CA1050212A - Method of increasing pulp yield and viscosity - Google Patents

Abstract

METHOD OF INCREASING PULP
YIELD AND VISCOSITY

Abstract of the Disclosure A process for increasing the yields of wood pulp and improving the viscosity of the pulp, employing-a sequence of an alkaline treatment of pulp with cyanide, followed by an alkaline digestion, and a delignification oxygen-alkali treat-ment.

Description

lOSOZ~Z
Rackground of_the Invention Since the kraft process provides a lower yields than those obtained by sulfite pulping, the kraft paper industry has continually sought means of increasing pulping yields.
To achieve this, pretreatments of wood chips prior to pulp-ing have been employed. These pretreatments typically em-ploy hydrogen sulfide, sodium polysulfide, sodium borohy-dride or sodium cyanide. Similarly, stabilizers have been added in the cooking treatment to try to improve yields.
Such stabilizers as sodium polysulfide, hydroxylamine and hydrazine have been employed. Yield improvements for a give lignin content have also been reported for multistage processes consisting of an alkaline digestion o~ wood chips, followed by breakdown of resulting softened chips 9 Wi th a further delignification with alkaline solutions of oxygen.
Processes have been described for protecting polysac-charides in the pulping of lignocellulosic materials agairst alkaline attack during processes for converting the lignoc ellulosic materials into paper products. One such proce-dure involves the addition of cyandies, such as sodium cyanide, to the cellulosic material prior to the pulping process to stabilize the polysaccarides against alkaline attack and thereby increase the pulp yields. One such sug-gestion is found in Bills et al~. U.S. patent 3,532,596, granted October 6, 1970. Another is the printed abstract of a paper presented at the 13th International Congress of Pure and Applied Chemistry, held in 5tockholm, Sweden, in 1953, by E Venemark, entitled "A Comparison ~etween ~he Influence of Cyanide and Sulfide On the Alkaline Delignifi-cation~," describing a treatrnent of cellulosic material with cyanide during alkaline delignification.

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Falkehag et al. U.S. patent 3,663,358, granted May 16 1972, concerns an alkaline pretreatment with cyanide fol-lowed by an acid bisulfite digestion.
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While these treatments with cyanide provides a means of increasing the yields of pulp by pulping processes, the magnitude of the increase is diminished at lower lignin contents. Thus, the paper industry has sought further means of providing substantial increases in the yields of pulp during pulping processes.
Worster et al. U.S. patent 3,691,008, German patent specification (Offenlegungsschrift) 2,123,542 of Samuelson and Noreus (1971), and an article by J. Nowakowski in Zeszyty Problemowe Postepow Nauk Rolniczych, No. 52, pp.
107-126 (1965), entitled "Role of Oxygen in Alkaline Dig-estion of Softwood Under Pressure," all disclose processes employing a mild alkali treatment followed by cooking with sodium hydroxide in the presence of oxygen. These procesces have been shown to provide yield increases over those ob-tained by the kraft process.
While these prior art processes provide advantages, still further improvement of pulp yields and viscosity have been long sought by the paper industry.
It is therefore an object of the present invention to provide a method of producing a pulp that is substantially high in yield, at a give Kappa No., than pulps produced by prior art methods.
- It is another object of the invention to provide a method of producing pulp having irnproved yields and viscos-ity over those of pulps produced either by cyanide treat~
ment along or by oxygen-alkali treatment along.
It is a further object of the present invention to produce pulp that is substantially higher in viscosity than is obtained from kraft/alkali or soda-oxygen/alkali proces- -~
ses.
In one aspect of the invention there is provided a ; ~# - 2 -.: .

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method of increasing yields and improving viscosity in the pulping of cellulosic material, which method com-prises treating - 2a ~

i cellulosic material in an alkaline aqueous solution having a pH of between about 7 and 13 and containing cyanide ions followed by a partial delignlfication coo~ing treatment at an ;i elevated temperature of from about 1609C to about 185C for a period of about 1 to 3 hours, employing an agueous solution of sodium hydroxide or sodium hydroxide and sodium sulfide, followed ~r by an additional delignification treatment employing an aqueous .
solution of sodium hydroxide in the presence of oxygen under pressure.
L0 Further objects and aspects of this invention will be apparent to those skilled in the art from the present description, token in conjunc~ion with the aDpended drawings, in which: ¦

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Figure 1 is a diagram of an apparatus desirable for conducting the oxygen-alkali delignification treatment of the method of the invention.
Figure 2 is a graph comparing the visosities of pulps at particular Kappa Nos. obtained by the present process employing a cyanide-soda cooking treatmen-t followed by an oxygen-alkali delignification treatment, with the viscosi-ties obtained employing a prior art process employing a soda cook followed by the oxygenalkali delignification treatment.
Figure 3 is a graph comparing the yields obtained by the process of the present invention, employing a cyanide-soda cooking treatment followed by an oxygen-alkali delig-nification treatment, with the results obtained by the conventional kraft and soda processes.
Figure 4 is a graph comparing the yields obtained employing the process of the invention, using a cyanide kraft cooking treatment followed by an oxygen alkali delig-nification treatment, with the yields obtained by the conventional kraft processes.
General Description of the Invention .
~We have now discovered that unexpectedly superior pulp :yields and high pulp viscosit;~es can be obtained by employ-ing a sequence of pulping employing a pretreatment step with a cyanide, followed by an alkaline cook and then followed by an oxygenalkali treatment. This sequence of steps provides the advantage over cyanide-kraft or cyanide-soda treatments in producing pulp in a substantially higher yield than is possible with a cyanide pretreatment in accordance with the prior art methods. The method of the invention has the advantage over soda-alkali-oxygen treat-ment alone in not only producing a pulp with a high yield $~ _3 ~osa)~z at a given Kappa No., but also with a substantially higher viscosity for each Kappa No. The higher viscosity values represent - 3a -.

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a higher degree of polymerization of the cellulose, or conversely, less degradation of the cellulose. This is, of course, highly desirable. Thus the method of the invention has the advantage over conventional kraft or conventional soda pulping in producing a pulp with a sub-stantially higher yield at a given Kappa No.
In accordance with out method, the wood chips are first subjected to an alkaline pulping pretreatment. In this pretreatment the chips are first introduced into alka-line digesters with the cu~tomary kraft or soda pulping constituents.
Either kraft or soda pulping media may be employed.
A weight ratio of pulping liquor to wood chips of between about 2.0 and 5.0 to 1 is desirable, with the best results obtained at a ratio of between 3.0 and 4.0 to 1. Set -forth below in Table 1 are some desirable alkaline pulping conditions which provide excellent results, although it will be appreciated that these conditions may be varied markedly without untoward results.
Table 1 Alkaline Pulping Conditions Linerboard Bleachable Soda Yield Kraft Yield Kraft Pulping Liquor: Wood Ratio 3:1 3:1 4:1 Time to maximum temperature~ min.65 90 90 Time at maximum temperature, min.35 80 60 Maximum temperature~ C.171 171 173 % Sulfidity - 22-31 22-31 0 In the alkaline pulping temperatures of about 140 to 20C C., preferably from about 165 C. to about 175 C., are suitable.

The chips in the pulping medium are first steamed to remove air from the chips, after which cyanide solution is added to the pulping liquor. The amounts of cyanide, pre-ferably introduced as alkali-metal cyanide, such as sodium ~#

:lV5(~Z~2 or potassium cyandies, may vary widely, but the amounts of`
between about 0.25 and 2.0% by weight of moisture-free cellulosic material are preferred, with the optimum being between about 1 and 1.5~

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Ad~ustment of the p~ of the pulplng solution cont?inin~
~ .. cyanide, to bet~reen about 7 and 13 is desirable, or preferably from bet~leen about 8 and 12.
The c~-?-nide treatment is desirably conducted for bettleen about ~0 and 60 minutes.
- In the treatment with tne cyanide, a broad range of temperatures, such as fro~ ambient temperzture to about 130C.
- is satisfactory. Temperatures in excess of 130C., while satis-factory, are less desirable than temperatures below 130C.
The preferred temperatures zre between about 75C. and 95C.
me pulping digester, containing chips, pulping liquor and cyanide, is then sub~ected to gaseous pressure, with an inert gas, such as nitrogen, at fr;~l apout 10 to 250 psig., preferably from about 50 to 150 psig.g in1order to force the pulping li~uor lnto the chips.
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Follotling the cyanide pulping pre~reatment, the cyanide s~lution is ~rithdra~ from the digester and replaced wi~h either (a) a solution of s~dium hydroxide, in the case of a soda cook, or tb) a solution of sodium hydroxide and sodium sulfide ~n the ~ case o~ a kraft cook. The soda or kraft cooking stage is conducted from about 1 to ~ hours at a temperature of fr~m ~60 to 185C. in order to partially delignify the chips.
~ The partially delignified chips are then removed from - the digester and given a m~ld mechanic21 treatment, preferably .25 w$th a disc attrition mill, t~ produce a co~rse, unrefined pulp.
This pulp is then sub~ected to further delignification in a reactor with oxygen and alkali.
. ~n the oxygen-al~ali delignifi-ation stage, pulp, ad~usted deslrably at a l,to 40~ by weight consistency, preferably 30 at a c~nsistency Or between about 1 and 10~ by ~eight~ is placed - into a treat~.ent vessel, where it is sub~ected t~ alkali spray containin6 dissolvcd oxy~en. This treatment may be conducted ln an app~ratu~ similar to that depicted in Figure 1 ~r the ... '~ . ., - , .............. . . .

-i 10502~2 nppended dr~win~s rn accordance ~ith that apparatlis, the pulp is ~lacecl in vessel (10), equlpped ~ith an internal ~rire pen-mesh b~sket (11). Treatment liquor containin~ alkali and oxy~en is constantly clrculated throu~h the pulp and through the open-mesh basket (11). The liquDr is pass~d throu~h he2t exchanger (12) ;rhich m~intains constant temperature of the treat-ment liquor. Circulation is maintained by means of pump (1~).
The liquor is ejected through nozzle (14), which causes the liquor spray to be constantly refortified ~ith oxygen, or other source of oxy~en such as air, at an elevated pressure of bet~een about 100 and 200 psig. oxygen par~ial pressure. me liquor - concentration of alkali is maintained at a desired constant level of such as between about 0.1 and 10.0 grams per liter, preferably about 0.25 grams per liter, of sodium hydr~xide. This is main~
tained by in~ecting additional sodium hydroxide from fresh ~lka}i reservoir (20) ~Jhere alkali solution is made up and heated by heat exchanger (21). Pump (22~ assists in circulating liquid --- through the heat exchange~ ) and reservoir (20). High pressure in~ection pump (2~) transfers liquor through line (24) into liquor circulation line (25) to be made available to the oxygenation treatment. -- During the oxygen treatment, a temperature range o~
between about 90 and 140C. is desirable, but preferably be~ween about 120 and 130C. - -?5 Deta _ed Description of the Pr~cess ~ -In order tD disclose more clearly the nature of the present invention9 the follo~ring examples illustratIng the lnvention are given. It should be understood, however, that this ~s done solely by way of example and is intended neither to delineate the scope Df the invention nor limit the ambit o~ the appended cl~ims. In the examples which ~ollow, and throughout the specifica~ion, the quantities of material are expressed in .... . . -- :
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term of parts or percent by weight, unless otherwise speci-fied.
E mples l_through 36 Examples 1 through 36 demonstrate pulping or cooking experiments employing either the soda process or the kraft process with or without -the addition of cyanide in the form of sodium cyanide, and the difference in results obtained.
In the Examples 1 through 14, the soda process of pulping was employed, while in Examples 15 through 36, inclusive the kraft process was employed. Examples 10 through 14, 19 through 23 and 32 through 36, employing sddium cyanide in the alkaline pulping pretreatment, constitute the first step of the process of the invention. The other examples are provided for comparison purposes.
In the process of Examples 1 through 26, Southern Pine wood chips were screened and analyzed for moisture content. As referred to herein, all calculations and re-sults are based on oven dried wood. The conditions employed for the alkaline pulping treatments are those set forth in Table 1, supra, except in those cases where sodium cyanide was employed, where the following treatment condi-tions were used.
Liquor: Wood ratio 6:1 Temperature 75C
Time 42 minutes % NaCN on Wood (as HCN) 1.40 Befor-e adding the cyanide solution, the chips were steamed. After adding the cyanide solution, 150 psig.
nitrogen pressure was used to force the liquor into the chips.

Table 2, on the following page, provides the data obtained in Examples 1-36 in tabular form:

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Examplcs 37 throu~h 63 Several Or the foregoing exa~ples or cooks set rorth ln Table 2 above were combined and mlxed and divlded lnto a lar~e nu~ber Or samples. These samples were then sub~ected to oxygen-alkali dellgnlrication treatment employing the apparatus Or Fl~ure 1 and the conditions descrlbed hereinabove. In essence, pulp o~ a conslstency Or 3.3~ was placed in vessel (10), equipped with the oven mesh-basket (11). The alkaline liquor contalning 0.25 grams/liter alkali (expressed as NaOH) was clrculated constantly through the pulp and through the open mesh basket tll), through heat exchanger ~12) where its temperature was maintained constantly at about 130 C. The heated liquor was e~ected through nozzle (14) where oxygen was supplled rrom a constant supply at 150 psig. oxygen partial pressure. The alkaline liquor concentration was maintained at a constant level of about 0.25 grams o~ sodium hydroxide per liter, provided from reservoir t20~.
The purpose Or Examples 37 through 63 was to compare the results obtained with regard to yield and viscosity for pulps sub~ected to the alkall-oxygen delignification treatment whlch had prevlously been sub~ected to the cyanide alkaline cooking pretre~tment as agalnst those which had not received the ~.~;cnide pretreatment, but only an alkaline cooking pretreat-ment. The comparlson is made both on pulps which had rece~ved an alkallne cook by the soda process or the krart process.
The conditlons o~ the aikall-~xygen delignirication tre~t~ent and results are set rorth ln Table 3 below. This table also deslgnates the examples employed ln pro~lding the alkaline cook ; - pretrcatment.
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: 1050212 The yield benefi~s of the c~mbined cy~nide-alkaline cool~ pretreatment with the oxygen-alkali delignification treat-ments are set forth in Table 4 below. Employing conventional kraft cooked pulp as a control, it ls ap~arent that at, eit~.er Kappa No, level sho~m in the table, neither cyanide alkaline - cooX pretreatment alone nor oxygen-al~ali delignification treatment alone provides a ~ield increase as large 2S that - obtained by the combination of both cyanide alkaline cook pretreat~ent and oxygen-alkali delignification treatment of the present invention. It is also apparenkthat the yield increase possible by the process ~f the invention is greater than the æum of the yield increases possible from each treatment ' separately. This benefit is unexpected and unobvious and of - great economic imiortance to the paper industry.
A second unexpected benefit of the combined process 3f the present invention is the lignin-free viscosity of the.treated pulp versus pulps which had been treated solely by the soda cGoklng process followed by oxygen-alkali delignification, , compared at the sa~.e Kappa No. These results are also set forth 20 in Table 3s supra and are sh~nn graphically in Figure 2 of !he ''' drawings.
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The fore~oing ex~mples demonstrzte the superiority - Or yield and viscosity provided by the process of the in/cntion.
It should ~e reco~nized that the magnitude of the yield increases vver controls e~ployine the kraft process depends on cert~n f~ctors such as (1) the yield of the starting cyanide soda or cyanide kraf~ pulp and (2) the ~ethod of the oxygen-al~;ali delignification. ~ith regard to (1), if one starts with æ
10~Jer yield pulp prior to the oxygen-alkali delignification treatment, the yield will be lower than that obtained in the foregoing exa~ples. Conversely, higher yield pulp will provide even greater yield benefits. ~Jith regard to (2), more drastic oxygen-alkali delignification treatment conditions provide ~ le ser yield benefits. I
- The benefits of the processes of the invention with regard to yield are also sho~n by the graphs of Figures ~ and 4 o~ the drawings. Plotted on these graphs are the yields on the basis of oven dried wood employing the process of the inven-~~ tlon compared with the customary kraft and soda processes.Flgure 3 compares with the results obtained employing a cyan~de-soda alkaline cooking followed by the oxygen-alkali delignifica-tion treat~ent as against customary kraft and soda processes.
Fieure 4 makes a similar comparison of cyanide-kraft alkaline - cooked pulp which is sub~ected to the~ oxygen alkali delignifica-tion treatment as against ordinary kraft process pulps. As can be seen, the yields obtained by the present process are substan-tially higher.
- The terms and expressions which have been employed are used in terms of description and not o~ limitation, and there is no intention ln the use sf such terms and expressions of excludin~ any equivalents of the features sho~ ard described or portions thereof, but it is recognized that various ~ .

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Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows;

1. A method of increasing yields and improving viscosity in the pulping of cellulosic material, which method comprises treating cellulosic material in an alkaline aqueous solution having a pH of between about 7 and 13 and containing cyanide ions followed by a partial delignification cooking treatment at an elevated temperature of from about 160°C to about 185°C for a period of about 1 to 3 hours, employing an aqueous solution of sodium hydroxide or sodium hydroxide and sodium sulfide, followed by an additional delignification treatment employing an aqueous solution of sodium hydroxide in the presence of oxygen under pressure.

2. A method in accordance with Claim 1, wherein the cellulosic material is in the form of wood chips.

3. A method in accordance with Claim 1, wherein said treatment with cyanide ion is conducted at temperatures of at least about room temperature.

4. A method in accordance with Claim 1, 2 or 3, wherein the cyanide ion is obtained from sodium cyanide.

5. A method in accordance with Claim 1, 2 or 3, wherein the cyanide ion containing solution shall contain at least about 0.25% by weight of cyanide based on the moisture-free weight of the cellulosic material.

6. A method according to Claim 1, 2 or 3, wherein said additional delignification treatment is conducted at a temperature between about 90 and 140°C.

7. A method in accordance with Claim 1, 2 or 3, wherein said treatment with cyanide ion is conducted at a pH of between about 8 and 12.

8. A method in accordance with Claim 1, 2 or 3, wherein the cyanide ion containing solution contains between about 0.25% and 2% by weight of cyanide based on the moisture-free weight of the cellulosic material.

9. A method in accordance with Claim 1, 2 or 3, wherein said additional delignification treatment is conducted at a pulp consistency of between about 1 and 40% by weight.

10. A method in accordance with Claim 1, 2 or 3, wherein said additional delignification treatment is conducted at a pulp consistency of between about 1 and 10% by weight.

11. A method according to Claim 1, 2 or 3, wherein said additional delignification treatment is conducted at an oxygen partial pressure of between about 100 and 200 psig.

12. A method according to Claim 1, 2 or 3, wherein said additional delignification treatment is conducted in an alkali medium containing between about 0.1 and 10.0 grams per liter of sodium hydroxide.