US1742219A - Pulp-treating process - Google Patents

Description

UNITED STATES PATENT OFFICE eEonqE A, nronrnn, or BERLIN, NEW Hamrsnmn,

ASSIGNOR TO BROWN COMPANY,

OF BERLIN, NEW HAMPSHIRE, A; GORROBATION OF MAINE PULP-TREATING PROCESS 1T0 Drawing.

This invention relates to the treatment of a sulphite pulp, which'has been prepared under special conditions, to yield a product having certain optimum characteristics to be hereinafter described.

In my application, Serial No. 749,564, filed November 12, 1924,. it is disclosed that when wood chips (spruce, for example) are cooked in an acid sulphite solution in which theffree S0 and the combined S0 are present in substantially equal proportions (or the free S0 less in proportion than combined S0 but are present in sufficient proportion to the weight of the solution to act uponthe encrustinfiior cementitious content of the wood for a su cient period of time in a closed digester, at a temperature higher than that ordinarily employed. in sulphite digestion, a pulp is produced which has unusual characteristics and which possesses features of advantage over the usual or commercial form "of sulphite pulp. First, the product has longer fibers and a much greater strength. than the usual commercial sulphite pulp; and, second, it is comparable in average length of fiber and strength with sulphate or kraft pulp, but has the advantages thereover, that it is not so dark in color and requires less beating to condition it for use in the manufacture of paper. A sheet formed of this pulp, with or without beating, has a shiny or glossy appearance and has a decided rattle when shaken. Its appearance is markedly different from that of either commercial sulphite or sulphate pulp, and as stated, it possesses the best characteristics of both sulphite and sulphate pulps in the particulars mentioned. Moreover, the new pulp contains a higher proportion of the wood than is contained in either sulphate or commercial sulphite pulp. Hence, in addition to cellulose fibers, the-product contains a large proportion of lignin and pentosan, which by their presence doubtless impart to the product certain identifying characteristics hereinbefore noted.

To produce pulp such as has beendescrlbed, the wood chips are cooked in an acid suiphite solution having a relatively high comblned SO content say, 3% to 4%) with free SO no greater in proportion than the combined Application filed December 1, 1925. Serial m, 72,602.

S0 at a temperature of about 320 F. and a total pressure of about to 95 pounds. 7 The cooking operation is carried out at this temperature and ressure for about ten toeleven hours, tests being made in the usual manner to determine the progress of the operation and to ascertain when the chips have been cooked to the desired extent. In such a process, the cellulose fibers are substantiallyuninjured, since the hydrolysis of the fibers is less than where there is an excess of free S0 and hence the average length of fiber is greater than when the acid cooking is carried on with an excess of, free S0 whereas, on the other hand, the pulp contains much of the fiber that in an'alkaline digestion would be dissolved in the alkaline liquor. I r

I present herein a table which indicates the characteristics which a sulphite pulp produced by the method described possesses, these characteristics beingrepresented by average figures obtained in testing samples from dif- U ferent cooks:

Uharactem'stics of 4-4 W2 Time of beating -50 minutes Strength (Mullen) 170-182 Tear 92-140, Pentosan 5.0% to 6.0% Alpha cellulose 88 7 -89 Copper number 2-3 Thus it will be seen that a surprising strength of pulp is secured, which is comparable to that of kraft or sulphate pulp, which usually possesses a Mullen strength (beaten) of about 155 to 165.

I have further discovered that When'such sulphite pulp, herein termed a 4-4 pulp, is treated with a concentrated solution of an alkaline compound, such as caustic soda, at

about room temperature, the alkaline solution containing sufficient alkali to react with the lignin, less-resistant celluloses, and certain too high a concentration of caustic soda must a be avolded, since otherwise the alpha cellulose pulp does not tend to hydrate or 1' ell, the beating operation merely tending to macerate or shred the fiber, and resulting in a paper of low strength. On' the other hand, if too' low soda solution,

a caustic .concentration is employed, a sufficient purification or reaction with the 1mpurities contained in the pulp, by the caustic will not take place and the resulting product will-not have undergone a suflicient purification and increase 1n tear test.

In other words, the concentration of caustic soda employed in the alkaline treatmentof the 4-4 pulp is preferably maintained between two limits, namely, etween a ma x1- mum concentration'at which mercerizat on takes place, and a minimum concentrat on at which a sufiicient increase 'in' tear test of the. pulp is not efiected. These limits will vary somewhat, depending upon the exact temperature or different characteristics of the raw 4-4 stock undergoing the treatment.

Afterthe purification treatment, the pulp is washed free from the alkaline treating agent and may then be bleached to a high'deree of whiteness with the usual amount f l ypochlorite bleach, without material 1njury to its strength or decrease in cellulose content. If a pulp of maximum whiteness is desired, the bleached pulp may finally be. I superbleached in a chlorine solution.

\Vhile I have indicated that the purification or refining treatment is ordinarily carried out at room temperature, I desire to have it clearly understood that lower than room temperature (say, 0 C. to 25 C.) ma be employed in the treatment of the-4-4 pulp. Likewise,

higher temperature (for example, 30 C. to A 50 C.) an a correspondingly suitable concentration of caustic soda, may be em 10 ed in the treatment. Moreover, an a ka ine liquor containing any other suitable alkaline compound or compounds, such as caustic potash and sodium sulphide, ma be employed in the purifying treatment, if esired, one or p more of these compounds being used to relace a portion or all of the caustic soda. For lnstance, substantially equivalent results to those previously outlined may be obtained by treating a 4-4 stock with a solution of sodium sulphide of somewhat higher concentration than a solution of caustic soda necessary for effecting the same results. Or white liquor such as utilized in a kraft mill for the roduction of kraft pulp,-and containing sodium sulphide, sodium hydroxide, and

a slight amount of sodium sulphate and sodium carbonate, may be frequently used with certain advantages.

Where the causticsoda is replaced by a substantial proportion of treatment employed and the mercerization of the alpha cellulose content of the pulp undergoing treatment is precluded, since sodium sulphide does not cause a mercerization of cellulose fiber. In certain cases it may be advantageous to incorporate into the cooking liquor, sulphur as such, or in the form of reducing sulphur compounds, such as sodium sulphide or sodium thiosulphate, in order to maintain a reducing condition in the mass during the purifying treatment. Such a condition, it is believed, tends to .improve the yield and the qualities desired in the final product. At any temperature, however, where a solution of'eaus'tic soda is em loyed for the purifying treatment, the"max1mum concentration of caustic soda which is preferably employed is below that at'which mercerization occurs, andthe minimum above that at-which a suflicient increase in tear test is not efiected. For optimum results in respect to alpha cellulose content in the finished product and in the tear test of the product, at a treating temperature 175% caustic soda based on the weight of dry I It may be advantageous to employ a relatively small proportion of oxidizing agent or bleach prior to or in the alkaline treatment or purification of the H pulp, especially where. the product is to be subsequently bleached. Such treatment renders the pulp more easily .bleachable in the" subsequent bleaching operation, and is especially desirable whe the 4-4; pulp is especially high in ligne Si and other coloring substance. Ordinarily, about 2% to 5% of 35% lime bleach (CaOCl based on the dry weight of pulpwhen used in the alkaline liquor is sufiicient to render the pulp easily bleachable in- .subsequent operation. If desired,-however, chlorine maybe introduced into the alkaline treating solution to form the desired amount of sodium bleach in solution. Or the pulp may be treated for a relatively short period of time prior to the alkaline treatment, with an oxldizing agent, preferably in the form of a bleach or chlorine solution, depending upon plant expediency, about 2% to 4% of. lime bleach, or 1% to 3% chlorine, being sufficient to render the difiiculty removable nonalpha cellulose constituents contained in the pulp susceptible to removal in the subsequent treatment with the alkaline liquor. Or the pul may be pretreated, after which it is prefera I washed, and then treated with an alkaline liquor, as reviously described. Of course, where an al aline liquor containing 'a reducing a ent such as'sodium sulphide is employed, leach or other oxidizing agent maynot be added thereto.

In carrying out the treatment in accordlab 16 content, inthe practice of the present invenance with this invention, a l4= pulp produced by digestion in an acid sulph te liquor under the conditions hereinbefore described is Washed substantially free of its digesting liquor and spent products of digestion and is preferably thenscreened prior to the alkaline treatment. It is then thickened and intimately admixed with an alkaline solution of caustic soda to a consistency of about 5% to 6%, the stock suspension containing about 100% to 175% of caustic soda, based on the weight ofdry fiber. While a lower percentage of caustic soda may be employed in cases Where the resulting pulp need not be of optimum tear test and of high alpha cellulosic tion it is preferable to employ a caustic soda concentration between the limits indicated, so that a suflicient increase in tear test and puri- .mercerization of the fiber.

fication of the fiber may be obtained without impairing or injuring "its paper-making. characteristics to any material extent, or in other words, without causing a substantial The alkaline treatment is carried out preferably while stirringor agitating the pulp so as to promote a'uniform reaction, say, at room temperatiirc' (3.), and the purification may require thirty minutes to two hours, depending upon the'characteristics 6f the particular stock undergoing treatment. The spent liquor contains not only a very high percentage of residual unconsumed causticsoda, but various organic substances removed from the sulphite pulp, as indicated. The large portion of spent liquor is reemployed in subsequent treatment of other pulp, and the chem icals in the remaining liquormay be recovere portion of such spent The washed pulp is of light color, not much different in appearance from the color of orstrong rag sheet.

'dinary unbleached sulphite pulp.- I present herewith a table-indicating certain characteristics of a pulp which has been treated as hereinbefore escribed:

.Time of beatin 100 minutes Strength (Mul en); 100-140 Tear 240270 Pentosan 1.8-3% Alpha cellulose -l v90-9et% From the foregoing table, which represents the average of a number of tests on samples obtained from different batches of pulp treated with liquors of different concentrations of caustic soda, it will be seen that the Mullen strength of the pul has undergone some reduction, but that t e tear test has increased considerably. The treated fiber is no longer harsh and rattly; it resembles the feel of a The purified fiber may now be bleached to r a hi h whiteness without materially affecting its strength or tear, by treating it at a consistency of about 10% to 16% with about weenie bleaching,

5% to 10% of 35% lime bleach (CaOClf is taken to maintain a temperature not ex ceeding 90 F. in the bleaching operation, so as to preserve the maximum strength of fiber. If desired, a certain small roportion of canstic soda liquor may be a ded to the bleach liquor and pulp in the bleaching operation, to retard the bleaching reaction and thereby el minate the possibility of lowering the alpha cellulose content or injuring the fiber strength of the stock, due to .too violent bleaching. The bleaching period ranges from eight to twenty hours, depending upon the characteristics of the fiber undergoing treatment, and producing stock having a color of 95 to 10 The stock thus bleached may be improved by a superbleaching operation. In superthe preferably washed and bleached high alpha cellulose stock is treated at a consistency of from 4% to 6% with about based on the dry weight of pulp. Precaution 0.2% to 0.5% chlorine, based on the weight of dry fiber. N o efl'ort need be made to control the temperature in superbleaching, which may approximate the temperature of the wateravailable at the time. If it is desired to increase the rate of superbleaching, the temperature may be increased to a point not higher than. F. After superbleaching, the stock is treated with an anti-chlor, as, for example a bisulphite solution, which serves to react with and to neutralize any residualtraces of chlorine or hydrochloric acid present in the stock. The pulp is then washed free from reaction products, and may thenbe sent to a wet machine, from which it is taken off in sheet form, ready for shipment.

Due to its properties and the purif *ing treatment which it has received, the high alpha cellulose may be employed instead o1 rag fiber in the manufacture of high grade 1 currence, since the tear test and its accompanyingquality of folding endurance is of extreme importance in high grade papers,

whereas the Mullen or strength test is usually of secondary importance. This feature may be better understood from the fact that it is relatively easy to increase the Mullen or strength test of a paper sheet by suitable sizing. The sizing operation, however, does not affect the tear test so appreciably, so that it is a distinct advantage to increase the tear test at the expense of the strength of the pulp,

inasmuch as it is possible to increase the strength in subsequent treatment.

Moreover, paper made from the unbleached purified pulp as herein roduced, because of its softness of sheet, is a mirably adapted for many uses to which a harsh or rattl paper could not be put. For'example, suc paper may be emplopled for the production of artificial reed fi er such as employed in the manufacture of bab carriages, furniture,

etc. Such a paper s ould have a high tear test, should be soft and yet firm, and should have sufficient porosity to permit its ready impregnation with glue or other chemical reagents.

Before concluding, it may be stated that, while I have specified certain concentrationsof stock and treating compounds in order to more full and adequately describe my invention, t ese concentrations may vary,'de-

pending upon plant equipment and conditions. That is to say, the plant equipment with which the practlce of the present invention was carried out, permitted the treatment of b-4 stock at a concentration of about 5% to 6%, most efiiciently .and successfully. On the other hand, a stock concentration of 10% or greater may be treated-under other plant conditions. In such cases, the percentage of caustic soda or other alkaline compounds utilized in the purification may be' correspondingly decreased. In other words, the determining factor in carrying out the treatment or purifying reaction resides in the concentration of the alkaline compounds in the liquor, even though for convenience these compounds may frequently be expressed in terms ofdry fiber weight. Thus, a 5% stock suspension treated with. 100% caustic soda based on dry fiber, represents a 5% caustic soda solution. Of course if sodium sulphide or some other reagent Whlcll isless drastic or effective in its reaction than caustic soda is used, the percenta e must be increased 1 in order to produce a product substantially equivalent to that obtained by the use of caustic soda.

I do not herein claim the subject matter claimed in my Patent No. 1,635,637, issued mgpulp from the spent iquor o digestion;

and treating such pulp, admixed with a solution of caustic soda to a stock consistencyJof about 5% to 6%, at about room temperature,

- for about thirty minutes to two hours, saidrocess which comprises digesting solutionv containing 100% to 175% caustic soda based on the wei ht of dry fiber.

2. A rocess whic comprises di esting wood ChlPS in an acid sulphite liquor aving a combined SO content appreciably above that of a saturated solution of calcium bisulphite, and treating the resulting pulp at about room temperature with an alkaline solution containing suflicient alkali to react with and dissolve from such pulp a substantial proportion of its non-alpha cellulose constituent.

3. A process whichrcomprises di esting wood chips in an acid sulphite liquor aving a combined S0 content appreclably above that of a saturated solution of calcium bisulphite and a freeSO, content approximately equal to'its combinedSO, content, treating the resulting pulp with an oxidant, redigesting the oxidant-treated pulp at about room temperature with an alkaline solution containmg sufficient alkali to react with and dissolve from such pulp a substantial pro ortion of its non-alpha cellulose content, and leaching the alkaline-digested pulp. v

4. A process which comprises pulping wood chips in an acid sulphite cooking liquor in which free S0 and combined S0 are present in approximately equal proportions of 3% and 4%, and treating the resulting pulp at about room temperature with an alkaline liquor containing suflicient alkali to react with and dissolve from such pul a substantial proportion of its nonalp a cellulose content.

In testimony whereof I have aflixed my signature. I GEORGE A. RICHTER.

of such alkaline reagent