US2885317A - Method in manufacturing chemical or semi-chemical pulp from heavily resinous wood - Google Patents

Description

United States Patent cc METHOD IN MANUFACTURING CHEMICAL 0R SEMI-CHEMICAL PULP FROM HEAVILY RES- INOUS WOOD Per Harald Collin, Upplands, Vasby, and Karl Esbjiirn Eftring and John Harald Jonsson, Skutskar, Sweden, assignors to Stora Kopparbergs Bergslags Aktiebolag, Skutskar, Sweden, a corporation of Sweden No Drawing. Application March 21, 1955 Serial No. 495,752

Claims priority, application Sweden March 30, 1954 13 Claims. (Cl. 16237) Since long ago it has been a desideratum in the cellulose industry to make use of the heavily resinous kinds of wood of the Pinus family as a raw material in the production of chemical or semi-chemical pulp according to the sulphite method. As is well known, it is possible with a normal sulphite method only to digest the sapwood but not the heartwood of the Pinus species. This depends on the contents of pinosylvin of the heartwood, a substance which with the pH-value prevailing at the digestion of a normal sulphiting batch condenses with the lignin so as to make it impossible to dissolve the same out.

A number of methods for the digestion of Pinus species have been patented. As examples might be mentioned, inter alia, the British Patent No. 5,365, the US. Patent No. 1,511,664 and the Swedish Patents Nos. 77,429, 79,840 and 82,751. As far as we are aware, none of the methods described in these patents have found any technical use. This is believed to depend, inter alia, on the fact that with said methods the digestion of the heartwood cannot be carried so far as the digestion of the sapwood, so that a pulp is obtained, which is not acceptable from the point of view of purity.

According to the methods described in said patents, the digestion takes place in two steps, the wood being sulphited in the first step with an aqueous solution containing sulphite and/ or bisulphite ions and alkali and/or alkaline earth metal ions, the temperature being augmented continuously or stepwise to the desired level during the sulphiting period, whereupon the sulphited wood is hydrolysed with an acid agent, generally sulphurdioxide, in a second step.

If such a two-step method is used on a laboratory scale for the digestion of Pinus wood and the sulphite treatment is effected at a pH-value between 4 and 7, and if large quantities of the base are used in the digester, such as 20-100 kilograms of Na O per kilogram of wood conceived of as being dry, pulps of relatively good properties can be manufactured. However, it will then be necessary at the same time to make use of very long digesting periods, generally 50-100% longer than those required in the digestion of ordinary spruce according to the calcium bisulphite method.

Endeavours to apply the above described method on a technical scale, while the same time, with a view to reducing the base consumption, tapping of sulphiting liquor is effected after the impregnation, will in spite of all result in an irregularly digested pulp. The irregularity of such a pulp clearly manifests itself, if the roe-number of the sapwood and heartwood portions is examined. Differences in the roe-number of up to units will then be found. I

The method according to the present invention, however, constitutes a complete solution of the problem of digesting heavily resinous kinds of wood and has been successfully tested on a technical scale. The method, where the digestion also takes place in two steps, the wood being sulphited in the first step upon steaming 2,885,317 Patented May 5, 1959 a second step, is distinguished, furthermore, by the features that the sulphite treatment is elfected with the digester filled with liquor under a hydraulic over-pressure during the greater portion of the sulphiting period, that sulphiting liquor adapted to be tapped is drawn oif entirely or approximately so and is used for the preparation of fresh sulphiting liquid, and that the hydrolysis is brought about by an adapted quantity of sulphur dioxide being introduced continuously or stepwise into the digester.

According to the invention, a filled digester is thus operated with during the sulphiting period, the liquor being held in the digester under a hydraulic over-pressure, resulting in that the total pressure in the digester considerably exceeds the steam pressure of the liquor. The favourable influence of the hydraulic over-pressure is believed to depend, first, on the fact that air enclosed in.

the chips is pressed together by the pressure to a fraction of its initial volume so as to cause the digesting liquor to come into contact with greater portions of the chips, and, second, on the fact that the solubility of the air in the digesting liquor is multiplied, whereby a large portion of the air enclosed in the chips can be released.

Experiments have shown that hydraulic over-pressures as high as possible should be used, that is to say, a pressure as high as allowed by the digester is preferably applied, involving that 14-112 p.s.i. over the steam pressure of the sulphiting liquor is made use of. Experiments have shown, furthermore, that the hydraulic pressure should be maintained during the greater portion of the sulphiting period. Thus two digester batches on a technical scale, which were performed identically save for the fact that the hydraulic over-pressure in one of the batches was maintained only one hour against 8 hours in the other batch, involved a difference of many hundred percent in the contents of splinters.

Experiments on a technical scale have shown that inert gases are accumulated by degrees in the top portion of the digester during the sulphiting period, and it will be found suitable to release these gases as they are formed, so that all chips in the digester are covered with liquor.

In addition to the abovementioned advantages, the hydraulic over-pressure results in that the temperature can be augmented very rapidly during the initial phase of the digesting process. Thus temperature increases up to C. per hour may be permitted, without the pulp becoming more uneven thereby. The rapid temperature increase in the initial phase of the digesting or cooking process brings about a shorter digesting period and thus a better utilization of the digesting plant.

Technically, the hydraulic over-pressure may be arranged in such manner that the circulation conduit of the digesters is connected 'via a valve to a steam conduit, wherein a pump maintains an over-pressure. With the aid of a regulator adapted to release liquor from the steam conduit the pressure in the latter is maintained at the correct value. The liquor released by the regulator is preferably returned to the pump vessel. The liquor under pressure may be constituted by sulphiting liquor from earlier batches, by weak liquor from the washing of the pulp, or by water. a

In digesting densely grown pinewood of a high age, for example, that is to say such pinewood as contains heartwood for the major portion thereof, it has been found suitable, with respect to the uniformity of the pulp, to supplement the pressure treatment by a preceding 'vacuum treatment. Technically, such treatment may be carried into efiect as follows: The digester is filled with chips with the use of a chip filling apparatus operating with steam. When the digester is filled, the cover of the same isput on and the digester is connected to a suitable vacuum-producing apparatus such as a jet pump. Hereby the pressure in the digester is lowered, until the wood water and the condensate formed in the chips during the steam process commence to boil. The steam then formed entrains air enclosed in the chips, when it is sucked out of the digester. Hereby the impregnation of the chips with sulphiting liquor is considerably facilitated. Furthermore, the vacuum treatment is believed to pull resin plugs out of the channels in the wood, which also contributes toward facilitating the impregnation.

It has been found to be suitable in connection with experiments on a technical scale to effect the vacuum treatment in such manner that the pressure in the digester is lowered below 0.6 atmosphere, preferably to 0.2-0.4 atmosphere, which corresponds to 60-75 C. At this latter pressure the temperature of, the digester contents has been lowered by approximately -30 C. through the vacuum treatment. By the negative pressure prevailing in the digester after the vacuum treatment, the intake of sulphiting liquor is considerably facilitated.

As stated hereinbefore, large quantities of base should be used during the sulphiting period in the digesting of heavily resinous kinds of wood according to the method as outlined above. However, if a hydraulic overpressure is used during the sulphiting period in accordance with the present invention, all sulphiting liquor adapted to be tapped may be drawn otf upon the termination of the sulphiting period and be used for the preparation of fresh digesting liquor. Hereby, it will be possible in the method according to the invention to attain a low net base consumption in spite of the large quantities of base having to be used during the sulphiting period. By reason of the fact that only an inconsiderable portion of the wood is dissolved during the sulphiting period, all sulphiting liquor drawn oiI may be used for the preparation of fresh sulphiting liquor, without the risk of burnt cook being incurred. Inasmuch as all sulphiting liquor permitting of being tapped is drawn olf upon the termination of the sulphiting period, very high dry substance percentages will be obtained in the liquor (up to 22%) after the hydrolysis. This is obviously a great advantage, inasmuch as the costs for the evaporation of the liquor are considerably reduced thereby.

At experiments with the method on a technical scale according to the present invention, it has been found that the sulphite treatment will have to be carried into eifect in different ways, depending on the kind of wood to be subjected to treatment, in order that pulps of the same strength shall be obtained. If Swedish pinewood (Pinus sylvestris) is being used, for example, such wood having been stored in log piles for 1 year, it has been found to be suitable to operate the sulphiting process at a maximum temperature of 135140 C., whereas fresh pinewood only requires 130-135 C. and a sulphiting period shortened by 1 hour. Should the more intense conditions be applied to the fresh wood, a pulp will be obtained, which is distinguished by a considerably lower strength than the pulp obtained with the stored wood under the same conditions.

To hydrolyse and to dissolve the lignin sulphited in the solid phase in connection with the method according to the present invention, sulphur-dioxide is made use of. By reason of the fact that all or approximately all sulphiting liquor permitting of being tapped is drawn oft" upon the termination of the sulphiting period, it is necessary to employ an acid, which is gaseous at the hydrolysis temperature, in order that the acid shall permit of being uniformly spread over the contents of the digester.

The sulphur-dioxide may be supplied to the digester in a gaseous or liquid state. In the latter case the sulphurdioxide is preferably supplied in the circulation conduit,

whereby it is rapidly evaporated and distributed over the digester. In this latter case it will thus be preferable to keep so much liquor in the digester that the circulation can be maintained.

In digesting Pinus sylvestris it has been found to be suitable to introduce so much sulphur-dioxide into the digester that 34% of free sulphur-dioxide is obtained in the liquid phase. This corresponds at 130 C. to a total digester pressure of 6-8 atmospheres overpressure. As could be expected, the hydrolysis proceeds more rapidly, the more sulphur-dioxide the digester is supplied with.

On account of the fact that the sulphiting process, especially in connection with stored wood of Pinus sylvestris, will have to be carried into eifect at a relatively high temperature, it may be found suitable, with respect to the strength of the pulp, to lower the temperature during the hydrolysis period below the temperature prevailing during the sulphiting period. A certain temperature decrease is obtained already at the tapping of the sulphiting liquor, since the latter is understood to boil oif and to form steam in the digester. If liquid sulphur-dioxide is made use of, a further temperature decrease will be obtained through the heat absorbed by the sulphur-dioxide at the evaporation of the latter. In connection with experiments on a technical scale it has now been found that in spite of no heat being supplied to the digester from the outside, the temperature is continuously augmented during the hydrolysis period. This raising of the temperature proceeds more rapidly in the beginning of the hydrolysis period than is the case during the latter part of the same.

If it is desired to maintain a constant temperature during the hydrolysis period, it will therefore be necessary to provide for cooling during the same. One may also proceed so that the temperature of the digester contents is lowered, prior to the introduction of the sulphur-dioxide, below the temperature at which the hydrolysis is intended to be carried into effect. By said temperature decrease, the temperature level is then obtained during the hydrolysis period. The simplest method of effecting the lowering of the temperature consists in blowing out steam from the digester. The heat contents of this steam may be utilized in various ways, for instance for preheating sulphiting liquor, as digesting steam, for the preparation of hot water, and so forth.

Pulp that has been manufactured in accordance with the method according to the invention must be relieved of resin by reason of its high percentage of resin, before it can be used to advantage in the paper mills. The removal of the resin may be elfected, for instance, by treating pulp of a concentration of 1020% with approximately 10 kg. of NaOH per ton of pulp in a defibrator, and by acidifying the pulp that has been subjected to an alkali treatment, optionally after a portion of the liquor having been pressed off, to a pH-value of below 6 either by treating the pulp with H at a high pulp concentration, or by mixing the same with acidified water, so that the pH-value of the suspension is below 6. Hereby resin percentages below 1% in the finished pulp are obtained, while the production of resin spots in the pulp is also prevented.

What is claimed is:

l. A process for the manufacture of pulp from heavy resinous wood in a digestion zone comprising the steps of sulphiting said wood by steaming with a liquor comprising an aqueous solution of at least one member of the group consisting of alkali and alkaline earth sulphites and bisulphites while maintaining a substantially constant hydraulic over-pressure of between 14 and 112 psi. over the steam pressure of said sulphiting liquor throughout said sulphiting step and maintaining a temperature sufficient to augment said sulphiting, and thereafter hydrolyzing the resulting sulphited wood.

2. The process of claim 1 including the step. of lowering the pressure by creating a vacuum in a mixture of said wood and liquor, thereby facilitating the intake by said wood of said liquor and thereafter sulphiting with said liquor while maintaining a hydraulic over-pressure.

3. The process of claim 2 wherein said pressure is lowered to below 0.6 atmosphere.

4. The process of claim 2 wherein said pressure is lowered to between 0.2 to 0.4 atmosphere.

5. The process of claim 1 wherein the step of sulphiting with said liquor while maintaining a hydraulic overpressure is carried out during a period of about one to eight hours.

6. The process of claim 1 wherein the step of hydrolyzing the resulting sulphited wood is carried out by continuously introducing sulphur dioxide into said digestion zone.

7. The process of claim 6 wherein the step of hydrolyzing the resulting sulphited wood by sulphur dioxide is carried out in the absence of external heat.

8. The processof claim 6 wherein the step of hydrolyzing the resulting sulphited wood by sulphur dioxide is carried out at a temperature less than the sulphiting temperature.

9. The process of claim 1 including the steps of drawing off said liquor after terminating said sulphiting and thereafter employing said liquor in the preparation of fresh liquor.

10. A process for the manufacture of pulp from heavy resinous wood in a digestion zone comprising the steps of sulphiting said wood by steaming with a liquor comprising an aqueous solution of at least one member of the group consisting of alkali and alkaline earth sulphites and bisulphites while maintaining a constant hydraulic over-pressure of at least 14 p.s.i. during a period of about 1 to 8 hours and maintaining a temperature sufficient to augment said sulphiting, and thereafter hydrolyzing the resulting sulphited wood with sulphur dioxide.

11. The process of claim 10 wherein said hydraulic over-pressure is 14 to 112 p.s.i. over the steam pressure of said sulphiting liquor.

12. A process for the manufacture of pulp from heavy resinous wood in a digestion zone comprising the steps of sulphiting said wood by steaming with a liquor comprising an aqueous solution of at least one member of the group consisting of alkali and alkaline earth sulphites and bisulphites while maintaining a substantially constant hydraulic over-pressure of at least 14 p.s.i. throughout said sulphiting step and maintaining a temperature willcient to augment said sulphiting, lowering the temperature of said digestion zone by blowing of steam therefrom and thereafter hydrolyzing the resulting sulphited wood by introducing sulphur dioxide into said digestion zone.

13. A process for the manufacture of pulp from heavy resinous wood in a digestion zone comprising the steps of sulphiting said wood by steaming with a liquor comprising an aqueous solution of at least one member of the group consisting of alkali and alkaline earth sulphites and bisulphites while maintaining a constant hydraulic over-pressure of at least 14 p.s.i. throughout said sulphiting step, and maintaining a temperature of from -l40 C., for a period of about 1 to 8 hours and and thereafter hydrolyzing the resulting sulphited wood with sulphur dioxide.

References Cited in the file of this patent UNITED STATES PATENTS 68,261 Tiifany Aug. 27, 1867 480,334 Maste Aug. 9, 1892 1,761,544 Spencer June 3, 1930 1,790,260 Cable Jan. 27, 1931 1,804,967 Wolf et a1 May 12, 1931 1,864,620 Richter June 28, 1932 1,903,962 Dreyfus Apr. 18, 1933 1,927,541 Bouvier et al Sept. 19, 1933 1,940,136 Haglund Dec. 19, 1933 1,986,760 Kreissler Ian. 1, 1935

Claims (1)

1. A PROCESS FOR THE MANUFACTURE OF PULP FROM HEAVY RESINOUS WOOD IN A DIGESTION ZONE COMPRISING THE STEPS OF SULPHITING SAID WOOD BY STEAMING WITH A LIQUOR COMPRISING AN AQUEOUS SOLUTION OF AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH SULPHITES AND BISULPHITES WHILE MAINTAINING A SUBSTANTIALLY CONSTANT HYDRAULIC OVER-PRESSURE OF BETWEEN 14 AND 112 P,S,I. OVER THE STEAM PRESSURE OF SAID SULPHITING LIQUOR THROUGHOUT SAID SULPHITING STEP AND MAINTAINING A TEMPERATURE SUFFICIENT TO AUGMENT SAID SULPHITING, AND THEREAFTER HYDROLYZING THE RESULTING SULPHITED WOOD