CA1082859A - Pulping process using oxygen and sodium tetraborate - Google Patents

Abstract

PULPING PROCESS USING OXYGEN AND SODIUM TETRABORATE

ABSTRACT OF THE DISCLOSURE

A process of oxygen pulping is disclosed wherein cellulosic material is digested with oxygen in the presence of borax in an amount between 12% and 22%, as Na2O on wood, at a temperature between on 120° and 155°C.

Description

108'~859 The present invention relates to the making of cellulo8ic pulp8, more particularly to the pulping of cellu-lc)8ic raw material by mean~ of oxygen in the presence of s~ium t~traborate.

S PRIOR_ART
The pulping of wood and other cellulosic raw mate-rial~ by ~eans of oxygen has been the sub~ect of considerable research in recent years not leaJt because of the environmen-tal advantages a pulping process based e~sentially on oxygen and alkali would represent in comparison with exi~ting sulfur using proce~se~. Various proposals have been made as to methods and conditions sultable for the carrying out of uch pulping. Thu~ in 1954 Harri~, U.S. Patent 2,673,1~8 disclo-~ed pulping a cellulosic raw material with oxygen at a p~ 7 -lS 9 u~ing a olution o~ odium blcarbonate or pho-phate or tetra-borate a~ a buffer, at a temperature between 125 - 175& , with oxygen pre~sure~ at least 800 p~ ater Grangaard and 8aunder~, Can. Patent 611,503 di~clo~ed pulping with oxygen at a p~ 7 - 9 in an alkaline solution with the solution buffer~d, e.g. ~ith ~odium bicarbonate. Landucci et al (Tappi, October 1974~ inve~tigated delignification of fiberi~ed wood with molecular oxygen in a mildly al~aline ~ystem u~ing, a Ag others, borate & boxic acid as a buffer. Mlnor and Sanyer ~Tappi, Hay 1974) ~tudled oxygen pulping using buffe-red systems, with very high chargos of borax or the orderof 32~ Na20 on wood ~16% ~a~ed on one active sodium per formula weight of borax). Inve~tigatio~s on oxygen pulping of ~pruce wafer~, using as alkali sodium hydroxide, sodium bicarbonate, sodium carbonate, ~odium tetraborate and ammonium hydroxide, were di~clo~ed by J.J. ~enard et al ~Tr3nsactions of the Technical Section C.P.P.A., ~ol. l, 1(~8Z8S9 Nol, March 1975). The investigator~ concluded that on spruce wafers ~optimum r~sult6 were obtained for a 9 -11~ charge of alkali on wood, expres~ed as Na20, irre~pective o~ the nature of the anion~.
BRIEF DESCRIP'rION OF T}~E INVENTION
I havo found, unexpectedly, that part~cularly high yield~ of chemical pulp are obtained by pulping in a ~ingle ~tage with oxygen in the presenee of sodium tetraborate (borax) as the alkaline reagent, when the charge of borax (as Na20 on o.d. wood) i8 bet~een 12% and 22% a~d the cooking temperature is not higher than 155C. Not only the yield, but also the properties of the pulp, notably viscosity, are improved when pulping i- carried out in the~e condition~, ;
and the pulp- are obtained in co~rcially acceptable oooking tlm~-.
Accordingly, the prosent invention providos a method of preparing a chomical pulp from oellulosic raw mate-rial, wherein said raw material in frag~ented form is treated in an aquoous solution of borax with oxygen under pressure at a temp~rature bet~een 120 and 155& , the charge of borax being bet~een 12~ and 22%, as Na20, relative to the raw materlal.
T~E DRA~INGS
The invention will be described in conjunction with the drawings the ~ingle Figure of which shows a compa-rison of the yields obta~ned on the same wood and in identical conditions u~ing as alkali reagent, respectively, 80dium carbonate and scdiu~ borate and varying the charge of alkali on wood.
DETAII.ED DESCRIPTION OF T~E INVENTION
In acoordance with the process of the invent~on ~e cellulosic raw material, such a~ wood chip~, flax, ortho like, i8 mixed with the borax solution in a pre~sure ~ssel to which oxygen or an oxygen - containing gas i~
~dmltted under presJure. 80dium tetraborate is readlly ~oluble in watsr at the temperatures w ed for cooklng and solutions of de~irable concentration can be eas~ly prepared.
As indicated, the charge of borax on the cellulosic raw ~aterial i8 an important feature of the invention and will be between 12% and 22%, the optimum charge of borax within this range varying ~omewhat depending on the raw material, e.g. being somewhat greater for ~oftwood than for hardwood. With the usual liquor to wood ratios employed in pulping being, e.g. betwee~ 4:1 and 15:1, the range of sodium tetraborate solutions will thus be approxlmately betweon 26 and 178 g/l. The borax 1~ b-lng u~ed up a~ the dige~tion proceed and thi- i- indicated by a gradual drop in the pH of the mixture from an initlal p~ of 9 to a final 6-7.5.
The mixture of wood and liquor is maintained in a ~tate of thorough agitation whlle it i8 expo~ed to oxygen gas under pressure. Variou~ techniques have be~n developed to facilitate the contact between the ~ood and the liquid and gas, ~o as to a~sure optimum penetration of the oxygen into the wood. For example, the mixture may be agltat~d by 8uitable means in the reactor, for example by means of a rot~ting digester. Alternatively, the mixture m~ybe with-drawn from the lower end of the reactor and returned through a recirculation line to the other end of the reactor where it will oome into intimate contact with the oxygen gas. Alternatively, only the liquor i8 80 withdraw~ and returned to the reactor through the recircu-lation line. It i~ generally believed that the oxygen enters the wood via th~ liquor hence che advantage of having condltion~ ~u¢h as will favour the dissolution or di.sp~rsion of the oxygen in the liquor. Ox~gen prea~ure iEI, of cour~e, a major factor determining the amount of oa~ygen dissolved and the rate of penetration into the wcK~d. Pressures of the order of 100 - 600 p~i constitute a practical range, but pressures both higher and lower can be used. Oxygen may be used as such; but mixtures of oxygen with other gases, sxemplified by air, may also be u~d with suitably increased pressures to ensuro that the partial pressure of oxygen iB within ~he operabl~
range.
The temperature influences the penetration of oxygen into the wood and the rate of reaction and hence is known to affect both the time of the ¢ook and the properties of the pulp, but it has not hltherto been ¢onsidered 1~ fac-tor affecting yield. I have found that optin~m yielas are obtained with a charge of borax ~ herein provided at tempe-ratures not higher than 155& and preferably between 120 and 150C.
Uniform penetration of the liquid and dissolv~d ga~ throughout the fragments of wood or like material has long been a problem in oxygen pulping particularly when conventional chips are used. If penetration is not uniform the ~urface portions of the chips generally become overcooked while the inner core remains substantially underoooked, giving simultaneously a high rejects level and a weakened pulp. UniforMity of penetration is improved by making the wood frag~ents relatively thin, preferably wafer s}laped, thus increasing substantially the area of contact and decrea-sing t~e depth to which penetration must occur. Use of low temperature and correspondingly longer cooking time also has a baneficial effect on reducing the level of rejects. The r~action of oxygen with the conJtituents of the wood results in carbon dioxide being generated and ~hile some of it remain~ in ~olution in the liquor or is combined with the 5 alkali present, sub~tantial quantitios of Co2 can accumulate in the gaseous atmos p ere having a diluting effect on the oxygen. Techniques have been developed for continuous or intermittent removal ~bleeding) of a portion of the ga~
present in the d~gester, the gas thus removed being then purged of its C0~ content (e.g. by passing through an alka-line ~olution) and returned to the reactor. These techniques may be used in the practice of this invention, however, they do not form a necessary part of it.
As will be ~een from Figure 1 oxygen pulping in the presence of borax result~ in highor yield~ at a given Kappa numbor compared with oxyg~n pulping in the pre~ence of sodium carbonate, provided the charge of the chemical on wood i8 greater than 11~. When sodium carbonate is w ed, a charge of 11% or les~ appears to provide optimum yield~t increasing the charge results in lower yields. When sodium tetraborate is used, the maximum yield and optimum vi~cosity, at commercially reasonable cooking times, is achiev~d, in the case of hardwood, with a chemical charge between 12%
and 18% (as Na20 on ~ood), and in the case of softwoods, with a borax charge between about 16% and 22%. Generally a charge between 12~ and 22% of borax on o.d. wood will be used. The yieldb obtained according to the invsntion at Kappa numbers usual for a chemical pulp, e.g. about 15 for a hardwood pulp and about 25 for a softwoDd pulp, are of the order of 58 - 59%, with minimal amounts of rejects.
The ~trength propsrti~s of th~ pulp8 obtained are substan-tially identical with those of oxygen pulp8 prepared using other alkali~ such as sodium carbonate.
The oellulo9ic raw material used will b~ primari-ly wood chips, preferably of a reduc~d thicknes~, such as wafers of a thi¢kness of 1.5 mm or le~s, or conventional chlp~ sllced, crushed or otherwl~e broken down to a thickness of the same order. Other cellulose ra~ materlals, such as flax, bagasse and the like, are also suitable. The term borax used herein stands for ~odium tetraborate and denotes the latter compound either in its coDmercial sr chemically pure form. Solutions of a composition substan-tially equivalent to sodium tetraborate can be prepared by mixing boric acid or sodium pentaborate with an alkali such ~ sodium hydroxide or ~odium carbonate, or by mixing sodium metaborate with borlc acid. Mixture~ of borax with minor ~mount8 of other boron compoundJ or with alkali- ~uch a~ sodiu~ bicarbonate, sodium carbonate or sodium hydroxide can also be used without ma~or change in the cooking results, provided the total chemical charge should be maintained in the range 12%-22% Na20 on wood. Potassium borate can also be used instead of sodium borate, but for economic reasons the sodium compound is preferred.
The chemical used in the pulping process can be reoovered, preferably by use of a wet oxidation technique to destroy the dissolved organic matter and ~imultaneously regene-rate the sodium tetraborate.
The following examples will further illustrate the invention but are not to be interpreted as limiting the inven-tion to the specific embodiment~ disclosed.
Exa~ple I
A. Poplar wafer~ of 1.0 mm thickness having an oven dry weight of 91.2 gm were placed into a chip basket in a one-gallon pressure reactor with 43.0 gm of sodium tetra-lOBZ859 borate ~Na2s407) and sufficient water to g~ve a lo:l liquor to wood r~tio. The alkali ch~rge was therefore 14.5 calculated as Na20 on o. d. wood. In separate runs, the alkali dosage was varied a~ indicated in Table I below, the charge belng expressed as per cent Na20 on o.d. wood. The reactor was heated using an electric heating ~acket to 145C
in 60 minutes, and pressurized oxygen was introduced to the reactor when the temperature reached 130C. The pres~ure at 145C ~including ~team pre~ure and oxygen pres~ure) was 300 psig. The oxygen wa~ continuously ~ixed with the oooking liguor by means of internal agitation of the liquor wi~hin the reactor. More oxygen was introduced to the reactor during the oooking period to compensate for the oxygen consumed and to maintain the pre~sure at 300 p8ig. No ga~ was bled from the reactor during the oooking perlod.
The temperature of l~s& wa~ malntalned for a time sufficient to give a Xappa number of about 13, and at the end of the cooking period the reactor was quickly cooled and the pressure reliev~d. The p~ of th- oooking liquor wa~ then measured after being allowed to cool to room temperature.
The cooked material was disintegrated in a conw ntional manner.
The level of rejects was negligible in all cases.
The results of thooe cooks obtained with the various borax charges are shown in series A of Table I. All the result~ refer to pulp8 of Kappa number 13.
B. Poplar wafers of the same origin and size were cooked in an identical manner as abovQ except that ~odium carbonate was used as the alkali charge and the charge was varied over ~ub~tantially the ~ame range as the borax in series A above; and except further th~t, in order to obtain pulp8 of the same Kappa number of about 13 as before, it wa~ necessary to increase the cooking time co~pared with 108285g tho times in ~eries A.
The results of those cooks with various charges of ~odium carbonate are shown in series B of Table ~. A~
in series A, the results refor to pulp8 of Kappa number 13.
C. For further comp~r~son the ~ame poplar wafers were cooksd using the conventional kraft proce~ undor ~ui-t~ble conditions to obtain a pulp of approximately ~e same degree of delignification (as indicated by the Xappa number) as the pulp8 in A and ~. This entailed cooking ~ith 16%
active alkali ~as Na20) on wood, with a 25% sulfldity llquor, at a liquor to wood ratio of ~bout 5:1, by heating th~
reactor contents to a temperature of 173C in 60 mlnutes and maintaining the ~ame temperature for 95 minute-. The results are shown in section C of Table I. The Kappa nu~ber of this pulp wa~ 15.8.

TABLE I

Charge of alkali as ~appi Run Na2O on Time Final visoosity Brightness 20 No. ~oad ~%) (Min) Yield p~~Cp8) ~Elrepho) Series A ~Borax as alkali charge in oxyqen cook) 1957.7 360 56.5 5.5 8.9 19711.0 200 57.6 6.412.3 58.5 19114.5 165 59.1 6.912.0 57.0 19018.1 145 58.3 7.210.6 58.0 19224.7 125 56.9 7.510.0 59.1 19330.8 110 56.1 8.3 8.6 57.7 19437.0 100 56.5 8.2 8.2 58.6 Chargs of alkall as Tappi R~ Na o on Time Final vi~cosity Brightne88 No wo~d ~) (Min) Yield p~ ~cps) (Elr~pho) 8erie~ B (Sodium carbonate as alkali charge in oxygen cook) 2t\2 11.0 280 56.8 5 2 8 7 50.4

2~1 14 5 230 55~7 7 4 9 3 53 5 1~8 18 1 210 56 2 7 4 7 7 55 3 200 30 8 140 48 0 8 2 4 9 s3.s Series C (Kraft) 21316.0 95 50~6 - 30 2 31 1 It will be evident that when borax i8 used as the alkali charge an optimum yield at the desired degree of deli-gnification i~ obtained when the charge on wood 18 betweenabout 12~ and 22~ In thi~ range of alkali charge th-re i~
a yield advantage of about 2 - 3 percentage point~ over the u e of carbonate in substantially identical conditions ~he comparative re~ults as to yield are shown graphically in Figure 1. Similarly, in thi~ range of alkali charge, there i8 a visco~ity advantage of about 3 cps and a brightness advantage of about 4 percentage point~, as well a~ a substan-tial advantage in cooking time.
Example 2 Spruce wafers of 0 8 mm thickness were oooked by the method described in Example IA with a charge of borax on wood varying as ahown in Table II The cooking temperature was, as before, 145& and total pressure in the reactor ~oxygen and water v~pour) 300 p~i To obtain the de~ired degree of delignific~tion somewhat longer times were needed in this case oompared to hardwood in the previous example, and the oooking times were adjusted for each alkali charge ~08Z859 to obtain the ~ame xappa number of about 25 ~which corres-pands to a cw tomary degree of chemical deligniflcation in the case of softwood pulp~).
For comparison the same spruce wafer~ were oooked uslng the conventional Xraft process, with 18% active alkali on wood, 25% sulfidity in the cooking liquor, and a h~ating schedule of 60 minute~ to a temperature of 173C and 95 minutes at temperature.
The results are shown in Table II. All the result~
refer to pulps of Kæppa number 25 except for the kraft pulp wh~ch was Xappa No. 29.~. The reject~ level for all c~oks was negligible, Table_II
Charge of alkali as Tappi Run Na 0 on Time Final visco~ity Brightne~
No. wo~d ~%) (Min) Yield pH ~Cp8) (~lre~ho) Series A ~Borax as alkali charge in oxygen oook) 2157.7 390 56.9 4.9 9.4 33.5 21211.0 285 59.2 6.2 14.0 33.7 21114.5 235 59.1 6.8 12.7 38.8 20618.1 158 59.2 7.5 12.7 40.2 21024.7 150 57.6 7.8 11.0 39.4 20930.8 135 57.6 7.8 9.8 39.1 20839.2 130 55.6 8.1 7.7 40.9 Series B (Kraft) 21418.0 95 47.5 - 23.9 32.2 Exam~le 3 Spruce wafers of 0.8 mm thicknes~ were cooked by the method described in Example IA with a charge of borax of 12% as Na20 on wood, a liguor to wood ratio of lO:lt at a .

1~8Z859 cooking temperature of 145C, and at a pressure, respective-ly~, of 300 p~i and 600 psi. The re~ults are shown in T~ble III series A. As can be seen, the variation in pressure h~d little effect on yield ~allowing for the Kappa number) but a higher pressure favour~ viscosity and brightness. For comparison, cooks were carried out with a charge of 11~ Na2O
at 160C and at a pressure of 600 ps~. The result~ are shown in Table III B. It can be seen that with the higher tempe-rature of cooking and a slightly lower alkali charge, the yield and viscosity are lower when compared at similar Rappa number.

Table III
Series A. (Temperature 145C) Run No. 258 261 263 Charge of alkali (as ~
Na2O on wood) 12 12 12 Pre~sure ~psi) 300 300 600 Time (min) 270 255 225 Yield ~%) 56.0 56.6 54.5 Kappa No. 16.6 23.9 11.5 Tappi viscosity (cps) 8.3 10.8 8.5 Brightness (Elrepho) 38.1 35.8 48.2 Series B.(Temperature 160C) Run No. 257 260 Charge of alkali (a~ ~
Na2O on wood) 11 11 Pre~ure (psi) 600 600 Time (min) 90 70 Yield (~) 49.2 53.1 Kappa No. 7.8 18.9 Tappi viscosity ~cps) 4.7 7.7 Brightness (Elrepho) 49.6 44.4 xamPle 4 A. Birch wafer3 of 1.5 mm thickness were diges-ted with oxygen in the presence of borax in the same manner a~ in Example lA in a 1 gallon pressure reactor. The charge of aodium tetraborate ~as Na20 on oven dried wood) wa8 18.1%, and the liguor to wood ratio wa~ 6:1. The reactor was main-tained at a total pres~ure ~of oxygen and steam) of 300 p~i and a tempsrature of 140& for 195 min., but the gas was continuously bled off and fresh oxygen was added to the reactor. Agitation was provided for the liquor within the ve~sel. The pulp was beaten in a PFI mill and formed into hand~heets. The results of the cook and the properties of the pulp are shown in Table IV, under A.
B. Birch wafers identical to those used in the present Example under A were cooked by the Xraft proce~s, with an active alkali charge ~on oven dried wood) of 16%
with a liquor of 25% sulfidity and with a liquor to wood ratio of 3.6. The heating to te~perature required 60 minutes and the temperature of 173 was ~aintained therea~ter for 90 minutes. The pulp was beaten and formed into handsheets and the properties measured as under A.
The results of thi~ kraft cook and the propertie~
of the pulp are ~hown in Table IV, under B.

T le IV
A B
(Borax oxygen)(Krat) Screened yield (%) 60.8 52.5 Kappa No. 14.6 17.2 Viscosity ~cps) 17.7 25.2 Brightnes~ (Elrepho) 62.4 25.7 Beating Revolutions 1500 3500 Can. Std. Freenea~ 411 404 Bulk cm3/g 1.38 1.35 Burst Factor 53 56 Te~r Factor 55 76 5 Te~8ile B.L.(km) 10 9.2 Elongation ~%) 2.4 2.5 M.I.T. Folds 200 208 ~xample 5 Decort~cated flax was cooked using the method of 10Example 1, with a charge of borax of 18.1% (as Na20 on o.d.
flax) and a liquor to wood ratio of 12.7: 1. The cooking conditions and re~ults are ahown in Table V under A. In the same table, under B, a compari~on is made with a labora-tory cook using a conventional soda-sulfur method.

15Table V
A B
(Borax-oxygen) (Soda-sulfur) Run No. 167 6532 Charge of alkali 18.1% 18.8% alkali 20 (% Na20 on wood) 4% sulfur Pressure (p~i) 300 60 Temperature ~C) 140 155 Time (min) 120 180 ~ield (%) 64.1 56.5 Kappa No. 9.4 20.1 Tappi viscosity (Cp9) 21.1 Brightness (Elrepho) 52.7 It will be apparent that pulping with oxygen in the presence of borax in accordance with the invention preqent~

- ~4 -~stantial advantage~ iA ter~s of yield of pulp and proper-tiLes of the pulp obtained. It i~ to be understood ~hat change~ and variations may be made without departing from the ~cope of the inventlon as defined in the appended claims.

Claims (10)

WHAT I CLAIM IS:

1. A method of producing a cheimcal pulp from cellulosic raw material comprising treating said raw material in fragmented form in an aqueous solution of sodium tetraborate with oxygen-containing gas under pressure at a temperature between 120°C and 155°C, the charge of said sodium tetrabo-rate as Na2O on said raw material being between 12% and 22%.

2. The method of Claim 1 wherein the combined pressure of steam and oxygen in said oxygen-containing gas is between 100 and 600 psi.

3. The method of claim 1 wherein said temperature is bet-ween 140°C and 150°C.

4. The method of Claim 1 wherein said raw material consists of wood chips of a thickness not greater than 1.5 mm.

5. The method of claims 1 or 2 or 4 wherein said raw material is hardwood chips and the charge of said sodium tetraborate is between 12 and 18%, as Na2O on wood.

6. The method of claims 1, 2 or 4 wherein said raw material is softwood chips and the charge of said sodium tetraborate is between 16% and 22%, as Na2O on wood.

7. The method of claim 1 wherein said raw material is fragmented non-wood cellulosic material.

8. The method of Claim 7 wherein said raw material is decorticated flax or bagasse.

9. The method of claims 1, 2 or 3 wherein said oxygen-con-taining gas is oxygen.

10. The method of claims 1, 2 or 3 wherein said oxygen-con-taining gas is air.