US2041666A - Cyclic process of treating alkaline pulps - Google Patents

Description

May 19, 1936. I G. A. RICHTER 2,041,666

CYCLIC PROCESS OF TREATING ALKALINE PULPS Original Filed Dec. 4, 1925 Patented May 19, 1936 UNITED STATES CYCLIC PROCESS OF TREATING. ALKALINE PULPS George A. Richter, Berlin, N. H., assignor to Brown Company, Berlin, N. H., a corporation of Maine Application December 4, 1925, Serial No. 73,193 Renewed September 26, 1930 23 Claims.

This invention has relation to the production of a.high alpha'cellulose fiber, that is, a fiber containing a high percentage of alpha or resistant cellulose and having certain optimum characteristics, from a pulp derived from the digestion of wood chips in an alkaline liquor, and more particularly in a kraft or sulphate liquor. n W

Wood pulp, aspidduced by an alkaline'diges tion, such as kraft, is especially characterized by its quality of high strength, which renders such pulp industrially valuable for use in the manufacture of strong papers, e. g., wrapping and bag papers. Such pulp, however, is inferior in respect of tear resistance and folding endurance,

to the high grade manila or rope pulps, so called, which find extensive use in cable insulation papers and in the manufacture of bags, and various other products, where the characteristics of'high tear resistance and folding endurance, as well as strength, are of prime importance.

' Moreover, such alkaline pulps are exceedingly difiicult to bleach. While it is possible to bleach kraft pulp with an excessive amount of bleach, I

say, about 50% of 35% lime bleach (CaOClz) based on the weight of dry pulp, the strength of the resultant bleached pulp is materially lowered and its alpha cellulose content undergoes a material decrease by conversion or degradation into oxycelluloses, so that the commercial value of v ,such pulp is lowered. This difliculty in bleaching 72,522 filed December 1, 1925, by Milton 0. Schur .and myself, as being successful in connection with the treatment of sulphite pulp.

In working with or producing various pulps, I

have discovered that if an alkaline digested pulp,

such as kraft, is treated with a relatively con-.-

centrated, cold alkaline liquor, e. g., a solution of caustic soda, I am enabled to produce a fiber having a very high tear resistance and. very good 50 folding endurance, and which may thus be employed as a manila fiber substitute. The change in the characteristics of the pulp, which has been efiected by the alkaline treatment, doubtless arises from the fact that the treatment purifies or frees the pulp from the pentosans, less-resistant celluloses, ligneous and other non-alpha cellulose constituents contained therein, without perceptibly injuringits alpha cellulose content, thereby converting it into a pulp which is especially high in alpha or resistant cellulose 5 content.

Even after such alkalinetreatment, however,

' tlipiilp does not yield to a bleaching treatment very readily, and requires an excessive amount of bleach to be converted into a white pulp, The 10 use of such excessive bleach materially aiiects the fiber strength of the pulp and the alpha cellulose content thereof, as previously stated. I have .further discovered, however, that the necessity for the use of excessive bleach may be avoided 15 by treating a thoroughly cooked raw pulp prior to the alkaline treatment, with a small quantity of oxidizing or chlorinating agent, such as a chlorine solution. This preliminary treatment of the pulp is not materially changed. The chlorine, it appears, reacts with and forms soluble compounds with ligneous and other coloring substances contained in the pulp, or conditions the pulp by rendering such substances more susceptible to the dissolving or removing reaction exercised thereupon by" the alkaline liquor in the subsequent treatment. After the alkaline treatment, the pulp may be bleached with a moderate amount of bleach into a white pulp of high alpha 30 cellulose content suitable for use in the manufacture of high grade writing papers, or forlother purposes where a pulp of high alpha cellulose content is required.

The kraft pulp employed in the present invention may be produced, as ordinarily, by the digestion of wood chips (spruce for-example) in a digester, under the requisite time, temperature, and pressure conditions in accordance with modern practice, in the usual kraft or sulphate liquor 40 containing sodium sulphide, sodium hydroxide, and a relatively slight or adventitious amount of sodium sulphate. Spruce, hemlock, pine, or any other raw cellulosic material suitable for the production of kraft pulp may be utilized. After the necessary period of digestion, the digester contents are blown and the kraft pulp is washed and separated from its accompanying black spent digesting liquor.

The washed pulp is then preferably screened to remove shi'ves, specks, and other contaminating matter, and, if desired, is partially bleached as hereinbefore described. It is then thickened to the consistency desired for admixture with a liquor containing the alkaline compounds em-- ployed for purification. The thickened pulp is then intimately mixed with a solution containing caustic soda or other equivalent alkaline compounds and is treated therewith, say at room temperature (30 C.), for a sufiicient period of time to permit the alkaline solution to react with and remove the non-alpha cellulose constituents of the pulp. In carrying out the treatment or purification only with caustic soda, too high a concentration of caustic soda must be avoided,

since otherwise the fiber of the pulp will become mercerized (such mercerization tending to lower the strength of the pulp), and subsequent beating merely macerates or shreds the pulp, without effecting its hydration, so that paper made therefrom is extremely weak. 0n the other hand, if too low a concentration of caustic soda is employed, a sufiicient purification or reaction upon the impurities contained in the pulp, by the caustic Soda solution, will nottake place and the resulting pulp will not be sufiicientlyhigh in alpha cellulose. In other. words, the concentration of caustic soda employed in the alkaline treatment is maintained between two limits, namely, be-

tween a maximum concentration at which merjeri aation takes place and a minimum concentra-. tion at which a sufi'minfpurification of the pulp is not effected. These limits will vary somewhat, depending upon the temperature or treatment employed and the differences in character of the initial raw stock. After the purification, the pulp containing a high percentage of alpha cellulose is washed free from its treating liquor and may be employed as a manila fiber substitute, as previously indicated. If desired, the treated or puri- ,40 (CaOC1z),based on the dry "weight of manila fiber.

fied fiber which still possesses the tan or brown.

ish color of the raw kraft pulp may be bleached at a stock consistency of about 10% to 16% with about 5%. to 19% of 35% lime bleach out materially afiecting its strength, tear resist ance, folding endurance, and alpha cellulose content, to a color "which closely approximates or simulates the light yellow tinge of high-grade In certain cases, however, it is desirable to produce from raw alkaline digested pulps a pure white product which is characterized by its high -trati on of alkali as to cause mercerization.

may be obtained by treating a raw alkaline digested pulp with a solution of sodium sulphide of somewhat higher concentration than that of a solution of caustic soda. When the caustic soda is replaced by a substantial proportion of 5 sodium sulphide or in entirety thereby, mercerization of the fiber of the pulp is generally precluded, since sodium sulphide does not readily cause a mercerization of cellulose fiber.

While I have indicated that the alkaline purii fying or refining treatment is ordinarily carried out at room temperature, it may be stated that lower than room temperature (say, 0C. to 25 C.) may be employed in such treatment. Likewise,'higher temperatures (for example, 25 C. 1

to 75 C.) and a correspondingly suitable concentration of alkaline compound may be employed in such treatment. Thus, in effecting the purification of kraft pulp with a white liquor utilized for digesting wood chips in a kraft mill, 29

a very good product was obtainedat a treating temperature of 75 C. In this connection, I have observed that a more highly concentrated alkaline solution should be associated with a higher treating temperature in order to avoid causing 25 a mercerization of the fiber and injury" to the papermaking--characteristics of the pulp For instance, in order to treat raw pulp successfully at 30 C. with the usual white liquor of a kraft mill without mercerizing the fiber, such liquor 30 must be diluted with about water, whereas the treatment, as indicated; may be carried out without the dilution of the liquor at 75 C. This observation is in line with the generally known principle that mercerization may be more readily 35 effected at low-temperatures. In actual practice, the lower the temperature employed in the purification treatment, the greater the care which must be exercised to avoid such a high concen- The presenthin vention may be practised advantageously in connection with a plant wherein raw alkaline digested pulp, such as either a kraft or soda pulp, is produced. The soda pulp herein referredhto is preferably a long-fibered pulp ap- 45 proximatin gthe-stmn gth of kraft, i. e., a pulp produced from the digestionoLraw oellulosic material, such as spruce or pine. SuclTkraitor soda pulp may be employed as the raw matefiahfor-- l the production of a high alpha cellulose fiber 5 possessing the properties of very high tear resistance, and folding endurance, and stability.

invention may be obtained from the following description, when considered in conjunction with 5:,

A more complete understanding of the present I dizing agent, such as a chlorine solution or bleach liquor, for a relatively sl i ort period of time prior to treatment with an alkalineilution, and is then treated with a solution of a suitablealkaline 0 compound, the pulp is sufficiently freed or purialpha cellulose content. If a pulp of exceptionfied from its non-alpha cellulose constituents to permit the purified pulp to be bleached with a moderate amount of bleach without perceptible injury to its strength or decrease in its ally high whiteness is desired, such pulp may, if desired, be superbleached in a chlorine or other suitable solution.

In certain cases, it may be of advantage to replace a portion or all of the caustic soda used in the alkaline treatment by a suitable amount of other alkaline compound, such as sodium sulphide, which exerts a similar purifying or refining action on the raw pulp. For instance, substantially equivalent results to those outlined the accompanying diagrammatic and conventional drawing, which represents what may be termed a'flow sheet showing the course or fiow of the various materials employed inthe process.

The invention will now be described as applied in on connection with the kraft process of fiber liberation, bu t it will be obvious that it may be, em-

ployed in connection with the soda process.

Referring to the draWing at l, I have indicated a digester in which raw cellulosic material in the 65 form of wood chips may be placed and digested in an alkaline liquor for the production of an alkaline pulp suitable for use as a raw material in the preparation of the finished high alpha cellulose fiber, herein desired. After the chips 70 have been cooked in an alkaline cooking liquor comprising a solution of caustic soda and sodium sulphide, under the necessary time, temperature, and pressure conditions in accordance with modern practice, to eifect a liberation of fiber, the

digester contents are blown into a suitable blow pit 2, wherein the pulp is separated from its spent black digesting liquor, which is delivered from the blow pit into a black liquor storage tank 3.

The alkaline raw pulp, together with its assoelated spent black liquor, is withdrawn from the blow pit by a pump 4 and delivered therefrom into a counter-current washer, consisting of a series of pulp washers indicated conventionally at 5. The washer is preferably of the counter-current type illustrated and described in U. S. Letters Patent No. 1,421,664, to Brown Martinson et al., although, if desired, the ordinary rotary vacuum type of washer may be employed. In the washer of the patent referred to, the pulp is carried continuously through tanks in one direction and the washing liquor is carried successively through such tanks in the opposite direction. The clean wash water is passed from a pipe I to the pulp delivered at the last washer of the series from which the washed pulp is delivered. The washed pulp may be delivered from the washer by a conduit 9 through the usual screens indicated conventionally at Ill, from which it is delivered to a mixing tank I I, wherein it is intimately mixed with a solution of alkaline compounds necessary for the purifying treatment. The spent black liquor which is associated with the pulp is passed from the first pulp washer of the series by a conduit I3 into the black liquor storage tank 3.

A part of the black liquor from the tank 3 may be withdrawn by a pump through a pipe I4 into the digester I, wherein it is reemployed, together with a freshly prepared cooking liquor, for the digestion of another batch of wood chips. The remaining portion is delivered through a pipe I5 into an evaporator I6 wherein a part of the aqueous content of the liquor is removed prior to the incineration of its combustible content and the recovery of its inorganic mineral content.

The concentrated liquor from the evaporator I6 is delivered through a pipe II into a recovery retort 20 and smelted under reducing conditions, thereby producing a smelt consisting essentially of sodium sulphide and sodium carbonate. For making up losses in the digesting cycle, sodium and sulphur in the form of salt cake may be introduced into the concentrated spent liquor from a storage tank I8 through a pipe I9 prior delivered into the water contained in the dissolving tank 2| from which the resultant heated smelt liquormay then be passed through the pipe 22 into a causticizing tank 23, wherein it may be treated, while being agitated, with sufi'icient lime to causticize or convert its carbonate content to caustic soda. The heated liquor is then filtered as usual, as by passage through a rotary filter 24,

tank 21 may, however, be used for admixture with I30 and by-passed the raw stock in the mixing tank to effect the purification of such stock, as will presently be described. I

Reverting now to the alkaline stock which has been delivered into the tank II, as shown, provision is made for uniformly or intimately mixing such stock with asolution containing alkaline compounds, e. g., caustic soda, for its purification. To this end, the stock in the tank is treated,

while being agitated, with a solution containing alkaline purifying compounds fed thereinto in controlled and regulated amount by way of a feeding device 30 from a pipe 36. The stock, together with the treating solution uniformly admixed therewith, is withdrawn from the mixing tank by a pump 3| and passed into a thickener 32, wherein it is concentrated or thickened to the consistency desired in the purifying treatment, say about 6% to 16%. The white water extracted from the stock in the thickening operation is passed from the thickener 32 through a pipe 33 into a storage tank 3i from which it is withdrawn by a pump 35 and passed through the pipe 63 into the mixing tank II, wherein it is reused for admixture with other raw alkaline stock to be purified. Thus the white water which is removed from the thickening operation and recirculated to the mixing tank merely serves as a vehicle or carrier .for effecting a thorough mixture and distribution of caustic soda throughout the stock. Such thorough mixing can only be obtained with a dilute stock containing excess carrier which is reemployed in the cycle. The stock is allowed to undergo the purification treatment at relatively low temperatures, which may range from lower than room temperatures up to C., as previously indicated. When a solution of only caustic soda is employed, the maximum concentration thereof is below that at which the mercerization of the alpha cellulose content occurs, and the minimum above that at which a suificient purification is not effected. For optimum results in respect to alpha cellulose content in the finished product, at a temperature of about 30 C. the minimum and maximum concentrations in a stock suspension of about 5% or 6% are, respectively, about and caustic soda based on the weight of dry fiber. The alkalinity of the caustic soda solution is thus about 5% to l caustic soda strength.

At this temperature the stock usually requires about one hour to undergo purification, .or, in other words to complete its passage through both treating tanks. Of course; the time of treatment or temperature may be varied, as desired, de-

pending upon the characteristics of the raw stock. The purified stock, together with the tremendous excess of unused caustic soda contained in the liquor, is passed through a counter-current washer 38 of a type similar to that previously described, wherein it is washed free from its alkaline constitutents. The washed and purified stock is withdrawn at one end of the washer, through the conduit 39.

The spent liquor which is withdrawn at the other end of the washer contains a large amount of free caustic. Thus, if a caustic solution contains 100%caustic soda based on the dry weight of stock at the time when it enters the reaction tank 34, approximately 90% of this caustic soda remains unconsumed or unconverted after the purifying treatment has been effected, so that, obviously, the larger portion of the liquor from the washer may be 'reutilized for the purification of other raw alkaline stock. As shown, such spent- '4 liquor is passed from the washer through the con duit 40 into an intermediate storage tank 4| from which-it is withdrawn by a pump 42 and deliveredwthrbugh a conduit 36 into the feeding device 307which serves to regulate the supply of purifying solution going to;the mixing tank I I. In order to avoid an accumulation of organic matter which has been removed from the raw alkaline stock,

however, provision is made to divert a certain small portion of spent liquor obtained from the counter-current washer 38, either to the evaporated liquor prior to its incineration, of for use together with alkaline liquor for the liberation passed into the digester I, if desired. This volume diverted for evaporation or use in the digester may approximate 10% of the total spent liquor.

For the replenishment of losses occasioned in the supply of alkaline compounds used in the purifying treatment, provision is made for continuously removing a certain small portion of strong, hot white liquor from the storage tank 21 to make up such losses. To this end, I have shown a pump 50 which withdraws a portion of such hot white liquor from the tank 21 and forces it through a pipe and a cooler 52, thence through a pipe 53 into the intermediate storage tank M, from which it is supplied to the feeding device 30 for admixture with raw alkaline stock. Such an arrangement -,allows the process to be carried out in a veryeconomical manner. Thus, when the purifying treatment is carried out in connection with a raw alkaline pulp derived from the digestion of wood chips by the kraft process, as outlined, the treating or purifying liquor for such alkaline pulp will contain a certain portion of sodium sulphide as well as sodium hydroxide. Sucha liquor has been found to serve the purpose of. converting kraft stock to a high alpha" cellulose containing fiber possessing the propertie spf very'high tear resistance and folding en- However, when the process is carried outpin a plantwherein chips are digestedin a solution of sodium hydroxide, obviously, then, the

"purifying treatmentis carried out with caustic soda alone as the purifying reagent.

However, in a kraft pulp plant favored by a cheap electrical supply so that sodium hydroxide maybe manufactured economically, that is, where local and economic conditions permit, a modification of the hereinbefore described procedure may be advantageous. The raw materials for carry ing out the process insuch case may consist of scdiumhydroxide and sulphur, rather than salt gether with the concentrated spent liquor. Of

' course, the amount of sodium constituent diverted to .the kraft mill from the purifying treatment will be-sufficient to make up sodium losses occasioned therein. With such an arrangement,

' even. though the purification treatment of the alkaline pulp is practised'in connection with the kraft process, no sodium sulphide will be present in the purifying liquor. 'Under conditions where the purifying treatment is carried out with sodium hydroxide alone, the resulting purified fiber is more easily bleachable, especially when it has been pretreated with an oxidizing'agent.

In certain other cases it may be. of advantage to change the described procedure to one wherein all the spent liquor obtained from-the washer 38 is employed, instead of the filtered hot white liquor, for the cooking of wood chips in the digester I. In other words, the procedure may be one wherein all of the liquor from the white liquor storage tank 21 is employed as a purifying liquor, and all of the liquor from the purifying treatment is utilized inthe digester I for the liberation of raw fiber from chips. Inasmuch as only a small proportion of the total alkaline content in the white liquor item the counter-current washer 38 has actually been converted to inert or inactive constituent in the purifying treatment, the strong liquor from the washer 38 has substantially the same concentration of sodium sulphide and canstic soda under certain conditions hereinbefore stated, as the white liquor in the storage tank 21. This being the case, it may be used in the digester I for cooking fresh chips, rather than recirculated to the mixing tank II, as shown in the drawing. With such an arrangement, the hot white liquor from the storage tank 21 may be passed through a cooler prior to its passage through the control feeding device 30 and into the mixing tank I I, if low temperature is employed in the purification treatment. On the other hand, if a full strength white liquor, with which a treating temperature of 75 C, should be associated, is used in the purification treatment, it is, of course, employed without cooling.

Among the advantagesflowing from the practice of the hereinbefore described procedure, is that I am able to produce, with a high yield, from raw alkaline digested pulp, a fiber having a high alpha cellulose content and possessing a very high tear resistance and folding endurance. The

Kraft pulp Purified pulp Mullen strength (beaten) 155-160 120-142. '1" 220-245 350-400. Alpha cellulose, 7 88-89 93-96. Pentosans, 7-10 3-5. Folding endurance 1800-2400 3000-4000 double fold Yield of process. 92-95.

From the foregoing table, it will be seen that ment prior to the alkaline treatment or purification, as hereinbefore indicated. Such pretreatment conditions the pulp for the alkaline treatment and allows the pulp to be bleached after the alkaline treatment with a moderate amount of bleach and without perceptible injury to its strengthor decrease in its alpha cellulose content. The pretreatment is, however, apt to lower the yield of final product somewhat. In'such case, the pulp is pretreated approximately at a.

stock density of about preferably with a solution containing 1% to 2% chlorine or 2% to 6% of 35% lime bleach (CaOCl-z) based on the dry weight of fiber. The oxidizing agent reacts with the difilcultly removable non-alpha cellulose constituents contained in the pulp, forming soluble reaction products, or products more easily removable by the subsequent alkaline treatment. The pretreated stock isthcn purified in .a solution of alkaline compounds, as hereinbefore described.

Since the pretreatment and subsequent alkaline purifying treatment have removed from the raw alkaline digested pulp a substantial amount of non-alpha cellulose constituents, the purified pulp can be bleached with materially less hypochlorite or bleach than is required without such treatments, to produce a pure white product; and this bleaching operation may be effected without material injury to the strength of the purified pulp, or reduction of its alpha cellulose content. Ordinarily, a bleaching treatment at a stock consistency of about to 16% with about 5% to 10% of 35% lime bleach (CaOClz) based on the dry weight of pulp, is sufficient to bring the pulp up to a whiteness equivalent to or approximating that of ordinary bleached sulphite pulp.

The stock thus bleached may be improved in color by a superbleaching operation, which may be carried out at a stock consistency 'of from 4% to 6%, with about 2% to .5% chlorine, based on the weight of dry fiber.

Owing to its properties resulting from the purifying treatment which it has received, the white high alpha cellulose fiber may be employed not only as a substitute for new rag fiber in the manufacture of high grade bond and ledger papers, but also in the manufacture of the cellulose esters and the cellulose ethcrs, and for other purposes where a substantially pure cellulose fiber is required.

Thus, by the practice of this invention, I am enabled to increase the tearresistance and folding endurance of the raw alkaline digested pulp at the expense of its Mullen strength, and also to purify it into a product which is especially high in alpha cellulose. This is highly desirable, since the tear resistance and its accompanying quality of folding endurance are of extreme importance in high grade papers, whereas the Mullen or bursting strength is usually ofsecondary importance. This may be better understood from the fact that it is relatively easy to increase the Mullen or bursting strength of a paper sheet by suitable beating and sizing. The heating and sizing operations, however, do not affect the tear test so appreciably, so that it is a distinct ad-- vantage to increase the tear test even at the expaper may be employed for the production 01' artificial reed fiber, such as serves in the manufacture of baby carriages, furniture, etc. Such a paper should have a high tear resistance, should be soft and yet firm, and should have sufficient porosity to permit its ready impregnation with glue, varnish, or other waterproofingreagents. Such paper can be produced from the product resulting from the herein described process.

Before concluding, it may be stated that, while I have specified certain concentrations of stock and treating compounds in order to more fully and adequately describe my invention, these concentrations may vary, depending upon plant equipment and other conditions. That is to say, the plant equipment with which the present invention was practiced permitted the treatment of an alkaline digested stock at a concentration of about 5% to 6%, most efliciently 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 adjusted. The determining factors in carrying out the purifying treatment or reaction are the concentration of the alkaline compounds in the liquor and the temperature at which the liquor is used. Of course, if sodium sulphide or some other reagent, which is less drastic or effective in its reaction than caustic soda, is used, the percentage of such reagent may be increased in order to produce a product substantially equivalent to that obtained by the use of caustic soda.

The change in the characteristics of the raw alkaline digested pulp is effected with a relatively high yield of finished high alpha cellulose product, the yield ranging from 93% to 95%, based on the weight of dry, raw pulp.

At the risk of repetition, it may be stated that the drawing is wholly diagrammatic and conventional, no attempt having been made to show the true size relationship between the various instrumentalities, or their proper location. For the sake of clarity in the drawing, certain details of construction have been omitted, but it will of course be understood that fittings, pipes, pumps, devices for controlling temperature conditions; or any other apparatus necessary for maintaining the flow of the various materials or for maintaining certain conditions, will be supplied wherever necessary.

I do not herein claim the subject matter of my Having thus described this invention and a method of practicing the same, it should be obvious to those skilled in the art that various changes and modifications in procedure might be resorted to without departing from its spirit or scope as defined by the appended claims.

What I claim is:

1. A process which comprises treating with a liquor containing fresh alkaline sodium compounds, pulp derived from the digestion of wood chips in a digesting liquor containing sodium compounds, reutilizing one portion of alkaline liquor resulting from such treatment in the alkaline treatment of other pulp,- and smelting and recovering the sodium compounds from the other portion of muchliquor, together with the sodium compounds of said spent digesting liquor, in the form of alkaline sodium compounds.

' 2. A process which comprises treating with a liquor containing fresh alkaline sodium compounds, pulp derived from the digestion of wood chips in a digesting liquor containing sodium compounds, reutilizing one portion of alkaline liquor resulting from such treatment in the alkaline treatment of other pulp, smelting and recovering the sodium compounds from the other poition of such liquor, together with the sodium compounds of said spent digesting liquor, and'adding suflicient alkaline sodium compounds to said first mentioned portion to carry out the alkaline treatment of other pulp.

3. A process which comprises treating with a liquor containing fresh alkaline sodium compounds, pulp derived from the digestion of wood chips in a digesting liquor containing sodium compounds, reutilizing one portion of alkaline liquor resulting from such treatment in the alkaline treatment of other pulp, smelting and recovering the sodium compounds from the other portion of such liquor, together with the sodium compounds of said spent digesting liquor, in the form of alkaline sodium compounds, and adding suflicient alkaline sodium compounds from the recovered alkaline sodium compounds to said first mentioned portion to carry outithe alkaline treatment of other pulp.

4. A-process which comprises treating with a liquor containing fresh alkaline sodium compounds, raw pulp derived from the digestion of wood chips in a digesting liquor containing alkaline sodium compounds, utilizing alkaline liquor resulting from such treatment as digesting liquor for the liberation of other raw pulp from wood chips, smelting and recovering the sodium compounds from said digesting liquor in the form of alkaline sodium compounds, and utilizing the recovered compoundsin an alkaline liquor for. the treatment of other raw pulp.

5. In purifying unbleached pulp preparedfrom wood by an alkaline process, the sequence of operations which comprises partially bleaching the pulp, treating the-partially bleached pulp with a solution of caustic alkali, the. pulp and solution being brought together-in such amounts and concentrations as to give a resulting suspension hav-.

ingan alkalinity equivalent to that of a caustic soda solution of at least about 5% strength but below mercerizing strength, continuing the treatment for the desiredperiod of time at a temperature less than the boiling point of water, separating caustic "alkali solution from, the sotreated pulp and washing the latter, subjectingthe treated and washed pulp in aqueous 'suspension to a second stage of bleaching treatment, and washing the resulting finally bleached pulp.

6. A process which comprises subjecting unbleached alkaline wood pulp to a preliminary bleaching treatment with a fraction of the total bleaching agent required, admixing the partially bleached pulp with a solution of caustic alkali of such concentration and in such' proportion that the resulting mixture has an alkalinity equivalent to a solution of caustic soda of at least about 5% strength but below mercerizing strength,

maintaining the mixture at a temperature below- 212 F. for the desired period of time, separating caustic alkali solution from said mixture and washing the pulp, subjecting the washed pulp to a secondary bleaching treatment with the restof the bleaching agent required, and washing the resulting pulp.

'7. A process which comprises subjectingun-j" bleached sulphate pulp obtained from coniferous.

wood to a preliminary bleaching treatment with a fraction of the total bleaching agent required,

period of time, separating caustic alkali solution from the mixture and. washing the pulp, subjecting the washed pulp to a secondary bleaching treatment with the rest of the bleaching agent required, and washing the resulting pulp.

8. A process which comprises subjecting unbleached sulphate pulp obtained from coniferous wood to a preliminary bleaching treatment with a fraction of the total bleaching agent required, thickening the partially bleached pulp, admixing therewith caustic soda solution in such amount and concentration as to bring the re- 'sulting mixture to a consistency of about 6% of' dry pulp substance with an alkalinity of the resulting solution equivalent to about 6.5% of caus-, tic soda, maintaining the mixtureat lessthan the boiling point of water for about one hour with agitation, separating caustic soda solution .from

the mixture and washing the pulp, subjecting the washed pulp to a secondary bleaching treatment with the rest of the bleaching agent required, and washing the resultingbleached pulp. I

9. A method for the treatment of. cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an -aqueous solution containing an, alkaline sulphide, removing resulting solution from the remaining pulp,

and treating thelatter with an aqueous alkaline solution of a reactive compound of sulphur.

.10. A method for the treatment of cellulosic' materials which comprises reducing the same to the condition of pulp by efiecting incomplete] cooking with anaqueous solution containing an alkaline sulphide, removing the resulting solution from the pulp and treating the latter with, 'a'

reactive compound of sulphur in alakaline s'olution.

11. In purifying unbleached chemical wood pulp, the sequence of operations which comprises partially bleaching-the pulp, washing the partially bleached pulp, treating the partially bleached and washed pulp with a solution of caustic alkali, the pulp and solution being brought together in such amounts and concentrations as to give a resulting suspension having an alkalinity equivalent to that of a caustic soda solution of from about 5% to about 12% strength and a pulp concentration of from, about 5% to about 10%, continuing the treatment'for from about 10 minutes to about 6- hours, separating caustic alkali solution, ,from the so-treated pulpv and washing the latter, subjecting the treated and washed pulp to a second stageof bleaching treatment, and washing the resulting finally bleached pulp.

12. A method for the treatment of cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an aqueous solution containing an alkaline sulphide, removing resulting solution from the remaining pulp, and treating the latter with an aqueous alkaline solution of sodium sulphide.

alkaline sulphide, removing the resulting solution from the pulp, and treating the latter with sodium sulphide in alkaline solution.

14. A method for the treatment of cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an aqueous solution containing an alkaline sulphide, remov ing resulting solution from the remaining pulp, treating the pulp with a bleaching agent, and then treating the pulp with an aqueous alkaline solution of a reactive compound of sulphur.

15. A method for-the treatment of cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an aqueous' solution containing an alkaline sulphide, removing resulting solution from the remaining pulp, treating the pulp with a bleaching agent in amount insuflicient to bring the pulp to substantial whiteness, then treating the pulp with an aqueous alkaline solution of a reactive compound of sulphur, and again treating the pulp with suflicient bleaching agent to bring it to substantial whiteness.

16. A method for the treatment of cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an aqueous solution containing an alkaline sulphide, removing the resulting solution from the remaining pulp, treating the pulp with a bleaching agent, and then treating the pulp with an aqueous alkaline solution 01' sodium sulphide.

17. A method for the treatment of cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an aqueous solution containing an alkaline sulphide, removing resulting solution from the remaining pulp, treating the pulp with a. bleaching agent in amount insufilcient to bring the pulp to substantial whiteness, then treating the pulp with an aqueous alkaline solutionoi sodium sulphide, and again treating the pulp with suflicient bleaching agent to bring it to substantial whiteness.

18. A method-for the treatment of cellulosic materials which comprises reducing the same to the condition otpulp by cooking with an alkaline liquor containing caustic soda, removing resulting liquor from the remaining pulp, and treating the latter with an aqueous alkaline solution of reactive compound of sulphur.

19. A method for the treatment of cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an alkaline liquor containing caustic soda, removing resulting liquor from the remaining pulp, and treating thelatter with an aqueous alkaline solution of sodium sulphide.

20. A method for the treatment of cellulosic materials which comprises reducing the same to die condition of pulp by cooking with an alkaline liquor containing caustic soda, removing resulting liquor from the remaining pulp, treating the pulp with a bleaching agent, and then-treating the pulp with an aqueous alkaline solution of a reactive compound of sulphur.

21. A method for the treatment of cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an alkaline liquor containing caustic soda, removing resulting liquor from the remaining pulp, treating the pulp with a bleaching agent in amount insumcient to bleach the pulp to substantial whiteness, then treating the pulp with an aqueous alkaline solution of a reactive compound of sulphur, and again treating the pulp with suflicient bleaching agent to bleach it to substantial whiteness.

22. A method for the treatment of cellulosic materials which comprises reducing the same to the condition of pulp by cooking with an alkaline liquor containing caustic soda, removing resulting liquor from the remaining pulp, treating the pulp with a bleaching agent, and then treating the pulp with an aqueous alkaline solution of GEORGE A. RICHTER.

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