US1849334A - Process for the production of chemical wood pulp - Google Patents

Description

March 15, 1932. G. A. RICHTER PROCESS FOR THE PRODUCTION OF CHEMICAL WOOD PULP Filed May 1, 1926 Patented Man 15, 1932 UNITED STATES PATENT OFFICE GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE PROCESS FOR THE PRODUCTION OF CHEMIOAL WOOD PULP Application filed May 1, 1926. Serial No. 105,995.

This invention relates to the production of chemical wood pulp as effected by wellknown chemical pulping processes. Inasmuch as the present invention is especially advantageous in the sulphite process, it will be described as applied thereto.

In producing sulphite pulp for use in the manufacture of bond, ledger and writing, papers, a prime desideratum is fiber of good strength and other physical characteristics. This is notably the case when the fiber is to serve as a raw material for'the production of a high alpha cellulose fiber which is intended for use as a new rag substitute in the manufacture of paper. I

In order to appreciate the results obtained by the practice of the present invention,- 1t is desirable preliminarily to consider certain factors that affect the production of sulphite pulp. Initially, the Wood, e. g., spruce, is cut into small, thin chips of more or less definite size. The chips, together with an acid sulphite liquor of the desired composition (usually a calcium bi-sulphite liquor), are

charged into a digester. The liquor usually contains about 5% free S0 and 1% combined S0 The charge is slowly and gradually heated, as it is necessary to avoid a too rapid increase in temperature. Thus,

the operator may reach the maximum temperature of digestion of, say, from 205 to 310 F., in about eight to twelve hours after digestion was commenced. The maximum pressure of digestion is usually controlled by the periodical opening of a relief valve at the top of the digester, which allows the escape of free sulphur dioxide and steam. Unless a time-temperature, curve determined from experience is carefully adhered to and 4 the temperature thereon indicated at any definite time not exceeded, a fiber of inferior quality is produced. Injury to the fiber ensues because the combined S0 is not given sufiicient time to penetrate into the chips,

whereas the free S0 diffuses readily thereinto and is present alone. In other words,

the combined SO, in the form of S O ions is-not present in the chips in suflicient amount 5 to repress the ionization of the I-LSO, into H ions, which hydrolyze and dissolve a portion of the fiber with an attendant low yield of short fiber pulp having low strength and tear resistance. By adhering to the predetermined time-temperature curve during the digestion, sufficient time is allowed for penetration of combined S0 into the chips.

Another factor which afiects the rate of penetration of combined S0 content into the chips is the concentration of combined S0 in the digesting liquor. Thus, by increasing the combined SO content of the digesting liquor, the rate of penetration of combined SO into the chips may be increased. In the' usual calcium base cooking liquor, however, the concentration of combined SO should not be raised above the concentration of a saturated solution thereof, which amounts to about 1%, for the reason that if saturation concentration is exceeded, a heavy precipitate of calcium monosulphite is present during digestion, and the resulting pulp becomes contaminated with the precipitate.

The object of the present invenion is to avoid the long time heretofore necessary to reach maximum digesting temperature, without, however, sacrificing the quality of fiber. I have found that this object may be attained if the chips are suitably pre-treated or chemically conditioned prior to digestion. I have further found that by suitable pretreatment much larger wood chips may be digested without extending the time of'digestion hertofore found necessary in producing pulp with chips of usual size.

Briefly stated, in accordance with the process of this invention, the chips are immersed or soaked prior to digestion in a concentrated solution until they contain a total quantity of combined 80,. or of a base chemical capable of reacting with free S0 to produce combined S0 even greater than that present in the standard acid cooking liquors employed in the sulphite cooking process, which standard liquors have a combined SO content of about 1% as hereinbefore described. The chips may then be charged in a digester together with the desired amount of only, Water and free S0 and cooked under pressure treated therewith an amount of combined S0 or of base chemical greater than that present in the usual or standard acid sulphite cooking liquor. Preferably, the pre-treating solution is highly concentrated or even saturated with the particular pre-treating chemical chosen, whereby the chips may acquire in a short period of time within their pores and interstices substantially all of the chemical other than S0 necessary for subsequent cooking to complete fiber liberation in the di gester. The pre-treating solution may, for example, contain 20% sodium sulphite (10% combined S0 and be prepared by combining a solution of sodium carbonate, caustic soda, or the like, with sulphur burner gas or with sulphur dioxide recovered from the blowpit or relief gases. Inasmuch as a sodium sulphite solution is neutral or faintly alkaline, ordinary apparatus is suitable for carrying'out a pre-treatment with the solu* tion. The pre-treatment should, however, be carried out in acid-proof apparatus when a sulphurous acid solution of sodium sulphite, say, a sodium bisulphite solution containing 10% free and 10% combined S0 is used. This latter procedure, using a liquor containing free S0 tends to speed up the rate of penetration of combined S0 into the chips,

' but the use of a neutral or alkaline solution is generally more economical and expedient.

The chips are immersed or soaked in'the pro-treating solution of sodium sulphite until they contain in their pores and interstices the desired combined S0 content, preferably substantially equal to that required for subsequent digestion and fiber liberation. The pre-treatment may be, and preferably is, carried on as a continuous operation at room or at elevated temperature (say, 200 to 212 F.), and under atmospheric pressure. If desired, however, a pressure somewhat above atmospheric may be applied to shorten the time of pro-treatment. Pretreatment under pressure, however, is generally less desirable, inasmuch as it calls for the use of stronger apparatus and ordinarily can be practiced only as a batch basis.

After pretreatment, the chips are removed from the solution and drained, whereupon they may then be stored in a chip loft or directly charged into a digcster. The liquor drained from the chips may be recovered and ire-employed in pretreating fresh chips after replenishment with an amount of Water and chemical equivalent to that absorbed by the chips. The drained liquor will vary from a pale yellow color to a dark brown or amber color, owing to partial solution or extraction of resinous and gummy materials from thechips. The coloring material, however, does not interfere with the pre-treatment of fresh chips.

The factors entering into the pre-treatment may, of course, be varied according to the type of pulp which it is desired to produce. Thus, in producing a 44 pulp such as described in my application, Serial No. 749,564, filed November 12, 1924, a quantity of chemical much higher than usual should be introduced into the chips by the pre-treatment. This may be accomplished by maintaining a suitable concentration of dissolved chemical in the pre-treating solution and by properly controlling the time and temperature of pretreatment.

When the chips have acquired during pretreatment the amount of,base chemical or combined SO necessary for digestion, only a sulphurous acid solution containing the desired S0 content is necessary for digestion. When the chips have acquired'both the free and combined S0 content necessary for di-- gestion, only water need be added to the digester to produce a cooking solution of the desired strength. The concentration of H ions Within the chips during digestion, in either case, is sufiicient to hydrolyze or dissolve the encrusting material or ligno-celluloses, but the chips also contain at the very start the necessary amount of-combined S0 to avoid injury to the pulp, so that maximum temperature of digestion may be reached as rapidly as plant conditions will allow.

After fiber liberation has been effected, the digester contents may be blown, and the pulp separated from the spent liquor. The spent liquor may then be concentrated, its inorganic content smelted, and the smelted compounds recovered in aqueous solution. If desired, instead of re-using the solution drained from the chips after pro-treatment, it may be co- -mingled with the spent liquor and the mixture subjected to recovery operations.

It should be noted that sodium'is selected as the pre-treating base because its compounds are highly soluble in water and can give a high rate of penetration into the chips. The rate of penetration possible with calcium bisulphite, on the other hand, is materially lower, because of its low solubility and the consequent low concentration obtainable in a saturated solution thereof. The sodium compounds may be replaced in part or in whole by compounds of other bases which are highly soluble in water, such, for example, as magnesium sulphite, magnesium bisulphite, magnesium sulphate, or the like.

Vilmn the chips are pre-treated with a base chemical, such as caustic soda or sodium carbonate, then, during subsequent acid digestion, the free SO introduced into the digester readily difiuses into the chips and immediately combines with the base chemical to produce combined S0 in situ in the chips. When the chips are pre-treated with a concentrated solution of sodium'sulphate (salt cake), subsequent digestion will take place in a sulphurous acid solution of sodium sulphate, as described in my Patent No. 1,427,- 125, dated August 29, 1922, should the added liquor consist of a sulphurous acid solution. When sodium sulphite or other suitable base chemical is employed in the pre-treatment of the chips, a sulphurous acid solution of calcium sulphite may subsequently be introduced with the pre-treated chips into the digester, in which case digestion takes place in a composite base liquor, as describedin my application for patent, Serial No. 94,621, filed March 13, 1926.

It should be observed that the chips, if pretreated at an elevated temperature, may be charged while in heated condition into the digester, thus conserving their heat content. Where storage of the chips is necessary, however, even if partial evaporation of water from the chips ensues, the chemical remains entrapped therein.

One of the great advantages in practicing a process such as hereinbefore described is that a material saving in time of digestion may be effected. The digesting period heretofore found necessary to produce a high quality sulphite fiber may be shortened to from two to four hours without sacrificing quality of fiber. Moreover, pre-treated chips of larger than usual size may be successfully digested, but, inasmuch as more time is required to heat such chips, the time of digestion may not be reduced below that heretofore found to be necessary. A pulp of exceedingly high strength and tear may be produced, however, by such latter practice, because the presence of short chips having short fibers and shives is largely eliminated. An increase of about 25% in strength and tear may be real ized in actual practice.

On the accompanying drawing, I have illustrated a schematic or conventional layout of apparatus which may be employed for continuously carrying out the process of the present invention.

Now, describing the process as carried out when sodium sulphite is employed as the specific pre-treating chemical, at 1.is shown a tower in which the pre-treating solution of sodium sulphite may be prepared. The tower may be filled with inert interstitial material, e. g., spiral brick, through which a solution of caustic soda or soda ash may slowly percolate in counter-current contact with sulphur dioxide passed upwardly there through. The alkaline solution may be introduced at the top of the tower through a valved pipe 2, and the sulphur dioxide may the tower. The sulphur dioxide employed may be sulphur burner gas or that recovered from the blowpit or relief gases," the inert gas carried along therewith into the tower being wasted through a vent 4. The sodium sulphite solution may collect as a pool at the bottom of the tower and may be withdrawn by a pump 5 and delivered through a valved pipe 6 into a storage tank 7 in amount and concentration sufficient to replace the sodium sulphite and water absorbed by the chips after pretreatment and drain- The pre-treatment of the chips may be carried out continuously in a rotary drum indicated conventionally as a whole at 8. The chips may be continuously carried in predetermined, measured amount from the chipping machine on a belt conveyor 9, and charged into a hopper 10 feeding into the intake end of the drum. The solution of sodium sulphite maintained at the proper concentration in the storage tank 7 may be withdrawn by a pump 11a and delivered at predetermined rate into the intake end of the drum, together with the chips. The chips may be continuously passed through the solution of sodium sulphite maintained in the drum at a rate such that when they issue from the drum they contain the combined S0 content necessary'for subsequent digestion.

The pre-t'reated chips may be continuously discharged through an inclined discharge nose 11 at the outlet end of the drum on an inclined foraminous platform 12, the solution associated with the pre-treated chips draining through the platform into a funnel 13 discharging into a pipe 14 leading to the storage tank 7 The solution draining from the chips is thus re-employed in the pretreatment of other chips after sufiicient fresh solution, which replaces the chemical and water absorbed by the chips after drainage, has been admixed therewith. It may be desired, as previously stated, to carry out the pretreatment of the chips at an elevated temperature, especially when a neutral or alkaline solution is employed in the pre-treatment. In this event, the treating liquor may be heated to the desired temperature while in storage tank 7 prior to delivery into the a loft 17, from which the chips may be removed, as required, for charging into a digester 18.

I claim:

1; A process which comprises treating chipped wood until it is completely permeated with a solution of'at least about 10% concentration of a highly soluble compound capable of being converted by sulphurous acid into bisulphite, treating the permeated Wood with only water and sulphur dioxide to produce a bisulphite cooking liquor of at least 1% strength in base, and cooking at elevated temperature and under pressure in the liquor to effect a liberation of fiber from the wood.

2. A process which comprises treating chipped wood until it is completely permeated with a solution of at least about 10% concentration of a sodium compound capable of being converted by sulphurous acid into bisulphite, treating the permeated wood with only water and sulphur dioxide to produce a bisulphite cooking liquor of at least 1% strength in sodium base, and cooking at elevated temperature and under pressure in the liquor to effect a liberation of fiber from the wood. v

3. A process which comprises treating chipped wood until it is completely permeated With a, solution of sodium sulphite of at least about 10% strength, treating the permeated wood with only water and sulphur dioxide to produe a bisulphite cooking liquor having a combined S0 content of at least about 1%, and cooking at elevated temperature and under pressure in the liquor to effect a liberation of fiber from the wood.

In testimony whereof I have affixed my signature.

I GEORGE A. RICHTER.

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