US1977879A - Process of manufacturing sulphite pulp - Google Patents

Description

Patented Oct. 23, 1934 PROCESS OF MANUFACTURING SULPHITE PULP Ernst Hochberger, Hawkesbury, Ontario, Canada, assignor to Canadianlnternational Paper Company, Montreal, Quebec, Canada No Drawing. Application June 3, 1933, Serial No. 674,223. In Canada July 4, 1932 4 Claims.

This invention relates to the manufacturing of sulphite pulp, a process which, incomparison with known methods, produces substantial improvements from the point ofview of economy 5 and the quality of the finished pulp, and which more particularly permits of better adjustment and control of cooking conditions than any of the processes now in use.

In the-manufacture of sulphite pulp, the process as generally applied at present involves cooking of chipped wood under pressure and at elevated temperature with a solution containing calcium bisulphite or a mixture of both calcium and magnesium bisulphite and a comparatively large excess of free sulphurous acid. Cooking by means of direct or indirect heating is continued until fibre liberation is attained. This procedure has many disadvantages from a chemical point of view as well as from the viewpoint of economy on raw 0 materials and heat. In cooking, the first step of the'many reactions occurring and finally resulting in fibre liberation, is penetration and impregnation of the chips by the cooking liquor. The next step is characterized as the sulphonation of '25 the lignin by means of the bisulphite ions of the cooking liquor and the iormation of insoluble lime or magnesia salts of ligno-sulphonic acid. 'The third step is the hydrolysis of these compounds by the hydrogen. ions of the cooking liquor and their decomposition into compounds of smaller molecular size, which are soluble in the cooking liquor. Thus, the incrusting material of the wood is removed and the cellulose fibres areliberated. The speed of all these reactions depends upon the concentration of the cooking liquonthe temperature prevailing and upon the actual acidity, the latter, inturn, largely depending upon the temperature andthe cooking degree already reached. Unfortunately, the three main reactions finally 4 leading to fibre liberation, do not proceed prop-- erly one after the other, but to a certain extent occur simultaneously, and, consequently, a certain quantity of organic matter is already dissolved before both penetration and sulphonation are completed. The concentration of this organic matter, of course, increases steadily with further progress of cooking, and finally, during the third period, i. e., hydrolysis of the sulphonated incrusting matter, the cooking liquor is heavily laden with dissolved organic material.

This high concentration of soluble organic compounds is the reason that, at the high temperature which during-this period has to be maintained, side reactions occur, of whichthenet re- 5 sult isanundueincreaseof acidity of the liquor,

(Cl. 92-11). I

the lignin left in the fibres, so that the bleach consumption of the pulp obtained remains relatively high. Furthermore, such an undue increase of acidity is connected with untimely decomposition of the cooking liquor, this reaction raising the consumption of sulphur per unit weight of pulpproduced. Another result of these conditions is the fact that the degree of purification of the pulp attainable is insufiicient for many technical uses, since substantial removal of the hemicelluloses is always accompanied by cellulose attack and a considerable loss in both alpha-cellulose and pulp yield. Within the scope of the sulphite cooking process, as carried out in commercial scale at present, it is possible to reduce both speed and magnitude of the side reactions by maintaining a lower'temperature during thelast period of cooking. This, however, also reduces the speed of fibre liberation and lengthens the cooking time to such an extent that additional digester space and farhigher investment costs arerequired for a given production. On the other hand, it is possible to regulate and decrease the acidity of the-cooking liquor during the last period by starting with a cooking liquor of high combined S02. This, however, again lengthens the cooking time, and, in :addition, increases the sulphur consumptionandnecessitates the use of costly sodium as-base of the cooking liquor instead of lime, since the latter may result in heavy precipitations of calcium monosulphite.

The object of this invention is to provide a process which will enable the manufacturer o'fsulphite pulp to overcome the difiiculties outlined above, to reduce the speed and magnitude of side reactions and to adjust the acidity of thelastperiod of cooking without lengthening the cook'- ing time, increasing sulphur consumptionor requiring the u se of costly-alkali salts. -A further object of the invention is to provide process which requires lower sulphur consumption, which permits the removal of dissolved organic substances so troublesome in the last period of cookingandwhich provides'ior control of acidityv in the last cooking period in ordertodevelop desirable characteristics inthe'pulp which requires :a smaller consumption of bleaching agent; 7 In accordance with the-invention, chippedwood or like cellulosic=material, is cooked in two stages, inthe first' of which fibre liberation is. avoided substantial lowering of the temperature or pressure. This is accomplished by introducing direct steam into the top portion of the .digester. Ordinarily drainage of the liquor in this way is sufficient to remove the bulk of the dissolved reaction products but it is sometimes preferable to wash the contents of the digest-er,'at'thispoint, with a hot liquor, such as water, S02 solution,

dilute cooking liquor or the like. .i i. I

The second stage coo-king liquor is now;intro-; duced into the digester.

The actual acidity of this liquor isadju'sted'in accordance with the characteristics .desiredrin the finished pulp in a manner to be later described. This cooking operation isv continued until fibre liberation is complete. The dissolved wastc'reace tion products having been earlier removed, the desiredreactions are not interfered with in this stage oicooking. T f Q This process gives a higher degree of both purification of the pulp'and hydrolysis of hemicellu loses than is possible by the usual cooking pro cesses and without degradation of the cellulose and, therefore, without loss or" alkali-resistant alpha cellulose. Moreover, the pulp produced requires less bleaching agent than pulps produced by the usual sulphite cooking process. Asillustrative of a specific operation of the process of the invention, thefollovving is given: Cooking of the wood chips or the like is started in the usual manner with respect to tempera-- ture, pressure, relief of gas pressure, etc.,and the first stage cooking liquor is of the usual bisulphite type, containing, for example, combined S02 in the proportion of 20 to 25% of the "total S02, with, for example} 5.20% Of total 502 and 1.20% combined S02. This cook is carried on for 5 to 11 hours until a temperature of 120 to 155 0. is reached. Thisliquor am n drained off without substantial lowering of the tempera, ture or pressure, as above described, and-the; sec 0nd stage cooking liquor, preheatedto the trans}. fer temperature, is forced into-the. digesteri'ri any desired known way.- T is liquorhas a relative acidity than the first stage liquor and may contain combined S02 in the proportion of 5 to 15% of the total S02, and preferably a lower concentration of total S02, the total 50 for example, being l.5-i.5 and the combined being 0.08-0.67%. This cook i continueduntil .fibre liberation is complete and the desireddegree of purification of the pulp isreached. Thetem- .perature and pressure of this stage as Well as the dump colour or dump test has to be adjusted according to the quality of pulp desired; Tnepressure is kept constant at, for example,x'75 lbsand is not lowered until. the blow back at the en'doi the cook. The temperature is raised gradually from the transfer temperature to 145-1521 ;C; which is reached in 3 to 6 hours after inti oduction of thisliquor. The twostage cookmay takedi) to 14 hours in all. r

This process may-be operated withoutany higher heatrequirements thanthose of the usual one stage cooking process and with a substantial sav This is a suiphiteliquor of higher relative acidity than the first liquor and is preheated before introduction to the digester.

ing in sulphur consumption. The drain liquor from the first stage cook with its content of useful chemicals and heat may be returned to the cycle after making up the volume required and strengthening by known methods, whereas the du npliquor frolnthe second stage cook may be discarded after'recovery' of its content of heat and sulphur dioxide, or it may be used in the same manner as any other sulphite waste liquor.

The higher relative acidity of the second stage liquor causes alowering of the viscosity of the pulp which is desirable for certain important uses of the pulp as is well known. This may be illustrated'by two pulps cooked to the same degree, the one cooked with a second stage liquor in "which the proportion of combined S02 is 40% less than that of the second stage liquor in which the other is cooked, has a solution viscosity onehalf that of the other.

Furthermore, the use of a second stage liquor having a concentration of S02 of the order given results inreducingthe amount of bleaching medium required to produce the desired bright white colour. .Higher S02 concentration may be used but it. is unnecessary: and unecomonical. A saving inbleach consumption as high as 60% may be made byrthus controlling theconcentrationof the second stage liquor. j I" Pulp. produced by this process, in comparison with, similar pulps produced by the ,usual sulphite process, has 2 to 3 higher alpha cellulose content and a solution viscosity of substantially one-fifth to one-tenth that of thenormal sulphite pulps, whereasits resin content is only substantially 50% cr'less of that of the'normal sulphite pulps. It will thus be apparent that the. pulp produced by this invention has substantial improvements in these most important characteristics of. sul-. phitepulp. l Y I ,WhatIclaimis: r

l. A twostage processfor manufacturing sul-.- phite pulp which-comprises cooking cellulosic material in a digester with a sulphite'liquori in which the combined S02 is 20 to'25% of the'total S02, discontinuing, this cook before liberating" the fibresr' removing the liquor-without substantially lowering the temperature or pressure, introducing a second heated sulphite liquor of higher acidity in. which 5 to 15% .of the S02 is combined and continuing thecookin'g operation until the fibre-liberatioh-andpurification of thepulp is completem 2.;A two stage process for manufacturing sulphite pulp which comprises. cooking cellulosic material with a sulphite liquor containing-sub-' stantially 5.20% total S02 and 1.20% combined S02, removing this liquor before liberating the fibres and without substantial lowering of the temperature or'pressure, and completing the cook with a-' sulphite liquor containing L d-4.5% total S02 and 0.0'8f0f67% combined S02.

'3 cellulose pulp produced by a two stage sulphitecooking process;':said pulp having. an alpha cellulose content of substantially 90%, a resin content not higher tlian 0.7% and a solution viscosity not more than one fifth that or normal sulphite pulp. Y I 'Acellulose pulp produced by a two stage sulphite cooking process characterizedby its low con-'- sumption of bleach, an alpha cellulosecontent of substantially 90% and having a solution viscosity substantially one-fifth to-one-tenth that of normalsulphite pulp. J

p r ERNST HQCHBERGER-