US3393122A - Pretreatment of green wood with reducing agent prior to storage - Google Patents

Description

United States Patent "ice 3,393,122 PRETREA'I'MENT 0F GREEN WOOD WITH REDUC- ING AGENT PRIOR TO STORAGE John L. Marshal Bellingham, Wash., assignor to Georgia-Pacific Corporation, Portland, 0reg., a corporation of Georgia No Drawing. Filed June 3, 1964, Ser. No. 372,368

Claims. (Cl. 162-27) ABSTRACT OF THE DISCLOSURE The treatment of green wood in a chemi-mechanical pulping process with a reducing agent prior to aging of the green wood to prevent the darkening of the pulp obtained from the wood upon storage or aging of the wood prior to pulping.

This invention pertains to a chemi-mechanical pulping process and more particularly to an improved process whereby a pulp of improved brightness is obtained.

In recent years, an increasing amount of wood is being pulped by means of cherni-mechanical processes. In a chemi-mechanical process, chips are impregnated with cooking liquor, given a mild digestion or cooking, and then mechanically reduced to the pulp or fibrous state. The process differs from the chemical pulping of wood wherein the wood chips are more drastically treated with chemicals to delignify the chips which results in the removal of some of the hemi-celluloses and other constituents in addition to the lignin. The removal of these constituents considerably lowers the yield of the pulp. In chemical pulping, yields in the order of to are realized, while in a chemi-mechanical pulping process, the yield of pulp is generally and above and at times as high as to Since, in chemi-mechanical pulp, greater portions of the lignin and other extractive components of the wood which influence color or brightness of the pulp are retained, usually a darker pulp is obtained than obtained by chemical pulping and bleaching. For many uses, a pulp of high brightness is required. Thus, an improvement in brightness of pulp markedly increases its utility and saleability.

The color of chemi-mechanical pulp is also affected by the greeness of the wood from which the pulp is prepared. A darker pulp is obtained with aging of the wood. In all pulping processes, some time delay takes place between the time the tree is cut in the forest, the log hauled to the mill, chipped, and the chips chemically subjected to the pulping reaction. Many times, due to weather conditions and other factors, it is necessary to store logs or the wood after it is chipped for a considerable length of time. It may be months before the product is subjected to pulping. As a result, the pulp obtained is relatively dark and must be bleached to improve the brightness to the extent that the pulp is acceptable for many applications. At times, even extensive bleaching will not produce an acceptable product and the addition of chemical pulp must be made to obtain the necessary brightness.

It is therefore an object of this invention to provide an improved chemi-mechanical pulping process wherein a pulp of improved brightness is obtained. A further object is to provide a method of brightening the pulp obtained in a sulfite chemi-mechanical pulping process. A further object is to provide a process for improving brightness of chemi-mechanical pulp obtained from stored or aged wood.

The above and other objects are obtained according to this invention which comprises contacting green wood which is to be used in the chemi-mechanical pulping 3,393,122 Patented July 16, 1968 process with a reducing agent selected from the group consisting of sulfur dioxide, sulfites, hydrosulfites, borohydrides and ascorbic acid to sufficiently impregnate the green wood with the reducing agent to inhibit the darkening of the wood upon aging. Upon pretreatment of the wood with one of the above agents, the wood may be stored for considerable lengths of time without obtaining a darker pulp due to the aging of the wood. The brightness of pulp obtained from wood from a freshly cut tree so treated is somewhat improved over the brightness of the pulp produced from the untreated wood pulped immediately. A further improvement in brightness may be obtained by bleaching after pulping. The extent of this improvement is about the same as that obtained by bleaching pulp from untreated, aged chips. Thus, the improvement in brightness obtained by the pretreatment of the wood to inhibit the darkening due to the aging is additive to the improvements in brightness obtained by other treatments to which the pulp may be subjected during and after pulping.

While it is not definitely known by what mechanism the pretreatment will prevent the darkening of pulp, it is believed that the pretreatment of green wood with the reducing agent inhibits certain reactions between particular constituents in the wood which take place upon aging, resulting in the darkening. The constituents formed by aging of the wood are probably dissolved in the chemical pulping processes, since the aging of the wood has no appreciable effect on the color of the pulp obtained. However, in the sulfite chemi-mechanical process, the mild digestion employed in the process possibly does not remove these constituents which remain to darken the pulp. Thus, the improvement in brightness is obtained by the pretreatment in preventing the formation of these darkening products.

The rate of darkening or loss of brightness of the pulp resulting from the aging of the wood is affected somewhat by the moisture content or dryness of the aging wood but is mainly dependent upon the amount of surface of the wood exposed to the atmosphere. For example, a darker pulp will be obtained upon aging of wood in log form but the rate of darkening is gradual compared to the darkening obtained from wood aged in particulate form where a large surface area is exposed. Eventually, the brightness of the pulp obtained from the wood aged as a log will be about the same as that obtained from wood aged in chip form. For example, hardwood chips, when exposed to the atmosphere, will age sufiiciently in four hours to result in a brightness loss in the resultant pulp of about one-half percent unit. Thereafter, the chips may darken at a rate such that the pulp produced will lose about one percent unit per day until about 70 to 80% of the total darkening is obtained. Substantially all of the darkening due to aging will be completed in one to three weeks. With the wood in log form, it may equire three to four months to attain 75% of the total darkening and possibly six months before substantially all of the darkening is obtained. The presence of moisture somewhat decreases the rate of darkening. However, wood or chips will darken even though stored in water. Thus, the term green wood, as used herein, means wood which has been cut or chemically barked while standing within six months and which has not been exposed to drying such as in a kiln, or otherwise so that the wood at least contains a portion of the original moisture of the tree above the equilibrium moisture content.

The term brightness, as used herein, means the color rating of the pulp as determined and expressed according to TAPPI Standard T-217.

To obtain the improvement in brightness, wood, while in a green state, must be treated with the reducing agent 3 to inhibit the darkening by possibly reacting the reducing agent with particular constituents in the wood. Any of the various known methods of treating wood may be used as long as the wood is impregnated and contacted with the reducing agent. Logs may be treated as such. However, in reducing the wood to a particulate form, the impregnation and contact of the reducing agent may be more effectively obtained. Generally, it is preferred to treat the wood just after it has been chipped or reduced to particulate form.

The wood may be treated with the reducing agent in gaseous or liquid state. When the agent is in gaseous state, the wood can be contacted with an atmosphere containing the particular agent. Often it may be desirable to treat the chips by maintaining an atmosphere containing the desired gas in the storage bins until the chips are processed. At other times, it may be more convenient to wash the chips with a solution containing the desired constituents. Generally, after the chips are washed with the solution, they are drained and stored. A sufiicient amount of the solution is retained by the chips to impregnate them and to inhibit the darkening upon storage.

While solutions to saturation and atmosphere containing 100% of the reducing agent may be used, generally, the treatment of the wood with the agent is carried out with relatively dilute concentrations of the agent. Solutions containing more than 10% of the reagent are seldom employed. Usually, the concentration of the solution is maintained in the range of .1 to 4% and at times may be less than .0l%. Likewise, for a gaseous reagent, the concentration of the reagent over 10 volume percent is seldom used. Usually, an atmosphere containing less than 1% is employed. The wood is contacted with the reducing agent, in the relatively dilute concentration, until a substantially uniform impregnation of not more than 0.5 weight percent of the chemical, based on dry wood, is obtained throughout. Generally, ambient temperature and pressure are employed, especially for the more reactive agents.

In treatment of the green wood with a more reactive reducing agent, such as sulfur dioxide, chips can be overtreated, especially at high concentrations and elevated temperatures and pressures. The improvement in the brightness of the resulting pulp obtained increases progressively with the time that the wood is subjected to the reducing agent until a maximum brightness is obtained. Further contact or reaction of the wood with the reducing agent generally results in decreasing the brightness of the pulp. The decrease in the brightness by overtreatment, for example, with sulfur dioxide, may reach a point that no advantage is obtained by the pretreatment. Thus, in treating the chips, the preferred operation is to treat the chips with the reactive reducing agents for a time which is within 10% of the time required to obtain the maximum brightness in the final pulp. The contact time to obtain the optimum brightness varies with the concentration as well as conditions of temperature and pressure. For example, in a sulfur dioxide-containing atmosphere which contains from .001 to about 0.1% sulfur dioxide, the wood or chips may be stored at ambient temperature indefinitely without overtreatment. However, with an atmosphere containing about 1% sufur dioxide, an optimum brightness may be obtained after about 30 to 60 hours of treatment and then the brightness will gradually decrease with additional treatment, while with a 10% S atmosphere, the maximum brightness may be obtained in about 5 to hours. Thus, with the more concentrated atmospheres, the optimum contact time is less and the rate of decrease in brightness due to overtreatment is greater. Generally, it is preferred to treat the wood under conditions such as to impregnate the wood substantially uniformly without impregnating any substantial portion of the wood with the reducing agent, for example, S0 or the chemical equivalent thereof, in an amount in excess of 0.5 weight percent, based upon the dry wood, for any appreciable length of time. When the wood is treated with a larger amount of the agent, generally the optimum brightness is not obtained and if 0.5% is exceeded no improvement in the final brightness may be obtained.

Although the pretreatment of the wood with sulfur dioxide or a sulfite solution may result in an overtreatment of the wood, these agents are preferred. The reaction obtained upon treatment with sulfite ions appears to differ somewhat from the other reducing agents, giving an enhanced result. In treating the wood with sulfur dioxide or a sulfite compound which forms sulfite ions in solution or upon contact with the moisture of the green wood, it is generally desirable to treat the wood under conditions such that the wood will absorb .005 to .15% of the chemcal, expressed as S0 based upon the weight of the wood in a dry state. Solutions of sulfur dioxide containing from about 0.1 weight perecnt to saturation of S0 as well as solutions of sulfite and bisulfite salts of metals such as alkali metals and alkaline earth metals and NH may be conveniently used for washing the chips. Also, other sulfite compounds which dissolve or decompose to form sulfite ions may be employed, such as zinc or sodium hydrosulfite, which may react as is or decompose to form sulfur dioxide. The chips are often treated by contacting the chips with an atmosphere containing sulfur dioxide. Generally, after chipping a log, the chips are stored until they are used in enclosed storage bins and thus may be easily subjected to an atmosphere containing sulfur dioxide. The injection of a small amount of sulfur dioxide into the storage bin will provide the required atmosphere. Since it is not essential to use high concentrations, usually an atmosphere in the storage bin of from 0.1 to 0.01% is employed. Concentrations as low as .0001% are eiiective.

The point of maximum improvement in brightness may, at times, be determined by observing the conditions of the wood chips after treatment but prior to pulping. The treatment of wood with sulfur dioxide, for example, results in a brightening of the chips. With continued treatment, the chips will progressively lighten and reach a point of maxi-mum brightnes above which very little improvement is obtained with additional treatment. This improvement in the chip may be easily noted by mechanically defibering the pretreated chip prior to contacting the chip with the pulping solution and determining the brightness of the groundwood. Although the effect of the brightness of the groundwood pulp thus obtained may not necessarily correspond to the brightness of the pulp obtained after the sulfite solution pulping, the point at which the maximum brightness of the groundwood is first obtained is generally close to the contact time at which the optimum brightness is obtained for sulfite chemi-rnechanical pulp.

The final brightness and the extent of improvement obtained by the pretreatment of the green wood with the reducing agent will depend upon the conditions employed in the chemi-mechanical pulping process. In the process, the wood chips are impregnated with a sulfite solution having a pH in the range of 4.5 to 9.5, and subjected to only a mild digestion in order to obtain a high yield of pulp. A pulp of optimum brightness is generally obtained when the impregnation or digestion liquor employed has a pH in the range of 6.0 to 7.5. A darker pulp is obtained with liquor of higher pH, even though the improvement in brightness obtained by the pretreatment may be greater.

The following examples further illustrate the invention:

EXAMPLE I A red alder log which had been cut two months previous was chipped and a portion of the chips treated prior to storage. One portion of the fresh chips was washed for about one minute with a sulfurous acid solution at a pH of 1.75, allowed to drain, placed in a plastic bag, and stored. A second portion was washed with a sodium sulfite-sodium bisulfite solution at a pH of 6.5 and a concentration of about 0.5 weight percent expressed as S A third portion of chips was placed in a plastic bag without any treatment.

After the chips had been stored for 70 hours, the chips were pulped by a neutral sulfite chemi-rnechanical process to a yield of approximately 90%. In the pulping process, the chips were steamed for 5 minutes at atmospheric pressure and impregnated with a sodium sulfite solution at a pH of 6.5 under a hydrostatic pressure of 135 p.s.i. for one hour and the excess liquor drained off. The impregnated chips were subjected to a mild digestion by heating with steam at sufficient pressure to obtain a temperature of 150 C. for about minutes. The chips were mechanically defibered, washed, and the brightness of the pulp determined according to TAPPI Standard T-2l7. Both of the samples which had been pretreated had a brightness of 75%. The wood chips which were stored in a plastic bag with no pretreatment had a brightness of 70.3%.

EXAMPLE II To illustrate the improvement in brightness obtained by treating the chips with sulfur dioxide gas and also the effect of overtreatrnent, fresh chips from a red alder log, cut one month before chipping, were treated with sulfur dioxide under different conditions. A portion of the chips was placed in a plastic bag and the chips treated by passing sulfur dioxide gas into the bag while the chips were agitated. The bag was then sealed and stored at room temperature for 70 hours. A second batch of chips was placed in a container and continuously subjected, for a period of 68 hours, to a gas stream containing 16 to 18% sulfur dioxide. The treated chips, as Well as a batch of chips which had been stored for 70 hours without being treated, were pulped in a manner similar to that described in Example I to obtain a yield of approximately 90%. The brightness of the pulp obtained from chips which had been purged with sulfur dioxide gas was 77.5% while the brightness of the pulp from chips which had been continuously subjected to the sulfur dioxide stream for 68 hours was 62%. Chips which were stored without any pretreatment had a brightness of 74.7%.

In another run, fresh chips from a green alder log were subjected continuously to an air stream containing low concentrations of sulfur dioxide for 66 hours. One portion was exposed to a gas stream containing 0.01% of sulfur dioxide and a second portion of the chips was exposed to a gas stream containing 1% sulfur dioxide.

The treated chips and a batch of untreated chips, which were stored in a plastic bag for 66 hours, were pulped in a manner similar to that above to obtain a yield of approximately 90%. The brightness of the pulp obtained was 76.9% for chips treated with the gas stream containing 0.01% of sulfur dioxide, 76.4% for the chips subjected to the 1% sulfur dioxide gas stream, and 73.1% for the untreated chips.

EXAMPLE III A series of batches of fresh alder chips, from a log cut about three months before chipping, was treated with various reducing agents prior to storing and pulping.

In each of the runs, the chips were washed with a solution of the reducing agent by dumping the chips into the solution and agitating to thoroughly wet the chips. The chips were then placed on a screen and allowed to drain, packed in plastic bags and stored 48 hours.

One batch of chips was washed with sulfurous acid solution having a concentration of about 1.0 weight percent, expressed as SO A second batch of chips was washed with a 1% solution of ascorbic acid, while a third batch was washed with a 1% solution of sodium hydrosulfite. After pulping in a manner similar to that described in Example I, the brightness of the pulp was found to be 78% for the sulfur dioxide pretreatment, 77.5%

for the ascorbic acid pretreatment, 77.8% for the sodium hydrosulfite pretreatment, and 74.5% for the chips which had not been treated but just stored in a plastic bag for 48 hours.

EXAMPLE IV Fresh alder chips from a log cut 3 /2 months previous to chipping were treated with a solution of sodium borohydride and sulfurous acid solutions of various concentrations. After the treatment, the chips'were stored in plastic bags and then pulped in a manner described in Example I. The brightness of the pulp was determined and compared to pulp obtained from untreated chips which had been stored in a similar manner.

One batch of chips was washed with a 1% solution of sodium borohydride, drained and stored for 4 hours. Another batch was Washed with a sulfurous acid solution having a concentration of 0.1 weight percent and stored for 40 hours. The third batch Was Washed with sulfurous acid solution having a concentration of 0.5 weight percent expressed as S0 and stored for 40 hours. The brightness of the pulps obtained was 77.8% for the sodium borohydride treated chips, 75.5% for the 0.1% S0 wash, 76.5% for the 0.5% S0 wash, and 73.4% for the chips receiving no pretreatment.

EXAMPLE V Fresh alder chips from a green log cut 3 /2 months prior to chipping were washed with a sulfurous acid solution having a concentration of about 1.0 weight percent, expressed as S0 until the chips had absorbed an amount of solution equivalent to 0.2%, based on the dry wood. The treated chips were then stored in a plastic bag for 42 hours with a control sample which had received no treatrnent.

After storage, a portion of the chips from each of the bags was subjected to a chemi-mechanical pulping process similar to that described in Example I except that the pH of the neutral sulfite liquors used for the digestion were 5.4, 6.4, and 8.6.

The brightness of the pulps obtained is shown in the table below.

EXAMPLE VI Various wood species were treated and the improvement in brightness of the pulp obtained was determined. Green logs of the species were chipped and the fresh chips treated by washing with a sulfurous acid solution having a concentration of 1.0%, expressed as S0 until about 0.2 weight percent S0 based on the dry wood, was retained by the chips. The treated chips and batches of untreated chips were then placed in plastic bags and stored for 42 hours. The chips were pulped in a manner similar to that described in Example I to obtain a yield of The brightness of the pulps obtained is shown below: 1

Pulp Brightness (percent) Wood Species sulfurous Acid Treated Untreated 72. 5 68. 5 79. 5 75. 0 80. 5 76. 5 Sugar Maple 78. 5 77. 1

I claim:

1. In a process for the preparation of a high yield :pulp in a sulfite chemi-mechanical pulping process wherein green wood is stored prior to pulping, the improvement which comprises contacting the green wood prior to the time the wood has appreciably darkened due to 7 aging, with a reducing agent selected from the group consisting of sulfites, hydrosulfites, borohydrides, sulfur dioxide and ascorbic acid to sufliciently impregnate the green wood with the reducing agent to prevent darkening of the wood upon aging and storing the so treated wood.

2. In a process for the preparation of a high yield pulp in a sulfite chemi-mechanical pulping process wherein green wood is stored .prior to being subjected to a chemical pulping reaction in particulate form, the improvement which comprises contacting the green wood prior to the time the wood has appreciably darkened due to aging, with sulfite ions under conditions of temperature and concentration to pretreat the wood to inhibit darkening sufficiently to obtain a pulp of improved brightness and storing the so treated wood.

3. A process according to claim 2 wherein the wood is reduced to particulate form and is washed with a solution of a sulfite compound, after the reduction to particulate form, prior to the time the wood has aged sufiiciently to reduce the brightness by 1 percent unit of the pulp produced therefrom.

4. A process according to claim 2 wherein the wood is reduced to particulate form and is contacted with a sulfur dioxide containing atmosphere prior to the time the wood has aged sufliciently to reduce the brightness by 1 percent unit of the pulp produced therefrom.

5. A process according to claim 2 wherein the wood is reduced to particulate form and is stored Within 4 hours after reduction to particulate form in a sulfur dioxide atmosphere, said sulfur dioxide atmosphere containing from -1 to 0.001% sulfur dioxide.

6. A process according to claim 5 wherein the green wood is a hardwood.

7. In a chemi-mechanical pulping process wherein green wood is chipped to obtain the wood in particulate form, and wherein the wood chips are pulped to a yield of at least 80% by treatment with a sulfite solution and cooked at a pH in the range of 9.5 to 4:5, and mechanically defibered, and wherein the Wood chips are stored after chipping for a sufficien-t time 'before being subjected to the chemical pulping reaction to obtain a substantial darkening of the freshly exposed surface of the chips, the step to improve the brightness of the pulp, which comprises contacting the green wood chips prior to substantial darkening of the surface of the chips with sulfite ions, said chips being contacted with the sulfite ions under conditions of time and concentration to pretreat the chips without impregnating the wood with sulfite ions, expressed as S0 of more than 0.5% of the weight of the wood in a dry state to inhibit the darkening of the chips.

8. A process according to claim 7 wherein the green wood is selected from the group consisting of alder, birch, maple, beech, spruce and mixtures thereof.

9. A process according to claim 8 wherein the green wood is contacted with the sulfite ions until the wood is impregnated with from 0.005 to 0.15 weight percent of the sulfite ions, expressed as S0 based upon the wood in a dry state.

10. A process according to claim 9 wherein the chips are Washed with a sulfite solution.

11. A process according to claim 10 wherein the chips are washed within four hours of chipping with a sulfurous acid solution having a concentration, expressed as S0 of from 0J1 weight percent to saturation.

12. A process according to claim 9 wherein the chips are contacted with a sulfur dioxide containing atmosphere.

13. A process according to claim 12 wherein the sulfur dioxide atmosphere contains from 25 to 0.0001% by weight of sulfur dioxide and the chips are contacted within 4 hours after the chipping.

14. A process according to claim 12 wherein the chips are subjected to a sulfur dioxide atmosphere containing from 0.01 to 0.1% by weight of sulfur dioxide immediately after chipping for at least 10 hours.

15. The process for the preparation of high yield pulp of improved brightness from green wood which comprises chipping the wood to obtain the wood in particulate form, immediately washing the chips with a sulfite solution at ambient temperature and pressure, storing the so-treated chips, and pulping the so-treated chips in from four hours to 30 days with a neutral sulfite solution at a pH in the range of 6 to 7.5 to obtain a yield of pulp of at least 16. A process according to claim 15 wherein the chips are contacted at ambient temperature with an aqueous sulfur dioxide solution containing from .1 weight percent to saturation of sulfur dioxide until the chips are impregnated with from 0.005 to 0:15 weight percent of sulfur dioxide, based upon the weight of the wood in a dry state.

17. A process for the preparation of high yield pulp of improved brightness from wood, which comprises chipping green wood to obtain a wood in particulate form, treating the chips with a sulfur dioxide atmosphere at about ambient temperature and pressure, storing the sotreated chips and pulping the so-treated chips in from four hours to 30 days with a neutral sulfite solution at a pH in the range of 4.5 to 9 to obtain a yield of pulp of at least 80%, said chips being treated with the sulfur dioxide atmosphere for a time within 10% of the time required to obtain the maximum brightness of a groundw=ood made from the chip-s without pulping with the sulfite solution.

18. A process according to claim 17 wherein the chips are subjected under ambient conditions to an atmosphere containing from 0.1 to 0.01 percent by weight of sulfur dioxide until the chips are pulped.

19. A process according to claim 2, wherein the wood is reduced to particulate form and is washed with a boro- -hydride solution, after the reduction to the particulate form, .prior to the time the wood has aged sufiiciently to reduce the brightness by 1% unit of the pulp produced therefrom.

20. A process according to claim 2, wherein the wood is reduced to particulate form and is washed with an ascorbic acid solution, after the reduction to particulate form, prior to the time the wood has aged sufficiently to reduce the brightness by 1% unit of the pulp produced therefrom.

References Cited UNITED STATES PATENTS 1,169,597 1/1916 Bache-Wiig 16227 1,904,894 4/1933 Westad 16264 X FOREIGN PATENTS 829,506 3/ 1960 Great Britain.

OTHER REFERENCES Du Pont, The Technical Bulletin, vol. 1 1, No. 3, pp. 13 6, 137, 138, Bleaching Groundwood Pulp With Du Pont Hydrosulfite Come. September 1955, 162-80.

DONALL H. SYLVESTER, Primary Examiner.

HOWARD R. CAINE, Examiner.