AU427219B2 - Method of preparing woodpulp - Google Patents

Description

frmIt is known tha 0tIsposil to make cellulose pulp frmwood by digesting the wood with an alkaline liquid in the presence of oxygen. The oxygen serves as an oxidant, which attacks the lignin of the wood and converts it into products soluble in the alkaline liquid. This process, however, is very difficult to control, and norm- ally the wood material is pulped only superficially which results in a considerable amount of slivers and a non-uni- form pulp containing excessively degraded carbo-hydrate substances. Surprisingly, it has now been established that it is possible to control the process and reduce the amount of slivers as well as to increase the uniformity of the pulp and to Improve its color and strength properties, by pre- paring the digestion liquid in a manner such that the car- bon dioxide formed in the gas phase during the process is removed and utilized, Also, a saving of both alkali and oxygen is obtained. Accordingly, the present invention relates to a method of making wood pulp by treating wood, preferably as chips, shavings or particles, e.g. sawdust with an alkaline liquid containing sodium hydrogen carbonate or sodium car- bonate or both In the presence of oxygen under pressure, said method comprising treating the oxygen- containing gas used In the pulping process during- at-re-ast a part of the pulping process with .an alkaline -absorption liquid In a separate apparatus or absorption zone to absorb carbon di- oxide formed in the pulping process and using the absorption liquid as the digestion liquid In the same or a different puljing process or to prepare digestion liquid for said pulping process. 3 0,25 0 /71 The absorption liquid used in the process of the present invention may advantageously consist, wholly or in part, of an aqueous solution containing sodium carbonate, resulting from the combustion of waste liquor obtained from a wood pulping process according to the invention. Pre- ferably, the waste liquor from the pulping process will be concentrated by evaporation to a solids content above by weight and then subjected to combustion to form a com- bustion residue comprisihg substantially sodium carbonate.

It is also possible, in a manner known per se, to make use

of so-called wet combustion, i.e. a combustion process util-

izing oxygen or air under high pressure. In this case, the

evaporation process may be omitted in part or entirely.

In another embodiment of the present. invention

the absorption liquid comprises or consists of a solution

containing sodium carbonate resulting from the combustion of

a waste liquor obtained from a process wherein wood pulp is

treated with/alkali, with or without introduction of oxygen.

Examples of such waste liquors include waste liquors

obtained from hot alkali treatment of e.g. sulphite celThlose

pulp, or from cold alkali treatment of sulphite or sulphate

cellulose pulp, as well as waste liquors obtained from so-

called oxygen bleaching of chemical or semichemical wood

pulp, i.e. a bleaclhng step carried out in an alkaline fluid

in the presence of oxygen, preferably under elevated pressure.

In the case of such liquors, too, conventional combustion or

wet combustion may be used.

Mixtures of liquors or solutions containing sodium

carbonate obtained from..the combustion of different types. of

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3 0,25 0 [71 waste liquors may be used. This results in a low chemical consumption and also minimizes the discharge of waste mat- erials from the plant to the environment. The method of this invention can be used to advant- age to make so-called chemical pulp, i.e. a pulp of low lignin content, which is readily defibrated, e.g. pulp which is d~fibrated merely by blowing It from the digestion vessel or by a mild mechanical treatment, as well as to make so- called semichemical pulp, i.e. a pulp which to liberate the fibers requires forceful mechanical treatment, e.g. In refiners or other defibrating equipment. The chemical reactions taking place when wood is

pulped with alkali in the presence of oxygen are not fully understood. Arart from the reactions taking place In the presence of alkali, oxidation reactions obviously occur to a considerable extent. Experiments have shown that the dissolution of the lignin as well as the yield of pulp and the degradation of the cellulose are influenced considerably by the composition of the digestion liquid as well as by the temperature and the oxygen pressure. At present, no certain theoretical explanation of the favourable results obtained according to the present invention can be given. It will be noted, however, that the considerable advantages obtained are due not only to the fact that removal of the carbon dioxide -increases the partial pressure of the oxygen. Practical tests have shown that advantages are obtained also with regard to the formation of slivers. The process is more readily controlled, possibly due to the fact that the alkalinity and the buffer-Ing capacity of the solution can be controlled In a simple manner throughout the pulping

4272 19 process. 301250 /71 The bsorption liquid used according to the invent- ion may be prepared by dissolving in water sodium carbonate or combustion residues comprising essentially sodium car- bonate. With regard to the economy of the process it Is suitable to replace the water, wholly or in part, by a liquid containing sodium salts and organic substance. Such liquid may comprise waste liquor, liquor withdrawn during the.process and/or washing liquids derived from alkaline treatment of cellulosic materials, e.g. liquids derived from the process of this invention, from alkaline refinement of cellulose and/or from the bleaching of wood pulp. The same llqiids may be supplied direct to the pulping process, e.g. by diluting the absorption liquid, before its use as a digestion liquid, by liquids of the types referred to. It has been found that the carbo-hydrates of the wood are protected by carrying out the process in the pres- ence of magnesium ions, while the dissolution of the lignin Is influenced only slightly by the addition of magnesium compounds. The addition results In an increase in the yield of carbo-hydrates and in a reduced degradation of the cellulose, which is manifested in improved strength prop- erties of paper produced from the pulp. Particularly sur- prising is the positive effect obtained by the presence of magnesium ions, even when the pulping process is carried out at a low pH value, namely .:.;thin.the pH range of 7 to 9, I.e. a range which is lower than that used in so-called oxygen bleaching where the action of magnesium compounds as inhibitors is previously known. As a source of magnesium Ions It is possible to use soluble magnesium salts, e.g.

5 4 27 2 magnesium~ supae 0,25 0 171 magesim slphtesparingly soluble magnesium compounds, e.g. magnesium carbonate, 'oxide and hydroxide, and complex magnesium compounds, includng magnesium complexes of organic acids, such as hydroxycarboxylic acids, e.g. glycollic. acid or dihydroxy butyric, acids, or dicarboxylic acids, e.g. oxalic acid or tartaric acid, or other magnesium complex compounds, e.g. complexes of polyphosphoric acids.- It is

In particular advantageous to dissolve magnesium compounds in waste liquor derived from an alkaline treatment of cell- ulosic material or In liquor withdrawn from such a treat- ment, e.g. In liquor obtained from the process accordirg to this invention, and to supply the liquor containing the magnesium Ions to the pulping process. A* During the pulping process/e11 pH value can be varied within a wide range depending on the desired type of pulp to be produced. Thus, the pH value during a

part of the process maybe as high as, 10 and during

another part It may be allowed to drop to about pH 7. For

some types of pulp It may be suitable to keep the pH of the digesting liquid at or below about 9. In such a case

the active alkali consists advantageously of sodium hydrogen carbonate obtained as a solution in the absorption liquid

according to the Invention. Thus, the invention permits an

Inexpensive preparation of pulping liquid of a suitable pH value and a simple control of the pH value during the process by supplying fresh pulping liquid and by controlling the carbon dioxide partial pressure In the pulping vessel. Regardless of how the process Is carried out, it is important to ensure an intbiate contact between the oxygen containing gas and the digestion liquid. Suitable measures

3 0,25 0 r71 to ensure this result comprise atomization of the digestion liquid in the~ps phase at the top of the digesting vessel by means of nozzles or other known devices and so-called gas phase digestion, in which the digestion liquid Is sprayed onto the wood material during a part of or the entire dig- estion period. It may also be suitable to disperse-the freshly supplied and/or circulating oxygen-contalfting gas in the digestion liquid, which may be done in the digestion vessel and/or in a separate branch circuit leading to the vessel, which circuit may include a container or other device through which the digestion liquid flows to be intimately contacted therein with the oxygen-containing gas. The following examples illustrate the invention as applied to batchwise digestion of the wood, I.e. under concdtions where the wood material is stationary throughout

the digestion period. In all examples the total amount of active alkali was supplied at the start of the digestion as sodium hydrogen carbonate. Batchwise digestion according to this invention may also be carried out In such a manner that during the digestion process the digestion liquid is periodically supplied with fresh active alkali added In the form of sodium hydrogen carbonate, sodium carbonate or sodium hydroxide or a combination thereof. This procedure is pre- ferable when the chemicals to be supplied include sodium carbonate and/or sodium hydroxide. It is also possible, during the digestion process to withdraw digestion liquid which may be supplied to some other digester or to the same digester In a subsequent digestion process. The Invention may also to advantage be applied to continuous pulping of wood material, wherein wood is

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3 02 5 0-/7 continuously fed to the digester and pulp continuously withdrawn therefrom by means of suitable devices, e.g. such as are known from continuous sulphate pulping processes. It will also be understood that In plants comn- prising several digestion vessels ("digesters") the means for absorbing carbon dioxide may be common to a number off digesters or to all the digesters of the plant. As will be apparent from the following examples, the resulting wood pulp has a high brightness value. It can

be used as such, without any bleaching step,. to make paper, cardboard or similar products, In cases where the require-

ments on brightness are not very high. If desired, however, the wood pulp obtained by the process of this invention may be subjected to a bleaching step to further enhance its brightness, e.g. with the aid of well known bleaching agents, such as chlorine, hypochiorite, chlorite, chlorine dioxide, peroxide, peracetate and/or oxygen in accordance with known bleaching methods. A particularly suitable bleaching sche- dule Includes bleaching with chlorine dioxide (with or without chlorine), extraction with alkali and bleaching iUth chlorine dioxide. The Invention is illustrated but not limited by the following examples, wherein parts and percentages are by weight. Example 1 A digestion vessel A (see Fig.' 1) was charged with 100 parts of birch chips whi ch were placed on a perforated bottom. The chip dimensions were approximately 3 x 6 x mm. At the bottom of the digestion vessel 500 parts of digestion liquid of 60 0 C were Introduced, said liquid being

-8-427.219-

3 02 50 in prepared from 450 parts of aqueous sodium hydrogen carbon- ate solution and .50 parts of spent liquor from a previous digestion process to which has been added a quantity of magnesium sulphate such that the amount of magnesium (cal- culated as 14g0) corresponded to 0.2% based on the dry wood. The introduction of digestion liquid was interrupted when the chip bed was covered by the liquid. After an'impreg- nation period of 30 minutes, digestion liquid was drawn off to lower the liquid level below the chip bed. The quantity of sodium hydrogen carbonate In the liquid remaining after the withdrawal corresponded to 20% of NaHCO based on the 3 weight of dry wood. The withdrawn liquid was used to pre- pare digestion liquid for a subsequent digestion. The system was then put under an oxygen pressure of 10 bars. This pressure was maintained during the process by supplying more oxygen, as required. The digestion liquid was circulatedly means of the circulation pump B from the bottom of the digestion vessel tn its top, the liquid pass- Ing through a heat exchanger C in which the liquid was heated to 120 0C, for one hour. At thestop of the digester, the circulation conduit was provided with atomizing devices, comprising nozzles atomizing the liquid to provide a large contact area with the oxygen, whereby dissolution of oxygen In the liquid was facilitated. During the process the- oxygen-containing digestion liquid passed over the chips so that they were In contact with a film Pf di gestion liquid which was progressively renewed In the course of the dig- estion process. A high rate of circulation of the digestion liquid promotes the pulping of-the wood. During the process, oxygen was consumed and at the

9 4 272 19 same time carbon dioxide 2e5olv/71 0,s The oxygen admixe d with carbon dioxide was circulated between the digestion vessel A and the absorption apparatus D by the aid of the circulating means E, which may be a blower-or an ejector device, or other known apparatus for conveyinig gases under pressure. In the absorption apparatus, which in this example was a pressure scrubber, but may be any known type of -absorption apparatus, an absorption liquid contsaning sodium carbonate and prepared by clssolving combustion residues from the combustion of waste liquor from the pro- cess, was introduced at F. The absorption liquid was atom- ized by means of nozzles In the absorption apparatus so that an efficient absorption of carbon dioxide was obtained with simultaneous formation of sodium hydrogen carbonate. The absorption liquid was circulated in the embodiment shown In the Figure by a pump G and a solution containing mainly sodium hydrogen carbonate was withdrawn from the absorption apparatus and used to prepare fresh digestion liquid. As the hydrogen carbonate was consumed during the digestion process fresh sodium hydrogen carbonate was added, so that the total charge amounted to 50% based on the dry wood. When the temperature of the. digestion liquid had been maintained at 12000C for 24~ hours, the pulping was completed. The oxygen was collected to be used for a sub- quent digestion -process and the digester was emptied, e by blowing the pulp.- In experiments carried out as set forth above, a pulp yield of 60% was obtained (dry pulp based on dry wood). The Kappa number of the pulp was 15, and its viscosity according to TAPPI was 40. The pulp brightness, measured accordin [O25 71 accodingto SCAN, was 62. Exam-ple 2 The procedure of Example 1 was repeated except that the temperature was 141000C instead of 120 OC and the time 6 hours instead of 24 hours. The pulp yield was .58%, the brightness of the pulp according to SCAN was 58 and its Kappa number was 21. Example 3 The procedure of Example 1 was repeated using a temperature of 148 0 C and a digestion period of 6 hours, resiting in a pulp yield of 55% and a brightness of 66. The

Kappa niumber was 9. Example 4j A digestion experiment carried out in the same

manner as In Example 3, but with no addition of magnesium sulphate gave a pulp yield of 5;0.5% and a brightness of The Kappa number of the pulp was 'Example A digestion vessel was charged with 100 parts of saw dust from birch and 1500 parts of cooking liquor con- taining 1000 parts of spent liquor from an earlier digest- ion carried out under the same conditions and 500 parts of wash water obtained from washing of the pulp produced in the earlier digestion. The cooking liquor which contained g per litre of sodium hydrogen carbonate was heatr, from 0C to 114.00C in one The temperature, was kept at

14.0PC for 6 hours to complete the cooking. Oxygen of 6 bar was introduced at the bottom of the digestion vessel and passed through the suspension from the beginning of the heating period to the end of the cooking. A side stream of

3 0,25 01~71 carbon dioxide containing oxygen was passed from the top of the digestion vessel'through a pressure scrubber In which the gas was washed continuously with sodium car- bonate solution to remove carbon dioxide and to obtain a sodium hydrogen carbonate solution saturated to about with sodium hydrogen carbonate solution at 950 C. The purified oxygen from the pressure scrubber was then returned to the bottom of the digestion vessel via a compressor. The sodium hydrogen carbonate content in the cooking liquor was determined analyticall y during the heating period and during the first 5 hours at the final temperature level and the concentration was kept at 8 12 g per litre by introducing nearly saturated sodium hydrogen carbonate solution into the digestion vessel. The sodium hydrogen carbonate solution was obtained from the pressure

scrubber where it was prepared from carbon dioxide evolved during the digestion and sodium carbonate produced by burning' the spent liquor after previous evaporation. After 6 hours at 1 4~0 0C the pressure was released and the pulp suspension was blown out and washed on filters to obtain spent liquor for burning and evaporation and dil- uted liquor which was used to prepare cooking liquor for a subsequent cooking. The pulp yield was.53% calculated as dry pulp on dry wood. The Kappa number of the pulp was 19 and the pulp brightness, determined according to SCAN, was

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Claims (1)

1. 3 0.2 5 0 il The claims defining the invention are as follows:-

   1. A method of making wood pulp by treating wood, ~roforal~ chps. Xvngs Grpa rtol with an alkaline liquid containing sodium hydrogen carbonate and/or sodium carbonate in the presence of oxygen containing gas under pressure, characterized by treating the oxygen containing gas used in the digestion process, during a part of or the entire digestion period, with an alkaline absorption liquid in a separate apparatus or absorption zone to absorb carbon dioxide formed in the digestion process and using the absorption liquid as digestion liquid In the same or another digestion process or-to prepare digestion liquid for such digestion process.

   2. -A method as claimed In claim 1 In which the absorption liquid comprises an aqueous solution containing sodium carbonate obtained from the combustion of spent digestion liquid from the digestion of wood by said dig- estion process.

   3. A method as claimed -in claim 1 in which the absorption liquid -comprises a solution containing sodium carbonate derived from the combustion ofl spent liqu~or from an alkaline treatment of wood pulp -with or without addition of oxygen.

   A method as claimed In any one of the preceding claims In which the absorption liquid comprises spent liquor, withdrawn liquor and/or wash liquid from an alkaline treatment

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   3 0,25 0 71, of cellulosic material.

   A method as claimed in any one of the preceding claims, in which the digestion process is carried out in the presence of added magnesium ions.

   6. A method as claimed in any one of claims 1 to in which the wood to be treated is in the form of chips, shavings or particles.

   7. A method as claimed in claim 4 in which the

   absorption liquid is spent liquor from a digestion process

   in accordance with claim 1 or from alkaline refining and/or

   bleaching of wood pulp.

   DATED this llth day of August, 1972.

   MO OCH DOMSJO AKTIEBOLAG

   -i c- Edwd. Waters Sons, Patent Attorneys, i 30 Russell Street, C I f MELBOURNE.. VIC.: AUST. t

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