CA2036311A1

CA2036311A1 - Process for producing pulp from a lignocellulosic-containing material

Info

Publication number: CA2036311A1
Application number: CA002036311A
Authority: CA
Inventors: Stephanus J. Raubenheimer
Original assignee: Individual
Current assignee: Sappi Ltd
Priority date: 1990-02-12
Filing date: 1991-02-12
Publication date: 1991-08-13
Also published as: MW191A1; RU2037594C1; EP0442806A1; AU7098791A; AU643746B2; PT96722A; FI910655A; ZW1191A1; NZ237098A; BR9100592A; FI910655A0; BR9100565A

Abstract

A PROCESS FOR PRODUCING PULP FROM A
LIGNOCELLULOSIC-CONTAINING MATERIAL

ABSTRACT

This invention provides a process for producing pulp from a lignocellulosic-containing material.

According to one aspect of the invention, there is provided a process for reducing the lignin content (as expressed by the Kappa number) of unbleached pulp intended for dissolving grade pu1p manufacture, the process including the step of applying an alkali extraction step to the pulp.

According to another aspect of the present invention, there is provided a process for producing dissolving grade pulp, the process including the following steps:

a pre-hydrolysing step;

a step of dissolving the lignin contained in the pre-hydrolysed material;
and carrying out an alkali extraction step.

The alkali extraction step may include using about 2% to 5%, preferablyabout 3%, alkali by mass of pulp; at a temperature of about 50°C to 100°C, preferably about 70°C to 80°C; for a period sufficient to give optimal lignin removal. The alkali used is preferably sodium hydroxide.

According to a further aspect of the present invention, there is provided a dissolving grade pulp when produced by a process described above.

Description

A PROCESS FOR PRODUCING ~ULP FROM A LIGNocELLuLost~-~o ~ [~ ~ G
-MATERIAL

This invention relates to a process for producing pulp from a lignocellulosic-containing material. More particularly, this invention relates to a process for producing dissolving pulp from a lignocellulosic-containing material.

Wood pulp containing high levels of alpha cellulose, which is used in the manufacture of various cellulose derivatives for various end uses or products, is known in the art as dissolving pulp. Other terms synonymous with dissolving pulp are chemical cellulose and special high alpha pulp. Two processes are in general use for the manufacture of dissolving pulp viz:

1) The acid sulphite process, the development of which started at about the beginning of this century (Rydholm, S.A., Pulping Processes, p. 280, Interscience Publishers, New York-London-Sydney 1~65); and 2) The prehydrolysis - Kraft process, the development of which started in 1929 (Rydholm referred to above - p 281) The latter process makes use of an acidic pretreatment ("prehydrolysis") step in order to remove hemicellulose prior to the alkaline pulping (delignification) step. South African Patent 88/4037 discloses the prehydrolysis - neutral sulphite - anthraquinone process (PH-NS-AO) for the manufacture of "hemicellulose hydrolysate and special pulp" (high alpha grade). This prehydrolysis step performs essentially the same function as the prehydrolysis step prior to the .~raft (Sulphate) pulping method for the manufacture of dissolving grade pulp; while the neutral sulphite-anthraquinone de1ignification steo is essentially the same as the process also known as the ,~ ~5 semi-alkaline sulphite-anthraquinone (SAS-AQ) process, first reported at a technical conference in 1979 by Raubenheimer, S and Eggers, S.H., (both being research workers employed by SAPPI LIMITED, the present applicant herein) viz the 11th European ESPRA Meeting, Maastricht, The S Netherlands, during May 1979.

In the aforementioned patent specification, the washing of the cooked chip material is reported to have taken place over a period of 10 hours, which is clearly impractical; and certain of the tests show a Kappa number which is not as low as could reasonably be required or expected.

It has been found by the inventor of the present invention that the aforementioned process could be improved when applied to timber presently used for dissolving pulp manufacture in South Africa viz Gum (various Eucalyptus species - mostly E. grandis) and Wattle (Acacia mearnsii) and it is therefore an object of the present invention to provide a process which overcomes, at least partly, the disadvantage(s) of the aforementioned process and hence of the prior art.

According to one aspect of the present invention, there is provided a 20 process for producing dissolving grade pulp, the process including -applying a prehydrolysing step to a lignocellulosic-containing material for example hardwood.

partly dissolving the lignin contained in the prehydrolysed material (- a pulping step); and carrying out an alkali extraction step.

About 2% to 5%, preferably about 3%, by mass of alkali may be used to carry out the alkali extraction step.

The alkali used may preferably be sodium hydroxide.

The alkali extraction step may be carried out at a temperature of about 50C to 100C, preferably about 70C to 80C, for a period sufficient to give optimal lignin removal.

According to another aspect of the present invention, there is provided a process for reducing the lignin content (as expressed by the Kappa number) of unbleached pulp intended for dissolving grade pulp manufacture, the process including applying an alkali extraction step to the pulp.

About 2% to 5%, preferably 3%, by mass of alkali may be used to carry out the alkali extraction step.

The alkali used may preferably be sodium hydroxide.

The alkali extraction step may be carried out at a temperature of about 50C to 100C, preferably 70C to 80C, for a period sufficient to give optimal lignin removal.

Spent liquor from the pulping step may be removed from a digester, treated with alkali, and re-introduced to the digester to lower the Kappa number of the pulp and/or to improve evaporation of the spent liquor.

After re-introduction of spent liquor to the digester, the digester may be operated at a temperature ~ 100C and at e1evated pressure.

About 10% to 15% a1kali by mass of pulp may be used.

The alkali used may preferably be sodium hydroxide.

The invention extends to a dissolving grade pulp whenever produced by a process as herein described and/or exemplified.

The invention will now be described and illustrated in greater detail with reference to the fol10wing non-limiting examples:

Prehydrolysis and pulping were carried out under laboratory conditions on run-of-mill (ENSTRA) Eucalyp-tus wood chips. The prehydrolysis conditions and yields are shown in Table 1 hereunder.

Table 1 ~ __ _.__ _ R~ l~io. ~ydrolysis agent Tlme e~ Time at Yield % on wood temp. (mins) temp. (hrs) (%) __ ~__ __ __, P1 2% - H2S04 ~0 to 15$ C 2 at lS5 C 73,1 P2 1~ - S2 60 to 16~ C 2 at 165 C 70,S
P3 0,2S~ - S0~ 60 to 165 C 2 a~ 165 C 80,4 P4 0.'53 - St 60 ~o 165 C 3 at 165 C 77,6 PS ___ 60 ~o 150 C 2 at 150 C 85,:5 (Pl - P4 used 4,0/1 L/W ratio; P5 used 5,0/1 L/W ratio) Thereafter pulping was carried out on material from three of the prehydrolysis runs, yielding the pulping results shown in Table 2 hereunder.

Table 2 .__ . _ ~
Cook Prehydrolysis C~ar~e as Na20( ~ ) Y~eld ( ~ ~ 1;appa No. Conditions ~ Screened Rejects No.
_ _ ~ __ ~ _ C1 AsP3- Ta~le 1 17 4 _ 51,97 0,1 ~2,5 C2 As P4 - ~able 1 20 2, 52, 5 ~9, 44 0, 02 87, 9 . C3 /~ ~7 4 _ 50,96 0,52 ~Z,9 The following process conditions applied:

0, 2% Anthraquinone in all cases 3,5/1 L/W ratio in all cases 85 mins to temperature (175C) in all cases 3 hours at 175C for Cl and C3 5 hours at 175C for C2 The Kappa number from cook no. Cl was relatively high.

Virkola has reported Kappa numbers in the low twenties after the pulping step but how such figures were obtained is uncertain.
IVirkola, N-E and Tikka, P.O. - Tappi Pulping Conference, 1988; New Orleans U.S.A. October 30 - November 3 p 255 . In South Africa patent No. 88/4037 (with Virkola and Tikka as inventors) continuous washing for 10 hours after pulping is reported.]

tJ fi,~'`'` ~3~ ~

For cook number C2 the chemical charge was increased~ caustic soda was added and the pulping time was increased significantly in order to reduce the Kappa number which, contrary to expectations, increased.
It is believed that this increase was due to the harsher prehydrolysis conditions resulting in higher deactivation of the lignin.

Cook number C3 gave a lower Kappa number than Cl possibly because of the milder prehydrolysis conditions.

In order to reduce the Kappa number a caustic extraction step was applied.

This yielded the results shown in Table 3 below.

Table 3 . _ .. __~
Cook E:str~ction I Extraction lCappa llo. Yield on unbl.
. . . . ___ __ No. i~o~ m~lu3 ~etore- After pulp (~) _ ~ ~
C2 E1 3~ ~laOH*87, g Z6, 6 N. D.
C2 EZ Wate- 87, 9 66, 6 ~.D.
C1 E3 3~ NaO~ 42tS 16,9 N.D.
C3 E4 3~ YaOH 3Z,9 17,6 95,71 The following process conditions applied:

* - Percentage on pulp Time - l hour Temperature - 70C
Consistency - lO%

It was apparent that some of the residual lignin in the unbleached pulp was rather loosely held and could be removed to a large extent with a caustic soda extraction step (followed by washing).

Two bleaching runs were done, the results of which are shown in the following table:

Table 4 ~ __ Unbl. pu1p IB1. No. 5equenco Bright- ~lpha Y1c1d % Visc.
~ ness ~) ellu- llm -r cm~/g ~___ ._ ___ C1 ¦ B1 C8,; - E4,~ - D~,5 87,6 ~S,S~ 89,37 37,~ 528 E4 1 82 Ci,; - ~-2, 7 - DO, 8 89,i 96,5~ 97,36 40,i 848 _~ ___ _. ~

Bleach number Bl was done before the positive effect of an extraction step after pulping was established. Because of the high unbleached Kappa number, bleach chemical charges were extremely high. Both the alpha cellulose content and the pulp yield looked promising. Bleach number B2 was done using an extracted unbleached pulp with a Kappa number of 17,6. Both the alpha cellulose content and pulp yield were high.

-(RUN-OF MILL - SAICCOR) PREHYDROLYSIS STEP

Table 5 hereunder sets out prehydrolysis conditions and results.

Table 5 __ __~ ~
Timber Rurl No. 52 charge L/~ Time to Tiale at Spent l.$q Yl~ld ( ~ ) ratio te~p Tcmp (Dlin~ ) pH (% ) . ~ins ) __ _ _ __ _ __ ~__ Gurn . P60,2 593/l 6~ 120 ~ lS0C 2,49 83,36 Gum P72,Z 5,3~1 6~ 120 ~ 150C Z~66 83,60 (~m P80,2 5,3/1 6~ 120 ~ 150C 2,64 83,gl Gum ?90,2 5,3/l 60 120 @ 150C 2~67 83tS2 Qn~ P1~~ Q,2 5~3/l 60 120 ~ 150~C 2,52 82,70 W.~ t t leP1 t 0, 2 S, 3/1 60 1 2D e 1 so~c 2, 78 7g, :i2 The spent hydrolysis liquor from run no. P8 ~as titrated against standard NaOH solution in order to get an indication of the caustic soda consumption of such 1iq~or. This would be relevant in the case where the prehydrolysis step is done in the same vessel or digester as the pulping step and the acidity of the liquor remaining after draining of -the bulk of the prehydrolysis liquor has to be neutralized when adding pulping chemicals. Neutralization required 2,49 NaOH per litre of prehydrolysis liquor.

~ ~ J ~
_ 9 _ PULPING STEP

Pulping conditions and results are given in Table 6 hereunder.

The initial concentration of NazSO3 (as NA20) at a charge of 17% at a L/W ratio of 3,5/l was 48,6 g/l. In order to reduce the Kappa number, cook number C5 was done at the same initial concentration as the other cooks but at a much higher charge (-L/W ratio increased to 5,3/1).
The anthraquinone charge was also increased to 0,2% (on timber) and the time at top temperature was increased to 4 hours. The Kappa number could be lowered to 28,7 but this is still relatively high for 20 a bleachable hardwood pulp. See hereunder under the heading "Extraction Step".

~ ~ ~ 9~
~ _ O~
- ~ - - - - ~
~ ~; ~ ~ In ~ ~ - - o~ o c~ ~ t~
:~ ~ " ~ n C O O O C~ ~ Q .
~ el~ o ~ o o o o ô
_ ~ ..
~ ~ ~ O
~ - _ ,r, a n~ a ~

~: _C ~
.. , = . ..... -.
i~`, _ ~ O O O O O O O

.n ,-, ~ ~ r'~
. . .
~ ô c~ ~^ ô o ~ -c~ ~ ~
~5 ~ z o _ ~ ~ _ ~ ~D ~ ~ ~ ~ er u ~ o æ z I ~ ~ cocn CQ~
~ - -a ~ W 'J' ~

THE EXTRACTION STEP

Table 7 -~ _ I
&t.ac~ion ~b. E~ E~ E7 E8 E9 ¦ E10 . Cook .~'o. C4 C5 C6 -C, ~8iC~ C10 Initial Kappa`~. 40,S 28,7 37,8 34,1 ~,7 31,7 .~aOH ~S) 3 ¦ 3 3 3 3 i l~m?era~uTe (C) 70 i 70 7a 7Q ~0 70 ¦! Consistency (S~ 10 1 10 10 10 10 10 .
l Re~er,~ion ~hr3 1 1 1 1 1 1 .
I ~inal pH 17,~0 12,44 12,~7 1Z,SO 1~,34 129iO
K2p?a .~o. 14,; lZ,4 15,2 14,0 ~0,3 16,0 Yic'd (~) ~3,.3 N.D. g4,~3 ~S,~1 95,26 96,7~
~'iscosity (c~3/g) 998,~ 1 - lQ42,~ 1095,g 102S,7 1184,&
_ ~ 1573 l663 154S 1812 . . .

The Kappa number drop during the extraction step was such that it was obviously more advantageous to use such a step than to try to reduce the Kappa number by using a more severe pulping step. This is clearly shown by comparing E5 and E6 in Table 7.

SINGLE VESSEL PROCEDURE

Cook number Cll ( see below) was done in order to simulate the procedure when doing prehydrolysis, pulping and the extraction step(s~
in a single vessel (- batch digester, no washing between steps), as follows:

- 1 2 - ~ ~ s i~

PREHYDROLYSIS STEP:

Wood charge - 1400 9 O.D.
L/W ratio - 5,3/1 S02 (%) - 0,2 Time to 150C (mins) - 60 Time at 150C (hrs) - 2 Liquor drained (ml) - 5500 Liquor remaining (ml/calculated) - 1920 Spent liquor pH - 2,66 PULPING STEP:

Assumed prehydrolysis yield (%) - 84 O.D. timber left (9) - 1176 NazSOl charge (% Na20) - 17%
Na2CO3 charge (% NazO) - 4%
NaOH charge for neutralization 1,920 x 2,4 9 - 4,608 9 A.Q. (%) - 0,15 L/W ratio - 3,5/1 Time to 175C (mins) - 80 Time at 175C (hours) - 3 Spent liquor residual sulphite (g/l NazO) - 9,05 Spent liquor pH - 8,89 Spent liquor volume (ml) - 1800 Spent liquor left (ml/calculated) - 2316 The amount of NaOH necessary to compensate for the acidity left after draining of the excess prehydrolysis liquor, came to 0,3% (as Na20) on O.D. - prehydrolised wood material.

f ~ 3 ~ ~

EXTRACTION STEP:

Spent pulping liquor from cook number C6 was titrated against NaOH in order to determine the amount of caustic soda necessary to raise the pH to 11 (the approximate pH at which all NaHCO3 will be converted to Na2 CO3). This figure came to 129 NaOH per litre of spent pulping liquor.

Assumed pulping yield (%) - 53 O.D. material left (9) - 623,28 (assumed) NaOH (/0) ~ 3 NaOH for raising pH of remaining pulping liquor to 11 = 2,316 x 12 = 27,792 9 = 4,46%
Consistency (%) - 10 Time at 80C (mins) - 20 final pH - 11,66 Kappa no. - 13,4 Yield % - Screened - 42,59 - Rejects - 0,02 The final Kappa number of 13,4 was in line with figures obtained doing the three steps separately with washing between each step. The Na'`H
necesscry tc raise the pH of the remaining pulping liqllor ~o 11 cam"e to about 4,5,'0 on O.D. pulp.

OXYGEN DELIGNIFICATION

- In order to further lower the Kappa number, four of the unbleached pulps were subjected to oxygen delignification. See Table 8 below.

Table 8 _ __ _ Oxygen bleach no. , 01 . .02 03 04 .
Startlng pulp no. EB C11-- ~,. ~70 Startin~ K~ppa No. 14,0 13,4 Zo,3 lb NaOH ~) l 1 2 1,5 Mg as MgS04 (~) O,05 Nil Nil Nil Consistency (~) lO 10 lO 10 Time to temp. (mins) 22 16 26 26 Time a~ tamp. (mins) ~0 3C 30 3n Top te~perature (~C) llO llS 118 llS
2 pr~ssure (kPa) 6gO 690 6gO 69~
Spent liquor pH 9,66 9,S8 9,57 9,88 Yield (%) 97,86 g8 9 25 9~,45 96,37 Viscosity (cm3/g) 973,6 g46,8 784,8 1037,8 D.P. 1459 1415 1lSU lS66 ~appa ~umber ¦ 6,6 S,l _ 3,6 4,2 These pulps delignified easily to low Kappa numbers during an oxygen delignification step.

BLEACHING

Some caustic soda extracted pulps and the four oxygen delignified pulps were subjected to full bleaching. See Table 9 below.

j _ _ _ _ _ _ _ . _ V~ ~ ~ ~-- O
O rl_ 0~ ~O Ln ~ ~ r_ I
.
_ _~

~: ~ oo r~ cn .~ C~J CO I ~ ~ C`
o ôôo_o~
_.

s o .~ I~
._. ~. _ ~ .
. .
o I_ r~ ~ . a-- co c~ ~ r~

1." i ~ ~
CJ'~ Cll ~) a7 o) , ____,_____ ~

~ôo^
~ ^ ^ ~ T
2 '-- ~ ~

~J ~O ~ O O I

O ~ O O
J N C~J --I al o ~ o ~ ~
O O
. -~

~. ¦
_ O ~> d' Ln ~ r~
c~ z ~n c~ c~ ~ ~ ~
,~"1 _.~

:
In order to further illustrate the effect of an alkali extraction step after the pulping step and its interaction with the prehydrolysis step, a separate set of experiments was done using a new sample of run-of-mill (SAICCOR) Eucalyptus chips.

Prehydrolysis conditions were varied keeping L/W ratio constant at 5,0/1 and temperature rise time at 1 hour. (Table 10) Table 10 Prehydrolysis I ~ydrolysis I Te~p~ ¦T1me at ¦Yield¦Effluent 10 No I agent (% on wood) tC1 ¦teEp (mins)l (O ¦ p~ !

,2 SO~ :~0 1120184,63 2,71 ~ ,3 SO~ 60 1',0 78,11 2,S0 2'' 3,~ SO~ ',0 ' ao 73,34 .,37 ~1_ û,' SO~ 0 120 77,14 2,30 215 ~ure water j150 ~20 88,53 3,24 ?1- 3,2 SO~ 0 60 ~0,08 ',74 Hydrolysed material was soaked f-or 30 minutes in water after treatment berore washing and spin drying.

Material originating from some of the prehydrolysis runs was subjected to pulping. (Table 11) f'~ s` ~ ~

The following process conditions applied except for C17 3,5/1 L/W ratio 0,1% Anthraquinone 16% Na2S03 as Na20 5 4% Na2C03 as Na20 80 minutes to temperature (175C) 3 hours at 175C

C17 was done using original wood material which had not been subjected to a prehydrolysis step. In this case the chemical charge was 10 increased to 18% Na2S03 (as Na20) and 4,5% Na2C03 (as Na20) keeping the anthraquinone charge and other conditions as above.

Table 11 _ .
Cook No.¦ Pre- ~ ~ield ~%) ' Spent Liquor ¦
hydrolysis Nb. i Screened ¦ Rejects 1 Re5~ Sul~hite P~ No as Na2O (g/l) _ '2 1 ?12 1-_,,2 1',04 3,07 ~3,83 35,.
¦ ~'3 1 ?'3 ,~2,:~ ll 0,80 16"~ 8,88 7;,3 ~7 1 ?1~ ,cl !" 81 '5,'3 3,~39 '70,:
C'~ I ?'5 i3"5 '',60 ~,~6 8,86 1 33,/
~'6 ?16 1-',01 ~0 6,~0 9,02 ! ~o,i 20C~, I9,08 ~_,02 1'3,~8 3,16 ~ 8~8 It was obvious that the prehydrolysis conditions had a large influence on delignification response in the pulping step. Harsher prehydrolysis conditions lead to higher pulp Kappa numbers and higher levels of residual sulphite in the spent liquor indicating greater ~.d '~ 3 ~.

deactivation of the lignin. This lignin had lost some of its fibre bonding properties however as the rejects obtained from pulping the prehydrolysis material were much lower than the rejects obtained from pulping the unhydrolised material in spite of the latter's lower Kappa 5 number compared to figures obtained from pulping some of the prehydrolised material.

The pulps obtained as above were subjected to an alkaline extraction step using sodium hydroxide as the alkali. Extraction was done in plastic bottles in a constant ternperature water bath. (See Table 12) The following constant process conditions applied:

60 minutes retention time 10~o consistency temperature 70C

Table 12 .
.. j .. j . , ---- 1 I E~traction, Ccok INaO~¦Effluent ¦ ~appa Rappa ND ~ield %
, ~0 i~ ~ ,D~ ' Drop Sta~e¦Screened on O.Dq ¦
! w~
. , i . .... , .. I
_ ~ 96,00¦ 42,02 Ii " ,_8 5~ a,o 97,,6¦ 42,70 ~ -,? :2,5 36,51~ '2,24 _~ '2 ~ "9 35,311 41,'2 Jq ~ ' 39r061 ~6,2S
, ~ 5 ~ ; ' ~ 3~,_ 35,o 32,91l 30,77 _ ,~ , '3,0 -~,/ 38,96 36,68 ~ ~ ^ ,O _ _ 97,77 14,15 1 _~9 , _ ~ '112,i/ _1,- ," 9,01 !

~ ? ~? ~ 3 Comparing Kappa numbers of Ell, El2, E13 and El4 shows that the Kappa number drop during the extraction step is dependent on the amount of alkali charged. In a separate experiment lOOg O.D. of prehydrolysis -S.A.S.-A.Q. gum pulp of 38,3 Kappa number was washed with 1 litre of a 3g/l NaOH solution at 70C on a Buchner funnel. After washing with water the Kappa number of the resulting pulp was 25 indicating that merely washing with alkaline wash water can also effect a substantial Kappa number drop.

Comparing other extraction step Kappa numbers in Table 12 also indicates that the extraction step brings widely varying pulping step Kappa numbers, originating from varying prehydrolysis conditions, much closer together. This is a positive result but yield figures indicate that large yield losses can result from too harsh a prehydrolysis step. This step, however, must be sufficiently harsh (pH, temperature and time) to effect adequate hemicellulose removal The Kappa number drop obtained from the unhydrolised pu1p (Cook C17) on extraction also indicates the different nature of the lignin in this pulp compared to the lignin in pulp which had been hydrolised.
If this material had been obtained by applying a hydrolysis step prior to the pulping step, the Kappa number drop on extraction would have been more pronounced considering the unextracted Kappa number of this pulp, The above results were obtained from pulp which had been well washed after the pulping step in order to demonstrate the positive effect of an alkaline extraction step.

When running batch digesters using this proces~ the preferred procedure would be to drain the digester after the prehydrolysis step of the prehydrolysis spent liquor. (This liquor would be used for recovery and beneficiation of products dissolved and extracted from the wood).

The pulping chemicals (Naz503, ~a2 C03, and Anthraquinone) would then be charged (possibly together with a small amount of NaOH to compensate for the residual acidity of the prehydrolysis step). After a penetration period and ?ulping ~ime, the spent liquor would be pumped from the digester. Alkali (,~aOH) would then be added to this liquor and reintroduced to the digester followed by circulation Cor a period. This would complete the cook.

Ihe addition of alkali to the spent liquor and reintroduction of this mixture to the digester serves two purposes.

a) An effective extraction is accomplished which lowers the Kappa number prior to the oxygen delignification step (or other step depending on the bleach sequence). The pH adjusted spent liquor can be introduced at nigh temperature because the free alkali concentration is low which otherwise would have resulted in cellulose yield loss at high temperature. The results appear to indicate that the lowering of the Kappa number is mostly the result of fibre swe11ing at the higher pH. Pathways are opened as a result of the swelling which allow removal of some of the lignin molecules which are loosely held.

bj Spent S.A.S.-A,Q. pulping liquor can be difficult to evaporate to high solids levels without forming troublesome precipitates.
his can be overcome by adjusting the pH of the spent liquor to about 11 prior to evaporation.

~ 3 In a continuous digester introduction of pH adjusted spent liquor to the wash zone can accomplish both the extraction step Kappa reduction benefit and the enhanced liquor evaporation properties resulting from the higher pH.

It appears from the above that the process of the present invention can produce high alpha-cellulose pulp at a high pulp yield. An alkaline extraction step following pulping proved to be satisfactorily effective in removing residual lignin.

An oxygen bleaching step also proved effective in further lowering the Kappa number to acceptably low levels prior to full bleaching. Full bleaching of such pulps required fairly low charges of chlorine-containing chemicals, thereby providing a process which has a low pollution potential. "Non polluting" bleach chemicals like ozone and peroxide could also be considered.

Although cer~ain embodiments only of the invention have been illustrated above, it will be appreciated by any person skilled in the art that other possibilities and variations of the invention are possible. Such possibilities and variations are therefore to be considered as falling within the spirit and scope of the present invention as defined in the appended claims.

Claims

l. A process for producing dissolving grade pulp, the process including -applying a prehydrolysing step to a lignocellulosic-containing material.

partly dissolving the lignin contained in the prehydrolysed material (- a pulping step); and carrying out an alkali extraction step.
2. A process as claimed in claim l wherein about 2% to 5%, by mass of alkali is used to carry out the alkali extraction step.
3. A process as claimed in claim 2, wherein about 3%, by mass of alkali is used to carry out the alkali extraction step.
4. A process as claimed in claim 3, wherein the alkali used is sodium hydroxide.
5. A process as claimed in any one of the preceding claims, wherein the alkali extraction step is carried out at a temperature of about 50°C to 100°C for a period sufficient to give optimal lignin removal.
6. A process as claimed in claim 5, wherein the alkali extraction step is carried out at a temperature of about 70°C to 80°C for a period sufficient to give optimal lignin removal.
7. A process for reducing the lignin content (as expressed by the Kappa number) of unbleached pulp intended for dissolving grade pulp manufacture, the process including applying an alkali extraction step to the pulp.
8. A process as claimed in claim 7 wherein about 2% to 5%, by mass of alkali is used to carry out the alkali extraction step.
9. A process as claimed in claim 8, wherein about 3%, by mass of alkali is used to carry out the alkali extraction step.
0. A process as claimed in any one of claims 7 to 9, wherein the alkali used is sodium hydroxide.
11. A process as claimed in any one of claims 7 to 10, wherein the alkali extraction step is carried out at a temperature of about 50°C to 100°C for a period sufficient to give optimal lignin removal.
12. A process as claimed in claim 11, wherein the alkali extraction step is carried out at a temperature of about 70°C to 80°C for a period sufficient to give optimal lignin removal.
13. A process as claimed in any one of the preceding claims, wherein spent liquor from the pulping step is removed from a digester, treated with alkali, and re-introduced to the digester to lower the Kappa number of the pulp and/or to improve evaporation of the spent liquor.
14. A process as claimed in claim 13, wherein after re-introduction of the s-pent liquor to the digester, the digester is operated at a temperature > 100°C and at elevated pressure.
15. A process as claimed in either claim 13 or claim 14, wherein about 10% to 15% alkali by mass of pulp is used.
16. A process as claimed in any one of claims 13 to 15, wherein the alkali used is sodium hydroxide.
17. A process as claimed in either claim 1 or claim 7, substantially as herein described and/or exemplified.
18. A dissolving grade pulp whenever produced by a process as claimed in any one of the preceding claims.