US1996363A - Method of bleaching cellulose materials - Google Patents

Description

April 2, 1935.

J. CAMPBELL ET AL METHOD OF BLEACHING CELLULO SE MATERIALS Filed Sept. 2'7, 1930 I ,1 iNVENTORS EVMVM@MZQ m? BY TOR Patented Apr. 2, 1935 UNITED STATES PATENT OFFICE METHOD OF BLEACHING CELLULOSE MATERIALS Delaware Application September 27, 1930, Serial No. 484,733

4 Claims.

This invention relates to the bleaching of cellulose materials such as cellulose pulp. It has reference to the type of bleaching agent employed and to the methods and apparatus used in the 5 treatment of the pulp or other cellulose material during the bleaching and washing stages. The treatment contemplated is particularly suitable for the bleaching of the pulp developed by the sulphate or alkaline process, commonly known as kraft pulp.

Heretofore it has been the common practice to bleach pulp by mixing with it a certain quantity of hypochlorite of lime, the mixture being agitated in a large vat until the .pulp has become white 15 or turned the desired shade. The process has been conducted as a batch operation and normally in an open vessel under atmospheric pressure.

According to the present invention simply chlorinein the liquid or gaseous state is employed for bleaching the pulp and the operation is made continuous and conducted under a suitable pressure. Improved methods are also contemplated, for the subsequent washing and other 25 treatment of the material. The expense involved in the bleaching and washing of a given quantity of pulp is greatly reduced and the time required for the treatment of the pulp is considerably lessened. At the same time the resulting product is more uniform in color and is left freer of impurities and harmful reagents which serve to discolor the paper formed from the pulp and weaken its texture.

Other features and advantages of the inven- 35 tion will be pointed out in the claims and will be set forth in the following detailed description of an illustrative embodiment of the same in conjunction with the accompanying drawing in which:

The figure is a diagrammatic view disclosing the arrangement of suitable apparatus to be employed in the bleaching. and washing of pulp and. the like.

Briefly the present process contemplates the continuous passage of a stream of pulp from a storage vessel through a comparatively long chamber or pipe under a pressure in the neighborhood of 70 lbs. per square inch and at a temperature of approximately 120 F. Chlorine in the liquid state being under a pressure in excess of that within the chamber, is introduced into the latter with the pulp or through an inlet adjacent the pulp inlet. The pulp and chlorine are continuously agitated as they are gradually advanced through the chamber to effect an intimate intermixture. For economical operation the pulp is preferably of high concentration so as to carry a minimum amount of water consistent with proper pumping of the material through the system. Maintenance of high concentration not only reduces the quantity of heat required in raising the temperature to the desired degree but it reduces the pumping costs and increases the useful capacity of equipment of given dimensions. Now chlorine is soluble in water to only 10 a definitely limited extent and since it must be forced into solution to enable it to perform its bleaching action on the pulp, the maintenance of pressure in the system is quite important. At a pressure of say 70 lbs. sufiicient chlorine will enter into solution in the water content of the pulp to effect the desired bleaching action.

In order that the condition of the pulp in its progress through the pipe or chamber may be noted the latter may advantageously be formed of'glass or at least be provided with transparent window portions at spaced intervals and particularly at the discharge end of the chamber. In any event the chamber should be formed of or lined with material which will not be, or only slightly, attacked by the chlorine. After a suitable reaction period of say half an hour the pulp will have worked its way through the chamber to the discharge outlet. An inspection of the pulp at this point will disclose whether it has been 30 sufflciently bleached. If it has not more chlorine should be introduced into the charge at the inlet of the chamber. Should it be observed that a desired final degree of bleaching is obtained at some point in advance of the discharge outlet, the amount of chlorine introduced may be reduced or the operation may be speeded up, if desired, in which event a certain wastage of chlorine would take place. During the operation of the system the chlorine first enters into solution in the water 9 content of the pulp and then in the course of the reaction it is removed from the. water and enters into the pulp to effect bleaching. The material which is discharged from the chamber will normally be found to carry very little chlorine in the water and most of it will be in the pulp. of

. course, in a commercial operation the advantage of increasing the capacity of a given unit by forcing the material more rapidly through the same with resultant wastage of chlorine must be balanced against the loss involved in the wasted chlorine or the expense involved in its recovery.

After the chlorine has completed its bleaching action the material may be discharged from the reaction chamber in a continuous stream and washing of the pulp with a solution of sulphur dioxide. It is important that prior to this treatment the free chlorine content of the pulp should have been removed so that there can be no re-. action between chlorine and sulphur dioxide in solution to produce sulphuric acid. This would of course, be quite harmful to the pulp tissue. Another simple water wash should follow the sulphur dioxide treatment. At the end of the washing treatments and preferably also at one or more intermediate stages the pulp is thickened by the removal of the water content in any suitableway as by the use of suction.

with this general explanation of the various stages of the process suitable equipment that may be employed in its conduct will now be dis-' cussed with reference to the diagrammatic illustration in the drawing.

Pulp which has been produced in any suitable way, as by the digestion of wood and similar cellulosic materials, in the usual digesters of a paper mill and which has been subsequently washed and treated in the usual way may be continuously fed to a large storage tank III. This pulp will preferably be highly concentrated, having a relatively low moisture content so is reduced to a minimum consistent with the easy flow of the material through the system. The pulp in the tank In may suitably be at a desirable temperature between 65 and 209 F., preferably about 120 F., or if it is below the desired temperature it may be passed through a suitable heater to raise it to the desired point and may then be forced by a pump ll into the bottom of a large chamber or pipe II. This chamber in a commercial installation may be suitably between 3 and 6 feet in diameter and have a height or length of 30 to 40 feet. Obviously. these dimensions may be varied to suit the particular requirements and they are merely stated by way of example. As illustrated, the chamber is preferably disposed vertically but if desired it may be employed in a horizontal position. It should be either formed completely of or lined with-material which is not affected or only slightly attacked by chlorine. Materials of this sort may include glass, stone, earthenware, rubber, chrome nickel steel, and the like. Small scale equipment may very well be formed entirely of glass but large apparatus, such as preferably employed on a commercial basis, should be formed of other materials. When non-transparent materials are employed in the construction of the chamber, a strip of transparent material, such as glass, may be provided along one side so as to permit inspection of the interior of the tower or a series of windows at different heights closed by transparent material may be provided. The purpose of the transparent construction, as will be understood, is-to permit inspection of the pulp at various stages in its movement through the chamber so that the extent of the bleaching acthat its bulk.

tion may be readily noted. In lieu of the transparent construction, valved outlets may, if desired, be provided at various points along the chamber for permitting the withdrawal of quantities of the pulp from time to time to allow inspection of the same.

The pump I I should be so constructed and operated as to supply the pulp to the tower at a suitable pressure of between 5 and pounds, preferably in the neighborhood of 30 pounds. The bleaching agent, which as already explained may be in the form of liquid chlorine, may be carried under a pressure in excess of that present in the chamber, in a reservoir I3 and may be led from the latter into the base of the chamher by means of a line l4. Chlorine under normal atmospheric conditions is in a gaseous state but when sufiiciently compressed may be reduced to a liquid. Where the chamber is operated under a pressure of the magnitude mentioned, the normal pressure of the liquid chlorine may be relied upon to force it into the chamber. Thus, a reservoir or tank containing liquid chlorine may be heated, for example, by the use of steam or hot water to supply the necessary heat of vaporization, and gaseous chlorine under a high pressure will be created. If desired a compressor l5 may be associated with the reservoir ii for continuously supplying liquid chlorine thereto. If desired the chlorine might be introduced into the pulp at a point between the pump II and the chamber but in that event the pipe line fromthe point of introduction to the chamber should be formed of material not affected by the chlorine. A' valve I provided in the line it may serve to control the quantity of chlorine introduced into the chamber. Preferably only so much of the chlorine will be introduced as may be absorbed or dissolved by the water and pulp material. Under the pressure prevailing in the system this will be found suflicient to effect the desired bleaching action. If it should be found insuflicient due to some unusual circumstance. the pressure may be increased to the extent required.

In order to provide for a thorough intermixture of the chlorine and pulp within the chamber, a series of agitator vanes or blades l6 are preferably provided on a shaft H which may be rotated in any suitable way, as by a motor 18 connected to varied to suit the particular requirements, de-

pending largely upon the consistency of the pulp in the chamber. The speed should normally be between 200 and 500 R. P. M.

Due to the continuous supply of the pulp and chlorine at the bottom of the chamber, the mixture will gradually be forced toward the top and will be discharged from the top through a conduit IS. The movement of the pulp through the chamber may be so regulated as to allow suflicient time for the bleaching action of the chlorine. If each portion of a quantity of pulp ispermitted to remain in the chambenfor a period of about half an hour in contact with an appropriate quantity of chlorine, it will be found that a desirable bleaching action is obtained. This is materially less than the time required in the normal operation of an ordinary batch bleaching process. The conduit l9 may conveniently be provided with a valve I9] which, in conjunction with the pump i I, may serve to regulate the pressure within the chamber II. If the pressure is to be raised the valve may simply be closed to a slight extent to create a greater back pressure.

From the end of the conduit IS the pulp, having a certain amount of free chlorine remaining, may be discharged into the upper end of a washing tower 20. This tower may be maintained at substantially atmospheric pressure so that the chlorine will be readily liberated and may be withdrawn from the top of the tower by a suitable suction fan through a vapor line 200.

Near the top of the tower there may be provided a trough 2! into which the pulp is discharged and against which streams of water may be forced from jets 22 connected with any suitable source of water supply by means of a pipe line 23. Water introduced in this way will serve to wash out a large portion of the impurities carried by the pulp after the bleaching reaction. The mixture of water and pulp may be made to flow over a series of baflie plates 24 within the tower to bring about a more intimate contact and better washing action. From the bottom of the tower the mixture may be discharged through an outlet 25 to a tank or vat 26 provided at one end with a rotary continuous filter 21, of well known construction. This filter may consist of a cylindrical screen member having a suitable filter surface and adapted to be rotated through the pulp in the tank. A suction will be created over the surface of the screen as it passes through any selected portion of its cycle by the usual valve construction. In the course of rotation of the screen, pulp will be picked up on its surface and due to the suction, a large portion of the moisture carried by the pulp will be withdrawn. The thickened, partially dried pulp will be carried around on the periphery of the screen until it reaches a doctor blade 28 which will remove it from the surface of the filter and discharge it into another vat or tank 29 through a funnel-shaped, repulping trough 300 at which point it may be sprayed with a solution of sulfur dioxide from a nozzle 30.

Within the tank 29 there may be provided a suitable agitator 3| which may also act to feed the pulp toward the opposite end of the tank. It may be driven in any suitable way, as by a motor 3"! at an appropriate speed to effect an intimate intermixture between the pulp and sulphur dioxide solution. At the end of the tank a second rotary filter 32 may be provided for the purpose of again thickening the pulp. This filter may conveniently be provided with several distinct suction sectors. For example suction may be created in any suitable way, as by the usual control of valves, over a sector 33 so that as each portion of the filter screen is rotated past this sector, pulp will be deposited against its surface and the solution of sulphur dioxide will be largely withdrawn through a discharge outlet 330. This solution may conveniently be replenished by the introduction of a fresh supply of sulphur dioxide to bring it up to the proper strength and it may then again be used at the nozzle 30. As the filter screen continues to rotate, it may carry the pulp on its surface beneath a series of sprays 34 supplied with water from any suitable source. In this way any remaining traces of sulphur dioxide, an other substances not desired in the final product, may be washed out. A second suction sector 35 may be created in the well known manner by proper construction and adjustment of the valves and may serve to withdraw from the pulp the greater portion of the water discharged by the sprays 34 so as to thicken the pulp to the desired extent. This water may be discharged from the filter through a separate outlet 350. As the screen now continues to rotate, the washed and treated pulp will be scraped from its surface by means of a doctor blade 36 and will be discharged into a vat 31 or it may, if desired, be discharged to any suitable conveyor system and carried to some other point for further disposition.

It will be seen that by the arrangement and operation of the equipment as explained, a continuous stream of pulp may be subjected to a bleaching action and may be washed to eliminate the impurities resulting from the bleaching operation so that pulp suitable for the production of pure white paper, or any desired shade of paper, may be continuously produced.

As previously stated, the process is particularly suited for the treatment of kraft pulp produced by the sulphate process, but it may be applied to the treatment of all forms of pulp and similar cellulose materials. While a suitable form and arrangement of apparatus has been set forth in considerable detail, and certain definite operating conditions have been specified, it is to be understood that various modifications may be made without departing from the general spirit and scope of the invention. The terms and expressions employed herein are to be regarded as merely descriptive of the illustrative embodiment of the invention and are not to be considered as imposing any limitations thereon.

What we claim is:

1.' A method of treating cellulosic material -which comprises intimately inter-mixing free chlorine therewith to effect a bleaching reaction, removing excess chlorine from said material, and then washing the material with a solution of sulphur dioxide to solubilize the chlorinated products of the bleaching action.

2. A method of treating cellulosic material which comprises intimately inter-mixing free chlorine therewith to effect a bleaching reaction, removing excess chlorine from said material, and then washing the material with a compound containing sulfur dioxide acting as a solvent for chlorinated impurities not removable by a water wash.

3. A method of treating cellulosic material which comprises continuously passing a mixture of said material and a bleaching agent including free chlorine through a reaction zone, subjecting the bleached material to a water wash, removing the wash water from said material and subsequently washing said material with an acid solu- CERTIFICATE or coaaacrrou.

Patent No. 1,996,363. April 2. 1935.

JOHN CAMPBELL, ET AL.

it is hereby certified that the state of incorporation of the assignee in the above numbered patent was erroneously written and printed as "Delaware" whereas said state of incorporation should have been written and printed as New York, as shown by the records of assignments in this office; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 25th day of June, A. D. 1935.

7 Bryan M. Battey (Seal) Acting Commissioner of Patents.

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