with a sulphite ion formed from carbonate and SO^ to produce another thiosulphate ion. T h u s , it i s evident that not only 40% of the smelt becomes Na^S^O^, but a l s o an-other 20%, so that at l e a s t over 50% of the sodium sulphite used in cooking i s recovered a s Na^S^O^ every time it p a s s e s through the cooking c y c l e . Na^S^O^ i s not only practically inert a s a cooking agent but, a s was shown by S C H E L H O R N ^, the presence of Na^S^O^ in a sodium monosulphite cooking liquor c a u s e s a decrease of the reaction rate while penetration of the cooking liquor in the wood chips is re-tarded.

Many patents have been issued about methods of processing this smelt in such a way a s to prevent the formation of Na^S^O^.

M o d i f i e d L e B l a n c p r o c e s s

In 1926 D R E W S E N ^ patented the treatment of the concentrated waste liquor with CaCO^. When burning the mixture under agitation in the presence of air, the following reactions are said to occur:

Na^SO^ + 2 C « Na^S + 2 CO^ Na^S + CaCOj = CaS • Na^COj

The double salt is obtained a s a fluid smelt at 900 to 1000° C . After grinding, Na^CO^ can be leached out and i s treated with SO^ to form N a ^ S O j . The residue of insoluble CaS can be converted to H^S and oxidized to SO^. In laboratory experiments R I T M A N * obtained very unsatisfactory yields of Na^CO^ and CaS, a s a result of side r e a c t i o n s . P r e c i p i t a t i o n of i n s o l u b l e s u l p h i d e s b y m e t a l o x i d e s

In 1926 R I N M A N * got patents on a process consisting of the addi-tion to the green liquor of one or more oxides of metals, forming unsolu-ble sulphides. In this way NajS i s converted to NaOH:

Na^S + CuO + HjO - CuS + 2 NaOH.

In the M O O R E ® patent (1939) — based on the same idea — iron and copper oxides are proposed.

C o n v e r s i o n of N a ^ S t o N a ^ C O ^

In a patent of R A W L I N G ^ of 1929 it is proposed to treat the ash with a mixture of air and steam at such a temperature a s to effect com-bustion of part of the carbon present in the a s h , the resulting CO^ reacting a s follows:

Na^S + CO^ + HjO = Na^COj + H^S . No mention i s made of any utilization of the H^S.

In 1934 B R A D L E Y and M C K E E F E * patented a treatment of the

green liquor with solid NaHCO^:

Na^S + 2 NaHCOj = 2 Na^COj + H^S .

T h e HjS liberated from the solution i s burned to SO which, together with the SOj recovered during evaporation and burning of the waste liquor, i s used in converting Na^COj to Na^SO and NaHCO^. This can be carried out at a temperature below 6U C, the precipitated NaHCOj being separated from the solution and reused to convert Na S to N a ^ C O j . BiLLiNGTON, C H I D E S T E R and C U R R A N ' concluded in 1935 that the following s t e p s might be most promising in this recover-ing operation: grindrecover-ing t h e smelt and treatrecover-ing it with moist NaHCO^ above 200° C or with steam at atmospheric pressure; dissolving the Na^COj formed in water and then p a s s i n g CO gas into the purified soda solution to precipitate part of the Na^CO^ as NaHCO^. The re-mainder of the carbonate i s sulphited by SO^ obtained by burning the HjS liberated before.

D i r e c t o x i d a t i o n of N a ^ S t o N a ^ S O ^

A patent of D R E W S E N ^° from 1928 d e a l s with a concentrated green

liquor which i s mixed with powdered Na^SO^ and heated in a current of air at a temperature of about 140° C. The following reactions are claim-ed:

2 Na^S + 2 O j + HjO = Na^S^Oj + 2 NaOH

3 N a ^ S j O j + 6 NaOH - 4 Na^SOg + 2 Na^S + 3 H^O.

T h e A R I E S - P O L LAK *^ patent of 1953 i s quite similar and claims that the Na^S formed in the second reaction, reacts again and all Na^S i s converted to N a ^ S O j . The reactions require large volumes of air and are highly exothermic. Therefore, to avoid the formation of unde-sirable by-products, the volume of air and the temperature must be controlled to a certain e x t e n t . In order to make all the air available to the Na S a s p e c i a l way of contacting i s developed yielding the end product - a mixture of Na^CO^ and Na^SO^ - in the form of a dry powder.

T h e a p p l i c a t i o n of c a t i o n e x c h a n g e r s

I , "ipa.a w i « l * B ^ ^ w ^ '- '"^w^ippipwni."'!mm^ fl

soda-base monosulphite waste liquor with a cation exchanger in the hydrogen form.

The exchanger retains the Na"*" ions from the green liquor and the H"*" ions set free form H^S which i s evolved a s a gas from the top of the column. The effluent from the column and the wash water can be passed back to the smelt dissolving tank. The exchanger i s regener-ated with a solution of 8 0 ^ which i s converted to a mixture of NaHS03 + S O j . TTie HjS liberated i s burned to 50^ and used to make the SO solution in the regeneration s t e p .

R A Y O N N I E R ' S patents ' ^ of 1953 are based on the same idea but the

capacity of the exchanger i s increased by increasing the temperature to 5 0 - 1 0 0 ° C .

b. Burning of the evaporated waste liquor with a full air sypply

R e c o v e r y of N a ^ C O ^ o n l y

R I C H T E R ^* in 1931 restricted himself to the recovery of the heat

and of NaHCOg by burning the evaporated waste liquor with a full air supply to produce a smelt containing Na^CO^ and Na^SO^. The smelt i s extracted with water and the aqueous solution i s carbonated to pre-cipitate NaHCOj.

In 1939 S i L v i o G I R O ' * treats the green liquor with Ca(HS03)2 ob-taining a solution of Na^SO^, together with a precipitate consisting of CaSO^, CaSOj and CaCO^. The solution can be used a s a cooking agent, while the suspended precipitate, with the exception of CaSO^, i s converted to Ca(HS0j)2 by treatment with SO^. The sulphate i s discarded and the solution of Ca(HS0j)2 can react again with the green liquor.

Also in the Zimmerman process * * the sulphur component gets lost; air i s passed into the waste liquor at elevated temperature and pres-sure, the organic matter i s oxidized completely to CO^ and H^O and the inorganic matter to Na^SO^. By application of the following reac-tions the sodium component of the waste liquor can be recovered:

Na^SO^ + Ca(HS03)2 - 2 NaHSOj + CaSO^; 2 NaHSOj + Ca(OH)j, «= Na^SOj + CaSOji CaSOg + SO^ + HjO « C a ( H S 0 3 ) 2 .

It i s claimed that the heat and power generated by this process will make it a practical solution of the pollution problem.

Though the foregoing shows that the problem of utilizing the waste liquor in the monosulphite process has been widely investigated and that several methods have been developed to solve t h i s problem, a general application on a technical s c a l e of one of these methods i s still l a r k i n g . Nearly all monosulphite waste liquor i s discharged into

p u b l i c w a t e r c o u r s e s . T h u s , t h e e x p a n s i o n of t h i s p r o c e s s i s s t i l l h a n d i c a p p e d by t h e l a c k of a s a t i s f a c t o r y e c o n o m i c a l p r o c e s s t o r e c o v e r t h e c h e m i c a l s from t h e r e s i d u a l l i q u o r s with c o n c o m i t a n t p r e v e n t i o n of p o l l u t i o n . L i t e r a t u r e c i t e d : I . W E L L S , SJD.: P a p e r Trade J . 119, no 2 1 : 107-111 (Nov. 2 3 , 1944); B A . 1 9 4 5 ^ , 11:77. 2 . S C H E L H O R N , F . B . : P a p e r Trade J . 119, no 23: 3 9 - 4 6 (Dec. 9, 1944); C . A . 39:1986. 3 . D R E W S E N , V.: U . S . p a t e n t 1^05,926 (Nov. 9, 1926); C A . 21:176.

4 . R I T M A N , E . L . : P u b l i c a t i e no 1 3 , p . 23—41, Netherlands Experiment Sta-tion for the UtilizaSta-tion of Straw, Groningen, N e t h e r l a n d s .

5 . R I N M A N , E J l . : Norwegian p a t e n t s 4 3 , 7 1 5 - 1 6 (April 1 3 , 1926). 6 . M O O R E : U.S. p a t e n t 2,161,141 (July 27, 1939).

7 . R A W L I N G , F.G.: U . S . P a t e n t 1,728^52 (Sept. 17, 1929).

8 . B R A D L E Y , L . and M C K E E F E , E J l . : U . S . patent 1.983.789 ( D e c . 1 1 , 1934). 9 . BiLLiNGTON, P . S . , C H I D E S T E R , G J H . and C U R R A N , C £ . : P a p e r Trade

J . 101, no 16: 44-46 (Sept. 12, 1935); C A . 29:7649.

10. D R E W S E N , V.: U . S . p a t e n t 1,659,193 (Febr. 14, 1928); C . A . 22:1445.

1 1 . A R I E S , BS., P O L L A K : T A P P I 3 5 , D e c . 1952, p . 142"^.

12. B I C K E L , T R E T H E W A Y : Report of the Meeting of the Canadian Pulp and P a p e r Association, Montreal, January 1951.

1 3 . R A Y O N N I E R : U.S. p a t e n t s 2,656,244, 245 and 249 (Oct. 10, 1953). 14. R I C H T E R , G A . : U . S . patent 1,815,328 (July 2 1 , 1931); CA. 25:5560. 1 5 . S I L V I O G I R O : Italian p a t e n t 368,170 (1939).

16. H A Y W O O D , G . : T A P P I 3 7 , F e b r . 1954, p . 1 3 6 ^ .

C h a p t e r V

THE USE OF MAGNESIA AS A BASE IN PULPING WITH SULPHUR DIOXIDE, BISULPHITE OR MONOSULPHITE *)

The digesting of cellulose-containing raw materials with sulphur dioxide in the presence of calcium bisulphite i s the oldest and most widely used sulphonation p r o c e s s . Therefore, it i s easily understood that this process was generally indicated a s "sulphite p r o c e s s " , al-though this name i s rather misleading, free sulphur dioxide being the principal digestive agent. However, notwithstanding the development of p r o c e s s e s utilizing compounds of tetravalent sulphur in l e s s acid, neutral and alkaline media, the former process i s still generally indi-cated as "sulphite" p r o c e s s .

In order to prevent confusion in distinguishing these p r o c e s s e s , a nomenclature will be used here based on the ratio of equivalents of acid in the form of sulphur dioxide and of alkali present:

Sulphur dioxide to alkali > 2 acid sulphite Sulphur dioxide to alkali > 1 < 2 bisulphite

Sulphur dioxide to alkali < 1 monosulphite.

Credit i s given to T I L G H M A N ^ for the invention of the method of pulping wood by the calcium-base acid sulphite p r o c e s s . He patented the process in 1866 but did not succeed in putting this method on a commercial b a s i s . However, T I L G H M A N ' S patent is often quoted be-cause this inventor s a y s : "1 have found that the addition of sulphite or bisulphite of lime or other suitable base, t o the acid solution, tends to make the fibrous product of a white colour and more easily bleached". T I L G H M A N chose calcium bisulphite because of its c h e a p n e s s .

D I E C K M A N N ^ s t a t e s that this patent covers the use of calcium a s well as magnesium and sodium.

E K M A N developed the acid sulphite process commercially and in 1874 pulp was produced by the calcium-base acid sulphite p r o c e s s . In 1881 E K M A N a l s o patented ^ a pulping solution containing approxi-mately 1.4% of magnesia and 4.4% of sulphurous acid, while the pulp-ing operatpulp-ing was carried out at a pressure of about 7 kg/cm^ a b s . for one to three hours.

A variant of t h i s pulping process was patented by G R A H A M in 1882. By his method the raw material was treated with the monosulphite of potash, soda, magnesia, lime or other suitable b a s e . When the g a s e s *) T h i s review is mainly extracted from the Bibliographic S e r i e s . Pulping P r o c e s s e s ,

P a r t III, compiled by C.J .WEST and edited by the Institute of P a p e r Chemistry,

contained in the vegetable s u b s t a n c e s were driven off by heat, sulphur dioxide in the g a s e o u s or liquid state was injected into the v e s s e l , either alone or in combination with one of the above b a s e s to give an e x c e s s of sulphur dioxide above that required to form a monosulphite, after which the materials were treated to liberate the fibers. In 1925 D R E W S E N * modified G R A H A M ' S process by cooking cereal straws in water for four to eight hours in magnesium, sodium or other soluble monosulphite under a pressure of 5 to 7 kg/cm^ a b s . The fibrous mate-rial obtained was suspended in water and then p a s s e d through a tower into which sulphur dioxide was introduced. This sulphur dioxide ment produced acid sulphites of magnesium or other base in the treat-ing liquor, so that the raw material was first subjected to an alkaline and then to an acid treatment.

T h e advantages in pulping with magnesium- instead of calciumbase acid sulphite were d i s c u s s e d by D E C E W *, H E Y E R D A L ^. K Y L A N -D E R *, H I L L E R ' , P O S S A N N E R V O N E H R E N T A L ***, C A L H O U N , C A N N O N , YoR S T O N and M A A S S ^^ and C H I D E S T E R and M C G O V E R N

1 2

T h e s e investigators indicate the following advantages:

1. As the s u l p h i t e s and sulphates of magnesium are more soluble in water in comparison with the respective calcium s a l t s , deposition of t h e s e s a l t s in d i g e s t e r s and blowpits i s prevented by the use of magnesium-base.

2 . Magnesium has more solvent action on the free resins of the wood; it seems advantageous to cook the more resistant and resin— con-taining woods with magnesia-base liquor.

3 . Thorough penetration of the chips i s readily attained by using mag-n e s i u m - b a s e .

4 . T h e optimum liquor composition with regard to rate of delignification and pulp yield of spruce wood appears to be a liquor containing 1.5% combined SO^. With a calcium-base liquor the use of this concentra-tion i s not feasible due to the low solubility of Ca()^S,0^)^.

5 . P u l p strength seems to differ little but in magnesium-base digestion the amount of screenings i s l e s s .

N E M E T H Y ' ^ proposed in 1906 to cook straw, hemp waste, and similar material which cannot be pulped satisfactorily by cooking with acid sulphite, by means of a solution of magnesium bisulphite and monosulphite. Also J A R D I N E and N E L S O N ^*' in 1915 patented a mag-nesium bisulphite process acting in i l e s s acid medium. They prepared a hot solution of magnesium bisulphite containing a predetermined greater proportion of base than would dissolve at normal temperature. Bamboo was cooked with a liquor containing 2.4% total sulphur dioxide 1.6% of which was combined as monosulphite and the remainder a s b i s u l p h i t e . In 1931 H A G I W A R E ^* obtained a patent on a method of manufacturing paper from b a g a s s e and other graminaceous plants with