Manufacture of papermaking pulp from lignocellulose fiber materials by alkaline delignification in digesters is successively being modified in regard to processing of chemicals. Black liquor (BL) is recovered for regenera¬ tion of alkali and sulphur compounds from white liquor ( ) which is distributed to various delignification stages. The present invention refers to kraft pulp cooking of wood chips and also applies to versions using sulphidic alkaline cooking liquors enhanced by various additions, such as polysulphide (PS) , sulphite, anthra- quinone, methanol and others.

Normal cooking yield of pulp, i.e. brown stock (BS) , ranges from 40 to 60% on wood, all calculated on a mois¬ ture-free basis. Bleached soft wood pulp was produced at an average yield of 45% before environmental regulations required extended cooking which decreases bleached yield to about 42% or lower. Hardwood yields bleached pulp on a higher level at an average of 50% on wood. Inclusion of PS in cooking liquor can give a pulp increment of 10-15% (on ordinary pulp) from a given quantity of wood but would require means to prevent emission of acidic and/or odorous sulphur compounds from BL processing. Too advan¬ ced yield increase of bleached pulp produces some non- desirable paper qualities, such as lower opacity and tear strength approaching that of (acid) sulphite pulp.

For some time, kraft pulp mills have reduced water pollu¬ tion by modification of cooking to produce low kappa brown stock for bleaching. This is accomplished with a certain selectivity in regard to concurrent degradation of cellulose, measured as viscosity on a solution there- of, for which originally a limit of 900 dm 3/kg was set but which is successively being lowered. However, yield of pulp, counted on wood, suffers.

A novel method is described here, giving improved pulp yield at any given brown stock kappa number by more efficient utilization of the sulphidic compounds as regards WL, commonly as Na ₂ S, together with wood he icel- 5 _. lulose. The successive steps in present pulping tech¬ nology and the novel improvement thereof is shown in Figure 1, wherein schemes I-V are as follows:

S = Steaming of chips 0 I = Impregnation of chips by process liquor

D = Digestion (cooking) of impregnated chips

DW = Digester washing comprising displacement in digester of ex-digestion liquor to be distinguished from pre¬ viously known displacement of BL (black liquor) from 5 BS (brown stock) to recovery plant

W = Any additional washing of pulp after DW, so far often on open drum filters, to produce BS

WL = White liquor (alkaline delignification liquor)

BL = Black liquor at pertinent concentration passed to 0 recovery plant for regeneration of WL alkali. Optional recycling of some portion of BL to digestion

DL = Ex-digestion liquor circulating within D-system and which is successively converted to BL Aq = Aqueous washing medium, e.g. condensate from process liquor evaporation and preferably filtrate from subsequent pulp washing stages such as from oxygen delignification or bleaching

BL+Aq = Regular BL at some dilution reclaimed from pulp and passed to recovery plant

Aq+ΔDL = Major portion of washing medium and dilute minor proportion of DL from pulp to recovery plant

BL from I = BL of DL from chips to recovery and essen¬ tially Aq-free.

For certain kraft process steps, the term reduction or reduction degree applies, which refers to sulphur equiva-

lents present as percentage of sulphide to total sulphur. According to the state of the art, reduction declines in the course of digestion to a major extent by dispropor- tioning reactions and is at its lowest level in evapora- ted BL. The highest level is reached in (by reductive combustion of BL recovered) soda product prior to its dissolving in green liquor preparation. When liquor is exposed to air, reduction declines down to about 85% when soda smelt is dissolved in green liquor and then in recausticizing plants of past design, depending on how well various forms of catalytic metals and contact to air oxygen are avoided. The admixture of BL from brown stock washing to WL at charge of regular cooks decreases reduc¬ tion under WL level. Although the BL/WL mixture occasio- nally may be tested on sulphidity, its reduction degree has previously not been found of interest but the WL reduction value could well be halved at said conditions. The standardized term sulphidity expresses Na_S as per¬ centage of active alkali = AA + Na_S, all calculated as equivalent NaOH (OH-) . This term is useful for fresh WL but somewhat misleading when applied to the progress of cooking when OH- from both AA components is consumed. Another common kraft liquor process term is oxidation degree which is related to oxidation of BL sulphide to produce more stable thiosulphate and prevent losses of sulphur compounds in non-condensable gases and is cal¬ culated as percentage on sulphide in non-oxidized BL.

Brown stock (BS) refers to fully washed pulp, discharged from digester and sometimes called "unbleached pulp".

Incompletely washed or non-washed pulp is, according to prevailing technique, obtained together with black liquor (BL) . Some pioneering technologists rightly applied the term "black stock" for its treatment in effluent-free systems. By now, the term "hot stock" refers to suspen¬ sion prior to the last washer in a series of washers delivering brown stock for bleaching. Novel features of

this invention necessitate that digested chips prior to defibering into a suspension are designated digester stock.

5. Scheme I of Figure 1 exhibits regular batch cooking with steaming (S) at digester chip filling, followed by charge of WL and BL filling to prescribed ratio liquor/wood. This ratio is in general 3/1 to 4/1, but may be lower, about 2/1, when vapour phase cooking is applied on parti- 0 cularly impregnated chips. Temperature rising schedule comprises initial phase impregnation, followed by, often stepwise, increase to maximum temperature. Appropriate combination of cooking variables is performed by means of the H-factor according to Vroom, formulated on kinetical 5 grounds.

Earlier versions of cooking featured transfer of BL at certain pressure to next digester charge, while other versions utilized alkali counter-currently. At the time, it was found that equilibrium concentration of delignifi- cation products formed by BL backfill at least did not impair pulp quality properties. When sulphidity was about or over 25%, no reason was found for practicing any modification, proposed at the time, involving multiple- digestion using white liquor injection. Reclaimed BL that is diluted by washing medium (Aq) is used for digester filling to pertinent liquor/wood ratio as well as for transport of black stock via a blow tank to a washing plant (W) .

Scheme II shows reclaiming of BL directly from batch digester stock by in-digester displacement (DW) and flushing stock with temperate filtrate from brown stock washing (SE patent 139 301, Mannbro, N.V.). Progress is rapidly made for modified cooking systems based on in- digester displacement of BL of which a portion is re¬ cycled for impregnation of chips to be digested by injec-

tion of WL. This type of modified cooking facilitates extended delignification to low kappa number, although the yield of pulp in percentage calculated on wood, decreases. Among the means to prevent this loss in yield, 5. an increase is proposed of not only sulphidity but also actual concentration of sulphide in the impregnation liquor, which may produce a gain in yield by 0.5-1%.

Scheme III refers to continuous cooking. For clarity, 0 directions of liquor flows through the material in the digester are omitted from the scheme. Presumably at most mills WL is still charged, wholly or to a major part, to impregnation liquor circulation. However, presumably in regard to sulphide-concentration and consequences on 5 sulphidity at modified WL alkali distribution, some recirculation of diluted BL from DW (indicated by hatched line) is considered elsewhere (SE patent application 8804578-6, Backlund, A.). Another but straight counter- current system employs total recycling of BL for impreg- 0 nation (SE patent application 9001783-1, Richter,

J.C.F.C.). For closed cycle operation between cooking and alkaline oxygen delignification (oxynation) a similar method has been described (US patent 4,155,806, Mannbro, N.V.). Both are, however, impaired by passing washing medium (AQ) through digestion (D) .

It is noted that for some cases of continuous cooking using WL in impregnation stages, the spent liquor is recycled as such through recausticizing plant. Digester heating is performed by vapour recompression. Another version employs green liquor in an initial stage of digestion and withdrawn liquor is causticized.

Scheme IV shows particular recycling of ex-digestion 5 liquor (DL) to the impregnation stage. The hemicellulose recycling increases somewhat pulp yield (SE patent from application 2823/66, Croon, Dillen and Noreus) . However,

all WL is also charged to the impregnation stage and all BL is reclaimed from DW diluted by Aq from brown stock washing. The patent discloses addition of separately prepared polysulphide solution for stabilizing hemicellu- lose in impregnation of softwood and birch chips. The latter gave a particularly increased yield of bleached pulp.

The present invention

10

Scheme V. Ex-digestion liquor (DL) is withdrawn and is split into two flows in connection with the chips impreg¬ nation stage. One flow is counter-current to chips from steaming (S) and reacts with chips, while the other dis-

15 places reacted impregnation liquor from chips and else recycles with or remains with chips for digestion (D) with WL. By impregnation reactions of wood components, the DL is converted to black liquor (BL) which is with¬ drawn directly from the wood chips, and passed to the

20 recovery plant for generation of alkali. Non-withdrawn DL is as residual DL (ΔDL) reclaimed from brown stock wash¬ ing (W) of digester stock and is hereby diluted by an ex¬ tensively larger volume of Aq. Hence Aq+ΔDL bypasses digestion to alkali regeneration plant.

_____

Continuous digesters of type Ka yr feature split charge of white liquor and application of zones for concurrent and counter-current flows, as regards movement of liquor relative to the fibrous material, as well as zones where

30 flows meet for withdrawal of liquor. According to the present invention, counter-current flow through layer of chips by BL produced from DL is preferred. Similarly, counter-current flow of washing medium (Aq) through digester pulp is used for displacement (DW) of digestion

35 liquor (DL) .

According to the invention, a volume of sulphidic white liquor, determined by its concentration of hydroxide (effective alkali) , is added to the impregnated chips and considered together with alkali in recycling DL in reac- tion with chips. The resulting sulphide equilibrium concentration is utilized by the recycled fraction reac¬ ted with feed of chips to the impregnation stage (I) . (In Scheme V, 0_ indicates addition of oxygen or oxygen containing gas to sulphidic impregnation liquor reacting with chips. This relates to an alternative oxidative conversion of sulphide to partial formation of poly¬ sulphide as dealt with further on.)

In Schemes I-V washing medium (Aq) is often used, compri- sing effluent filtrate from alkaline oxygen delignifica- tion stage in an effluent-free brown stock treatment system between digestion and O-stage (US patent 3,830,688, Mannbro, N.V. - Validity confirmed by owner's request for re-examination) . O-stage filtrate is commonly used for displacement of BL from brown stock and is then included in BL passed to alkali regeneration plant. Effluent-free operation is facilitated when carry-over with washed brown stock is derived from ex-digestion liquor (DL) rather than from black liquor (BL) . Alkali required in the O-stage is mostly prepared by WL oxida¬ tion using oxygen containing gas to such an extent that reduction degree approaches zero. Regeneration of alkali free from almost any sulphur compounds is being developed and such alkali may also be used for final stage cooking and extraction stages of the bleaching sequence.

Abatement of odorous gases emitted from digestion and from BL suspensions prior to O-stage, differs according to condensing and alternative blow and flushing system and some cooling of digester discharge liquor may be arranged for WL pre-heating. However, according to Scheme

V any such condensing or pre-heating system is moved to impregnation system.

The term iso-thermal cooking previously denoted pre- 5. heating of liquor for continuous cooking at a constant temperature, although it is now applied to extended delignification at relatively maintained temperature and alkalinity in a zone of continuous digester previously used for displacement washing. The DW zone of Scheme V 0 improves the extended delignification because of the early removal from the impregnation stage of BL solids. Originally, the term extended cooking referred to methods for bringing down brown stock kappa prior to oxidative bleaching, in order to avoid or reduce formation of 5 organic chlorine compounds in bleach plant effluent. When this has been accomplished, the new issue now reduces costs for oxygen based bleaching agents, such as peroxi¬ de. Scheme V is instrumental in fulfilling this aim, although separate combustion of bleachery filtrate in- eluded in the final filtrate (Aq+ΔDL) facilitates hand¬ ling of chlorine compounds.

The impregnation stage (I) shown in Scheme V is not limited to physical impregnation of liquor in the voids of the material but comprises chemical reactions signifi¬ cant to the initial phase of cooking. For the progess of a cook, three phases can usually be distinguished: 1) initial phase, 2) bulk delignification and 3) final delignification. Initial impregnation is optionally performed in a digester vessel or separate vessels are used as an alternative. Impregnation for batch cooking is preferably performed in each digester. Continuous cooking has developed into use of a separate zone or vessel for impregnation and sometimes an additional vessel for the initial phase.

In the impregnation/initial stages about 60% or more of the total charge of alkali (hydroxide) is consumed under a relatively short time and mainly by reactions dissol¬ ving carbohydrate (hemicellulose) . Dissolution of lignin is hereby non-selective and the wood ratio lignin/carbo- hydrate remains until bulk delignification. The lignin reacts, however, initially with sulphide (hydrosulphide and hydrogen sulphide respectively) in such a way that its dissolution by hydroxide in the later stage is pro- moted (Olm, L. and Tistad, G. : Svensk Papperstidning

1959, nr 15, pages 458-464) . It is noted that impregna¬ tion may be performed by addition of H_S according to known methods. Oxidation of sulphidic compounds prevents their emission to atmosphere and polysulphide can be produced, as dealt with further on.

In any impregnation stage alkali reacts with acidic groups of pertinent wood material, i.a. under formation of acetate, and with saponificable extractives. According to presently practiced technique, all these products pass through digestion stages to be removed with black liquor reclaimed from digested stock. Many brown stock washing investigations also prove slow removal of soap from brown stock. According to the present invention, the major part of said dissolved wood products is removed from chips prior to delignification which efficiently contributes to selectivity of final purification for bleaching and avoidance of pitch in stock systems.

Manifold observations show the importance of final delig¬ nification holding low concentration of dissolved wood components, essentially lignin, and of Na ' while con¬ centration of OH- should be sufficient for delignifica¬ tion. This applies to preventing lignin absorption on fiber as well as simultaneous cellulose degradation. Hereby bleached pulp cellulose viscocity is impaired, which indicates loss of potential paper strength pro-

perties. Recent environmental regulations lead to modifi¬ cations of digestion to further lowered kappa but O-stage delignification is, in this respect, very sensitive to carry-over of dissolved wood products with brown stock, 5 _. which, however, according to the invention, to a major part never entrain the carry-over contributing digestion liquor.

Digestion heat economy is favoured by the direct charge 0 of ex-digestion liquor (DL) to impregnated chips which join with hot white liquor (WL) . Less steam is required for heating because resulting cooking liquor is not subjected to dilution by washing medium (Aq) which, as far as volume balance of digester system allows, is 5 passed directly to plant for concentration, evaporation and alkali regeneration.

The cooking mixture of chips and new cooking liquor will have a temperature that, besides any pre-warming and 0 steaming of chips, depends on the withdrawal of heat with BL from the impregnation stage which derives from diges¬ tion. BL heat can be directly utilized for multiple effect evaporation or optionally flashed for steaming of chips. When thin liquor of washing medium (Aq+ΔDL) is 5 sufficiently free from chloride forming substance, it may be mixed with black liquor for common evaporation.

Some versions of batch cooking, e.g. so called low energy cooking, are designed for pre-warming of chips by some used displacement liquor. Alternatively, this liquor could be subjected to separate pre-evaporation, i.a. to thermo-compression evaporation within the limits for acceptable boiling point rise. Mechanical compression can possibly be gas turbine powered.

It is feasible to use thin liquor as solvent for recove¬ red soda smelt and causticize the solution to form "whi-

te" liquor (WL) . Optional evaporation of WL for separa¬ tion of dead-load Na-salts would be within the frame of heat economy. Anyhow, when facing huge investment in a soda recovery boiler, an alternative would be an additio¬ nal, preferably oxidative, combustion of evaporated Aq+ΔDL. Hereby produced combustion residue of soda, sulphate etc. could be passed to the kraft recovery process. When the soda content of the residue so qualifi¬ es, it may be causticized to liquor free from sulphide and other reducing matter and thus useful for bleaching. Gassification of black liquor for combi-power generation based on energy-rich BL and mechanical thermo-compression can be attractive in the future.

Impregnation together with conversion of sulphide to polysulphide

Impregnation in the presence of polysulphide (PS) is practiced according to various methods. Thus, it is disclosed that sulphide supplied as Na_S in white liquor (WL) contributed to delignification of increment PS pulp at given kappa number and denoted as a threshold effect. Elsewhere anthraquinone was introduced as a delignifica¬ tion accelerating agent while PS stabilizes hemicellulose and their combination is considered to give synergistic effect and the addition should be made for long residence in the digestion system by avoiding dilution, which otherwise is caused by a washing medium.

Hemicellulose stabilization is, as is well-known, attai¬ ned by reacting polysulphide (PS) according to Scheme IV. Separately prepared PS, either from elemental sulphur or PS essentially deriving from sulphide, is used. Elimina¬ tion of particular sulphur compounds to generate PS is developed according to other methods. Hence, oxidation of sulphidic liquor or mixture of liquors by oxygen contai¬ ning gas and in the presence of chips proved simple and

satisfactorily efficient. The kraft process produces fundamentally white liquor (WL) at a high reduction degree to form cooking liquor of high delignification efficiency which, however, is more or less lost by any 5. prior PS oxidation of WL sulphide as such or as a cooking liquor component.

The present invention features application of ex-diges¬ tion liquor (DL) for impregnation of chips which, in 0 turn, can be combined with oxidative PS reaction with no harm attaching to the reduction degree and sulphidic values contributed by fresh white liquor (WL) . Alkali remaining in ex-digestion liquor (DL) is utilized for BL forming reactions with wood and at a pertinent temperatu- 5 re after partial ex-liquor recycling determined by the ratio volume of cooking liquor/wood required at the digestion stage.

Black liquor viscosity during evaporation is primarily 0 related to its alkalinity, and some addition of alkali, during or after impregnation, could be justified. Alter¬ natively, the viscosity may be lowered by pressure hea¬ ting of the black liquor, whereby, in some cases, sulphi¬ dic gaseous compounds are expelled. These techniques are currently being practiced for evaporation of up to 80-85% solids on total liquor.

It is commonly acknowledged that lignin in pulping li¬ quors via quinone reactions oxidizes sulphide, i.e. hydrosulphide ions, to polysulphide (ions) which stabili¬ ze hemicellulose in the lignocellulose material as well as some hemicellulose dissolved at certain phases of digestion. When polysulphide is formed in the presence of wood components, optimization is reached by readings of the equilibrium expressed as redox-potential and pH for the purpose of which sonds are found on the market. They are designed for on-line application and redox sonds are

practiced for regular thiosulphate oxidation of BL. Such full oxidation may be performed on BL discharged from impregnation or alternatively, as an additional stage to PS oxidation in the presence of chips.

In-line oxygen injection to the total volume of ex-diges¬ tion liquor in a closed system prior to impregnation is disclosed by SE patent application 9102834-1 (Mannbro, N.V.). Such a closed system PS-oxidation requires con- centrated oxygen, preferably over 90% in volume. Recently developed ozone bleaching uses oxygen as carrier gas which may then be utilized in the pulping process and eventually for PS generation according to the invention.

Sulphidic impregnation according to the invention may, in its PS version, use air in agitators such as at screw feeding of chips through certain horizontal or inclined digester tubes of known design. These are also suitable for flow of air or oxygen containing vapour through the connecting chip steaming stage. Low concentration of oxidizing agent but still almost closed system is accomplished by injection of concentrated oxygen and optional degassing to some combustion stage. For safety reasons, it is important that turpentine and volatile combustibles from wood and vapour condensate are treated separately from oxygen. Sulphur compounds present in odorous process gases are, according to certain publica¬ tions (Teder and Tormund, Svensk Papperstidning 1983, R 198) , reacted by polysulphide. The possible use of vapour from oxidized BL for steaming of chips without odour emission is noted.

Some previously disclosed systems for multiple operation of batch digesters feature successive displacement of cooking liquor and displacement liquors via accumulators and allow for polysulphide pre-treatment of chips (US patent 4,595,455, Mannbro, N.V.). Continuous digesters

require modified extraction screens for transfer of ex- digestion liquor (DL) to the impregnation stage and separate discharge of surplus washing medium to plant for chemical recovery. The figure shown in SE patent applica- 5 _. tion 9102824-1 exhibits a suitable continuous digester system except that recycling ex-digestion liquor admitted to digestion is oxidized. This will, of course, impair the reduction degree of cooking liquor mixture.

0 Example

Bleached softwood kraft pulp was produced by typical Kamyr-type impregnation and MCC (Modified Continuous Cooking) to brown stock kappa number 25 and oxygen delig- nification to kappa number 18, followed by bleaching according to the sequence (C+D) (E0)D for brightness 90% ISO. To reduce pollution from the mill effluent trials were performed to evaluate lowering the kappa number of stock from oxygen delignification stage to 14. This, according to parallel laboratory measurements, decreased the yield of bleached pulp from 43 to 42% on wood and viscosity from 900 to 830 dm 3/kg. To establish means for improvement of said values, investigations within the frame of the present invention were performed by labora- tory cooking and bleaching, simulating mill conditions after installation of impregnation stage using ex-diges¬ tion liquor as follows:

a) using at the mill established WL sulphidity = 35%. b) according to presumptions increased WL sulphidity = 45% resulting from improved sulphur balance by oxygen injection to impregnation stage with redox control.

Each series comprised a succession of 5 recycling batch cooks simulating equilibrium at MCC continuous cooking. The following results were obtained:

a) at brown stock kappa 20 and kappa 12 after oxygen delignification, bleached pulp was obtained at a yield of 42% on wood and viscosity 890 dm 3/kg, b) at brown stock kappa 18 and kappa 12 after oxygen delignification, the yield of bleached pulp was 44% and the viscosity was 980 dm 3/kg.

The charge of effective alkali in WL calculated in per¬ centage on wood, moisture-free basis, was according to b) reduced by 5% as compared to operations established at the mill. Atmospheric emission of carbon dioxide is reduced in proportion to less dissolved wood substance and demand of regenerated alkali.

Results from the series of test cooks apply, on the choice of principle for bleaching for sequence, either ECF or TCF, whereby the latter encompasses stages for additional oxygen based bleaching agent such as ozone and peroxide. As to the TEF process, either an existent alkali recovery capacity for combustion of TCF substance or a separate treatment of diluted residue of ex-diges¬ tion liquor containing Aq from washing medium deriving from bleachery filtrate is required. When treatment of the latter features separate evaporation, concentrated filtrate may be combusted oxidatively as earlier dis¬ closed by the present inventor (Mannbro, N. , Nordic Pulp and Paper Journal Research Journal, 1989, pages 192-196 ibid. 1990, pages 134-141) . Such TEF process again allows a total environmental concept for brown stock kappa number about 35 and bleaching with efficient chemicals. These chemicals, particularly elemental oxygen and chlo¬ rine, are being manufactured at low energy demand and the pulp is produced at a bleached pulp yield of up to 45% or, according to the PS version, with 48% or higher within the limits for potential paper strength.