US2431478A - Bleaching fibrous material - Google Patents

Description

Nov. z5, 1947. l R. P. HM 2,431,478

l BLEACHING FIBROUS MATERIAL Filed July 25, 1942 BY '4, f. (g4-@4W ATTORNEYQS.

Patented Nov. 25, 1947 UNITED STATES PATENT OFFICE BLEACHING FIBROUS MATERIAL Raymond P. Hill, Wausau, Wis.

Application July 25, 1942, Serial No. 452,329

Claims.

The present invention relates generally to improvements in the art of treating brous material or the like, and relates more specifically to an improved method of and apparatus for bleaching and purifying cellulosic pulps.

An object of my invention is to provide an improved method of and apparatus for carrying out in a continuous iiow operation the chemical reaction phase of any process, whether in singleor multiple-stage, for bleaching or purifying cellulosc pulps or the like.

The basic operation in the bleaching of cellulosic pulps for the preparation of paper making stocks, and in the purification of such pulps for the preparation of cellulose derivatives, consists in treating a slurry of the fibrous material and water with a chemical reagent, under predetermined conditions of time, temperature and concentration, and then washing the brous material free from the products of the chemical reaction. Nearly al1 modern systems of bleaching and purication comprise a plurality of such steps, Whereby the fibrous material is subjected to treatment by a sequence of chemical reagents adapted to produce the desired characteritsics of product with the greatest chemical efficiency. Many of these systems employ up to seven or more such steps in sequence.

In the previous design of such systems the pulp slurry under treatment is often erroneously considered as being a substantially homogeneous mass, whereas it is in reality an aggregate of a vast number of non-identical fibres possessing unequal or different demand for and reactivity to the various chemical agents employed. This fact imposes the requirement that the fibre and reagent be not only well mixed initially, but also periodically re-mixed at suitable intervals throughout the reaction cycle. If this requirement is not adequately met and if chemical dosage is based on average demand, some of the bres will naturally be incompletely treated. Or, if chemical dosage is made sufficient to completely treat the bres having individual maximum demand, other fibres will obviously be overtreated and degraded, and a substantial part of the chemical agent will be wastefully consumed. I have discovered that in a typical commercial operation of bleaching with calcium hypochlorite the use of suitable mechanical means to give a periodic re-distribution and re-mixing of the chemical solution and fibre will effect a saving of ten to twelve percent in the consumption of chemicals and an improvement of fteen to twenty-,live percent in the paper-making quality of the product, as compared to an otherwise identical operation which depends wholly on initial mixing of the bre and chemical. Production of paper making stock having a high degree of brightness combined with maximum physical strength, and also the preparation of purified cellulose for the production of explosives, photographic lm, rayon, and other related products, further requires that the factors of chemical dosage, temperature, pulp consistence and reaction time, be controlled and uniformly maintained with a high degree of precision, in each of the several stages of the bleaching system employed.

All these requirements have been quite successfully met in commercial operation by the use of batch systems in which, in each stage of the process, a definite quantity of pulp slurry of predetermined consistence, together with measured quantities of chemical reagent, are placed in a reaction vessel having suitable mechanical means for circulating the pulp mass in a closed circuit with periodic re-mixing of the nbre and solution. After processing for a denite period of time at controlled temperature each batch of material is discharged from the reaction vessel, and the cycle is repeated with a new batch of material. While this prior batch method of treatment has been found relatively satisfactory so far as ultimate results are concerned, it requires a plant which i-s complex and costly, and its operation is tedious and exacting. The time consumed in filling and discharging the reaction vessels is non-productive, resulting in the necessity for a very large number of such units in a multi-stage system of even moderate capacity, and in complicated and cumbersome equipment for transporting the pulp between stages and selectively delivering it to and receiving it from the numerous raction vessels. These elements in turn require a heavy investment in building structures, power equipment and other auxiliaries.

It is elementary in theory to overcome these disadvantages by employing a system based on the principle of continuous flow, wherein a constant flow of pulp slurry passes continuously into and out of one or more retention Vessels whose volume, in relation to the rate of ilow, is such as to give the desired period of reaction time. Several such installations have heretofore been constructed, employing reaction vessels of various contours and proportions and with various types of mechanical auxiliaries; but in all cases serious difficulties have been encountered in operation, and such installations have been found unsuitable for the higher grades of product. One 0f the most serious difliculties encountered has been the impossibility of obtaining uniform treatment of the material passing through the reaction vessels, because of certain inherent characteristics of a suspension of brous material in an aqueous medium. Such a suspension can be treated neither as a subdivided solid nor a viscous liquid, and may be readily conveyed as a fiuent slurry only within a narrow range of velocity. Fibres will not remain uniformly dispersed in a iiuid medium, but tend to agglomerate in various ways according to their basic type, the degree of treatment they have received, and other variables. The fibres and fluid tend to separate, sometimes by precipitation and sometimes by flotation, and therefore tend to travel at different velocities under the influence of gravity or propulsion. When it is attempted to pass such a mixture in a constantly advancing stream through a reaction vessel at the relatively slow velocity required by processing time, the flow will behave much like a natural stream flowing through swampy ground, in that certain limited areas will pipe or channel at much higher than average velocity, while greater areas will remain sluggish or even coinpletely stagnant. In this manner portions of the pulp will receive far different treatment than others, and none will receive the correct average time for which the installation was designed. If mechanical agitators are employed rto place the pulp mass in circulation sucient to inhibit stagnation, uniformity is again defeated, in that a portion of the bre will take a direct path from inlet to outlet in much less than calculated reaction time, while other portions will circulate internally for much longer than the calculated time. The result is both impaired quality of product and waste of chemicals.

It is therefore a more specic object of the present invention to provide a simple and efficient method of effecting uniform and continuous bleaching, or similar treatment, of cellulose pulp or the like, with maximum retention of pulp quality and at minimum cost.

Another specific object of my invention is to provide new and useful apparatus for most eiectively treating cellulose pulp or the like in continuous flow formation by the improved method.

Still another specific object of this invention is to provide an improved method of and apparatus for effectiing continuous and uniform treatment of a fibre suspension, or any material having similar fluent characteristics, with a chemical reagent or other fluid medium.

These and other specific objects and advantages of the invention will be apparent from the following detailed description.

A clear conception of the several steps constituting my improved method of treating brous material, and of one specific type of apparatus for carrying on the method commercially, may be had by referring to the drawing accompanying and forming a part of this specification, wherein like reference characters designate the same or similar parts in the various views.

Fig. 1 is a central vertical section through a typical pulp bleaching reaction cell embodying my invention;

Fig. 2 is a. transverse horizontal section through the reaction cell of Fig. l, taken along the line 2 2; and

Fig. 3 is a perspective view of a fragment of the improved mixing rotor comprising a part of the installation of Figs, l and 2.

The improved method constituting one portion of the present invention, comprises in general producing a constant displacement in one direction of a mass consisting of a mixture of fibrous material and a liquid treating medium, by continuously introducing new mixture at one end of the mass and continuously discharging treated material from the opposite end, and at periodic intervals along the path of displacement, re-distributing and intermixing the brous material and liquid in a plane transverse to direction of advancement, by means which do not affect the displacement of the mass and cause no movement of material outside the limited zones of re-distribution.

Referring to the drawing showing one embodiment of apparatus for carrying on the improved method which has been successfully applied to commercial operation, a cylindrical casing 6, having a lower inlet 'I and an upper outlet 8, provides an intervening vertically elongated reaction chamber or space 9 of approximately circular cross-section.` A centrifugal pump I0 or other suitable means is provided for constantly forcing regulated quantities of fibrous material and treating agent into the bottom of reaction space 9 through the inlet 'I. After passing upwardly through the entire vertical length of reaction space 9 by the displacement effected by the incoming mixture, the treated material is discharged over dam 2| to outlet 8. The dimensions of the unit are made such that the volume of reaction space 9 in relation to the volume of incoming mixture per unit of time, will cause any given horizontal stratum of material to rise through the reaction space in a period of time equal to the optimum reaction cycle of the process being carried on in the unit. In commercial practice, units have been built with internal diameters of from nine to sixteen feet, and a vertical height of approximately fty feet; while the rate of vertical displacement has been on the order of four to six inches per minute.

Located centrally within the reaction space 9 is an upright shaft I I which is rotatable in lower and upper bearings I2, I3 respectively, by means of an electric motor I4 which coacts with shaft II through speed reducing mechanism I5; and the shaft II carries a series of radial arms I8 forming rotor divisions spaced at suitable intervals along the shaft length, and at approximately the level of dam 2l this shaft carries a discharge impeller I8. Each of the radial arms I6 has streamlined transverse cross-section, and is provided with two series of vanes I9 and 2U, projecting vertically downward and upward from the lower and upper surfaces, respectively, and is also provided with an inverting blade I'I, projecting radially at an angle to the main arms. The vanes IS, 20 have increasing pitch or angularity proceeding toward the rotor axis, and the lower vanes I9 are adapted to act upon the particles composing a shallow stratum of material immediately adjacent to the corresponding arms I6 and to move such particles inwardly toward the axis of shaft II; while the upper vanes 20 are similarly adapted to act upon adjacent particles, but are adapted to move such parti-cles outwardly away from the axis of the shaft. The inverting blades II are short as compared to arms I6, and have surfaces disposed at an oblique angle relative to adjacent horizontal planes; and these blades I 'I are adapted to deflect particles which have been brought near the axis by the action of lower vanes I9, upwardly into the Zone of action of upper vanes 20. In general the system of vanes associated with arms I6 is so formed and proportioned that it does not impart motion to the pulp mass as a whole but acts only to move toward and away from the axis, such fibrous particles as lie Within the zone of action of the vanes at any given time.

The pulp discharge impeller i8, which is 1ocated above the uppermost set of arms I6, has two radial arms which are curved longitudinally and Vary in depth at their leadingfaces, in such proportion that a uniform section or transverse slice of the vertical pulp, column is swept over dam 2l and discharged to outlet 8 at each revolution of the rotor. The lower end of casing 6 may also be provided with a drain 22 to facilitate fiushing and cleaning of the casing interior.

During normal operation of this improved reaction unit, for example in the bleaching of paper pulp, the centrifugal pump l0 is operating to introduce regulated quantities of pulp and bleaching agent into the lower extremity of the cell, and the material thus admitted rises slowly through the reaction chamber or space S during a predetermined period of time, and is ultimately discharged by the impeller i8 over dam 2| and through the outlet 8. The motor lil should be operating to slowly revolve the shaft il and its rotor assemblies or divisions through speed reducing mechanism l5. As any given particle of pulp reaches the level of the bottom of vanes i9 of any set of arms i6 it will be acted upon by these vanes and moved inwardly toward the axis of the corresponding rotor assembly. Upon reaching a point near the shaft the particle will be engaged by blade l1 and deflected upwardly into the Zone of action of vanes 20, by which it will be moved outwardly away from the axis of the rotor. The action hereinbefore described in relation to a discrete particle, is in practice exerted upon a stratum of approximately two inches vertical depth; and the rate of radial displacement inwardly and outwardly produced by the action of the rotor is substantially in excess of the relative axial displacement of the stratum produced by the supply pump I0. Therefore the outermost upper vanes 20 tend to produce a slight positive pressure by centrifugal force and the outermost lower vanes I9 produce a slight negative pressure by cavitation, causing a portion of the material in each active stratum to return into the zone of lower blades I9 and repeat the cycle of radial displacement. Under average operating conditions a typical particle will thus cycle some four or iive times before being axially displaced out of the zone of action of each rotor division. Between the upper boundary of the active zone of one rotor assembly and the lower boundary of the active Zone of the next higher rotor division the pulp moves upwardly in a quiescent mass.

The action of the rotor assemblies or divisions upon the fibrous material as herein described, performs two distinct functions, both of which are indispensable to uniformity of treatment with minimum chemical consumption. First, each rotor division renders the stratum of pulpslurry within its zone of action physically homogeneous, and this periodic combing of the advancing column overcomes the characteristic tendency of a fibrous suspension to channel and stagnate as previously described in this specification. Thus all pulp receives equal retention time in the reaction chamber. Second, in each rotor division zone the fibres and treating agent are mutually 6 re-distributed and intermixed, thus enabling each bre to satisfy its optimum reactivity with a total chemical supply to the system based on average fibre demand, whereas in the absence of periodic redistribution the chemical dosage must be based on maximum fibre demand, with resultant waste of chemicals and degradation of the softer libres.

From the foregoing detailed description of the construction of my improved apparatus and of the successive steps of the improved process, it should be apparent that the present invention provides a method of and apparatus for most effectively bleaching or otherwise treating fibrous material in a continuous manner and While the bre stock is travelling along a path in suspension. With the aid of relatively simple and durable apparatus the improved method can be carried on automatically. The present improved process and apparatus have proven highly successful in actual commercial use, and have produced the desired results with both a maximum chemical efficiency and extremely low power consumption per unit of product. For example, an installation employing a cell of sixteen feet diameter has handled a pulp production of 650,000 pounds dry weight per twenty-four hours with an input of only sixteen horsepower; whereas other attempted constructions have consumed more than ten times as much power with a less satisfactory result.

It should be understood that it is not desired to limit this invention to the exact details of construction of the apparatus, or to the precise steps of the method, herein shown and described, for various modifications within the scope of the claims may occur to persons skilled in the art.

I claim:

1. The method of treating fibrous material resorting to the use of an aqueous medium giving fluidity to a slurry of the two and effective when brought into intimate association with the iibers of the material to chemically act upon the latter, and which comprises: giving travelling energy to a continuous stream of the slurry and working the same in an upward direction at a sluggish rate of advance, and re-distributing and intermixing the fibers within the slurry by forcing a horizontal stratum of the upwardly moving stream to move in a direction which is generally radial to the vertical stream axis and while maintaining the advancing stream adjoining said stratum in a state of comparative quiescence, there being a succession of said re-distributing and intermixing treatments performed upon the stream at separated intervals within the length of the streams upward travel.

2. The method of treating fibrous material resorting to the use of a liquid chemical giving fluidity to a slurry of the two and by intimate .association with the fibers of the material acting upon the latter, and which comprises: working a constantly advancing stream of the slurry at a sluggish rate of travel along a definite vertical path, and successively at separated intervals along the prescribed path re-distributing and intermixing the fibers within the slurry by causing the stream in course of its traversal of limited transverse Zones to be diverted from said vertical path and move generally radial to the axial line of advancement, and while maintaining the advancing stream adjoining said limited transverse zones in a state of comparative quiescence.

3. The method of treating chemically-reactive fibers contained in a liquid body of the functioning chemical, and which comprises, giving travelling energy to a continuous streamvof the mixture and working the samel upwardly' at a sluggish rate of` travel along a denite path, and in course of the traversal oflongitudinally separated but immediately adjacent transverse zones lying within the length of said prescribed path causing the stream to pursue routes of travel generally radial to the central axis, giving to the stream a directional travel, radially considered, in one said zone opposite to that of the other and neighboring zone, and while maintaining the advancing stream in a state of relative torpidity as it is brought to and as it thereafter leaves said immediately adjacent transverse Zones.

4, The method of treating a chemically-reactive iibrous material, and which comprises the steps of giving travelling energy to a continuous stream of the material mixed with a liquid bodyv of the functioning chemical and working the same upwardly at a sluggish rate of travel along;

a denite vertical pat-h; ige-distributing, and intermiXing the fibers and liquid chemical by causing the stream, in course of its traversal of neighboring transverse zones lying within the length of said prescribed path, to pursue generally radial routes of travel, giving to the stream a directional travel in one said zone opposite to that of the other and neighboring zone; allowing the advancing stream to assume a state of relative torpidity following said re-distributing and intermixing; and repeating said re-distributing and intermixing of the fibers and liquid chemical in course of traversal by the stream of other neigh- 8. boring transverse Zones lying within the length of said prescribed path and longitudinally removed from the iirst-mentioned zones.

5-. The method of claim 4 in which the directional travel of the stream in its radial movement within the said neighboring transverse zones is rst inwardly toward the central axis and thereafter outwardly away from said central axis, being made to accelerate its travelling speed in moving toward and decelerate its travelling speed in moving from said central axis.

RAYMOND P. HILL.

REFERENCES CITED- l5; The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 211,772 OHarra Jan. 28, 1879 525,095 Detwiler Aug. 28, 1894 989,126 Currie Apr. 11, 1911 1,231,499 Crossley June 26, 1917 1,768,955 Johnson July 1, 1930 245 1,915,812 Wollenberg June 27, 1933 1,943,031' Meissner Jan. 9, 1934 1,976,955 MacLean Oct. 16, 1934 FOREIGN PATENTS soi Number Country Date 517,797 Great Britain Feb. 8, 1940 7,872 Great Britain 1913

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