NZ237098A - Process for producing dissolving pulp (high alpha-cellulose pulp) - Google Patents

Description

New Zealand Paient Spedficaiion for Paient Number £37098 23 7 0 9 8 No.: Date: r,.;.. , . .t2^.cto, Co.upioJti Specification f- rtd: • Pubticc'.lcn Qste: . J99$..., P.O. Journal, Mc: . ...IvOJo v 3 •<n:. '2FEBIJ9id % %<• (: i v*V 1 NEW ZEALAND PATENTS ACT. 1953 COMPLETE SPECIFICATION A PROCESS FOR PRODUCING PULP FROM A LIGNOCELLULOSIC—CONTAINING MATERIAL SAPPI LIMITED, of 48 Ameshoff Street, Braamfontein, Johannesburg, Transvaal, Republic of South Africa, a company incorporated under the laws of the Republic of South Africa, hereby declare the invention for which I / we pray that a patent may be granted to me/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- (Followed by Page la) — I o. 23 7 0 A PROCESS FOR PRODUCING PULP FROM A LIGNOCELLULOSIC-CONTAINING MATERIAL This invention relates to a process for producing pulp from a 1ignocellulosic-containing material. More particularly, this invention relates to a process for producing dissolving pulp from a 1ignocellulosic-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 1965); 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 Kraft (Sulphate) pulping method for the manufacture of dissolving grade pulp; while the neutral sulphite-anthraquinone delignification step is essentially the same as the process also known as' the 23 7 0 9 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 5 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 10 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 £. grandis) and Wattle (Acacia 15 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 prehydrolyzing step to a lignocellulosic-containing material to obtain a prehydroiyzed composition; subjecting saidprehydroiyzedcomposition to a SAS-AQ pulping step to partly dissolve the lignin contained in said prehydroiyzed composition 25 and to obtain a pulp composition; <!- optionally washing said pulp composition; and 2DEC1993S / / directly after washing said composition or directly after said SAS-AQ pulping step, conducting an alkali extraction step on said pulp composition by means of an alkali composition to lower the Kappa number of said pulp composition. 23 7 About 1% 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 50°C to 100°C, preferably about 70°C to 80°C, for a period sufficient to give optimal lignin removal.

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 50°C to 100°C, preferably 70°C to 80°C, 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 1iquor.

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

About 10% to 15% alkali by mass of pulp may be used. 23/0 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 5 with reference to the following non-limiting examples: EXAMPLE 1 Prehydrolysis and pulping were carried out under laboratory conditions on run-of-mill (ENSTRA) Eucalyptus wood chips. The prehydrolysis conditions and yields are shown in Table 1 hereunder.

Table 1 Run No.

Hydrolysis agent Time to Time at Yield X on wood temp, (mins) tenp. (hrs) Ct) pi l\ - h2so4 90 to 1SS °C 2 at 155 #C 73,1 P2 1* - so2 60 to 165 °C 2 at 165 aC 70,5 P3 0,25t - s02 60 to 165 °C 2 at 165 °C 80,4 P4 0,25% - S02 60 to 165 °C 3 at 165 °C 77,6 P5 0,2* - s02 60 to ISO °C 2 at 150 °C 85,3 (PI - P4 used 4,0/1 L/W ratio; P5 used 5,0/1 L/W ratio) ihwi 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 NO.

Prehydrolysis Conditions Charge as Na20(i) Yield (%) Kappa No.

NajSOj Na2C03 NaOH Screened Rejects C1 Asp3 - Table 1 17 4 - 51,97 0,1 42,5 C2 Asp4 - Table 1 2,5 2,5 49,44 0,02 87,9 C3 As PS - Table 1 17 4 50,96 0,52 32,9 The following process conditions applied: 0,2% Anthraquinone in all cases 3,5/1 L/W ratio in all cases 85 mins to temperature (175°C) in all cases 3 hours at 175°C for CI and C3 hours at 175°C for C2 The Kappa number from cook no. CI was relatively high.

Virkola has reported Kappa numbers in the low twenties after the pulping step but how such figures were obtained is uncertain. [Virkola, 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.] 22>"K*g 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 CI 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 No.

Extraction No.

Extraction oedius Kappa No.

Yield on unbl. pulp (*) Before- After C2 E1 1\ NaOH* 87,9 26,6 N.D.

C2 E2 Water 87,9 66,6 N.D.

C1 E3 3t NaOH 42, S* 16,9 N.D.

C3 E4 3t NaOH 32,9 • 17,6 95,71 The fol lowing process conditions applied: Time Temperature Consistency - Percentage on pulp - 1 hour - 70°C - 10% 2 7FEB199131 231W 6 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 Jnbl. pulp Bl. No.

Secusr.cs Brightness (1) Alpha CellUT lose (%) Yield \ Vise, cafyg Bl.

On Timber m o •T* —* B1 B2 C8,5 - Ed,5 - Dl,5 C2,5 - HI,2 - DO,8 87,6 89, S 95,50 96,56 89,37 97,36 37,3 40,5 528 848 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. 13 ion ■8 ■ EXAMPLE 2 RESULTS OBTAINED WITH EUCALYPTUS AND WATTLE MOOD (RUN-OF-MILL - SAICCOR) PREHYDROLYSIS STEP Table 5 hereunder sets out prehydrolysis conditions and results. Table 5 Timber Run No.

SO2 charge (*) L/W ratio Tine to tenp Coins) Time at Temp (mins) Spent Uq. PH Yield (*) Gum . P6 0,2 ,3/1 60 120 § 150°C 2,49 83,36 Gum P7 0,2 ,3/1 60 120 0 150°C 2,66 83,60 Gum P8 0,2 ,3/1 60 120 § 150*C 2,64 83,91 Gum P9 0,2 ,3/1 60 120 6 150"C 2,67 83,52 Gam P10 • 0,2 ,3/1 60 120 0 150°C 2,52 82,70 Wattle P11 0,2 ,3/1 60 120 § 1S0°C 2,78 79,32 The spent hydrolysis liquor from run no. P8 was titrated against standard NaOH solution in order to get an indication of the caustic soda consumption of such liquor. 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^4gr=J{^0H litre of prehydrolysis liquor.

D \\ * 27FEB 1991. 237 0 9 8 PULPING STEP Pulping conditions and results are given in Table 6 hereunder.

The initial concentration of Na2S03 (as NA2Q) at a charge of 17% at a L/W ratio of 3,5/1 was 48,6 g/1. In order to reduce the Kappa number, 5 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".

Table 6 Cook Prehy Cheaical □large (t) L/W Time Yield Ct) Kappa Spent Liquor No. drolysis Na2SOs Na2C03 &.Q. ratio To 17S°C At 17S°C Screened Rej.

No.

^SOj P" No. as NazO as Na2c ninutes hours as Na20 (g/1) C4 PG 17 4 0,15 3,5/1 80 3 53,02 0,02 40,6 11,32 9,21 C5 P6 ,74 6,075 0,2 ,3/1 80 4 49,02 0,01 28,7 19,43 9,16 C6 P7 17 4 0,15 3,5/1 80 3 53,26 0,01 37,8 ,73 9,33 C7 P8 17 4 0,15 3,5/1 80 3 53,23 0,01 34,1 ,28 9,21 C8 P9 17 4 0,1 3,5/1 80 3 53,63 0,02 37,4 9,45 8,84 C9 P10 17 4 0,1 3,5/1 80 3 53,74 0,03 41.1 ,33 9,10 CIO P11 17 4 0,1 3,5/1 80 1,5 57,94 0,50 31,7 ,15 9,0 ro CM O CO 09 237 098 " 11 " THE EXTRACTION STEP Table 7 Extraction No.

E5 E6 E7 E8 E9 E10 Cook No.

C4 C5 C6 -C7 C8+C9 - C10 Initial Kappa'So. 40,5 28,7 37,3 34,1 38,7 31,7 NaOH (*) 3 3 3 3 3 3 Temperature (®C) 70 70 70 70 70 70 Consistency (t) Retention (hr) 1 1 1 1 1 1 Final pH 12, :o 12,44 12,37 12,50 12,34 12,30 Kappa No. 14,S 12,4 ,2 14,0 ,3 16,0 Yield (0 93,73 N.D. 94,93 95,31 95, 86 96,79 Viscosity (cn^/g) 993,4 - 1042,4 1095,9 1025,7 1184,8 D.?. 1SC0 1S73 1663 1545 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: 23 7 0 9 8 PREHYDROLYSIS STEP: Wood charge - 1400 g O.D.

L/W ratio - 5,3/1 0 S02 (%) - 0,2 Time to 150°C (mins) - 60 Time at 150°C (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 (g) - 1176 Na2SOa charge (% Na2 0) - 17% Na2C03 charge (% NazO) - 4% 15 NaOH charge for neutralization 0 1 ,920 x 2,4 g - 4,608 g A.Q. {%) - 0,15 L/W ratio - 3,5/1 Time to 175°C (mins) - 80 Time at 175°C (hours) - 3 Spent liquor residual sulphite (g/1 Na20) - 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 Na*0) on O.D. - prehydrolised wood material. 237 098 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 NaHCOs will be converted to Na2 COs). This figure came to 12g NaOH per litre of spent pulping 1iquor.

Assumed pulping yield (%) - 53 O.D. material left (g) - 623,28 (assumed) NaOH (%) - 3 NaOH for raising pH of remaining pulping liquor to 11 = 2,316 x 12 = 27,792 g = 4,46% Consistency (%) - 10 Time at 80°C (mins) - 20 6 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 NaOH necessery tc raise the pH of the remaining pulping liquor to 11 came to about 4,5% 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. ami Table 8 Oxygen bleach no. ,01 .02 .. 03 04 . v Starting pulp no.

E8 C11- £10 Starting Kappa No. 14,0 13.4 ,3 16,0 NaOH (t) 1 1 2 1,5 Mg as MgSO^ (I) 0,05 Nil Nil Nil Consistency (%) ' Time to temp, (mins) 22 16 • 26 26 Time at tenp. (mins) Top temperature (°C) 110 115 118 115 O2 pressure (kPa) 690 690 690 690 Spent liquor pH 9,66 9,68 9,57 9,88 Yield (t) 97,86 98,25 95,45 96,37 Viscosity (ca3/g) 973,6 946,8 784,8 1037,8 D.P. 14S9 1415 1150 1566 Kappa Number 6,6 ,1 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 Tablrg^^e.lflw. fable 9 Starting Bleach Sequence Brightness Pulp No. 2 Xiso E5 B3 C2,9-E2,0-D0,6 90,3 E7 B4 C3,04-E2,0-D0,6-H0,6 91,7 01 B5 CI ,32-El ,16-00,4-110,6 91,8 02 B6 CI,02-El,01-DO,4-HO,6 91,6 03 B7 C0,726-E0,863-D0,3-H0,6 93,3 04 B8 DO, 316-EO,916-DO,3-HO,6 93,5 Alpha Cellulose % Yield % Vi scosi ty cm3 /g Bleached On timber 97,79 97,10 40,22 840 96,78 96,72 40,88 654 96,57 98,11 40,87 561 96,14 98,67 41,29 620 97,93 98,93 40,39 467 94,88 98,57 42,26 718 £3 CD r£> C«> 23 70 98 EXAMPLE 3 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 Hydrolysis Temp.

Time at Yield Effluent No. agent (% on wood) (°C) temp (mins) (%) PH P12 0,2 S02 150 120 84,63 2,71 P13 0,3 S02 160 150 78,11 2,50 P14 0,4 S02 170 180 73,34 2,37 P15 0,4 S02 150 120 77,14 2,30 P16 Pure water 150 120 88,53 3,24 P17 0,2 S02 150 60 90,08 .., 2,74 Hydrolysed material was soaked for 30 minutes in water after treatment before washing and spin drying.

Material originating from some of the prehydrolysis runs was subjected to pulping. (Table 11) "2 DEC 1993 <o. 23 7 0 9 8 - 17 The following process conditions applied except for C17 3,5/1 L/W ratio 0,1" Anthraquinone 16% NazSOa as NazO 4% NazCOs as NazO 80 minutes to temperature (175°C) 3 hours at 175°C .

C17 was done using original wood material which had not been subjected to a prehydrolysis step. In this case the chemical charge was increased to 18% NazSOs (as Naz0) and 4,5% NazC03 (as NazO) keeping the anthraquinone charge and other conditions as above.

Table 11 .

. Cook No.

Prehydrolysis No.

Yield (%) Spent Liquor Res. Sulphite as Na£0 (g/1) PH Kappa No.

Screened Rejects C12 P12 51,72 1,04 8,07 8,83 ,4 C13 P13 52,11 0,80 16,24 8,88 75,3 C14 P14 50,61 6,81 16,43 8,89 170,1 C15 P15 53,45 3,60 14,46 8,86 83,7 C16 P16 51,01 0,40 6,20 9,02 ,1 C17 49,08 11,02 13,48 9,16 58,8 .

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 W ' ^\\ 2 DEC 1993 m' •iv'* 23 7 0 98 18 - 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 temperature water bath. (See Table 12) . The following constant process conditions applied: 60 minutes retention time 10" consistency temperature 70°C Table 12 .

Extraction No Cook No NaOH (%) Effluent pH Kappa No Kappa No Drop Yield % Stage Screened on O.D. wood E11 C12 3 12,07 ,0 ,4 96,00 42,02 E12 C12 1 11,38 26,4 9,0 97,56 42,70 .

E13 C12 2 11,92 22,9 12,5 96,51 42,24 E14 C12 4 12,12 17,5 17,9 95,31 41,72 E15 C13 3 11,89 23,3 52,0 89,06 36,25 E16 C14 3 11,40 84,5 85,6 82,91 ,77 E17 C15 3 11,87 29,0 54,7 88,96 36,68 .

E18 C16 3 12,32 22,6 7,5 97,77 44,15 E19 C17 4 12,37 51,1 7,7 99,01 ! -2 DEC 5993 * *, / . • f, ' ■ J V ' 237 0 Comparing Kappa numbers of £11, El2, El3 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 5 3g/l NaOH solution at 70°C 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 10 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 15 and time) to effect adequate hemicellulose removal.

The Kappa number drop obtained from the unhydrolised pulp (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 20 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 25 an alkaline extraction step.

When running batch digesters using this process the preferred procedure would be to drain the digester after the prehydrolysis step of the prehydrolysis spent liquor. (This liquor would be used for 237098 recovery and beneficiation of products dissolved and extracted from the wood).

The pulping chemicals (NazSOj, Na2 CO3, 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 pulping ;ime, the spent liquor would be pumped from the digester. Alkali (NaOH) would then be added to this liquor and reintroduced to the digester followed by circulation ror a period. This would complete the cook.

The 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 swelling 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. b) Spent S.A.S.-A.Q. pulping liquor can be difficult to evaporate to high solids levels without forming troublesome precipitates. This can be overcome by adjusting the pH of the spent liquor to about 11 prior to evaporation. 237 098 - 21 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 10 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 certain 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 20 invention as defined in the appended claims. 23 7 22

Claims (8)

1. WHAT WE CLAIM IS: A process for producing dissolving grade pulp, the process including: applying a prehydrolyzing step to a lignocellulosic-containing material to obtain a prehydroiyzed composition; subjecting said prehydroiyzed composition to a SAS-AQ pulping step to partly dissolve the lignin contained in said prehydroiyzed composition and to obtain a pulp composition; optionally washing said pulp composition; and directly after washing said composition or directly after said SAS-AQ pulping step, conducting an alkali extraction step on said pulp composition by means of an alkali composition to lower the Kappa number of said pulp composition.
2. The process as claimed in claim 1 comprising conducting said alkali extraction step directly after said SAS-AQ pulping step, further including: separating spent liquor from said pulp composition after said pulping step; adjusting the pH of said spent liquor to prevent formation of precipitates in said spent liquor upon subsequent evaporation and thereby obtain a treated spent liquor; and adding said treated spent liquor to said pulp composition to conduct said alkali extraction step to lower the Kappa number of said pulp composition. -2DEC199J/I / 23 23 7 0
3. 3. The process as claimed in claim 1 wherein said pulp is washed and wherein 2% to 5% by weight of alkali is used to carry out the alkali extraction step.
4. 4. The process as claimed in claim 3 wherein 3% by weight of alkali is used to carry out the alkali extraction step.
5. 5. The process as claimed in claim 4- wherein the alkali used is sodium hydroxide.
6. 6. The process as claimed in claim 1 wherein the alkali extraction step is carried out at a temperature of 50 "C to 100 °C for a period of time sufficient to give optimal lignin removal.
7. 7. The process as claimed in claim 6 wherein the alkali extraction step is carried out at a temperature of 70 °C to 80 °C for a period sufficient to give optimal lignin removal.
8. 8. The process as claimed in claim 2 wherein said pulping step and said extraction step is carried out in a digester, including: removing said spent liquor from said digester after completing said pulping; adjusting the pH of said spent liquor to prevent formation of precipitates in said spent liquor upon subsequent evaporation and to obtain a treated spent liquor; and introducing said treated spent liquor to said digester to conduct said alkali extraction step and to lower the Kappa number of said pulp composition. r • ' W -2DECI993S . / 24 237098 11. 12. 13. The process as claimed in claim 8 wherein after introduction of said treated spent liquor to said digester, said digester is operated at a temperature greater than 100°C and at elevated pressures. The process as claimed in claim 8 or 9 wherein 10% to 15% alkali by weight of pulp is used. The process as claimed in claim 2, 8 or 9 wherein the pH of said spent liquor is adjusted to give a value of about 11 after said alkali extraction step. A process as claimed in any one of claims 1 to 11 substantially as herein described and/or exemplified. A dissolving grade pulp whenever produced by a process as claimed in any one of claims 1 to 12. DATED THIS DAT OfTXC^ Aj v 2DKl9935l! / ... •,