Reverse cleaning and de-inking of paper stock

Abstract

A slurry consisting of water, fibers and contaminants is fed tangentially into the top of a centrifugal cleaner having bottom and top outlets, thereby creating a vortex in the cleaner. The outer part of the vortex moves downwardly and most of the inner part of the vortex moves upwardly. Large percentages of contaminants are thrown into the inner part of the vortex and exhausted from the top of the cleaner, while most of the fibers and small percentages of the contaminants move to the outer part of the vortex and are exhausted through the bottom outlet.

Parent Case Text

RELATED APPLICATION

This application is a continuation-in-part of application Ser. No. 830,395 filed June 4, 1969 now abandoned by Harry J. Braun for REVERSE CLEANING AND DE-INKING OF RECLAIMED PAPER STOCK.

Description

FIELD OF INVENTION

Classifying, Separating And Assorting Solids; Grading Deposition, Vortical.

PATENT BACKGROUND

Fontein U.S. Pat. Nos. 2,550,341 and 2,649,963; Hirsh U.S. Pat. No. 2,975,896; Grundelius U.S. Pat. No. 3,486,619.

SUMMARY OF THE INVENTION

The invention is directed to a method of liberating an aqueous fibrous suspension from shives and/or other undesirable particles which, due to shape and density or shape, density and size, are difficult to separate. The other undesirable particles are regarded as lightweight particles since they normally appear with the desired fibers when these fibers are removed from a centricleaner as the light fraction, as opposed to the heavier rejects.

The method comprises feeding the suspension to a cyclone separator (centricleaner) having two outlets for separating the suspension into a first fraction containing a preponderance of shives and/or other undesirable lightweight particles and a minimum of acceptable fibers and a second fraction containing a preponderance of acceptable fibers and a minimum of shives and other undesirable lightweight particles. Of the two outlets, one is a light fraction outlet and the other is a heavy fraction outlet, the preponderance of acceptable fibers (second fraction) being discharged through the heavy fraction outlet and the preponderance of undesirable lightweight particles (first fraction) being discharged through the light fraction outlet.

Relative flow conditions are maintained in the outlets so that the suspension separates into the first and second fractions while maintaining a through-flow volume ranging from one which is substantially normal for the separator to one which is materially greater.

OBJECTS

In the field of papermaking, wastepapers constitute a valuable source of fibers, provided they can be decontaminated or de-inked. Up until comparatively recent times, the contaminants, which consisted largely of carbon black inks and clay and pigment fillers, could be removed reasonably well by conventional methods of defibering, solvating, washing, screening and other cleaning processes, including conventional centrifugal cleaning. The specific gravities of many of the contaminants ranged from about 2.5 to 8.0, and the specific gravities of the fibers ranged from about 0.9 to 1.4.

Centrifugal cleaning, as heretofore practiced, was reasonably successful for removing various particulate contaminants which lent themselves, because of their specific gravity, to rejection as so-called "heavies". Examples of useful types of centrifugal cleaners, so-called "cyclone separators", are those various sized devices manufactured by Bauer Brothers, Bird Machinery Company, Nichols Engineering, and others, and which consisted essentially in a cylindrical and/or cone shape vessel having a tangential inlet at the top, and axial outlet at the top and bottom. A slurry, of about 0.5 percent consistency, was fed in under pressure through the inlet to form a vortex, having an outer, downwardly spiraling portion (hereafter called "outer vortex") and an inner upwardly spiraling portion (hereinafter called "inner vortex"). Generally speaking, the heavy particles, i.e., those with radially outward settling velocities substantially greater than the radially outward settling velocities of fibers and "light" contaminants moved to the outer vortex and exhausted, as "rejects", through the bottom outlet; and the fibers, having radially outward settling velocities less than the radially outward settling velocities of the heavy contaminants were carried to the inner vortex and were exhausted as " accepts" through the top outlet.

While specific gravity is an important factor in centrifugal separation, other factors which militated against normal separation by centrifugation, as previously practiced, come into play. It was known that particle size and shape, and the effect of hydraulic drag thereon are significant. In conventional centrifugal cleaner operation, fibers failed to migrate to the outer vortex, apparently because the hydraulic drag coefficient thereof results in a negligible settling velocity. Unfortunately, a substantial amount of high density contaminants such as clay or pigment filler which would have been expected to go to the outer vortex also went to the inner vortex and out through the upper accept outlet despite their specific gravity being generally greater than the medium, apparently because of their size and shape which affects the hydraulic drag coefficient. So also with what is known as "ink pepper", i.e., extremely small particles of ink which had escaped preceding washing and screening processes.

Difficult as these problems were when conventional centrifugal separation was practiced, they became intolerable with the advent of a new class of contaminants, i.e., materials of plastic, polymeric, adhesive, rubbery, asphaltic or synthetic nature, now used in coating, printing, laminating and binding. Papers having one or more of these contaminants find their way into the wastepaper markets, and, since their specific gravities are generally close to and less than that of water, they, too, tend to migrate to the inner vortex of a conventionally operated centrifugal cleaner and are exhausted through the upper outlet along with the fiber accepts.

The object now is to provide a centrifugal separation process for de-inking and otherwise de-contaminating reclaimed paper stock, wherein the former centrifugal process is completely reversed, wherein the accept material, consisting predominantly of water and fiber, is taken off from the outer vortex and discharged through the heavy fraction bottom outlet of a centrifugal cleaner, and the reject material, consisting predominantly of water and contaminants of both heavy and light varieties, are taken from the inner vortex and out from the top of the cleaner. Among the contaminants so taken off from the inner vortex and discharged through the light fraction top outlet are ink pepper, clays and pigment fillers, fiber fines, and other minute solids, in aggregate, those which have densities greater than that of water, but which, because of size, shape or size and shape, are retained in the inner vortex, and also among the contaminants which are taken off via the top outlet are those of plastic, polymeric, adhesive, rubber, asphaltic or synthetic nature, in aggregate, those which have densities less than the density of water, and shives, whose densities are about the same as the desired fibers, but whose shapes are generally more extensive in one or more dimension than those of fiber.

These and other objects will be apparent from the following specification and drawing, in which the sole FIGURE is a diagrammatic representation of a typical centrifugal cleaner reversely operated according to the subject process.

Referring now to the drawing, which diagrammatically illustrates a typical centrifugal cleaner 2, which, for specific example, may be a standard Bauer Brothers 3-inch type with a conical side wall 4, a bottom nozzle 6, a top nozzle 8, a closed top 9, and a tangential inlet pipe 10 adjacent the top. In a cleaner of given size, the relative diameters of the top and bottom nozzles determine the flow splits, and, in part, the solids splits, which are critical to the improved results of the subject process, as will be discussed later.

In describing both the new "reverse" method and the old "normal" method, it will be understood that at this stage of the de-inking and decontaminating process, the stock has been defibered and solvated, washed, screened or otherwise de-inked by conventional methods, but that it is still contaminated with one or more of a variety of contaminants of greater density than water, such as clay or pigment, ink pepper, and contaminants of lesser density than water, such as adhesive, plastic, etc., materials. However, at any given time of an operation of the centricleaner, the contaminants may tend to consist of one variety, while at another time they may tend to consist of the other variety.

As the stock enters the top of vessel 4, an outer downwardly-spiraling portion OV of the vortex is formed, and the liquid and solids therein are exhausted out bottom nozzle 6. Within the outer portion of the vortex, an inner, upwardly spiraling portion IV of the vortex forms, and the liquid and solids therein are exhausted out the top nozzle 8. As previously normally operated, the liquid and solids exhausted via bottom nozzle 6 were the rejects, i.e., water and some of the heavy contaminants, and the liquid and solids exhausted out the top nozzle 8 were the accepts, i.e., water and fibers, but also the light contaminants, and some of the contaminants having densities greater than water which were present because of small size or shape and attendant large hydraulic drag coefficients. Because no separation is perfect, some fibers also went out the bottom nozzle 6 with the rejects.

The stock input through input pipe 10 should have a consistency of about 0.50 percent, consisting of water, fibers and contaminants, although the consistency may range from 0.10 to 1.50 percent. The most desired pressure of the stock fed to the cleaner according to the present process should be about 90 lbs. psig, but may range from 40 lbs. to 100 lbs., and preferably without back pressure. The input pressure of feed stock into the cleaner may thus range from that which is about normal for the separator to that which is considerably greater than normal; and, hence, the through flow may range from that which is about normal to that which is considerably greater than normal. Separation improves with increase in pressure, but in decreasing degree towards the higher end of the range, with diminishing return due to higher costs of pumping at the higher pressures.

The specific data for the necessary effective area and/or pressure relations can be easily determined for each type of separator with which the method of the invention is to be carried out as the calculation necessary for the purpose are well known to men skilled in the art. It is common practice to regulate the concentrations of the fractions obtained from cyclone separators by varying the area and/or pressure relations at the outlets as is described, for instance, in U.S. Pat. Nos. 2,550,341 and 2,649,963.

Among the unexpected results of the subject process wherein the fiber solids are exhausted with the underflow and the contaminants are exhausted with the overflow is that the temperature seems to make little appreciable difference in the fiber solids splits. However, when the fiber solids were exhausted with the overflow, significantly more fiber solids were rejected in the underflow with increase in water temperature. This was apparently due to decrease of water viscosity as it increased in temperature, with resultant decrease in hydraulic drag on the fiber solids, thereby permitting them to migrate towards the cleaner wall in the outer vortex so that more fibers were exhausted with the underflow rejects. The significant implication of temperature effects, however, is that the subject process is less temperature-sensitive than was the conventional operation of the cleaner.

The following is a compilation of two runs, denoting in the column entitled "Reverse" the cleaning operation of the subject process, and in the column entitled "Normal" a cleaning operation according to prior art practices. Both runs were made with the same conventional 3-inch Bauer Brothers centrifugal cleaner, with the slurries at approximately the same room temperature, i.e., unheated. In both runs, the cleaner had a five-eighths inch overflow nozzle. In the Normal run the cleaner had a one-eighth inch underflow nozzle, whereas in the Reverse run the cleaner had a one-half inch underflow nozzle. "Total Solids" means fiber and ash. "Ash Solids" means those solids, primarily clay and pigment, which remained after placing a sample of Total Solids in a muffle furnace and burning off all or substantially all the organic matter, including the fibers. Unrepresented in these figures was a small quantity of red rubbery natural latex adhesive which was added to the feed stock in both runs. This adhesive was prepared for these tests as follows: a sandwich of adhesive between two sheets of fiber, as commonly found in flying splices, was fed to a hydropulper and reduced to size representative of mill operation. This sandwiched adhesive was selected as representative of a light contaminant which normally has been particularly difficult to separate from fibers, and because of its easy identification in hand sheets made from the solids in the overflow and underflow exhausts. Its specific gravity is about 0.9 to 1.1, as compared with from about 0.9 to 1.4 fibers. In the Reverse run, about 99 percent of the adhesive was exhausted with the overflow reject, whereas in the Normal run about 99 percent of the adhesive was exhausted with the overflow accept.

In production runs, many contaminants have been identified in hand sheets made from the solids content of the overflow. These vary according to the make-up of the feed stock, and include rubbery adhesive particles of various colors, plastic film ranging from clear to various deep shades, strings from re-inforcing tape, large ink balls, finely divided ink pepper, chips of various sizes and colors, wood shives, fluorescence coating chips and plastic, and other extraneous material. The sizes of the contaminants ranged from 0.03 sq. mm (TAPPI Standard T-213m and T-347m) upward for the rubbery or adhesive contaminants to 1 inch .times. 1 inch for plastic films, and varied in specific gravity from somewhat less than that of the fibers in the feed stock to somewhat greater than that of the fibers. Some, such as the shives, were of about the same specific gravity as the desired fibers.

______________________________________ NORMAL REVERSE Feed Total Gallons 846 1189 Total Soliids 33.69 50.80 Ash % 18.64 20.40 Ash Solids Lbs. 6.28 10.20 Flow G.P.M. 21.84 33.5 Consistency % 0.481 0.512 Underflow total Gallons 29.0 611 Total Solids 4.67 42.70 Ash % 10.9 13.2 Ash Solids Lbs. 0.51 5.64 Flow G.P.M. 0.745 17.2 Consistency % 1.929 0.838 Overflow Total Gallons 817 578 Total Solids 29.02 8.10 Ash % 19.9 56.3 Ash Solids Lbs 5.77 4.56 Flow Gg.P.M. 20.9 16.3 Consistency % 0.426 0.168 Flow Split Underflow % 3.43 51.39 Overflow % 96.57 48.61 Solids Split Underflow % 13.86 84.05 Overflow % 86.14 15.95 Ash Split Underflow % 8.12 55.29 Overflow % 91.88 44.71 Fiber Split Underflow % 15.18 (Reject) 91.51 (Accept Overflow % 84.82 (Accept 8.49 (Reject) Psig Lbs. Inlet 45 90 Overflow Nozzle 5/8" 5/8" Underflow Nozzle 1/8" 1/2" ______________________________________

In addition to the almost total rejection of the light contaminant trace, the rejection of 44.71 percent of the ash by the Reverse process, as contrasted with rejection of 8.12 percent of the ash by the Normal process is significant. Also significant is the underflow acceptance of 91.51 percent of the fibers in the Reverse process, as compared with the overflow acceptance of only 84.82 percent of the fibers in the normal process.

As to the explanation of why the contaminants, with densities greater than that of water, such as clay and pigment and ink pepper, with specific gravities of about 2.6, 3.0 and 2.0, respectively, are found in the overflow, it is believed that they, because of their extremely minute particle size and possibly their platelet shape, and their concommitment much higher coefficients of hydraulic drag, as compared with the fibers, therefore hold back in greater degree than do the fibers from their natural tendency to migrate to the outer portion of the vortex.

In the foregoing examples, the "Flow G.P.M. 21.84" and "Psig Lbs. Inlet 45" approximate the normal (20 gpm and 40 Psig Lbs. Inlet) through-flow and inlet pressure recommended by the manufacturer for the 3-inch cleaner to obtain optimal separation results, and from which derives the following formula for determining normal through-flow for different-size centricleaners operating in normal mode and at the same efficiencies:

Q .apprxeq. 0.741 d.sub.c.sup.3 where

Q = through flow in gpm, and

D.sub.c = diameter of cone base in inches

The above process results in marked improvement in the separation of contaminants from fiber solids, recognizing that there still remains some undesirable overlap of contaminants in the fiber exhausted from one end of the cleaner, and of fiber in the contaminants exhausted from the other end of the cleaner.

As to the contaminants which because of their size, shape and specific gravity, exhibit normal behavior and therefore are exhausted through the bottom outlet with the fibers, these are subsequently separated from the fibers by passing them and the fibers through a cleaner which is normally operated according to prior practices.

Claims

I claim:

1. In the recovery of paper fibers for reuse from waste paper material, the process of separating the reusable paper fibers from certain contaminants, a first type of which have specific gravities lower than that of water, and a second type including ink peppers, clays, pigment and fillers, which have specific gravities substantially greater than that of water but which, because of size, shape or size and shape, have a high hydraulic drag coefficient, which comprises;

a. supplying said slurry to a closed cylindricalconical vessel having a tangentially arranged inlet port adjacent the base thereof and having an outlet port in the base thereof and an outlet port in the apex thereof,

b. maintaining the slurry flow through the vessel at a substantially higher rate than that which is normal for the vessel operating in normal fashion at nominal capacity, which normal rate is determined by the equation

Q .apprxeq. 0.741 d.sub.c.sup.3

where

Q = through flow in gpm

D.sub.c = diameter of cone base in inches

while maintaining in said outlet ports a flow relation wherein the flow through the apical outlet port is at least as great as the flow through the base outlet port to develope centrifugal force conditions causing vortical separation of said slurry with increased concentration of fibers in an outer fraction of the vortex which moves towards the apical outlet thereby increasing in an inner fraction of the vortex which moves towards the base outlet the ratio of the second type of contaminants to fiber above that which prevails with normal through flow and while increasing in said inner fraction the concentration of the first type of contaminants above that which prevails with normal through flow

c. exhausting through the base outlet the contents of the inner fraction consisting essentially of water, said contaminants and a minor portion of said fibers, and

d. exhausting through the apical outlet the contents of the outer fraction consisting essentially of water, the great predominance of said fibers, and a minor portion of said contaminants.

2. The process recited in claim 1, wherein the through flow rate through the vessel is maintained at about one and one-half times the rate which is normal for the vessel.

3. The process recited in claim 1 wherein the specific gravities of the second type of contaminants range from about two to four times that of water.