US2046551A - Process of bleaching chemical wood pulp - Google Patents

Description

D. B; DAVIES PROCESS ,OF 'BLEACHING CHEMICAL WOOD mm?- July 7, 1936 Filed Sept. 8, 1951 4 A'h-oRNEY PROCESS OF BLEACHING CHEMICAL WOOD PULP David B. Davies, Shelton, Wash.

Application September 8, 1931, Serial No. 561,723

28 Claims.

My invention relates to the process of bleaching chemical wood pulp such as sulphite, sulphate or soda wood pulp, to produce a better pulp product, to the product of said process and to the apparatus for carrying out said process.

In general, my invention relates to a preliminary treatment of such pulp with chlorine and then following such preliminary treatment by subjecting such pulp to the action of commonly used pulp bleaching reagents. Referring, for example, to the treatment of the sulphite chemical wood pulp, my invention is concerned with the treatment of the pulp after it has left the digesters and passed. over the usual knotters, rifliers, screens and deckers, (that is, a pulp thickening means) or other suitable mechanical means of cleaning the pulp of foreign matters of relatively large size. Such pulp has lignin and other encrusting foreign substances enveloping the fibre v which must be removed.

I use the termbleaching actionhere'n as commonly employed in the pulp art. That is, to refer particularly to the purification of the pulp by removing encrusting. materials, including 5 lignin which envelops the fibre or is disposed throughout the fibre in the form of a stain. That is, the term bleaching is not confined to the action which results in changing the color of the pulp to whiteness. Briefly stated, my invention 30 comprises the discovery that chlorine, other than in a, previously prepared solution form, may be introduced directly into the chemical pulp of low density when in a stream, that is, when the pulpstock suspension is under' motion, to remove 5 lignin and other foreign encrusting substances of the fibre to the extent of 30 to 50% of the total bleaching action; and my discovery includes the fact that the said pulp may be washed in water at normal pulp suspension temperature for the 40 separation of the compounds rendered soluble by the chlorine. The figures herein given are particularly concerned with sulphite pulp. A'second feature of my discovery relates to how the chemical activity of chlorine may be advantageously 45 employed to thus remove the lignin and other encrusting foreign substances without injury to the fibre, namely by providing for the temporary retarding of a part of its chemical activity, causing its thorough mixture with the pulp without 50 agitating means in the cell, and in keeping it in contact with the pulp for the progressive action of the chlorine until its full effect is obtained. A third feature of my invention comprises the discovery of how to very materially reduce the cost, 55 of the bleaching action,-namely, purifying the pulp fibre and changing the pulp color by providing for: 1-A continuous process, 2-Highly efiicient chemical action, 3Reducing by to 35% the period normally involved in performing the bleaching action, 4-Saving of chemicals nor- 5 mally employed, and 5-Avoiding destruction and deterioration of the alpha cellulose content. The preliminary treatment constituting my discovery is advantageously employed previous to subjectreagents, as calcium hypochlorite, may be efiected with better control of the viscosity factors of the pulp. Such improved viscosity results in a better j pulp for rayon and cellophane manufacture characterized by greater uniformity, lower ash 25 content and uniform viscosity whereby expensive filtration difliculties arising from filtering the dispersion of cellulose obtained from the pulp are avoided. Viscosity of the dispersion of cellulose afi'ects the'spinning process used in rayon manufacture very remarkably because of the fact that the dispersion must be forced through minute spinneret openings into the coagulating bath. Any change in the viscosity of the dispersion of the cellulose, seriously affects this spinning process and by virtue of this, afiects .directly the quality and properties of the resulting rayon yam. Change in the viscosity destroys the uniformity of the yarn, and uniformity of product in rayon manufacture is a prime requisite. A pulp for paper manufacture using this preliminary treatment results in an unexpected action in that a better paper. product may be made therefrom, which paper is characterized by having better physical properties, such as improved bursting, tearing and folding properties.

In the two stage bleaching systems now commonly, commercially employed, it is during the first stage that the viscosity of the cellulose dispersion is primarily, if not entirely, determined. Accordingly, the action of the chemicals during .the first stage of the presently, commercially emtion with chlorine, how to treat the pulp very rapidly and at the same time have discovered a way of treatment characterized by not adversely afiecting the viscosity.

Heretofore, the methods in common use of bleaching sulphite wood pulp generally werebased on the use of calcium or sodium hypochlorite in contact with the pulp. The solution of these chemicals is made up by causing the absorption of chlorine gas in a milk of lime solution or a caustic soda solution,usually, due to the cost, a milk of lime solution. The amount of lime employed in this solution is regulated according to the strength of chlorine desired. Ordinarily, these solutions are comparatively strong. Two general types of bleaching cells are in commercial use. The first and older of the two is the low density bleaching system which means a pulp suspension in water, wherein the pulp is 4 to 6% of the total weight. The newer system is knownas the high density system and the-concentration of the suspension of the pulp in water in this case is from 14 to 18% by weight. In both instances, suitable means have to .be provided for continuously agitating the pulp in the cells during the bleaching period. A modification of these processes developed in the last .few years, consists in carrying, the bleaching through in two stages. Sometimes intwp stage bleaching, two high density systems are used, while in other cases, one high and one low density is used. When two stages are employed, a predetermined amount of bleaching is done in the first stage, ordinarily, about seventy-five percent.

The pulp is then washed and all dissolved andencrusting matter is washed out and the pulp thickened and put into the second stage. After the second stage bleaching it is then washed and considered as fully. bleached. The chemical reaction which takes place whether the low density or the high density system is used, brings about the freeing of oxygen from the water present and this oxygen is generally stated to be responsible for the bleaching action.

While it has long been known that a' solution of chlorine in water, previously prepared and then added to the pulp, or chlorine gas at normal temperature, would bring about a good bleaching action at a very fast rate, it has not been successful, among other reasons, (11) because of thedrastic nature of the action of chlorine resulting in such a destruction of the fibre and'in such a pronounced deterioration. of .the cellulose fibre undestroyed, as to be economically. prohibitive in commercial operation; (b) because of the problem of keeping the chlorine in contact with the pulp in a proper Way for a predetermined period, and (c) because of the difiiculty of the pulp equipment confining the gas, i. e., equipment for processing pulp must be specially designed to meet the inherent properties of pulp and the meeting of these requirements renders it diflicult to confine the gas, as illustrating some of these difliculties, channeling tendency of the pulp; necessity of avoiding contamination and undue concentration of chemicals locally. Ac-' cordingly, instead of the solution of chlorine in water being applied directly to the pulp, the practice has been, as explained above, to use calcium hypochlorite or other hypochlorites because the action was comparatively slowsince in the decomposition of the hypochlorite, the production of oxygen was slow.

I have discovered,-as the result of extended experimentation, that a greatly improved pulp sion of the chlorine.

without any lime directly in contact with the rim in liquid form, as herein set forth, the procif its chemical activity is retarded. Such re- 5.

tarding is accomplished by lowering the temperature of the chlorine and this may be done by using chlorine in the liquid form and allowing it to expand in direct contact with the pulp, or by refrigerating the chlorine gas, or by compressing the chlorine gas and allowing it to expand while in direct contact with the pulp. Preferably, I employ chlorine in the liquid form and lead such chlorine in a feed pipe into the body of the stream of pulp and preferably after exposing such pipe to the temperature of the pulp in the stream. Thereupon, I introduce the chlorine directly into the pulp of low density while said pulp is in motion through a conduit enroute to the bleaching cell. The motion of flow of the suspension is necessary to prevent drastic action of the chlorine, that is, so the chlorine is continuously contacting with new material. Such motion is also necessary to get the proper dispersaid chlorine is being used 25 pulp and without injury to the pulp-that is, I have discovered that said chlorine may be used without any lime without injury to the pulp. Also, I have discovered that it is not necessary 3 to have agitation of the pulp as was heretofore the case when calcium or sodium hypochlorite was employed. Since it is not necessary to con; tinuously agitate the pulp when using the chloess which I have discovered is more easily adapted to continuous operation and with material at a comparatively low density, about 3%. The degree of density preferred is that which will per mit a ready flow of the pulp in suspension so that the material will be subject to beingreadily operated upon by the usual pulp pump means. It is not necessary in the device embodying my' invention to have separate and distinct agitating means in the body of the cell which is power 45 driven by means exterior to the cell. It will be understood that it is particularly advantageous to employ pulp of low density, as its transfer through the various stages of treatment is great ly facilitated. During this preliminary treatment with said chlorine, I accomplish 30 to 50% of the total bleaching action. After the pre-,

liminary' treatment, I proceedwith the bleaching process proper, which may be a two stage system.

by employing calcium hypochlorite in the first 55 stage to effect about 20 to 61% of the total bleaching action. In the second stage of bleaching proper, I employ calcium hypochlorite to ac- I complish about 9 to 30% of the total bleaching action. This pulp thus processed results in a 60 color valuation of 84 to 89. on the color scale. Heretofore, in all attempts to employ chlorine in solution or as a gas at normal temperature, such chlorine has been employed to accomplish only 1 to 5% of the total bleaching action, as against 65 30 to 50% in the process of my discovery. This was due, in part, to the difilculty or. confining the chlorine in the equipment, as well as keeping the chlorine in contact with the pulp for a. suflicient period of time. The ordinary bleaching cells are 70 charged from the top, while I provide a cell in which the material is charged into the cell by way of the bottom portion. a My process prevents damage being done by too long contact and this is due largely to providing for a continuous process and a most thorough inter-mixture and release of the mixture of chlorine and pulp into a large body of the pulp in suspension.

The primary'ob'jects of my invention are: to produce a better pulp product from the chemically prepared woodpulp; to produce a bleached pulp having an alpha cellulose content around 88 to 91%; and a color characteristic of 84 to 89 onthe color scale; to produce a pulp of said alpha cellulose content which is characterized by its uniformity of chemical content, lower ash content, and uniformity of viscosity, the uniformity of content being ,of the utmost importance in rayon manufacture, due to the fact that it bears directly upon the proper dyeing results and spinning properties; to provide said better pulp more economically than has heretofore been done by providing a preliminary treatment of a continuous process, making, as one contributing factor, reduction of the time presently employed in purifying and bleaching the pulp about 25 to 35%; and making a reduction in the amount of chemicals employed, by increasing the efficiency of such chemicals as are used. A further object of my invention is to provide a process for the direct introduction of chlorine into the pulp without the deterioration of the cellulose fibre, which has heretofore occurred in attempts at such direct chlorine treatment, due to the drastic actionof the chlorine, and to provide, an apparatus for carrying out the process characterized by fulfilling the objects above set forth.

The above mentioned general objects of .my invention, together with others inherent in the same, are attained by the invention illustrated in the following drawing, the same being preferred exemplary forms of embodiment of the apparatus part of my invention, throughout which drawing like reference numerals indicate like parts:

' 10 feet in diameter.

Figure l is a view in longitudinal section of the bleaching cell embodying my invention;

Fig. 2 is a view in cross-section of said cell on dotted line 2, 2 of Fig. 1;

Fig. 3 is a view of a detail part of the device,

namely the spinner located in close proximity to the discharge end of. the feed conduit for the wood pulp; v

Fig. 4 is a view of a modified form of the discharge outlet conduit part of my invention;

Fig. 5 is a view of 'amodified formof chlorine feed'pipe provided with a refrigerating coil. v

A bleaching cell 6 of cylindrical form is provided which may be formed of wooden walls having a. working capacity-of about 3% tons, which preferably means a cell of about 50 feet high and A pulp conveying conduit I having a gas trap means, preferably by having an inverted U shape section overthe top of the cell 6, comprising a pipe which may be of 10 inches in diameter, leads into the cell in the bottom portion. A spinner 8, which may be of rectangular contour and of lignum vitae material, is mounted, in close proximityto the discharge outlet of conduit 1, said spinner being mounted on shaft 9' having pulley I0 by means of which, power may be applied through shaft 9 to the spinner 8. Spinner 8 functions to thoroughly mix all portions of the stream of incoming pulp and break up any small masses of pulp which may be present-in the stream of pulp. Intermediate the length of the cell 6, outlet ports II are arranged in angular, spaced relation and these angles may be 90?. Said ports are preferthe pressure of the liquid chlorine in the cylintion of cell 6 and leads to a washing means (not shown). Said conduit may be 10 inches in diameter. A pipe IQ for the admission of chlorine, preferably in the liquid, form, extends through the wall of pulp feed conduit 1 and extends down for some distance axially in said conduit 7 so that the discharge outlet 20, having preferably a plurality, as 60 or 70 minute openings, is in spaced relation to the point 2| where the pipe l9 enters conduit 1. Chlorine gas escape pipe 22 connects with the cell 6 in the top portion. Needle valve 23, gauge.24, pressure reducing valve 25, globe valve 26 and gauge 21 are all operatively mounted 20 in chlorine feed pipe l9 and each has its obvious function. A by-pass pipe 28 with valve 29 operatively connects the conduit 1 with pipe 19 whereby all chlorine gas which may be left in conduit 1 when the operation of the cell is stopped for 25 any reason, may be withdrawn and its escape into the plant avoided.

In Fig. 4, a modified form of the discharge outlet is shown, wherein the discharge conduit 30 does not have a L' shape trap, but has a chlorine 30 gas collector pipe 3| so that when the level of the pulp is at '32 to form a seal for the discharge outlet pipe 30 if any gas should escape through the discharge conduit, it will be collected in pipe 3| and its escape into the plant avoided.

In Fig. 5, a modified form of the chlorine feed pipe isshown wherein feed pipe 33 has an enlarged chamber 34. In the chamber 36 provided in the pipe 33, refrigerating coils 35 are located, whereby chlorine gas, in passing through chamber 34, may have its temperature reduced, so that its chemical activity with the pulp stock is retarded. After having the temperature reduced, the gas is conducted by the feed pipe 33 into conduit 1, precisely as by feed pipe l9. 45 The mode of operation of said bleaching cell embodying my invention is as follows: Wood pulp of low density, approximately 3%, or of such density as will permit ready fiow, which in general, will not be over 6%, is conveyed by pipe 1 to the cell 6 from the blow-pit (not shown) by way of the usual knotters, rifliers, screens and decker, or other suitable mechanical means of cleaning the pulp of relatively large foreign parl ticles. The liquid chlorine is conducted to conduit I by pipe I9 from standard liquid chlorine cylinders (not shown), past pressure gauge 21,

ders is about pounds and is reduced to ten to twenty pounds pressure in pipe I!) by pressure gauge 24. This chlorine in liquid form, accordingly is discharged from the discharge outlet 20 directly into the stream of pulp after said chlorine 5 has been exposed in pipe I9 to the relatively warm wood pulp in conduit I, which wood pulp ordinarily has a temperature of 60 F. The exposure of the pulp to the pipe I!) cools the pulp due to the heat absorbed by the chlorine, and in the 7 operation of my'invention, 'the temperature of the pulp in conduit 1 is lowered to a. temperature below the freezing point of'water. The length of the pipe l9 within conduit 1 may be varied. This pipe may be of the metal alloy known as KA, 7'5

. dispersing means.

that is, a chrome nickel and refined steel alloy which is particularly resistant to the action of chlorine under the conditions involved. The stream of the mixture upon being discharged into the cell 6, encounters spinner 8 which operates to thoroughly inter-mix the components of the stream and breaks up any lumps of wood pulp which might be present. Also, said spinner operates to break up any particles of ice that might be present in the stream derived from the discharge outlet 25 of the liquid chlorine pipe iii. The pulp rises in the cell 6 uniformly throughout the cell cross-sectionally'considered, This important result is provided without having any beater or agitating means in the body proper of the'cell, by means of the recirculating means comprising conduits l2, l3, l4 and pump l5 and conduit Hi. This recirculating means provides for thepulp within the peripheral portions of the cell to rise uniformly about the walls and then as the pulp is pumped into the bottom portion of the cell under pressure, a flow is caused axially of thecell and upwardly directed. The recirculating means operates to prevent the pulp from channeling or forming stream through the pulp within the cell 6. Suchchanneling is a charac-' teristic tendency of pulp in suspension. The recirculation of the pulp thus provided also secures the further thorough and uniform and exposure of the pulp to the action of the chlorine before the wood pulp is discharged through the discharge pipe II. The rate of pulp feed is about nine or ten tons per hour and the recirculating means has a capacity of about three and one half tons of pulp per hour. The chlorine is held in contact with the pulp in the' cell for about; twenty minutes. This retention is not only attained by means provided for the uniform flow of the pulp through the cell, but also by means of the thorough inter-mixing secured by the spinner 8. Without the thorough inter-mixing by means of the spinner 8, it was found that ,the chlorine passes through the cell in about three minutes a distinct meaning and results in hydrationv of the pulp, and for a beater to have this result it is necessary that friction or a rubbing action be involved which is not the action of the spinner 8. i

If the liquid chlorine or refrigerated chlorine gas is introduced directly into the cell, beneficial the cell proper the pulp was found to be overtreated in part and other parts very much undertreated.

Trap l,8 functions to prevent any escape of chlorine gas through the" pulp' discharge outlet ipe and the free chlorine gas which'may collect in the'top portion of the cell may be collected by pipe 22. It will'be understood that the utmost care is necessary to prevent chlorine gas which may not have been, completely consumed in the process, from escaping into the plant and the cell embodying my invention provides against this difficulty, not only by providing forits eflfi- I cient consumptiombut also providing for the 001- .lection of any that escapes being consumed. Ac-

Beater in the pulp art has tual operation has shown such free chlorine to be very small in volume.

The apparatus thus described, constitutes 7 means for carrying out the process of my invention. Manifestly, this apparatus is designed for 5 continuous operation as distinguished from the usual batch form of standard bleaching cell now commonly employed, having agitating or heater .means within the cell and power means outside form of a gas at reduced temperature. Obvious- 15.

ly, instead of employing refrigerating means to reduce the temperature of the gas to provide for retarding chemical activity of the chlorine upon escape from outlet 20, the gas-could be compressed by any standard compression means and 20 introduced at a temperature comparable to the temperature of the stream of wood pulp in conduit 1, so that upon expansion during the escape from the discharge outlet 20 of the chlorine feed pipe Hi, there would be an absorption of heat 25 from the wood pulp resulting in a reduced temperature, which would likewise operate to retard the chemical activity of the chlorine. It will be understood that where the refrigerating means, illustrated in 5, is employed, to reduce the 30 temperature of the chlorine gas, that the gas. is under such pressure as to overcome the pressure in the feed conduit I, which, in the apparatus illustrated, provides for a hydrostatic pressure due to the head in the inverted U.

Heretofore, a solution of chlorine gas in water has been-provided and this has been found to have such a destructive and deteriorating effect upon the wood pulp that it has never been successful, and is nowhere presently in use in commercial practice for the purpose herein set forth so faras I know. It will be understood that the lignin compounds of wood pulp are exceedingly reactive. to chlorine or to hypochlorous acid and other oxidizing agents. The action of the chlorine has proven so drastic when directly applied to the pulp that its use long since has been abandoned commercially, so far as I know.

Whenattempts have been made to apply chlorine to the pulp, it has been after first preparing the chlorine as a solution in water, or it has been applied in gaseous form at normal temperature and pressure. In all such attempts, com mercially, only 1 to 5% of the total bleaching action was achieved However, I have discovered how chlorine. may be employed to perform 30 to 50% of the total bleaching action. I do not know positively the theoretical explanation of -just why the chlorine applied directly, as in my discovery, does not injure the pulp, as has been the uniform experience heretofore. One possible explanation, and the one which I hold as is best explained by myexperiments, is that the chlorine in liquid form uponbeing transformed from the liquid to the. gaseous form in the stream of pulp, absorbs heat in order to expand to form a gas, and in so doing reducesthe temperature of the immediate pulp. The reduction in temperature thus brought about provides for a greater quantity of the gas going into solution. However, the

expected usual drastic action of chlorine and the expected even greater drastic action of this stronger solution thus developed seems, in connection with wood pulp and under the conditions of my-discovery, to be sufiiciently off-set by the supply of the hypochlorous acid. In this wise,

lower chemical activity of the chlorine at the lowered temperature. Thus, the lowered temperature operates in my discovery, it would seem, to sufficiently moderat'e'or otherwise affect the ordinary drastic action of the chlorine upon the pulp and by the time its temperature rises so that its action would'be drastic, it is thoroughly mixed with the pulp in suspension in-the stream and is discharged into the contents of the cell where it has a very large body of pulp upon which to act. Its action is therefore spread over a great body of pulp, so that its drastic action on any given unit is impossible. Unquestionably, this result is greatly assisted by the uniform dispersion and thorough mixing in the pulp stream together with the uniform movement through the cell of the pulp throughout all parts of the cell.

Furthermore, a portion of the chlorine in liquid form, upon being discharged into the pulp at the ordinary temperature of 60 F. obtaining for the.

pulp at the lowered temperature, forms solid masses of chlorine hydrate. This portion of the chlorine would also have its action deferred until -,the temperature became such as to permit the chlorine hydrate to become hypochlorous acid.

Thus, the chlorine would seem to be present after introduction into the pulp in several forms, or conditions, namely:

l. Chlorine which ,is ready 'to dissolve in the water to form chlorine solution; 2. Chlorine in solution; 3. Chlorine hydrate; 4. Hypochlorous acid; and 5. Hydrochloric acid.

These various forms of the chlorine, excepting the hydrochloric acid, have different degrees of chemical activity and become successively efl'ective. The pulp stream in the conduit 1 at the point of discharge into the cell is under pressure this hypochlorous acid breaks down, by giving up its oxygen, into hydrochloric acid. The oxygen thus liberated oxidizes the lignin content of thewood pulpand other encrusting matter of the pulp fibre and forms soluble compounds which may be freed from the pulp fibre by subsequent washing. The purposein my invention in subjecting the pulp in the first stage of bleaching to the action of chlorine in liquid form is to provide a more efficient oxidizing agent. Obviously, it is highly advantageous to havepresent chlorine in a form which may immediately form hydrochlorous acid so soon as the foreign matter on the pulp fibre depletes theconcentration of hypochlorous acid in its immediate vicinity. It will be understood that as the hypochlorous acid breaks down to hydrochloric acid by virtue of the reaction of the hypochlorous acid with the impurities present on the fibre, that no further oxidizing effect is possible from that particular portion of the chlorine. Manifestly, by providing for the presence of the chlorine in a form and under lowered temperature conditions such that its full action is deferred, I provide for a supply, as it were in situ, of chlorine material to be immediately available to continually form hypochlorous acid and thus keep up a continuous ess proper and next preferably after separating an exceedingly high degree of efficiency is provided during the first stage of bleaching, or as herein termed the preliminary treatment, that is, a high degree of efficiency is provided for the 5 purification of the pulp during this preliminary treatment. Although my process involves the fact that the drastic action which results in treating the fibre is overcome, whatever the real reason may be, nevertheless, it is not possible to entirely complete the bleaching action by my process or by its repetition exclusively. During the first stage of bleaching the pulp fibre obviously has a greater quantity of lignin and other encrusting foreign matter enveloping thesame. 15 My discovery thus provides for disposing of the relatively large quantity of lignin or other fibre encrusting matter present in the initial treatment to an extent of to 50% and by a chemical which has heretofore been'adjudged in commer- 20 cial production too drastic in its action, and at the same time my discovery gives the unexpected result of not deteriorating the fibre but rather it produces a fibre having greatly improved color characteristics and higher alpha content and a 25 cellulose dispersion of improved viscosity.

While it is thus necessaryto have the chlorine present in a form to readily .produce hypochlorous acid promptly upon the depletion of the concentration as expiained, nevertheless, it is no less 30 important to provide for the control of the duration of the action. This control is accomplished in my invention by the definite control of the amount of the chlorine initially admitted, and by the thorough dispersion of the same and byits retardment of action and by the continuous process which provides for its prompt withdrawal.

Furthermore, I provide for overcoming the drastic action of chlorine by providing a method of dispersing the same uniformly through the wood pulp. This uniformity of inter-mixing or dispersion results from the step characterizing my invention of introducing the chlorine in liquid form into a stream of the pulp, portions of which stream are subjected to varying velocities of flow. This varying velocity results in part from portions of the material alternately finding its way to the part of the stream next to the wall of the conduit where its flow is retarded and then shortly finding itself in the center portion of the conduit where the velocity of flow is greater. Therefore, a most thorough inter-mixing results at a time when the chemical action is retarded. This thorough inter-mixing is further assisted by the action of the spinner mounted in close proximity to the discharge end of the conduit 1 supplying, or feeding the mixture to the cell.

After the preliminary treatment, above set forth, I preferably separate and wash the pulp 0 to remove the soluble compounds formed of the lignin and other encrusting matter and then subject the pulp to the usual calciumhypochlorite solution for the first stage of the bleaching procand washing the pulp, I subject the pulp to a further bleaching action of the usual calcium hypochlorite solution for the second stage of bleaching, when it is usually separated and washed again. This completes the second stage of bleaching process proper.

The second and third stages are in batch treatments and may be carried out in standard bleaching cells, said standard'bleaching cells not being gas tight and being provided with inlets for the introduction of pulp, chemicals and water. Aftersuitable washing the pulp is delivered continually to a plurality of bleaching cells so that while batch treatment is adopted, yet there is always a bleaching cell available to receive the stock from the continuous process of the preliminary treatment under conditions of normal operation.-

I have found that 30 to 50% of the total bleaching action maybe performed during the preliminary treatment with chlorine when applied to the pulp in accordance with-my invention. In the manufacture of pulp for rayon manufacture, I have found that it is preferable to perform about 35% of the total bleaching action during the preliminary treatment, about 55% during the first stage of bleaching with calcium hypochlorite and about 10% during the second stage of bleaching with calcium hypochlorite solution. This results in a pulp having an 89 color characteristic on the color scale commonly employed in grading pulp. In the manufacture of paper pulp, I have found that about 40% of the total bleaching action may be advantageously accomplished during the preliminary treatment when my process is employed, while about 30% of the total bleaching action 'may be accomplished in the first stage of bleaching with calcium hypochlorite and about 30% in the second stage of bleaching'using cal- ,cium hypochlorite. This results in a pulp having a color characteristic of 84.

The average concentration ofchlorine in 'the' conduit in my invention is preferably from oneone thousandth to three-one thousandths parts by weight. I

My discovery is characterized by dispensing with the step of agitating the pulp during the prelininary treatment so that a continuous method of treatment at a comparative low density,

namely about 3% is provided. All this results in increased efiiciency in operation, so that 25% to 35% of the time normally employed is saved.

In addition to the advantage of continuous operation, my process is characterized by the ease of control over the introduction of the chlorine and,

the importance of this control is manifest when it is stated that the output of a single unit may vary for a givenperiod from 90 to tons of pulp. Besides this control, my discovery facilitates control of the chemical action during the bleaching process proper as elsewhere herein set forth. A further advantage of my process is that it provides for the saving of the use of the chem ical lime and this saving means that the lime compounds or reaction products do not need to be removed from the pulp by washing, so that a double saving results. Further, because of the large amount of bleaching accomplished in my preliminary treatment and the low cost \of installation of the same, I have found that most ordinary existing bleaching plants may approximately double their output, while adding only approximately twenty percent to their cost of equipment. v

The above preliminary treatment and two stage bleaching makes it possible to provide a pulp containing 88.5% to 89.2% alpha cellulose, and so complete are the control features provided that an alpha content within an exceedingly narrow' able to increase the alpha cellulose content to 91%. Higheralpha pulp than 91% is. produced by using closed vessels and boiling with caustic soda or ammonia. An alpha pulp is thus produced around 98 to 94%. However, for some unknown reason this high alpha pulp thus produced in closed vessels under pressure is not as desirable as it was formerly thought'to be for the manufactureof rayon, and a pulp around 90 to 91 alpha cellulose content is preferable at prescut for such purpose. Accordingly, my process is adapted to produce in a most eflicient manner,

, a pulp for rayon purposes characterized by having the necessary uniform character as .well as other properties such as stable color, i. e., low recessionof color and low ash content, and a uni form viscosity in the rayon spinning solution produced from this pulp.

My invention, as established by actual commercial production in regular manufacture, results'in the production of a pulp that is characterized by having an alpha cellulose content averaging about 88.5% to 89.2%, said product being characterized by having a color characteristic averaging about 89 according to the ordinary methods of pulp grading, in contrast to the ordinary rayon pulp which has a low color characteristic ranging from .80 to 82, if the alpha content average'is over 88%, or if a high color characteristic such'as 89 is obtained, the alpha content is near 86.5%: to 87% in the pulp producedby meth ods and apparatus commonly used. My said product is furthercharacterized by being more stable when exposed to outside influences, such as moisture, dry heat, sunlight, etc., i. e., it has a remarkably low recession of color, and a further characteristic of my product is thatithas a low ash content averaging about 0.12 of 1% in contrast .with what is considered high grade rayon pulpwhich has about 0.15 of 1% and in contrast to many other rayon pulps which have an ash content as high as 0.3 (100.35 of 1%.

Referring specifically to sulphite pulp subjected to my bleaching processgthe lignin content of my product-is.0.1 of 1% to 0.5 of 1% in contrast to pulps subjected to other bleaching processes of approximately 0.5 of 1% to 1.5%.

The significance of the ash content is this: In the manufacture of rayon yarn all of the cellulose material that is used must be put into solution or into colloidal suspension. During this process of solution and dispersion part of the ash content of the pulp maynot go into solution. This insoluble portion must therefore be filtered out in order that it may not clog the spinnerets and cause other trouble. Filtration is an expensive and very troublesome process, particularly so in the manufacture of rayon, and therefore any reduction of insoluble materials in the pulp greatly lessens the filtration problem.

Part ofthe ash-content of the pulp to be used for rayon manufacture does dissolve during the course of manufacture. Compounds of iron, manganese, copper andnickel contained in said ash usually have a decidedly bad effect on the quality of the yarn that is produced. If the pulp is not of a uniform character, then the color effects obtained upon dyeing the manufactured rayon .threads varies, and manifestly, such re-' sult positively prevents any decided unique or predetermined color effects because-one thread will result,- upon dyeing, in having one shade, while another thread will be of still another shade, all as a result of the same dyeing treatment,i. e., shiner thread results.

ing qualities as shown by standard tests developed for the measurement of these properties. The product of my process results in the fibre being in better condition, while the usual two stage bleaching process results in the loss of about 10% in the physical properties, that is, bursting strength, folding and tearing properties and having a color characteristic of about 84. By alpha cellulose .or fibre, it is herein meant that cel- 'lulose which is resistant to 18%-caustic soda solution'at 68 F. for a period of one half hour. For the high alpha content in the product oi my invention, there are two possible explanations. First, due to the discovery of how to employ chlorine without either destroying or deteriorating the pulp'fibre, excepting to a very slight degree,

if at all, and due to the discovery of how toefliciently apply the said chlorine, a greater purification of the pulp results so that the percentage of alpha cellulose is' raised by the elimination of the impurities.

Second, on account of the large percent of bleaching action, which my discovery renders possible during the preliminary treatment, the amount of bleaching necessary with calcium hypochlorite solution is lessened and this means that very much less of the alpha cellulose is destroyed or deteriorated in the first and second stages of bleaching. It willl be remembered that calcium hypochlorite, although less active than chlorine in usual processes, is still a very strongly acting chemical on wood pulp and a strong chemical under ordinary methods of treatment is necessary to perform bleaching action and at the same time too great a quantity of the fibre must .not be destroyed. That is, the amount of alpha cellulose resulting must not be reduced unduly and the quality of such cellulose as obtained, mustnot be injured or deteriorated.

Accordingly, either a very dilute solution of the calcium hypochlorite must'be used, or. a small amount must be employed, to reduce its action either of which courses of procedure result in an extended time of treatment to get the desired and necessary efiect of bleaching. The less action on the pulp fibre, the better is the product.

Manifestly, since the preliminary treatment or my discovery performs such a large proportion of the bleaching action, it is possible to control much more. easily the strongly acting calcium hypochlorite, during the first and second stages of the bleaching operation proper.-

'It is also to be remembered that the temperature for cooking the wood pulp differs on the same pulp and differs for different varieties of wood. Preliminary tests quickly reveal the amount of bleaching which may be advantageously performed during the preliminary treatment of. my invention.

A-cel1ulose product possesses a valuable char acteristic if it showsa high alpha cellulose content and a small loss of weight when a given sample is repeatedly submitted to the alpha cellulose test. The test for determining the endurance quality of cellulose consists of subjecting samples of various brands of pulp to repeated treatments with l8%'caustic soda at 20". C. in

accordance with the standard alpha cellulose test, following each treatment with washing and drying'.

The pulp product of my invention in a comparative test, of pulps of various standard processes .regularly furnished for rayon manufacture, exhibited greater resistance to repeated treatment with caustic soda solution, as indicated by a smaller loss in weight, than any other sulphite pulp tested. A similar test on a number of sampies of paper grade, sulphite pulps gave the same result. The product of my invention was whiter than any of the other pulps of said test. In commercial operation, the pulp of my discovery can be removed in sheets from the press after steeping and pressing and can be shaken without destroying the sheet formation. This is in contrast to other pulps commonly used in ray'on manufacture which quickly break when similarly handled, furnishing additional evidence of the toughness and resistance of the cellulose in this pulp.

' I claim:

1. The process of bleaching a chemical wood pulp, comprising the step of introducing chlorine directly into a stream of said wood pulp of low density, the temperature of the chlorine and pulp being sufiiciently cold that the initial chemical activity between-the two is retarded.

2. The process of bleaching a chemical wood pulp, comprising the step of introducing chlorine in the liquid form directly into said wood pulp of low density.

3. The process of bleaching a chemical wood pulp, comprising the step of introducing chlorine in the liquid form directly into a stream of the pulp of low density; permitting the liquid chlorine to expand to form a gas while directly contacting the said pulp in the stream; and thoroughly inter-mixing the chlorine and said pulp.

4. The process of bleaching a chemical wood pulp, comprising the'step of cooling a stream of pulp of low density by means of liquid chlorine; introducing the chlorine into the body of the stream of the pulp; and thoroughly intermixing the chlorine and said pulp.

5. The process of bleaching a chemical wood pulp, comprising subjecting the pulp, to a preliminary treatment, that is, preliminary to the treatment with commonly used pulp bleaching reagents, which preliminary treatment consists in introducing chlorine directly into a stream of the pulp, the temperature of the .chlorine and pulp being sufiiciently cold that the initial chemical activity between the two is retarded 6. The process of bleaching a chemical wood pulp, comprising subjecting the pulp to a preliminary treatment,,that is, preliminary to the treatment with commonly used pulp bleaching reagents which preliminary treatment consists in introducing chlorine in the liquid form directly 55 into a stream of the pulp and performing 30 to 50% of the total bleaching action during this preliminary treatment.

7. The process of bleaching a chemical wood pulp, comprising subjecting the pulp to a preliminary treatment, that is, preliminary to the treatment with commonly used pulp bleaching reagents, which preliminary treatment comprises the step of introducing chlorine directly into a stream of'the pulp, the temperature of the chlorine and pulp being sufficiently cold that the initial chemical activity between the two is retarded separating said pulp from the liquid portion of the stream and washing the pulp; subjecting said pulp to calcium hypochlorite solution for the first stage of the bleaching process proper; separating said pulp from the liquid portion of the suspension and washing the same; subjecting said pulp to calcium hypochlorite solution for the second stage of the bleaching proc- 7 5 ess proper; and separating the pulp from the liquid portion and washing the said pulp.

8. The process of bleaching a chemical wood 'pulp, comprising subjecting the pulp to a preliminary treatment, that is, preliminary to the treatment with commonly '-used pulp bleaching reagents, which preliminary treatment consists in introducing chlorine directly into a stream of the pulp, the temperature of the chlorine and pulp being sufficiently cold that initial chemical activity between the two is retarded and bleaching the pulp to the extent of 30 to 50% of the total bleaching action; separating said pulp from the liquid portion of the stream and washing the same; subjecting said pulp to calcium hypochlorite solution for the first stage of bleaching proper, in which first stage to 61% of the is performed; and separating said stock from the liquid portion and washing the same.

9. The process of bleaching a chemical wood pulp, comprising subjecting the pulp to a pre liminary treatment, that is, preliminary to. thetreatment with commonly used pulp bleaching reagents, which preliminary treatment consists in cooling astream of wood pulp of low density by means of chlorine and introducing the chlorine directly into thebody of the stream of pulp, the temperature of the chlorine and pulp being sufficiently cold that initial chemical activity-between the two is retarded and bleaching the pulp to the extent'of 30 to 50% ofthe total bleaching action; separating the pulp from the liquid portion of the stream and washing said pulp; subjecting said pulpto hypochlorite solution for the first stage of bleaching proper in which first stage-20 to 61% of the total bleaching action is performed; separating the pulp from the liquid portion and washing the same; subjecting the said pulp to hypochlorite solution for the second stage of bleaching proper in which second stage of bleaching ,9 to 30% of the total bleaching action is performed; and separating the pulp from the liquid portion and washing said pulp. I

10. The process of bleaching a chemical wood pulp, comprising subjecting the pulp to a preliminary treatment, that is, preliminary to the treatment with commonly used pulp bleaching reagents, which preliminary treatment consists in cooling a stream of wood pulp of low density by means of chlorine and introducing-the chlorine directly into the body of the stream of pulp, the temperature of the chlorine and 'pulp being sufficiently cold that initial chemical activity between the two is retarded; and bleaching the pulp tothe extent of ,30 to 50% of the total bleaching action; separating the pulp from the liquid portion of the stream and wash-ing said pulp; subjecting said pulp to hypochlorite solution for the first stage of bleaching proper in which first stage 20 to 61% of t e total bleaching action is performed; separating the pulp from the liquid portion and'washing the same; subjecting the said pulp to hypochlorite solution for the second stage of bleaching proper in' which second stage of bleaching 9 to 30% of the total bleaching action is performed; and separating the pulp from the liquid portion and washing said pulp.

. into the wood pulp in suspension, the temperaof the chlorine and the pulp being sufliciently cold that the initial chemical activity between the two-is retarded.

' and intermixed chlorine into the bottom por- 11. The process of bleaching a chemical wood pulp, comprising the step of introducing chlorine into the wood pulp in suspension, the temperature of said chlorine being less than that of the pulp stream, at which lowered temperature chem- 5 ical activity of the chlorine is lessened.

12. The process of bleaching a chemical wood 'pulp, comprising the step of introducing chlorine into the wood pulp in suspension, the temperature of said chlorine being less than that of the pulp stream, at which lowered temperature chlorine hydrate is formed 13. The process of bleaching a chemical wood pulp, comprising the step of introducing chlorine 15 ture of said chlorine being less than that of the pulp stream, and at such lowered temperature that deleterious action of the chlorine on the wood pulp' is eliminated.

14. The process of bleaching pulp, comprising subjecting the pulp to a preliminary treatment, that is preliminary to the treatment with commonly used pulp. bleaching reagents, which preliminary treatment comprises the step ofintroducing chlorine direct- 1y into a stream of the pulp, the temperature 'of the chlorine and pulp being sumciently cold that the initial chemical activity between the two is retarded; separating said pulp from the liquid portion of the stream and washing the pulp; subjecting said pulp to calcium hypochlorite and alkali solution in an open bleaching cell without bringing the pulp 'tothe boiling point for the first stage of the bleaching process-proper; separating said pulp from theliquid portion of the suspension and washing the same; subjecting said pulp to calcium hypochlorite for the second stage of the bleaching process proper; and separating the pulp from the liquid portion and washing the said pulp. 40 15. In the process of bleaching a chemical wood pulp of a low density, the step of releasing chlorine directly in a body of said wood pulp, while said wood pulp is in motion, the temperature 16. The process of bleaching a chemical wood pulp comprising the steps of introducing chlorine directly into a stream under pressure of pulp in suspension, said stream having a velocity whichenables said stream to carry the chlorine along with it; discharging said stream of pulp tion of said pulp in a cell; and causing said pulp and intermixed chlorine to move upwardly in said cell substantially uniformly cross-sectionally considered.

-17. The process of bleaching a chemical wood pulp comprising the steps of introducing chlorine directly into a stream under pressure of pulp in suspension, said stream having a velocity which enables said stream to carry the chlorine along with it; discharging said stream of pulp and intermixed chlorine continuously into the bottom portion of a mass of said pulp in a cell;

- causing said pulp and intermixed chlorine to move upwardly in said cell'substantially uniformly cross-sectionally considered; and continuously discharging said pulp at the top of said cell. 18. The process of bleaching a chemical wood pulp-comprising the steps of introducing chlorine, directly into a stream of said wood pulp of low density, the temperature of the chlorine and pulp being sufiiciently cold that initial chemical a chemical wood 20 Y aocaesi on the pulp is prevented. 20. The process of bleaching a chemical wood pulp comprising introducing chlorine, directly into'a stream of the pulp, the temperature of the chlorine and pulp being sumciently cold that initial chemical activity between the two is retarded; discharging said stream into the bottom portion of a cell; causing said pulp and intermined chlorine to, move upwardly in said cell substantially uniformly cross-sectionally considered; discharging said pulp at the top of said cell; and causing that portion of the chlorine uncombined with the pulp to.escape from the top of said cell.

2l. The processor bleaching a chemical wood pulp comprising introducing chlorine, directly into a stream of the pulp, the temperature of the chlorine and pulp being suficiently cold that initial chemical activity between the two is retarded; discharging said stream into the bottom portion of a cell causing said pulp to rise'uniiormly in said cell while exposed to said chlorine; withdrawing from the body of the pulp, intermediate the length of the body of pulp, portions of said pulp; and discharging said portions directly into the body of the pulp at the bottom of the cell, thereby producing adequate reaction between the chlorine and the pulp as the two flow upwardly.

22. The process of bleaching a chemical wood pulp comprising the steps of introducing chlorine directly into a stream under pressure of said rine directly into a stream under pressure or pulp in suspension, said stream having a velocity which enables said stream to carry the chlorine along with it; discharging said stream of Pulp and intermixed chlorine into the bottom portion of said pulp in a cell; causing said pulp and intermixed chlorine to move upwardly in said cell substantially uniformly cross-sectionally considered; and causing the ,uncombined portion of the chlorine to pass from the cell as it escapes from the pulp.

24. The process or bleaching a-cheniical wood pulp, comprising chlorinatlng thepulp in aqueous suspension by introducing chlorine directly into a stream or said pulp in aqueous suspension, the temperature of the chlorine and the pulp being sufficiently cold that the initial chemical activity between the two is retarded; vigorously intermixing the chlorine and the pulp while said stream is confined in and moved under pressure through a pipe; and subjecting said stream, as it is released from said pressure, to a violent dispersion into the bottom portion of a relatively large upwardly slowly moving body of said pulp where thelargeness of the body of pulp reduces the drastic chemical activity of the chlorine towards the pulp.

25. In the process of bleaching a chemical wood pulp, the steps of causing pulp in an aqueous suspension to how upwardly for a substantial distance and then downwardly in a confined stream and. introducing chlorine directly into said pulp stream as it flows downwardly, the temperature of the chlorine and the pulp being sumciently cold that the initial chemical activity between the two is retarded, said.upwardly directing of the stream serving to seal the chlorine against backward escape of the chlorine.

26. The process of bleaching a chemical wood pulp, comprising subjecting the pulp to a preliminary treatment, that is. preliminary to the treatment with commonly used pulp bleaching reagents, which preliminary treatment, consists .in introducing chlorine in the liquid form directly into a stream of the pulp, the temperaliminary' treatment, that is, preliminary to the treatment with commonly used pulp bleaching reagents, which preliminary treatment consists in introducing chlorine directly into a stream of the pulp, the temperature of the chlorine and the pulp being sufliclently cold that the initial chemical activitybetween the two is retarded,

separating said pulp from the liquid portion of the stream and washing the pulp and then sub- Jecting said pulp to calcium hypochlorite solution, the extent of-the. bleaching by the chlorine andthe extent of the bleaching by the hypo-- .ical activity-between the two is retarded; causingsaid pulp'to rise uniformly in said cell while exposed to said chlorine; and continuouslywithdrawing said pulp' after exposure of the same to said chlorine.

' DAVID B. DAVIES.

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