US3567572A - Polysulfide liquor impregnation of lignocellulose materials in a multistage pulping process - Google Patents

Abstract

A PROCESS FOR THE PRODUCTION OF CELLULOISIC PULP FROM LIGNOCELLULOSIC MATERIALS INCLUDING THE STEPS OF IMPREGNATING THE MATERIALS WITH A POLYSULFIDE LIQUOR AT A TEMPERATURE BELOW THAT AT WHICH SUBSTANTIAL DECOMPOSITION OF THE POLYSULFIDE OCCURS, REMOVING EXCESS OF POLYSULFIDE LIQUOR FROM THE IMPREGNATED MATERIALS, INCREASING THE TEMPERATURE AND ADJUSTING THE PH FROM THE IMPREGNATION STEP TO PREVENT ALKALINE DEGRADATION OF THE MATERIAL, AND COOKING THE IMPREGNATED MATERIAL WITH A COOKING LIQUOR CONTAINING SODIUM HYDROXIDE TO SUBSTANTIALLY DELIGNIFY SAID MATERIAL.

Description

United States Patent 3,567,572 POLYSULFIDE LIQUGR IMPREGNATION OF LIGNOCELLULOSE MATERIALS IN A MULTI- STAGE PULPING PROCESS David W. Clayton, Hudson, Quebec, and Asahi Sakai, Cite de Lasalle, Quebec, Canada, assignors to Pulp and Paper Research Institute of Canada, Pointe Claire, Quebec, Canada No Drawing. Filed Sept. 6, 1967, Ser. No. 665,710

Int. Cl. D21c 3/26 US. Cl. 162-49 17 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to the production of pulp in improved yield from lignocellulosic materials such as wood, straws and grasses by the polysulphide process. In particular, the present invention relates to the production of cellulosic pulp in increased yield by a modification of the aforesaid polysulphide process in which the amount of polysulphide and alkali required is substantially reduced.

In the conventional kraft or sulphate process for the production of cellulosic pulp, in particular wood pulp, the subdivided wood or other lignocellulosic material is cooked in an aqueous solution of sodium hydroxide-sodium sulphide at a temperature of the order of 170 C. for the time required to produce a pulp in the required yield. The resulting pulp contains lignin and carbohydrate in a ratio which is determined by the specific conditions of the pulping process i.e., the temperature cycle, the time, the liquor to wood ratio and the ratio of chemical to wood. In general, the ratio of carbohydrate to lignin in the pulp from a given material at a given yield is a quantity which varies within very narrow limits being virtually fixed by the nature of the process.

In the polysulphide process the aforesaid cooking is effected in the presence of polysulphide ion, suitably effected by adding sodium polysulphide or elemental sulfur to the cooking liquor and the pulping operation is carried out under appropriately modified conditions which are well known to those skilled in the art. The polysulphide pulps which are obtained have a higher ratio of carbohydrate to lignin than the pulps obtained in the kraft or sulphate process at the same yield, due to the stabilization of the wood polysaccharides against alkaline degradation by the polysulphide ion. Thus in the conven tional polysulphide pulping process it is believed that there are at least two reactions occurring which compete for the polysulphide ion including, firstly, the reaction of the polysulphide ion with the reducing end groups of the wood polysaccharides to yield oxidized polysaccharides which are more stable to hot alkali than the original polysaccharides and thus, in the polysulphide process the carbohydrate portion of the wood is not degraded to the same extent as in the kraft process during the time re quired to produce pulp of a similar lignin content. The peeling reaction "which normally occurs in alkaline pulpice ing which results in a loss of carbohydrate is avoided to a great extent in the polysulphide process and this is one of the main purposes of the use of polysulphide in the cooking process. Secondly, there is the reaction of the polysulphide ion with hydroxyl ions in the cooking liquor to produce hydrosulphide and thiosulphate ions. This is an undesirable side reaction because it destroys the polysulphide ion without any benefit to the pulping operation and alkali is consumed in the decomposition of the polysulphide ion and thus, more alkali must be added in order to make up the loss to achieve the require degree of delignification in the pulp.

Thus, while the pulp products of the aforesaid polysulphide process compare favourably with the pulps obtained in the conventional kraft or sulphate process and, in particular, pulps having the same kappa number produced by the polysulphide process are usually darker in color, have easier beatability, have higher strength in burst and tensile, slightly lower strength in double fold and tear and have similar bleachability, the conventional single stage polysulphide process is subject to several problems. Thus, as the polysulphide sulfur charge is increased a much higher alkali charge is required for the required delignification because hydroxyl ions are consumed by the decomposition of the polysulphide ion to hydrosulphide and thiosulphate ions. Further, the rate of decomposition of the polysulphide ion increases with the temperature and becomes appreciable above C. in the presence of hydroxyl ions and in a typical liquid phase pulping cycle most of the polysulphide is destroyed before the maximum temperature is reached. Whereas it is obvious that the polysulphide charge can be better used by lowering the maximum temperature the practical minimum in a single stage process is about C. below which the delignification rate is too slow to be of commercial use.

In order to overcome the aforesaid problem of the decomposition of the polysulphide ion and reduce the requirements for more alkali in the single stage polysulphide process, it has been proposed in Japanese Pat. No. 7501 (1963) to effect the process in two stages in which the polysulphide is separated from the alkali to reduce the decomposition of the polysulphide. Thus, in the process of the Japanese patent the treatment with polysulphide to stabilize the polysaccharides against degradation in the lignocellulosic material is effected in the absence of alkali and at a temperature of about 130 C. over a period of about an hour and subsequently, the excess of polysulphide liquor is withdrawn and as it contains residual polysulphide ion can be re-used in the process. The stabilized lignocellosic material is then cooked at about C. for about an hour with sodium hydroxide or kraft liquor with a 15% sulphidity to obtain the required pulp. It is found in the two-stage process of this Japanese patent, that pulp yield increases of 5-7% based on wood were obtained when compared with kraft pulps of the same lignin content. However, the two-stage process of the Japanese patent, although an improvement upon the single stage conventional polysulphide pulping procedure, still has the disadvantage that a substantial quantity of the polysulphide is destroyed in the stabilization stage without it reacting with the polysaccharides in the lignocellulosic materials. This is due to the relatively high temperature of 130 C. used in the stabilization and, although polysulphide decomposes more slowly at lower temperatures, it still decomposes fairly rapidly at 130 C. and if lower temperatures were used in the stabilization stage of the process of the Japanese patent the reaction between the polysulphide and the polysaccharides would proceed too slowly to achieve maximum carbohydrate yield. As will be realized the decomposition of the polysulphide means a direct loss of chemical to the process and necessitates the installation of an expensive recovery system to regenerate the polysulphide for re-use in the stabilization of fresh lignocellulosic material and thus adds considerably to the cost of the pulp.

The present invention provides a process for the production of pulp from lignocellulosic materials by the polysulphide process which is effected in three stages and in which the decomposition of the polysulphide during the impregnation of the polysaccharides in the lignocellulosic material is substantially reduced and at the same time, an increase in pulp yield is obtained at essentially the same kappa number over that obtained by the conventional kraft or single stage polysulphide process.

It has now been found that when the lignocellulosic material in subdivided form is subjected to impregnation in a separate stage from the stabilization stage and in the impregnation stage, the temperature is maintained so that essentially no polysulphide ion is decomposed and the excess polysulphide liquor containing no added sodium hydroxide is removed from the material between the impregnation and stabilization stages, it is possible to effect both the impregnation stages and the stabilization stages under conditions which are optimum for impregnation and stabilization and thus avoid the problem which has occurred heretofore, particularly in the process of the Japanese patent namely, that the conditions for stabilization are directly opposed to the conditions desirable for impregnation.

It has been proposed in Canadian Pat. No. 759,361 to increase the pulp yield in the conventional polysulphide process for the same polysulphide consumption by modifying the polysulphide process such that a major part of the polysulphide consumed in the process, is already absorbed by the wood chips when the first stage of the digestion is finished before the temperature has reached 130 C. and in the second stage of the digestion is charged with a cooking liquor to which no sulphur is added to complete the digestion. In particular, in Example of this patent, it is disclosed that the excess polysulphide impregnation liquor is drawn off at 100 C. and cooking liquor, i.e. sodium hydroxide is added thereto, the temperature increased to 170 C. over 130 minutes and the cooking time at this temperature maintained for between 30 and 65 minutes. However, in this process the impregnating polysulphide liquor contains alkali, i.e., sodium hydroxide and as aforesaid the sodium hydroxide causes some decomposition of the polysulphide ions in the liquor by direct reaction of the hydroxyl groups of the sodium hydroxide with polysulphide ions. Further, the sodium hydroxide in the impregnation liquor at the temperatures specified, dissolves part of the wood chips leading to the presence of organic material in the polysulphide liquor, which causes further decomposition of the polysulphide ions. As a result of this decomposition, particularly when operating on a continuous basis in which the wood chips are continuously passed from a bath of the polysulphide liquor, the presence of sodium hydroxide in the liquor leads to a relatively rapid decomposition of the polysulphide ions with consequent loss of chemical to the process and necessitating repeated renewal of the liquor in the bath.

Further, the process set forth in Example 5 of the Canadian patent is essentially a two stage process, there being no separate stabilization stage, this being combined with the cooking stage with the slow heating in the presence of sodium hydroxide to 170 C. over 130 minutes. Thus, in contrast to the present invention, the optimum conditions for stabilization are not readily achievable and there is no teaching of this in the Canadian patent and, therefore, it will be seen from the example that, whereas in the process of the present invention the overall period is of the order of 170 minutes, that in Example 5 of the Canadian patent is of the order of 300 to 315 minutes.

According to the present invention, therefore, there is provided a process for the production of cellulosic pulp from lignocellulosic materials which comprises substantially completely impregnating said materials in subdivided form with a polysulphide liquor containing no added sodium hydroxide at a temperature below that at which substantial decomposition of the polysulphide occurs, removing excess polysulphide liquor from the impregnated materials, stabilizing the impregnated materials against alkaline degradation by increasing the temperature of said materials and subsequently delignifying said stabilized materials by cooking said material in a cooking liquor containing sodium hydroxide.

The impregnation is carried out so that the required amount of polysulphide ion is uniformly taken up in the subdivided lignocellulosic material without any substantial loss by decomposition. After the excess of polysulphide liquor is withdrawn, the temperature and usually the pH of the material can be adjusted in order to effect the stabilization reaction for which optimum conditions depend upon the nature of lignocellulosic material, e.g. the wood species.

Thus, because the excess of polysulphide liquor is withdrawn before the stabilization reaction is etfected, the conditions for stabilization are not limited by the necessity of protecting polysulphide ion from decomposition. The essence of the present invention, therefore, is the capability of the process for the establishment of optimum conditions for a complete and uniform impregnation of the lignocellulosic material with the least loss of polysulphide decomposition followed by the maximum possible stabilization of polysaccharides before the alkaline delignification stage.

While the present invention has application to any lignocellulosic material, particularly those in the production of paper products, it has particular application to the treatment of wood such as softwood and thus, the present invention will be further described with reference to wood as the lignocellulosic material and, in particular, to wood chips representing the lignocellulosic mateiral in subdivided form. However, it is pointed out that the present invention is not limited to the treatment of wood chips.

The impregnation of the wood chips with the polysulphide liquor is according to the present invention effected at a temperature below that at which substantial decomposition of the polysulphide ion takes place in the olysulphide liquor. It has been found experimentally that the polysulphide ion, particularly when in contact with wood decomposes to a substantial extent in the abence of alkali at temperatures above 110 C. and it is, therefore, according to the present invention suitable to effect the impregnation at temperatures below 110 C. and suitably below 100 C. in order to avoid the necessity of using pressure vessels in the impregnation process which would be required when working at temperatures above the boiling point of the polysulphide liquor, and, further, to avoid any significant decomposition of the polysulphide ion. In a preferred embodiment of the present invention the temperature is suitably in the range 90 C. and, more preferably in the range -90 C.

The polysulphide present in the polysulphide liquor as is known to persons skilled in the art, is a mixture of polysulphides of the formula Na S in which It may vary from 2-5 and may reach 6 together with the monosulphide Na S and the small concentration of ions produced by decomposition of which the most significant is thiosulphate. When such a solution is analyzed for its polysulphide content the result represents the average number of polysulphide sulfur atoms per molecule in the mixture and it may not be and frequently is not an integral number. Thus, the general formula of such solutions can be expressed as Na s; wherein R is the average number of sulfur atoms per molecule of polysulphide including monosulphide sulfur. It is preferred according to the present invention, that H is as high as possible because the sulphidity, i.e., the monosulphide content for a given polysulphide charge decreases as E increases, thus reducing the total amount of sulfur in the system. Preferably, according to the present invention, is greater than 3 and is suitably around 4.5. It has further been found, according to the present invention, that the impregnation is best carried out with a pure polysulphide solution as such a solution has a low hydroxyl ion concentration which increases the stability of the polysulphide ion and retards the dissolution and degradation of the wood polysaccharides. Thus, a sufficient amount of polysulphide sulfur can be introduced uniformly at a relatively mild temperature in a comparatively short time under atmospheric pressure provided the liquor has a high concentration. This is particularly important when a .pure polysulphide solution with a high value of in the range 3-4.5 and consequently, a lower pH, is used for impregnation. Such solutions are preferable since, as aforesaid, the total sulfur in the system can be kept lower than when is low.

The impregnation of the chips with the liquor can be effected on either dry chips or water saturated chips, the latter being the preferred procedure. If the chips are dry, impregnation can be brought about by forcing liquid into the empty capillaries in the fibers, carrying the chemicals with it, this being referred to as penetration. If the chips are moisture saturated no further entry of liquid is possible and ions must enter by diffusion after the liquid has enveloped the chips. In intermediate moisture contents both mechanisms contribute to the impregnation.

While it might have been expected that the diffusion rate would be much lower than the penetration rate it was, in actual fact, found that the diffusion rate of the liquid into wet chips was comparatively rapid and thus while dry chips took up a substantial amount of sulfur at 450 p.s.i.g. after one hour with diffusion presumably contributing little to the uptake, after the same time at atmospheric pressure wet chips took up a comparable amount of sulfur with diffusion supplying the sole driving force. Further, with wet chips, i.e., chips having a moisture content in excess of 100% based on dry wood, the results were reproducible whereas with dry chips the minimum pressure required to impregnate the liquor fully into the dry chips varied from 50 to 400 pounds per square inch from one sample to another of the same wood species. It is therefore, preferred according to the present invention that the impregnation of the polysulphide be effected upon wet chips of moisture content in excess of 100% based on dry wood and this high moisture content is preferably produced, when the moisture content of the chips is initially too low, by soaking the chips in warm 'water for some time. The impregnation of wet chips with the polysulphide can, of course, be effected at substantially atmospheric pressure whereas in using pressure in combination with dry chips it is necessary to use complicated apparatus involving pressure vessels, which, of course, is from a commercial point of view to be avoided. While it is obvious that as high a chip moisture content as possible is preferable, moisture contents in excess of 170% give difficulties in operating a continuous process and for practical purposes a moisture content in the range 100160% is preferred for impregnation under mild conditions. The impregnation of the wet chips is suitably effected for a period of the order /2 hour to 1 hour as it is found that in this range oftimes the wet chips can be impregnated with of the quantity of the chemicals which can be introduced after four days. Further, after /2 hour, the unimpregnated area in wet chips disappeared in the cross section thereof. Accordingly, when a sufficient amount of chemicals can be introduced within a short time by means of a high concentration the size of the impregnation vessel can be reduced because the throughput is higher.

The concentration of the polysulphide sulphur in the liquor is suitably in the range of 5-40 grams per liter for below 5 grams per liter impregnation with polysulphide ions is not generally sufficient for the process of the present invention and above 40 grams per liter there is no substantial advantage to be gained. Thus, the amounts of sodium and total polysulphide sulfur introduced into moisture saturated chips Within a given time at C. and at atmospheric pressure are proportional to the concentrations because the rate of diffusion is dependent upon the concentration gradient.

The liquor-to-wood ratio in the impregnation stage is suitably in the range of 2.5 :1 to 4:1 as above the ratio of 4:1 of infinite bath effect is approached where a two-fold increase in the ratio leads only to a 10% increase in impregnation under the same conditions. A high liquor volume places a great load on the circulation system and inherently exposes a large quantity of polysulphide to the risk of decomposition in a given time. Accordingly, to obtain efficient impregnation it is preferable to keep the liquid-to-wood ratio as low as possible and to increase the polysulphide concentration.

The impregnation of chips with a high moisture content further facilitates the entry of chemicals into the chips at low temperature when the liquor has a pH lower than about 13.5. This is the case with pure polysulphide solutions in which the value of i is greater than 2, since the pH falls with increase in Ft; these are precisely the polysuphide solutions which it is preferred to use in the present invention.

Subsequent to the impregnation of the Wood chips which, from a practical point of view, must be as complete and uniform as possible, in order to produce a useful pulp, the excess of liquor is removed from contact with the chips and may be recycled for further use in impregnation of fresh chips. The impregnated chips are then subjected to stabilization which necessitates increasing the temperature above C., suitably above C. and optionally in the range 120 C. to C. in order to cause reaction to occur between the polysulphide and the chips. It has further been found that the rate at which polysulphide stabilizes polysaccharides in the wood forming the chips is generally much lower at a low pH of about 11 than at a high pH of about 14, and thus it may also be desirable to adjust the pH of the impregnated chips 'by adding a basic material such as ammonia or sodium hydroxide. In certain special cases, when the pH is comparatively high, the rate of degradation of some polysaccharide components of the Wood may exceed the rate at which they are oxidized (and hence stabilized) by the polysulphide. In such a case it may be necessary to reduce the pH of the system in order to carry out the stabilization stage: this can be achieved by injection of an acidic material such as hydrogen sulphide dependent upon the wood species used and the pH of the impregnant polysulphide liquor. Thus, the stabilization may be efiected by heating the impregnated chips after Withdrawal of the excess of polysulphide liquor with direct steam to a temperature above 100 C. and preferably above 120 C. for the time necessary to stabilize the chips.

Alternatively, after Withdrawal of the excess. polysulphide liquor-impregnated chips by hydrogen sulphide or ammonia, gas is added and the impregnated chips are heated with direct steam in the presence of the added gas at a temperature above 100 C. for the required time and suitably above 130 C. As aforesaid the reason for adding one of these gases is to adjust the pH within the chips to a value at which the net loss of polysaccharides during this stage is at a minimum and for certain wood species a reduction of pH is required such as by the addition of hydrogen sulphide and for other wood species an increase of pH is required such as is achieved by the addition of ammonia.

In yet further alternative procedure in order to alter the pH, a solution of soda or kraft liquor is introduced and the mixture is heated to a temperature in the range IOU- C. the temperature being maintained below the maximum to ensure that delignificanation of the chips does not proceed at a high rate during this stage and 8 and 12.7% of effective alkali was consumed. The above results are given in the following table.

only stabilization of the polysaccharides in the wood is achieved.

The delignification of the stabilized chips is suitably carried out by adding soda or kraft liquor to the stabilized chips and cooking the chips at a temperature of 160 C. or higher. A minimum temperature of 160 as aforesaid is usually required for the delignification but suitably this is around 170 C. in the liquid phase.

Thus, in one embodiment of the present invention, the delignification may be effected by adding soda or kraft liquor and heating at a temperature above 160 C. and suitably around 170 C. for the time required to produce the desired degree of delignification. It will be seen that if soda or kraft liquor was present during stabilization at 100-150 C. then the delignification will merely entail raising the temperature of the system to the required value.

In an alternative delignification process the delignification may be effected by adding soda or kraft liquor, retaining the liquor with the chips at a temperature between 100-150 C. until the chips are sufficiently impregnated with alkali then withdrawing the liquor and heating the impregnated chips at or near 185 C. with direct steam for the time required to produce the desired degree of delignification. The delignification will effectively take place in the vapor phase and again, if the stabilization has been effected in the presence of soda or kraft liquor, the delignification will merely involve the withdrawal of the liquor added for the stabilization followed by heating the chips as described above.

The present invention will be further illustrated by way of the following examples.

EXAMPLE I 4.2 kilograms of black spruce chips with a moisture content of 176% based on dry wood were impregnated in a digester, with 20 liters of a solution of sodium polysulphide at a polysulphide-sulfur concentration of 19.2 grams per liter (expressed as sulfur), for 60 minutes at 80 C. and the excess of liquor was withdrawn leaving a charge of 1.85% polysulphide sulfur on the wood. The digester was then filled with hydrogen sulphide gas at a pressure of 30 lbs. per square inch gauge at 80 C. and

the impregnated chips were steamed for 20 minutes at 170 C. in the presence of hydrogen sulphide gas. 16 liters of sodium hydroxide solution at a concentration of 37 grams per liter effective alkali as Na O was added and the mixture was maintained at 170 C. for 90 minutes. The chips were washed, and defibrated by 2 minutes action of an 8 propeller revolving at 2300 rpm. The total yield of pulp which was obtained was 54% with screen rejects 0.16%, leaving a screened yield of 53.84%. The kappa number of the screened pulp was 36.9 and the consumption of polysulphide sulfur was 1.95%. The consumption of effective alkali was 12.5% on wood. In a conventional kraft cook at 30% sulphidity carried out by conventional methods, and in the liquid phase, the screened yield of pulp at a kappa number of 36 was 48.5% (total yield 49.6%, rejects 1.1%) and the effective alkali consumption was 11.8% based on wood. In a comparable pulp prepared according to the aforesaid Japanese patent, a screened yield of 53.5% was obtained at kappa number of 36.1 (total yield 53.5%, rejects 0.2%) and 4.74% of polysulphide sulfur based on wood It will be noted that at a kappa number of approximately 36, the screened yield of the polysulphide pulps is about 5% higher than for the kraft pulp. The pulp prepared by the process of the present invention has the same screened yield and substantially the same kappa number as the pulp prepared by the process of the Japanese patent but the consumption of polysulphide sulfur is substantially less than half which leads to a low odor and greater chemical economy.

The following Examples II and III illustrate the use of the process with comparatively thick (MW) chips: consequently, a long impregnation time of two hours was used. With thinner chips (Ar an impregnation time of one hour or less would suffice.

EXAMPLE II 2.5 kilograms of black spruce chips with a moisture content of 168% based on dry wood were impregnated with 16 liters of a solution of sodium polysulphide at a polysulphide concentration of 19.2 g./l. (expressed as sulphur) for minutes at 80 C., and the liquor was withdrawn. The digester was then filled with ammonia gas at a pressure of 30 p.s.i.g. at 80 C. and impregnated chips were kept at 80 C. for 15 minutes, then heated to C. in five minutes and kept at 160 C. for 30 minutes in the presence of the ammonia gas. 15 liters of kraft liquor (effective alkali 37.6 g./l., sulphide 12.0 g./l., all as Na O), preheated to 120 C., was then added. The mixture of chips and liquor was heated to C. in 30 minutes and kept at this temperature for 90 minutes. The pulp, isolated as described in Example I, had a total yield of 50.0% with screen rejects 0.03%, leaving a screened yield substantially of 50%. The kappa number of the screened pulp was 28.9 and the consumption of polysulphide sulphur 1.96%. The consumption of effective alkali was 12.7% on wood. In a kraft cook at 30% sulphidity carried out by conventional methods in the liquid phase, the screened yield of pulp at a kappa number of 29 was 47.4% (total yield 47.7%, rejects 0.3%) and the effective alkali consumption was 12.6% based on wood.

EXAMPLE III In this third example, 2.5 kilograms of black spruce chips (moisture content 161% based on dry wood) were impregnated with a polysulphide solution under the same conditions as in Example II except that the concentration of the solution was 14.3 g./l. After withdrawing the excess of impregnation liquor a kraft liquor, preheated to 90 C. was added (effective alkali 60.3 g./l., sulphide 12.1 g./l., all as Na O). The mixture of chips and liquor was heated to 120 C. in 10 minutes and kept at this temperature for 90 minutes. The liquor was then withdrawn and the chips were heated to C. in 2 minutes using direct steam, and kept at this temperature for 30 minutes. The pulp, isolated as in the previous examples, had a screened yield of 48.8% (total yield 49.0%, screened rejects 0.2%) with a kappa number of 25.1. Polysulphide sulphur and effective alkali consumption were respectively 2.08% and 12.4% based on Wood. A conventional liquid phase kraft pulp had a screened yield of 46.4% (total yield 46.9%, rejects 0.3%) at a kappa number of 25, and the effective alkali consumption was 12.9% based on wood.

The results described in Examples II and III are summarized in Table II.

*All as percent based on wood.

It will be seen that the process of the present invention has the following advantages:

(1) The polysulphide consumption in the process is substantially reduced;

(2) The polysulphide liquor can be continuously reused;

(3) Moreover, when the deligniiication stage is carried out in the vapour-phase after removal of the excess of alkaline cooking liquor used for impregnation; the said excess of alkaline liquor can also be re-used continuously;

(4) The conditions in the stabilization stage can be adjusted so that they are optimum for a given wood species; and

(5) Because any excess of polysulphide is withdrawn after the impregnation stage the charge is sulfur exposed to subsequent high temperature stages is kept to a minimum, which reduces the formation of odor in comparison with the normal polysulphide process or the polysulphide process of the Japanese patent.

We claim:

1. A process for the production of cellulosic pulp from lignocellulosic materials which comprises impregnating said materials in subdivided form with a polysulphide liqour at a temperature below that at which substantial decomposition of the polysulphide occurs and for a time sufficient for substantially complete impregnation of said lignocellulosic material, removing excess of polysulphide liquor from the impregnated materials, increasing the temperature and adjusting the pH from the impregnation step to cause a complete reaction between the polysulphide liquor and the cellulosic materials without substantial delignification of the material and to prevent alkaline degradation of the material, which stabilizes the material, and subsequently adding cooking liquor containing sodium hydrox' ide to the material and then cooking the material for a time suflicient to substantially delignify said material.

2. A process as claimed in claim 1 wherein said impregnating the lignocellulosic material is at a temperature below 100 C.

3. A process as claimed in claim 1 wherein said impregnating the lignocellulosic materials is at a temperature in the range of 7090 C.

4. A process as claimed in claim 1 wherein said impregnating the lignocellulosic materials is with a pure polysulphide liquor.

5. The process as claimed in claim 1 in which the polysulphide anion contains at least 3 sulphur atoms.

6. The process as claimed in claim 1 in which the subdivided material is wood chips.

7. The process as claimed in claim 1 in which the subdivided material is softwood chips.

8. The process as claimed in claim 1 in which the subdivided material has a moisture content in excess of based on dry wood at substantially atmospheric pressure prior to impregnation.

9. The process as claimed in claim 1 in which the subdivided material has a moisture content between 100 and 160% based on dry wood at substantially atmospheric pressure prior to impregnation.

10. The process as claimed in claim 1 in which the impregnation time is from /2 to 1 hour.

11. The process as claimed in claim 1 in which the polysulphide liquor contains 5-40 grams per litre of polysulphide sulphur.

12. The process as claimed in claim 1 in which the liquor-to-wood ratio in the impregnational stage is from 2.5 :l to 4: 1.

13. The process as claimed in claim 1 wherein the temperature of the impregnated materials is raised to at least C. to stabilize the material.

14. The process as claimed in claim 1 which comprises contacting the cooking liquor with the lignocellulosic material at a temperature of at least C. to delignify said material.

15. The process as claimed in claim 1 which comprises contacting the cooking liquor with the lignocellulosic material at a temperature of at least to delignify said material.

16. The process as claimed in claim 1 in which the cooking of the lignocellulosic material is with a kraft or soda liquor.

17. A process as claimed in claim 1 in which the temperature of the impregnated material is in the range of 120 C. to 130 C. for stabilization.

References Cited UNITED STATES PATENTS 2,944,928 7/1960 Kibrick et al. 16282X FOREIGN PATENTS 759,361 5/1967 Canada 23-49 7,501 5/1963 Japan.

0 S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner