US3663358A - A METHOD FOR INCREASING BISULFITE PULP YIELD COMPRISING TREATING LIGNOCELLULOSIC MATERIAL WITH CYANIDE IONS AT A pH OF 7{14 12 PRIOR TO DIGESTION - Google Patents

Abstract

A process for increasing pulp yield from lignocellulosic material consists of treating the lignocellulosic material with either a solution of cyanide ions at a temperature below about 130* C., preferably about 75* C., or treating the material with a mixture of cyanide-bisulfite ions in solution at a temperature below 130* C. preferably about 100* C., and digesting at conventional neutral or acid bisulfite conditions. The treatment of wood prior to neutral or acid bisulfite digestion with either cyanide ion alone or a combination of cyanide-bisulfite results in pulps having increased yield and improved brightness stability over untreated bisulfite cooks.

Description

United States Patent Falkehag et al.

[ 1 May 16, 1972 [54] A METHOD FOR INCREASING BISULFITE PULP YIELD COMPRISING TREATING LIGNOCELLULOSIC MATERIAL WITH CYANIDE IONS AT A PH OF 7-12 PRIOR TO DIGESTION [72] Inventors: Sten I. Falkehag, Charleston; Alan M.

Bills, Summerville, both of SC; Harold L. Hintz, Covington, Va.

[73] Assignee: Westvaco Corporation, New York, NY.

[22] Filed: Apr. 29, 1970 [21] Appl. No.: 33,044

[56] References Cited UNITED STATES PATENTS 3,532,596 10/1970 Bills et al. ..l62/70 Primary Examiner-S. Leon Bashore Assistant Examiner-Frederick Frei Attorney--Ernest B. Lipscomb and Robert S. Grimshaw [57] ABSTRACT A process for increasing pulp yield from lignocellulosic material consists of treating the lignocellulosic material with either a solution of cyanide ions at a temperature below about 130 C., preferably about 75 C., or treating the material with a mixture of cyanide-bisulfite ions in solution at a temperature below 130 C. preferably about 100 C., and digesting at conventional neutral or acid bisulfite conditions. The treatment of wood prior to neutral or acid bisulfite digestion with either cyanide ion alone or a combination of cyanide-bisulfite results in pulps having increased yield and improved brightness stability over untreated bisulfite cooks.

6 Claims, No Drawings A METHOD FOR INCREASING BISULFITE PULP YIELD COMPRISING TREATING LIGNOCELLULOSIC MATERIAL WITH CYANIDE IONS AT A PI'I OF 7l2 PRIOR TO DIGESTION BACKGROUND OF THE INVENTION This invention relates to a process for increasing the yield of neutral or acid bisulfite pulps. More particularly, this invention is directed to a process for improving pulp yield of neutral or acid bisulfite pulp by reacting lignocellulosic material with either cyanide ions alone or cyanide ions in combination with bisulfite prior to digestion to stabilize the lignocellulosic material from degradation during pulping.

Lignocellulosic materials (such as wood from coniferous and deciduous species of trees, the stalks of bamboo and reeds, bagasse, and cotton fibers) principally contain lignin, cellulose, and hemicelluloses. The lignin and hemicelluloses are located both in the cell wall of the fiber and in the inter fiber spaces, generally termed the middle lamellae. In chemical pulping processes utilizing lignocellulosic materials, lignin in the inter-fiber spaces must be dissolved, with as little damage as possible to the fibers, so that the fibers are readily separable. I-Iemicelluloses can be retained on the surface of the fiber, in which case they can influence the bonding properties of the fibers, and they are also retained in the cell wall of the fiber, improving the flexibility and thereby the papermaking properties of the pulp. It is consequently generally desirable to retain the hemicelluloses in order to produce paper products having superior strength characteristics. In acid and neutral bisulfite pulping, liquors directed toward dissolving lignin attack the cellulose very little, but hemicelluloses are attacked to a high degree, especially when the cook is greatly prolonged in order to remove as much of the lignin as possible before bleaching.

In attempting to prevent degradation of the cellulosic and hemicellulosic components, it was found that the terminal carbohydrate group may be changed by converting the aldehyde end group to a carboxylic unit. The selected reaction for doing so is the well known cyanohydrin synthesis or Kiliani-Fischer reaction for increasing the length of the carbon chain of the sugars by addition of hydrocyanic acid to aldehyde or ketones, as follows:

The original procedure devised by Kiliani for the sugars required adding a sugar to an aqueous solution of hydrogen cyanide in the presence of a little ammonia. A study of the effect of pH on the addition of cyanide to sugars was reported by Noji [52 Nippon Yakurigaku Zasshi 271-96 (1956)]. No cyanohydrin formation occurred with both glucose and fructose at pH 5.45-2.65 In a modification of the cyanide addition, Noji combined the cyanide solution with bisulfite addition and reported that bisulfite adds more readily than cyanide to sugars and then cyanide may more easily displace the bisulfite from the addition product.

Application of the Kiliani-Fischer reaction to treatment of lignocellulose material prior to alkaline pulping has resulted in US. Pat. No. 3,532,596 which issued Oct. 6, 1970.

This invention depends upon preventing degradation of the cellulosic and hemicellulosic components of bisulfite pulp. It is further successful because the pulping solution is able to more effectively contact the lignin. Consequently, a lower concentration of pulping solution can initially be mixed with the lignocellulosic material or a higher concentration of pulping solution is present when steady cooking temperatures are reached. The term yield as used herein is the dry weight of washed fibers which are recovered after pulping as a percentage of the weight of dry lignocellulosic material which was charged to the digestion process. Any decrease in yield caused by loss of lignocellulosic material is generally undesirable in papermaking.

It is the principal object of this invention to provide a process for increasing the yield in acid and neutral bisulfite pulps. A further object is to produce a pulp having good brightness stability while maintaining increased yield without substantially impairing other desirable properties. Other objects will be evident from the following disclosure.

SUMMARY OF THE INVENTION It has been found that pulp yield produced under acid and neutral bisulfite pulping conditions may be increased by treating the lignocellulosic material, e.g., wood chips, with a solution of cyanide ions at a temperature below about 130 C. and preferably below about 75 C., withdrawing the excess cyanide solution and digesting with bisulfite liquor under normal digesting conditions, or treating the chips with a mixture of cyanide-bisulfite ions in solution at a temperature below 130 C. preferably about C., until the reaction is complete before raising the temperature to digesting conditions. It was found the treatment of chips prior to digestion with both solutions of cyanide ions alone or a combination of cyanidebisulfite ions results in yield increases in acid and neutral bisulfite cooks over the untreated acid and neutral bisulfite cooks. Additionally, brightness stability is greater for cyanide treated unbleached pulps than for untreated pulps.

DETAILED DESCRIPTION OF THE INVENTION The process of this invention comprises the steps of treating lignocellulosic material at a pH between 7.0 and 12.0, with a solution of cyanide ions and hydroxyl ions the forming of cyanohydrin groups by reaction of the cyanide ion with aldehyde end groups of the lignocellulosic material and hydrolyzing the cyano portion of the thus formed cyanohydrin group into a carboxyl group whereby the lignocellulosic material is protected against degradation during acid and neutral bisulfite pulping. To maintain satisfactory contact between the cyanide solution and the lignocellulosic material, a solution to material ratio of 4 to l is preferred but is not critical as long as sufficient contact is made.

Cyanohydrin is formed more rapidly at higher temperatures, but hydrolysis of cyanide ion directly into ammonia is also accelerated at higher temperatures. To avoid competition between these reactions for the available cyanide ion, contact preferably occurs at relatively low temperatures such as about 75 C. However, the reaction to form cyanohydrin can take place at higher temperatures if desired and there is no bisulfite present to compete with the reaction. To achieve the most favorable treatment conditions, the cyanide ions should be contacted at a concentration of 0.3 grams of cyanide ions per liter of the alkaline solution. As a practical digestion procedure for wood chips, diffusion of cyanide throughout the cellulosic material and cyanohydrin formation generally proceed simultaneously. If wood chips are presteamed to drive out air and thoroughly wet the wood structure, about 70 percent of the possible reaction occurs in three hours at a temperature of 95 C. At a temperature of C., approximately 5 minutes is sufficient, but control and uniformity of cooking conditions are not reliable. An economic penalty must also be paid for cyanide ion which becomes unavailable for cyanohydrin formation because of hydrolysis directly into ammonia. When cyanide alone is used, the hydrolysis step takes place when contact is made with the bisulfite pulping liquor.

As another embodiment of this process, it is quite practical to include bisulfite with the cyanide solution to immediately form the hydrolysis reaction. In such cases the aqueous alkaline cyanide ion containing solution should have at least 2.0 grams of bisulfite per liter. preferably sodium bisulfite for the most favorable reaction.

In the practice of this invention, any aqueous solution containing cyanide ions may be used. When KCN, for example, is added to water, it readily dissociates into K" ion and CN ion. The cyanide ion then reacts with water:

The treatment conditions consisted of treating the chips with a mixture of cyanide-bisulfite at 100 C. before subsequently raising the temperature to 170 C.; or treating the chips with cyanide alone at 75 C., draining the excess cyanide CN H O HCN OH and addition of bisulfite liquor followed by subsequent cook- Whether derived from l-lCN gas, liquid hydrogen cyanide, an ing at 170 C. The cyanide solution was made by dissolving 2.5 inorganic compound or an organic compound, any treatment grams of potassium cyanide in 1 liter of water, as a convenient solution therefore contains reactive CN ions, so the source of source of cyanide ions and was used at a 4:1 liquor-to-chip the cyanide ion is immaterial. ratio. The cyanide bisulfite solutions were made by adding This invention may be practiced in continuous or bat h 16.6 grams of sodium bisulfite to acyanide solution having 2.5 digester operations. A typical process for carrying out the ingrams of potassium cyanide dissolved therein per liter. The vention, which is suitable for batch or continuous operations, same liquor to chip ratio was used. The total pulp yields were is: (a) presteam chips to drive out air, soften wood, and fill then determined and the results are shown in the table.

Type of treatment Untreated neu- Cyanide Cyanide Untreated acid tral bisulfite treated treated neutral Cyanide bisulfite bisulfite pulp pulp acid pulp pulp treated neutral pulp Cook number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Cyanide consumed (percent on 0.1). wood) 3.0 3.0 3.0 3.0 3.0 3. 0 3. O 3. 0 3.0 Tinu to maximum temperature, min l l, 25 25 30 25 20 3O 30 30 25 30 25 25 25 Tinlt zit-170 (3., min A. 120 00 (i0 30 120 no 30 120 30 120 90 30 120 90 60 30 pllutbeginningofcook. H. 13.5! (5..1 4.1) 4.1 6.7 6.7 6.7 6.7 6.7 6.7 111] utvmlol'mok. 3.0 3.1 3.3 3.4 6.8 6.8 (5,8 5,3 5.4 7.0 (1.4 6.4 7.1! 7.3 7.4 7.3 'lotul pulp yivld (perm-111.011 0.1 wnml) 51. .2 54. 5 62.3 71.3 77. 7 82.7 84. 1 79. 7 85. 0 81.4 81.0 811. 2 80.11 8 1.7 85.0 83.2 Total lignin (percent on 0.1). wootl).1l 13.8 18.13 22.1 11).) 21.6 22.5 21.5 22,6 18.5 18.1 22.3 .r 19.6

pores with liquid; (b) introduce a solution of sodium cyanide at a convenient concentration and at a temperature below 300 C. for a treatment which varies inversely in duration with the temperatures; (c) withdraw unreacted cyanide solution; and (d) introduce cooking liquor at a pH below 7 and digest at conventional bisulfite conditions.

It is therefore an advantage of this process that treatment of lignocellulosic material with cyanide ions gives increases in pulp yields when subsequently digested by the bisulfite process at neutral or acid conditions. Another advantage in producing unbleached pulp from lignocellulosic materials in accordance. with this invention, in addition to increased yield, is the brightness stability of the resulting pulp.

The practice of this invention may clearly be seen in the following example.

Pulping cooks were carried out in a multiple unit rotating digester using loblolly pine chips which had been screened to reject all chips passing the VB inch screen and all retained by the 1 inch screen. The cooks were designed to determine the effect of treatment of the chips with cyanide and cyanidebisulfite mixtures on the pulp and paper properties of the pulps resulting from acid and neutral bisulfite cooking of the chips. Several comparison cooks were made using a normal acid (pl-l 4) bisulfite process and also a neutral (pl-l 6.5) bisulfite process. The neutral pH was maintained using a phosphate buffer. The bisulfite cooking liquor contained 15 percent effective bisulfite (applied Nal-ISO as S0 on 0D. wood).

ln a typical cook, chips were placed in a filter flask and evacuated with a vacuum pump for 5 minutes. In the treatment cooks, the solution was admitted through a separatory funnel and the mixture was further evacuated for 15 minutes. The mixture was transferred to the digester, heated to the treatment temperature and held there for the prescribed time. The treatment liquor was withdrawn after treatment, the cooking liquor at a 4:1 liquor-to-solids ratio added and the temperature raised to 170 C. After the time at the cooking temperature was completed, the digester was immediately cooled. The pulps were fiberized by a single pass through the laboratory single disc Bauer refiner with the plate clearance set at 0.031 inch. After washing, the pulp was pressed to remove excess water and mixed in a Hobart mixer. Samples were taken to determine the oven-dry weight of pulp for the total pulp yield determination. The remaining pulp was screened in the laboratory flat screen with a 14 cut screen. The rejects were collected, dried at 105 C., and weighed. The accepts were pressed, mixed and samples were removed for oven-dry weight determination.

The results show that the pulp yield of the four acid hisulfite cooks of non-treated pulp [cooks 1-4] ranged from 51.2 to 71.3 percent depending upon the length of cooling time which varied from 2 hours to 30 minutes. The yield of acid bisulfite pulp cooks [cooks 8-9] treated .with cyanide alone varied from 79.7 percent at 2 hours to 85.0 percent at 30 minutes which is a substantial increase in yield. Yields of cyanide treated neutral pulps showed 4-5 percent increases. The treatment of chips with a combination of cyanide-bisulfite results in yield increases of 2-3 percent in the neutral bisulfite cooks over the untreated neutral bisulfite cooks.

The initial brightness of cyanide treated pulps was lower than the normal acid and neutral bisulfite pulps. However, the brightness stability after 2 hours heating at C. was much greater for the pulps from the cyanide pretreated chips. The pulps from the untreated acid and neutral cooks lost approximately 45 GE. brightness units; whereas the both cyanide treated pulps and the cyanide bisulfite treated pulps lost only 1-1.5 G.E. units. Also, the appearance of the pulps was very different. The normal acid bisulfite pulps had the familiar reddish appearance normally associated with sulfite pulps. The neutral bisulfite pulps were very pale yellow colored immediately following the cook, but quickly reverted to a medium brown color. The pulps from the cyanide treated chips were initially a light brown color and remain so upon heating.

While the invention has been described and illustrated herein by references to various specific materials, procedures and examples, it is understood that the invention is not restricted to the particular materials, combinations of materials, and procedures selected for that purpose. Numerous variations of such details can be employed, as will be appreciated by those skilled in the art. below C. to convert the terminal carbohydrate unit of the lignocellulose chain to cyanohydrin group, withdrawing the excess solution, and digesting the thus treated lignocellulosic material with a bisulfite pulping solution having a pH below 7 whereby the cyano portion of the cyanohydrin group is hydrolyzed to a carboxyl group.

2. The process of claim 1 wherein the lignocellulosic material is in the form of wood chips.

3. The process of claim 1 wherein said treating is conducted at a temperature about 75 C.

We claim cyanide ions at a pH between 7.0-12.0 and a temperature also 4. The process of claim 1 wherein said cyanide ion is obtained from potassium cyanide.

5. The process of claim 1 wherein said cyanide ion is at a concentration of at least 0.3 gram per liter in said solution,

mg? UNETED STATES PATENT QFFICE @ER'HNCATE GE RRETWN Patent No. 63,358 Dated May 16, 1972 Inventor(s) Sten I. Falkehag et al It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, In the Table, last line thereof, delete "M".

Column 4, lines 60-69 beginning with "below l30C" and concluding with "temperature about 7 C." are misplaced and should follow line 75 of column 4.

Signed and sealed this ZLLth day of Qotober 1972.

(SEAL) Attest:

EDWARD MQFLETCHERJ'R, ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents

Claims (5)

1. 2. The process of claim 1 wherein the lignocellulosic material is in the form of wood chips.
2. 3. The process of claim 1 wherein said treating is conducted at a temperature about 75* C.
3. 4. The process of claim 1 wherein said cyanide ion is obtained from potassium cyanide.
4. 5. The process of claim 1 wherein said cyanide ion is at a concentration of at least 0.3 gram per liter in said solution, said solution to said lignocellulosic material being at least 4 to 1 by weight, and said lignocellulosic material is treated for at least 5 minutes.
5. 6. The pulp produced according to the process of claim 1.