US1953076A - Apparatus for process of bleaching chemical wood pulp - Google Patents

Description

April 3, 1934.

D. a. DAVIES APPARATUS FOR PROCESS OF BLEAGHING CHEMICAL WOOD PULP Original Filed Sept. 8,-1931 A TTORNEY mvmox fiat/1a E .170

Patented Apr. 3, 1934 UNITED STATES PATE NT OFFICE David B. Davies, Shelton, Wash.

Original application September 8, 1931, Serial No.

7 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 actionof commonly used pulp 10 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, rifiiers, 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 lgnin and other encrusting foreign substances enveloping the fibre which must be re moved.

I use the term bleaching action herein as commonly employed in-the pulp art. That is, to refer particularly to the purification of the pulp by removing encrusting materials, including lig- 35 nin which envelopes 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 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 pulp stock suspension is under motion, to remove lignin and other foreign encrusting substances of the fibre to the extent of 30 to of the total bleaching actiontand my discovery includes thefact that the said pulp may be washed in water at normal pulp suspension temperature for the separation of the compounds rendered soluble by the chlor'ne. 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 43 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, caus- :ng itsthorough mixture with the pulp without 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 of the bleaching action, namely, purifying the Divided and this application March 13, 1933, Serial No. 660,511

pulp fibre and changing thepulp color by providing for: 1-a continuous process, 2-highly efficient chemical action, 3reducing by 25 to 35% the period normally involved in performing the bleaching action, 4--saving of chemicals normally employed, and 5avoiding destruction anddeterioration of the alpha cellulose content. liminary treatment constituting my discovery is advantageously employed previous to subjecting the pulp to any standard bleaching methods or systems now commonly, commercially employed. A fourth feature of my invention relates to the d'scovery that by the employment of chlorine in the preliminary treatment, that the chlorine,. as herein used in my discovery, not only expedites such a percentage of bleaching or purification as is performed during said preliminary treatment,

but that it results in unexpected action, namely, that the action of the chlorine is such that subsequent treatment with bleaching reagents, as calcium hypochlorite, may be effected with better control of the viscosity factors of the pulp. Such improved viscosity results in a better pulp for rayon and cellophane manufacture characterized by greater uniformity, lower ash content and uniform viscosity whereby expensive filtration difficulties arising from filtering the dispersion of cellulose obtained from the pulp are avoided. Viscosity of the dispersion of cellulose affects 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 vir tue of this, affects directly the quality and properties of the resulting rayon yarn. 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 thecellulose dispersion is primarily, if not entirely, determined. Accordingly, the action of the chemicals during the first stage of the presently, commercially employed bleaching methods must be relatively very slow and very carefully controlled. I have discovered by the preliminary treatment of my The preinvention with chlorine, how to treat the pulp very rapidly and at the same time have discovered a way of treatment characterized by not adversely affecting the viscosity.

Heretofore, the ethods in common use of bleaching sulphite wood pulp generally were based 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. a 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 known as the high density system and the concentration of the suspensionof 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 in two 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 pre-determined amount of bleaching is done in the first stage, ordinarily, about seventy-five percent. The pulp is then washed and all dissolved and encrusting 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, (a) because of the drastic 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; (1)) because of the problem of keeping the chlorine in contact with the pulp in a proper way for a predetermined period, and (0) 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 diificulties, channeling tendency of the pulp; necessity of avoiding contamination and undue concentration of chemicals locally. Accordingly, instead of the solution ofchlorine in water being applied directly to the pulp, the practice has been, as ex plained above, to use calcium hypochlorite or other hypochlorites because the action was comparatively slow since 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 product may be provided by subjecting the pulp to a preliminary treatment with chlorine. Such preliminary treatment involves the following: That chlorine may be employed advantageously if its chemical activity is retarded. Such retarding is accomplished by lowering the temperature of the chlorine and this may be done by us ng chlorine in the liquid form and allowing it to ex pand 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 temperatureof the pulp in the stream. Thereupon, I introduce the chlorinedirectly 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 chlor'ne is continuously contacting with new material. Such motion is also necessary to get the proper dispersion of the chlorine. Said chlorine is being used without any lime d'rectly in contact with thepulp 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 to have agitation of the, pulp as was heretofore the case when calc um or sodium hypochlorite was employed Since it is not necessary to continuously agitate the pulp when using the chlorine in liquid form, as herein set forth, the process which I have discovered is more easily adapted to continuous operaton and with'material at a comparatively low density, about 3%. The degree of density preferred-is that which will permit a ready flow of the pulp in suspension so that the material will be subject to being readily 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 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 varous stages of treatment is greatly facilitated. During this preliminary treatment with said chlorine, I accomplish 30 to 50% of the total bleaching action. After the preliminary treatment, I proceed with the bleaching process proper, which may be a two stage system, by employing calcum hypochlorite in the first stage to effect about 20 to 61% of the total bleaching action. In the second stage of bleaching proper, I employ calcium hypochlorite to accomplish about 9 to 30% of the total bleaching action. This pulp thus processed results in a 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 chlorFne has been employed to accomplish only 1 to 5% of the total bleaching action, as against 30 to 50% in the process of my discovery. This was due, in part, to the difliculty of confining the chlorine in the equipment, as well as keeping the chlorine in contact w'th the pulp for a sufficient period of time. The ordinary bleaching cells are charged from the top, while I provide a cell in which the material is charged into the cell by way of the bottom portion. 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 objects of my invention are: To produce a better pulp product from the chemically prepared Wood pulp; to produce a bleached pulp having an alpha cellulose content around 88 to 91%; and a color characteristic of 84 to 89 on the color scale; to produce a pulp of sad 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 to and making a reduction in the amount of chemicals employed, by increasing the efliciency 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 action of the chlorine, and to provide an apparatus for carrying out the process characterized by fulfilling same, are attained by the invention illustrated in J the following drawing, the same being preferred exemplary forms of embodiment of the apparatus part of my invention, throughout which drawings like reference numerals indicate like parts:

Figure 1 is a view in longitudinal section of the bleachingcell 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 close proximity to the discharge end of the feed conduit for the wood pulp;

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

Fig 5 is a view of a modified form of chlorine feed pipe provided with a refrigerating coil.

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 feet high and 10 feet in diameter. A pulp conveying conduit '7 having a gas trap means, preferably by having an inverted U shape section over the 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 proximity to the discharge outlet of conduit '7, said spinner being mounted on shaft 9 having pulley 10 by means of which, power may be applied through shaft 9 to the spinner 8. Spinner 8 functions to thoroughly m x 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 11 are arranged in angular, spaced relation and these angles may be 90. Said ports are preferably located in the upper half of the cell. From each of these ,ports a conduit 12 extends to a collector conduit 13, and from this collector conduit '13, a conduit 14 leads to pump 15 and from pump 15 a conduit 16 leads to the bottom portion of cell 6,-said ports, conduits and pump constituting a recirculating means. A pulp discharge outlet conduit 17, preferably having a U shaped trap portion 18, communicates with the top portion of cell 6 and leads to a washing means (not shown). Said conduit may be 10 inches in diameter. A pipe 19 for the admission of chlorine, preferably in the liquid form, extends through the wall of pulp feed conduit 7 and extends down for some distance axially in said conduit '7 so that the discharge outlet 20, having preferably a plurality, as or '70 minute openings, is in spaced relation to the point 21 where the pipe 19 enters conduit 7. Chlorine gas escape pipe 22 connects withthe cell 6 in the top portion. Needle valve 23, gauge 24. pressure reducing valve 25, globe valve 26 and gauge 27 are all operatively mounted in chlorine feed pipe 19 and eachhas its obvious function. A by-pass pipe 28 with valve 29 operatively connects the conduit 7 with pipe 19 whereby all chlorine gas which may be left in conduit 7 when the operation of the cell is stopped for 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 U shape trap, but has a chlorine gas collector p'pe'31 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 willbe collected in pipe 31 and its escape into the plant avoided.

In Fig. 5, a modified formof the chlorine feed pipe is shown wherefn feed pipe 33 has an en- -arged chamber 34. In the chamber 34 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 0011- J duit 7, precisely as by feed pipe 19.

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 flow, which in general, will notbe over 6%, is conveyed by pipe 7 to the cell 6 from the blow-pit (not shown) by way of the usual knotters, rifilers, screens and decker, or

other suitable mechanical means of cleaning the pulp of relatively large foreign particles. The liquid chlorine is conducted to conduit 7 by pipe 19 from standard lquid chlorine cylinders (not shown), past pressure gauge 27, valve 26, reducing valve 25, pressure gauge 24 and needle valve 23. It will be understood that the pressure of the liquid chlorine in the cylinders is about 80 pounds and is reduced to ten to twenty pounds pressure in pipe 19 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 has been exposed in pipe 19 to the relatively warm wood pulp in conduit'l, which wood pulp ordinarily has a temperature of 60 F. The exposure of the pulp to the pipe 19 cools the pulp due to the heat absorbed by the chlorine, and in the operation of my invention, the temperature of the pulp in concluit '7 is lowered to a temperature below the freezing point of ,water'. The length of the pipe lillfvv'ithin conduit '7 may be varied. This pipe they be of the metal alloy known as KA 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 5 6. encounters spinner 8 which operates to thoroughly inter-mix the components of the stream and breaks up any lumps of wood pu p which might be present. Also, said spinner o erates to break up any particles of ice that might be pres- 10 ent in the stream derived from the discharge outlet 20 of the liquid chlorine pipe 19. 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 12, 13, 14 and pump 15 and conduit 16. This recirculating means provides for the pulp 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 the cell and upwardly directed. The recirculating means operates to prevent the pulp from chan:

neling or forming streams through the pulp within the cell 6. Such channeling is a characteristic tendency of pulp in suspension. The recirculation of the pulp thus provided also secures the further thorough and uniform mixing and exposure of the pulp to the action of the chlorine before the wood pulp is discharged through the discharge pipe 17. The rate of pulp feed is about nine or ten tons per hour and the recirculating 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 onlyattai'ned by means provided for the uniform flow of the pulp through the cell, but also by means of the thor- 40 ough 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 time.

If the liquid chlorine or refrigerated chlorine gas is introduced directly into the cell, beneficial results are obtained, but such procedure lacks the very advantageous results secured by the cell illustrated, in that uniformity of dispersion and inter-mixingof the chlorine through the pulp is lacking. Without the spinner and without introducing the liquid chlorine or refrigerated gas in the pulp conduit previous to discharge of the cell proper, the pulp was found to be over-treated in part and other parts very much under-treated. Trap 18 functions to prevent any escape of chlorine gas through the pulp discharge outlet pipe 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 difiiculty, not only by providing for its efficient consumption, but also providing for the collection \of any that escapes beingco'nsumed. Actual operation has shown such free chlorine to be very small in volume.

{The apparatus thus described, constitutes means for carrying out the process of my invention. Manifestly, this apparatus is designed for continuous operation as distinguished from the usual batch form of standard bleaching cell now commonly employed, having agitating or beater means within the cell and power means outside the cell to drive the same.

The process which I have discovered and which constitutes part of my invention herein set forth, comprises subjecting the wood pulp to the direct 39 action of chlorine in the liquid form or in the form of a gas at reduced temperature. Obviously, 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 introduced at a temperature comparable to the temperature of the stream of wood pulp in conduit 7, so that upon expansion during the escape from the discharge outlet 20 of the chlorine feed pipe 19, there would be an absorption of heat 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 meansf illustrated in Fig. 5, is employed, to reduce the temperature of the chlorine gas, that the gas is under such pressure as to overcome the pressure in the feed conduit 7; 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 far as 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.

When attempts 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, commercially, 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 my experiments, is that the chlorine in liquid form upon being 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 reduces thetemperature 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 s'ufiicieritly off-set by the lowerchemical activity of the chlorine at the lowered temperature. Thus, the lowered temperature operates in my discovery, it would seem, to sufficiently moderate 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 overa great body of pulp, so that its drastic action on any'given unit is impossible. Unquestionably, this result is greatly assisted by'theunifo'rm 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 dischargedinto 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 itsaction 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:

1. 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 effective. The pulp streamin the conduit '7 at the point of discharge into the cell is under pressure due to the hydrostatic head of the mixture. The pulp stream receives the chlorine at about 15 pounds gauge pressure average, and about 20 C. lower temperature on an average than the pulp suspension. I

The desired chemical action is a selective one by which the impurities, which contaminate the fibre, are in part advantageously removed by this process. According to the commonly held idea the action is the production of hypochlorous acid by the action of chlorine on the water and then this hypochlorous acid breaks down, by giving up its oxygen, into hydrochloric acid. The oxygen thus liberated oxidizes the lignin content of the 7 wood pulp and other. encrusting matter of the l the chlorine.

pulp fibre and forms soluble compounds which may be freed from the pulp fibre by subsequent washing. The purpose in my invention insubjecting the pulp in the first stage of bleaching to the action of chlorine in liquid form is to provide a more eflicient oxidizing agent. Obviously, it is highly advantageous to have present chlorine in a form which may immediately form hypochlorous acid so soon as the foreign matter on the pulp fibre depletes the concentration 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 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 chlorinematerial to be immediately available to continually formhypochlorous acid and thus keep up a continuous supply of the hypochlorous acid. In this wise, an exceedingly high degree of eificiency is provided during the first stage of bleaching, or as herein termed the preliminary treatment, that is, a high degree of efiiciency is provided for the purification of the pulp during this preliminary treatment. Although my process involves .the fact that the drastic action which results in treating the'flbre 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 I quantity of lignin and other encrusting foreign matter enveloping the same. My discovery thus provides for disposing of the relatively largequantity of lignin or other fibre encrusting matter present in the initial treatment to an extent of 30 to 50% and by a chemical which has here-. tofore been adjudged in commercial 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 cellulose dispersion of improved viscosity.

While it is thus necessary to have the chlorine present in a form to readily produce hypochlorous acid promptly upon the depletion of the concention 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 by its 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 intermixing or dispersion results frpm 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 fiow. 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 or 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 7 supplying, or feeding themixture to the cell.

After the preliminary treatment, above set forth, I preferably separate andwash the pulp to remove the soluble compounds formed'ofithe lignin and other encrusting matter and then subject the pulp to the usual calcium hypochlorite solution for the first stage of the bleaching process proper and next preferably after separating and washing the pulp, I subject the pulp to a further bleaching action of the usual calcium hypohlorite solution for the second stage of bleaching, when it is usuallyfseparatedand washed again. This completes the second stage of bleaching process proper. r

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. After suitablewashing the pulp isdelivered 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 may be performed during the pre- 150 liminary 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 hypo.- chlorite and about 30% in the second stage of bleaching using calcium hypochlorite. This results in a pulp having a color characteristic of 84.

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

My discovery is characterized by dispensing with the step of agitating the pulp during the preliminary treatment so that a continuous method of treatment at a comparatively low density, namely about 3% is provided. All this results in increased efficiency 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 given period from 90 to 150 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 chemical 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.

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 range may safely be guaranteed. I have found that a modification of the above system of process is possible as follows: By using lime, caustic soda or ammonia in the first stage of bleaching with the calcium hypochlorite without bringing the pulp to a boiling point which is commonly done and this in a closed vessel under pressure, I am able to increase the alpha cellulose content to 91%. Higher alpha pulp than 91% is produced by using closed vessels and boiling with caustic soda or ammonia. An alpha pulp is thus produced around 93 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 manufacture of rayon, and a pulp around 90 to 91% alpha cellulose content is preferable at present for such purpose. Accordingly, my process is adapted to produce in a most efilcient 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 recession of color and low ash content, and a uniform 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 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 produced by methods and apparatus commonly used. My said product is further characterized 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 that it has a low ash content averaging about 0.12 of 1% in contrast with what is considered high grade rayon pulp which has about 0.15 of 1% and in contrast to many other rayon pulps which have an ash content as high as 0.3 to 0.35 of 1%.

Referring specifically to sulphite pulp subjected to my bleaching process, the 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 solu tion or into colloidal suspension. During this process of solution and dispersion part of the ash content of the pulp may not 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 of the ash content of the pulp to be used for rayon manufacture does dissolve during the course of manufacture. Compounds of iron. manganese, copper and nickel 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 result 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.

For paper pulp my process results in a product characterized by having better physical properties, namely, better bursting, tearing, and folding 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 i strongchemical under ordinary methods of treat 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 cellulose 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 of my invention, there are two possible explanableaching 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 ofbleaching. It will-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 ment 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, must not 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 inan extended time of treatmentto get the desired and necessary effect of bleaching. The less action on the pulp fibre, the better is the product. Manifestly, since the preliminary treatment of 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 tests quickly reveal the wood. Preliminary amount of bleaching which may be advantageously performed during the preliminary treatment of my invention.

A cellulose product possesses a valuable characteristic if it shows a 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 18% 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 procthan 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 destraying the sheet formation. This is in con trast' to other pulps commonly used in rayon manufacture which quickly break when similarly handled, furnishing additional evidence of the toughness and resistance of the cellulose in this pulp.

This application is a division of my application, Serial No. 561,723, filed September 8, 1931.

I claim:

1. The device of the character described, e'lnbodying a cell;' a pulp feed conduit leading intosaid cell near its bottom portion; a chlorine feed pipe communicating with said pulp feed conduit;

a pulp discharge conduit leading out of the top portion of' said-cell; and a recirculating means comprising conduits leading from outlet means disposed intermediate the length of the cell to a pump and from thepurnp to the bottom portion of the cell.

2. The device of the character described, em-

bodying a cell; a pulp feed conduit leadinginto said cell near its bottom portion, having a sealing means to provide against the escape of gas; a

being of inverted U form to provide a seal against the escape of gas; a chlorine feed pipe communicating with said pulp feed conduit; a pulp discharge conduit leading out of the top portion of said cell; and a recirculating means comprising conduits leading from outlet means disposed intermediate the length of the cell to a pump and from the pump to the bottom portion of the cell. 4. The device of the character described, embodying a cell; a pulp feed conduit leading into said cell near its bottom portion; a spinner rotatively mounted in said cell in close proximity to the discharge end of said pulp feed conduit; a chlorine feed pipe communicating with said pulp feed conduit; a pulp discharge conduit leading out of the top portion of said cell; and. a recirculating means comprising conduits leading from outlet means disposed intermediate the length of the cell to a pump and from the pump to the bottom portion of the cell. Y

5. The device of the character described, em-. bodying a cell; a pulp feed conduit leading into said cell near its bottom portion; a chlorine feed pipe extending through said pulp feed conduit and having its discharge end in spaced relation to the point of entering the conduit, a pulp discharge conduit leading out of the top portion or said cell; and a. recirculating means comprising conduits leading from outlet means disposed intermediate the length of the cell to a pump and from the pump to the bottom portion of the cell.

6. The'device of the character described embodying a cell; a pulp feed conduit leading into said cell near its bottom portion; a chlorine feed pipe extending through said pulp feed conduit and having its discharge end in spaced relationand having its discharge end in spaced relation to the point of entering the conduit; a pulp discharge conduit leading out of the top portion of said cell; a recirculating means comprising conduits leading from; outlet means disposed intermediate the length of the cell to a pump and from the pump to the bottom portion of the cell; and a .gas collecting conduit communicating with the top portion of said cell.

- DAVID B. DAVIES.

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