CA1087892A - Alkaline pulping process - Google Patents

Abstract

ABSTRACT

An improved alkaline lignocellulosic pulping process wherein improved colour in the resultant pulp is obtained by using an excess of alkali based on the lignocellulosic material.
Improved pulp strength is obtained by operating the delignifi-cation process at lower temperatures than normal generally from 100°C to 160°C. Both of the above improvements are enhanced and pulping times are reduced if the pulping liquors additionally contain a quinonoid or hydroquinonoid compound or tautomer thereof.

Description

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The presen~ in~ention relates to cellulosic pulping processes in which delignification o~ lignocellulosic material is ef~ected for the production of cellulosic pulps of the type which may be used in the manuEacture o~ paper or paperboard.
In particular this invention relates to high ~ield semichemical type pulps although it is also applicable to ~ull chemical pulp ` ~`
manufacture.
The kraft process is widely used, due to the excell~
ent properties of the pulp it affords. However, for some pur-poses, such as the manufacture of corrugating paper and somecomponent pulps for linerboards, the high strengths obtainable with kraft pulping are not necessary and the relatively low yield and consequent high cost of kraft pulps is a disadvantage.
Accordingly, variants such as "high yield kraft" and various sulphite processes have been recommended and used for these purposes. The most widely applied of these higher yield pro-cesses is the so-called neutral sulphite semichemical (NSSC) process which is capable of giving pulps with yields in the range 65-85~, and with properties suitable for use as the principal component in the manufacture of corrugating paper and as an important component of linerboards and bag and wrap-ping papers.
The increasing stringenc~ of environmental standards has placed greater emphasis on the need for chemical recovery systems which will allow recovery of the pulping chemicals and destruction of dissolved wood substances which could otherwise impose an undesirable burden on the environment. Recovery processes suitable for NSSC pulping are complex and expensive.
One solution sometimes used is to burn the NSSC
spent liquor in a kraft recovery system. However, this

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requires ~hat the NSSC mill is adjacent to a kra~t mill Which may often not be the case~ Apart from this, the chemicals obtained from the kraft recovery system are not suitable for use in a NSSC mill and must first be reprocessed at substantial ~ ~
cost. If environmental standards are so stringent that a sul- ; ;
phur-free process must be used then the NSSC process is in any case inapplicable. ~-It is an object of the present invention to provide a pulping process which will allow of a simpLi~ied chemical re- ~ -covery while obtaining results similar to those of the NSSC
process, in terms of high yield, good colour and adequate strength properties.
Semichemical pulps can be prepared by the soda process which employs sodium hydroxide as the active pulping chemical but this is rarely, if ever, used because pulp strengths and colour are not as good as for the NSSC process and chemical cost is higher than for the sodium carbonate, or sodium carb~
onate-sodium hydroxide semichemical processes.
The importance of colour cannot be over-stressed as this factor at present hinders or prevents the use of semi-chemical soda pulps in products such as linerboards and bag and wrapping papers.
One aspect of the present invention resides in a dis- ~ ;
covery that good colour in chemical or semichemicaL pulps is dependent on a hiqh residual content of alkali in the spent liquor resulting in a high residual pH at the end of the cook-ing period. This can be obtained by using an excess of strong ~ ;~
alkali in the pulping process. It is already known that there is a correlation between good colour and yield but the present invention resides in the discovery that at the

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~0~78~2 same yield an excess of alkali results in better colour. Like-wise where yields are different, a pulp of lower yield can have equivalent colour properties to a pulp of higher yield if excess alkali has been used in -the production of the lower yield pulp. Although an excess of alkali will result in a high residual pH in buffered pulping systems a large difference in the amount of alkali may not be reflected in the residual pH
values particularly where the pH i9 high. Previous processes using sodium sulphite, sodium carbonate or sodium carbonate- ;' sodium hydroxide could not have obtained the high residual pH
nor did the high yield soda pulp processes give a high residual `
pH as the first two do not use a sufficiently strong alkali while the latter two normally use insufficient alkali. This is especially true of the high-yield or semichemical process in which the aim has been to consume as much as possible of the applied alkali and leaves a low residual pH. In the present invention the use of excess strong alkalis gives a high res-idual pH of from 9 to 14 preferably 12 to 13.
To this end the present invention provides a process of delignifying lignocellulosic material which comprises digesting the lignocellulosic material with a pulping liquor containing primarily an hydroxide or carbonate of sodium, potassium, magnesium, calcium or ammonia wherein an excess of the hydroxide or carbonate based on the lignocellulosic material is used.
The process of the invention can be applied in both batch and continuous digesters irrespective of whether the latter are operated concurrently or countercurrently. However it is not limited to conventional digesting equipment but can also be applied to so-called "explosion" pulping in which

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chemicall~ treated chips are de~ibered b~ the rapid or explosive relief o~ preSsure.
The process is applicable to all t~pes of wood including both hardwoods and soEtwoods and to non-woody mater-ials such as straw, bagasse etc. Where wood is used it may be barked or unbarked and may include branches, roots, twigs and leaves as in the case of so-called "whole tree chips".
Conventional semi-chemical soda pulping processes usually employ temperatures of 165-180C and the pH of the spent liquor is normally in the range of 9 - 10. Also, U.S.
Patent No~ 3,954,553 which relates to a (similar) process designed to produce pulp similar to that from the conventional semichemical sulphite processes, discloses a process of using mixtures of sodium hydroxide and sodium carbonate in which a cooking temperature of 375F (190C) is used.
In another aspect of this invention it has been discovered that alkaline pulping at lower temperatures than normal can result in improved strength of the pulp. To this end the present invention provides a process of delignifying lignocellulosic material which comprises digesting the ligno-cellulosic material with a pulping liquor containing primarily an hydroxide or carbonate of sodium, potassium, magnesium, calcium or ammonia at a temperature up to 160C.
In the process according to this aspect of the invention cooking temperatures in the range of up to 160C
can be used aLthough temperatures above 100C are preferred and temperatures up to 120-150C are most preferred.
Operation at temperatures up to 160~C has a number of advan-tages. The unexpected advantage is that the strength of the ~;
resultant pulp is higher when lower cooking temperatures

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are used. There is also an indica-tion th~t in some cases colour is improved and of course the lower cookin~ temperatures results in an overall energy saving. A pre~erred form of the invention combines the use of excess alkali and low process ~, temperatures to giVe pulps af high residual pH having good colour and strength properties.
This process of the invention has been found to ~ ~
give pulps with properties, including colour, similar to those ~ ;
of NSSC pulps. In employing this process we have found in a further preferred aspect of the invention that further accel- ;
eration of the rate of cooking, and thereby either decrease in reaction temperature or reaction time can be obtained by addition to the chips or to the cooking or makeup liquor of small quantities of quinonoid or hydroquinonoid compounds.
Such additions may be made as the chips and/or liquor are being introduced into the digester or as a separate pretreatment.
The addition of the quinonoid or hydroquinonoid compounds is particularly useful where low cooking temperatures are used as increased strength due to the lower temperatures can be obtained in shorter cooking times due to the addition of the compounds. , ~`
A further benefit of the use of the additives described is an additional improvement in the strengths of the pulps produced. As may be seen from the examples, the strengths of pulps prepared by the process of the invention are superior to those of the conventional NSSC pulps at the same yield.
The quinonoid or hydroquinonoid compounds which we have found beneficial include anthraquinones (~Q's) and anthrahydroquinones and~their tautomers and derivatives, :-:.......... . . . ~ . , - ~. :: -- ~ :

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naphthoquinones and naphthohydroquinones and their kautomers and derivatives and benzoquinones and benz4h~droquinones and their tautomers and derivatives. Examples of a tautomer o anthrahydroquinone are 10-hydroxyanthrone and 1, 4 dihydro-anthraquinone and an example of a tautomer of a naphthohydro-quinone derivative is l, 4, 4a, 9a - tetrahydroanthraquinone.
Such compounds are described in more detail in our correspond-ing Australian Applications 25519/77 and PC 9939/77 which are incorporated herein by re~erence.
Preferably, the compounds are employed in an amount -of from 0.001 to 1.0% and most preferably from 0.01 to 0.3%
by weight based on the weight of dry wood used. As several of the preferred compounds are substantially insoluble in water, ~-it is preferred to use them inia finely gro~nd condition, i.e.
a condition of small particle size. In some cases the use of a dispersing agent or surfactant such as Teepol or Comprox (Trade Names) is beneficial.
Treatment of the lignocellulosic raw material with `
the quinonoid/hydroquinonoid compound or tautomer in accord-ance with the invention may be varied to suit the requirement of the particular process. For instance, the quinonoid/
hydroquinonoid compound or tautomer may be present in a pre-treatment liquor in which the lignocellulosic raw material is soaked or impregnated beore addition to the pulping liquor in a digester for completion of the delignification process: or the compound may be pre-mi~ed with the pulping liquor and lignocellulosic raw material before addition to ?
the digester or cooking under variable conditions; or the ~ ~;
compound may be added directly to the pulping liquor and ':

-' ,' ~LO 8 lignocellulosic raw material in the dlgester, either in a sin~le char~e or in severaL charges at di~erent stages of the digestion or continuously throuyhout the digestion.

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In practising the process o~ the present invention, it may prove advanta~eous to use the soluble or more~soluble hydroquinone compound(s) which can be generated in situ by reaction of the corresponding quinone compound(s) with a reducing agent in a solution which is added to the pulping liquor or which is subsequently u~ed as the pulping liquor.
Inorganic or organic reducing agents may be used for the pur-pose, with a preference for organic compounds or compositions.
Inorganic reducing agents which may be so used incLude sodium or zinc dithionite (hydrosulphite), sodium borohydride, or zinc powder and sodium hydroxide. Organic reducing agents, which it is preferred to use, include carbo-hydrates such as glucose, xylose, mannose, or other mono-saccharides, sucrose, cellobiose, maltose, or other dis-accharides, oligosaccharides such as ra~finose, or poly-saccharides such as starch, dextrin, oxidised starche~ (e.g.
"dialdehyde starch") or xylan; amines or alkanolamines, such as ethylene diamine or diethylene triamine or ethanolamines;
or aldehydes such as vanillin.
Reducing agents present in the cooking liquor may in some cases become exhausted or destroyed as the cooking process proceeds, resulting in the reduaing ef~ect ~eing sub-stantially diminished or entirely lost. We have Eound that in such cases it is advantageous to add increments o~ the reducing agent periodically by injection into the lignocellu-losic cooking digester in order to maintain a sufficient amount o~ the hydroquinone compound in the cooking liquor throughout the cooking period. ~;
Pretreatment of the ceIlulosic raw material in a ;~
soaking liquor containing the quinonoid or hydroquinonoid ;

8g2 compound for a preLiminar~ impregnation of the cellulosic raw ma-terial with said compounds may be carried. out before the introduction of the cellulosic raw material into a digester for completion o~ the delignification process. This pretreat-ment or preparatory pulping of the lignocellulosic raw material aims at obtaining a better penetration and diffusion of the quinonoid compound into the lignocellulosic raw material before the pulp is subjected to cooking, in order to enhance the beneficial ef~ects of the quinonoid or hydroquinonoid compound in the delignification cooking of the lignocellulosic raw material.
Such pretreatment or preparatory pulping of the :
lignocellulosic raw material may be in accordance with any one or any combination of steps (A), (B), (C) and (D) below:
(A) normal pressure or positive pressure (hydraulically or pneumatically applied) or negative pressure ~vacuum) impregnation of the lignocellulosic raw ~ . .
material with an alkaline solution of the quinonoid or hydroquinonoid compound at temperatures from ambient to 120C, which solution may be the normal cooking li~uor or a liquor of another suitable compo-sition which is drained off following impregnation and then replaced with normal cooking liquor;
or (B) prolongation of the time normally taken to raise the temperature of the lignocellulosic raw material and cooking liquor containing the quinone or hydroquinone compound from ambient to maximum cooking temperature ~ of up to about 150C; or : 30 (C) maintaining the lignocellulosic raw material and _ g _ 78~

cookin~ liquor containing the quinonoid or h~dro~
quinonoid compound at a temperature within the range of 100C - 120C for a period from 15 to 60 minutes and then continuiny the normal rate of temperature increase to the maximum cooking temperature of up to about 150C.
~D) recycling of spent or partially spent liquor from a previous cook or ~rom one or more stages of a multi-stage process or from some point in a continuous digester usually near the top of such digester, to a preimpregnation stage which may be operated either batchwise or continuously.
In operation step (A) above, the impregnation period may extend up to 1 hour, before proceeding with the cooking process, which may be conducted at a temperature up to 150C
for a period of 0.5-5 hours; in operating step (B) above, the prolongation period may extend up to 2-3 hours, before proceed-ing with the cooking process as in step (A) above; and in operating step (C) above, the period taken to reach the temperature o~ 100 - 120C may be from 30 minutes to 2 hours, whilst the period of cooking after the 15-60 minutes delay at ;~
100 - 120C may be from 0.5-5 hours at temperatures as in step (A) above. In some cases, the time taken to reach the required cooking temperature is sufficient to achieve a satisfactory pulp yield so that zero cooking time is required.
Pretreatments of the type described above improve ~ -~
the quality of the pulp and recycling of additive as in treat-ment (D) allows economies in the use of the additives.
Where reduction o~ insoluble or sparingly soluble ;
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quinone compounds to the hydroqulnorle form is effe~tecl as yenerally inclicated above and when operating at low li~uor circulation rates, the presence of a surfactant may keep the quinone compounds in suspension, pending reduction to the hydroquinone form, thereby decreasing the level of shives or insufficiently cooked fragments in the resulting pulp.
In the some cases, especially where batch digesters are employed, it is advantageous to operate with higher than normal pulping liquor circulation rates in the digester, for ;~
instance, pulping liquor circulation rates up to 10:1 related to the normal rate. Thus, under normal operating conditions the pulping liquor circulation rate is usually 6-10 ;
circulations of total liquor per hour however we have found that the process of the invention makes it beneficially poss-ible to employ pumps which operate at liquor circulation rates up to 60-100 circulations o total liquor per hour. However, this is not essential in all cases and excellent results may be obtained with normal circulation rates.
The absence of sulphur compounds in the process of the invention means not only that the objectionable odours often associated with the presence oE such compounds in a recovery system are eliminated, but that relatively simply recovery systems are applicable. Among suitable systems may be mentioned the normal recovery ~urnacer fluidized bed com-bustion and wet combustion.
The practical examples set out in the Table below demonstrate the preferred process of the invention. The table, graphs and further examples below demonstrate the ~

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87~92 pref'erred proces~es o:L' the -i.nvention. In Qll ca~ es7 a charge of ~00 g Qd chips (ei-ther eucalyp-t or pine) was pulped in a rota-ting eLectricall~ heated digester using a liquor -to wood ra-tio of 3.5 -to 1 and a perlod of :L.5 ho-urs from ambien-t -teMpera-ture to cooking -temp~rature. O-ther conditions are a~
shown in the -table and individual examples. Pulp properties were determined using Appita Standard Me-thods after beating in the ~ampen Mill. For pulps prepared ~rom mixed species and from Ash-type`eucalypts, evalua-tions were a-t 400 and 200 Canadian Sta~dard Freeness respec-ti~ely.
i Example 1 uses an NSSC pulping liquor instead of NaOH liquor and the liquor used in example 1 is 9.8% sodium sulphi-te plus 2.3~o sodium bicarbonate on O.D. wood.
In the table and els~where in this specification colour means tris-timulus green re~lectance fac-tor.

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'llhe ~`ollowing 12 comparisons of data from -the table ~,v~ll se~rve to clarify -the application o~ the invention.
1. Compari~on of Exampl~ 3 wi-th Example 4 shows -the narked ef'~ect of excess alkali on pulp colo~r. Example 3, with its h:igher re~idual pH resuLt:i~g from ex~e~s alkali has the be-tter colour -than Example 4.
2. Example 3 compared with Example 5 con~irms the effec-t of high residual pH. Example 3 (high pH) has a much better colour -than Example 5 (low pH) despite the slightly lower pulp ~ield in the former case.
3. Example 6 compared with Example 8 again confirms -tha-t the use of a higher alkali charge with consequently ;~
higher resid~lal pH gives a better colour despite a lower yield.
The differences in measured pH values at -these alkali levels are not grea-t owing to the bufferlng o~ the spen-t liquor at the end of the cook, but the alkalinity of ~xample 6 must clearly be higher than tha-t of Example 8.
4, Example 6 with Example 7 shows the combined effect of increased alkali charge and reduced tempera-ture (Ex. 6) at the same pulp yield. ~arge differences in colour would not be expected as the residual pH is high in both cases but an appreciable effect is observed.
5. ~xample 7 with Example 8 shows the favourable ef~ect of lower cooking temperature on pulp strength~. r~hese pulps were cooked with the same amount of alkali at different i temperatures and despite the fact that Example 7 (150C) is .
at the lower yield, Example 8 (140C) has the higher s-trengths. I~ pulps are cooked a-t -the same temperature under otherwise similar conditions, the pulp of lower yield normally has the higher strength. ;

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6. Example 8 with Example 9 f'urther confirms this poin-t.
A-t -the same pul,p yield, the pulp cooked a-t lower temperature (Ex. 8) ha~ -the hig'ner s-trength.

7. Example 5 compared wi-th Example 14 shows the ~ ' accelerating a~fect of anthraquinone addition. Using the same alkali charge and tempera-ture, yields of 73% and 69~ were obtained in the absence of, and presence of, anthraquinone respec-tively. It sho-uld be no-ted tha-t wi-th -the addition of anthraquinone~ only half the cook time is required.

8. Example 10 with Example 11, improvement in pulp colour ob-tained in the presence of anthraquinone by increase in alkali charge and residual pH at the same pulp yield and Kappa number.

9. Example 10 with Example 14 shows tha-t under otherwise similar cooking conditions, a lower -tempera-ture results in improved pulp colour at the same pulp yield. The residual pH is a little higher in Example 14 than in Example 10 but at values below 11 this should have li-ttle effect on colour.

10. Example 2 with Examples10 -to 12 shows the marked , improvement in pulp strengths obtained by addi-tion of anthra-; quinone.

11. Example 15 illustrates a preferred combination of pulping condi-tions using a high alkali charge and residual pH, a low cooking temperature and anthraquinone. ~he resul-ting pulp displays excellent colour and higher pulp strengths.

12. Example 7 compared with Examples 16 & 17, shows the ~' considera'bly increase in pulp strength obtained by the use of anthraquinone, together with a marked increase in pulping rate.
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The Table above showcJ the advantages available from the process ot -the inven^tion. As no-ted above, one aclvantage is the produc-tion of light-coloured pulps by con-trolling residual p~-l. The eff`ect of pH on colour is further illustrated in ~ig. 1 which shows a plot of colour agains-t residual pH
for pulps prepared at -the same cooking temperatures and similar yield. There is a clear improvement in colour as residual pH rises. ~he overriding effect of residual pH on the colour of semichemical soda pulps does not appear to have been recognized in -the technical literature hitherto. ¦~
A further advantage of -the process of the invention ¦-~
made clear by the examples is -the use of low pulping temperat- ¦~
ures. The invention makes possible the use of temperatures well below -those used hitherto or considered prac-ticable for production of semichemical pulps of the type descrihed. The use of such low temperatures no-t only resul~ts in energy savings but also results in an unexpected improvement in -the s-trength of the paper produced from the pulp (commonly referred to as "pulp s-trength"). This is further illustrated in Figure 2 in ~
which burst index is plotted against pulp yield for two ~; `
different pulping temperatures. Compared wi-th the use of 140C, use of 130C gives an increase in burs-t index of 0.1 - 0.4 units depending on pulp yield.
~IGURE 2 Examples 18 and 19 show the effec-t of pulping with an-thrahydroquinone. ~he results were generally similar to those ob-tained using anthraquinone (Example 17).
EXAMP~E 18 .
SEMICHEMICAL SODA ULPING WI AN~HRAHYDRO~INONE
Ash type;eucalypt chips were pulped using 15~ total ~
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~odi~ hydroxicle and 0.1% anthrahycLroquinone on o,d wood for 0.25 hr ~-t 150C. Anthrahydroquinone w~s prepared by dissol~-ing 0.~ g an-thr.Lquinone in 200 rnl. water con-taining ~.0 g glucose and 30 g sodi~l hydroxide and boiling under an a-tmosphere o~ ni-trogen.
Total yield 72%
Kappa no. 120 Spent liquor pH 12.9 Colour 40 Burst Index 5.Q
(kPa m2/g) Breaking length 8.7 (km) SEMICHEMICAL SODA PU~PING WITH ANTHRAHYDROQUINONE
Wood and pulping conditions were as ~or Example 18 except that anthrahydroquinone was prepared by dissolving 0.4 g anthraquinone in 200 ml water containing 0.5 g sodium dithionite and 30 g sodium hydroxide and boiling under an atmosphere of nitrogen.
To-tal yield 71%
Kappa no. 110 Spent liquor pH 12.9 Colour 41 Burst Index 5.2 (kPa an2/g) Breaking length (km) 8.7 Examples 20 and 21 show the ef~ect of pulping with IO-hydroxyan-throne and 1,4-dihydroanthraquinone respectively bbth -tautomers of anthrahydroquinone. These compounds are : ::
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~lkali-soluble and are believed to be converted ta anthra-hydroquinone in alkaline solution. They ~re at least as effective ~s anthraquinone (compare Ex. 12) and n~y be used beneficially where it is desirable to use a soluble additive which can be readily impregnated into the wood prior to the ~ ;
co~encement of pulping.

Mixed species eucalypt chips were pulped using 15% l ;
sodium hydroxide on od wood and 0.1% 10-hydroxyanthrone on od wood. Time at temperature (140C) was 1 hour.
- Total yield 67% ;
Kappa number 122 Spent liquor pH 12.0 Colour 29 :
SEMICHEMICAL SOD~ PULPING WITH 1,4-DIHYDROANT~AQUINONE
Wood and pulping conditions were the same as for Example 20.
Total yield 65.5% `;
Kappa number 121 Spent liquor pH 12.1 Colour 28 Example 22 illustrates the effect of pulping with 1, 4, 4a, 9a - tetrahydroanthraquinone. This compound can tautomerize in aIkaline solution to 1, 4-tetrahydroanthrahydro- -quinone which can also be considered as a substituted naph-thohydroquinone.

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EXAMPI,E 22 SE~IIC~IEMICAL SOD~ PULPING WI'r~l_L, 4, 4a, 9a - 'I'ETRAHYDRO-AN'~HRAQUINONE
Wood and pulping condi-tions were the ~ame as for Example 21.
Total yield65.1%
Kappa number122 Spent liquor pH 12.0 ~Colour 29 Examples 23 and 24 illustrate the present invention as applied -to Pine chip~. The pulp prepared with -the higher alkali charge and residual pH (Ex. 23) has the better colour ~`.
at the same yield.
Example 23 Example 24 ~. .
% Caustic Soda13~o 11% e Digestion -time 1.5 hours 2.5 hours Yield 74.5% 75.1 Kappa number 168 164 Residual pH 12.9 12.4 Colour ~ 35 30 . .
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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process of delignifying lignocellulosic material which comprises digesting the lignocellulosic material with a non sulfur containing pulping liquor containing primarily an hydroxide or carbonate of sodium, potassium, magnesium, calcium or ammonia, wherein an excess of the hydroxide or carbonate based on the lignocellulosic material is used and the pH of the spent liquor at the end of the digestion is from 12 to 13.

2. A process as claimed in claim 1, wherein the pulping liquor contains primarily sodium hydroxide, sodium carbonate or sodium hydroxide and sodium carbonate.

3. A process as claimed in claim 1, wherein the digestion temperatures are from 120 to 150°C.

4. A process as claimed in claim 1, wherein a quinonoid or hydroquinonoid compound or a tautomer of such a compound is added in an amount of 0.01 to 0.3% by weight based on dry ligno-cellulosic material.

5. A process as claimed in claim 4, wherein the additive compound is selected from anthrquinones, anthrahydroquinones, naphthaquinones, naphthahydroquinones, benzoquinones or benzo-hydroquinones or tautomers of such compounds.

6. A process as claimed in claim 5, wherein the additive is 9,10-anthrquinone, 9,10-anthrahydroquinone, 1,4,4a,9a-tetra-hydroanthraquinone or 1,4-dihydroanthraquinone.