GB2157722A - Agglomerated cellulosic particles - Google Patents

Abstract

A cellulosic particle, especially useful as a cat litter, is manufactured by agglomerating a fibrous cellulosic feed material in the presence of water, compacting the surface of the agglomerated particles, and drying the particles. The water causes hydrogen bonding between the films. Paper mill sludge or biberized waste paper is moistened with water in blender 33, agglomerated in rotating drum 41 or a rotating disc agglomeration to form spherical particles which are then compacted to remove projecting films by the action of fluidized bed dryer 46 or a further rotating drum. In the embodiment of Figure 4 agglomeration compaction and drying are accomplished in a single long rotating drum. <IMAGE>

Description

SPECIFICATION Agglomerated cellulosic particles and method of making same This invention relates to agglomerated cellulosic particles, and especially to those adapted for use as an animal litter, and to a method of making such agglomerated particles.

Commercially available cat litters frequently contain clay. Clay litters have disadvantages in that they are heavy, dusty, and stick together in the bottom of the litter box after use. In addition, because clay litters have low absorbancy, cat urine tends to pool on the bottom of the litter box and creates an odor problem as bacterial growth increases.

In an attempt to overcome the disadvantages of clay litters, other cat litters have been proposed made from cellulosic materials such as newsprint and alfalfa. These products contain water-soluble binders and are produced by extrusion and pelletization of the extrudate. However, such products also suffer from disadvantages. In particular, during use the pellets swell and break apart, resulting in a mess in the litter box. Also, these products are dusty in spite of the presence of binders because the exposed ends of the pellets are not protected by a hardened skin.

It is a general object of this invention to produce an agglomeration of particles which resolve or mitigate these problems.

An agglomerated cellulosic particle in accordance with the invention is particularly useful as an animal litter, comprises agglomerated fibres, aggregates of fibres, and/or fibre-sized pieces of a fibrous cellulosic material, said particle having a densified or compacted outer surface or skin which is substantially free of protruding fibre ends and/or fibrils. (For purposes herein, "fibrils" are irregular aggregates of fibres and fibrous material. (see Figure 9 hereafter).

Because the particle is formed by agglomeration in the presence of water, it is held together essentially by hydrogen bonds. A minor degree of adhesive bonding can also be present due to the inherent presence of binders in the fibrous cellulosic waste materials. Also, the addition of a small amount (less than about 10 weight percent) of a binder such as starch can be tolerated. Preferably, however, the addition of binders is kept to a minimum because of cost and because the particles of this invention remarkably are relatively dust-free and maintain their integrity sufficiently without binders. The shape of the particle is substantially irregular, but is preferably platelet-like as will hereinafter be described. Considering the fact that the particle is formed from fibrous cellulosic materials, the compacted surface is relatively smooth.Depending on the nature of the fibrous cellulosic material used to form the particle, the internal structure of the particle can vary. If, for example, shredded waste paper having pieces of a size on the order of about 41 inch is agglomerated, the particle will have a discernable layered or lamellar structure. On the other hand, if a more finely divided material is used, such as paper mill sludge, a more homogenous internal structure results.

Nevertheless, the rolling action of the agglomeration process still imparts a slight degree of internal orientation to the particle even when very finely divided feed material is used. This orientation appears to be an inherent characteristic of the agglomeration method used to make the particles.

The particles of this invention can be made from fibrous cellulosic waste materials, such as waste paper, newsprint, paper mill sludge, etc., and combinations of these cellulosic waste materials, and therefore provide a means for turning a waste material into a useful product. The particles of this invention are absorbant, light in weight, easily disposable, and exhibit very low dust generation.

When heavily moistened, the particles of this invention still maintain their integrity and do not tend to agglomerate to each other. Even when immersed in water, they retain their integrity after becoming saturated. This is surprising since the particles are primarily or substantially solely held together by hydrogen bonds. This characteristic is particularly advantageous when the particles are used as a cat litter.

A method for making agglomerated cellulosic particles in accordance with the invention comprises (a) agglomerating a moist blend of fibres, aggregates of fibres, and/or fibre-sized pieces of a fibrous cellulosic feed material, such as waste paper and/or paper mill sludge, to form individual agglomerated particles; (b) compacting the surface of the agglomerated particles to form a densified skin substantially free of protruding fibre ends and/or fibrils; and (c) drying the agglomerated particles.

Preferably, the particles are slightly flattened prior to final drying to form platelet-like particles which have less tendency to roll. This is particularly advantageous when the particles are used as an animal litter.

A bed of particles within a container may be formed to provide a litter, said particles consisting essentially of agglomerated fibres, aggregates of fibres, and/or fibre-sized pieces of a fibrous cellulosic material wherein the outer surfaces of the particles have a densified or compacted skin which is substantially free of protruding fibre ends and/or fibrils. Preferably the particles have a platelet-like shape. As a cat litter, the particles of this invention exhibit a very low level of dust generation and minimal sticking to the bottom of the litter box in use.

The invention will now be further described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic flow diagram illustrating how a raw feed material can be prepared for use in the method of this invention.

Figure 2 is a schematic flow diagram of one embodiment of this invention illustrating the essential agglomeration, surface compaction, and drying steps of this invention.

Figure 3 illustrates a process similar to the process of Figure 2, but additionally including an op tional step wherein the particles are flattened to form platelets.

Figure 4 illustrates a simplified process wherein a single rotating drum is used to agglomerate the particles and also compact their surfaces with partial drying.

Figure 5 is a photograph of several representative dried particles of this invention which are irregular, generally spherical in shape.

Figure 6 is a photograph of several representative particles of this invention which have been flattened into platelets.

Figure 7 is a photograph of cross-sections of representative particles of this invention.

Figure 8 is a photograph of cross-sections of representative particles of this invention formed solely from a paper mill sludge feedstock.

Figure 9 is a photograph of material taken from the blender prior to the primary agglomeration step, illustrating the appearance of protruding fibrils and fibre ends.

Referring to Figure 1, preparation of feed material for the method of this invention will be described in greater detail. Figure 1 shows a means for providing a fibrous cellulosic material suitable as a feed material for agglomeration. "Fibrous cellulosic materials", for purposes herein, are fibrous cellulosic materials which are substantially free of natural binders such as lignin, tars, and pitches naturally present in plant life. These natural binders cause the cellulosic fibres to be stiff and inflexible, which is undesirable for purposes of this invention. These binders are normally removed by extraction in a papermaking pulping process.

Therefore, cellulosic materials which have undergone digestion in a pulping operation are particularly suitable. Such suitable materials specifically include waste paper (including newsprint, cardboard, etc.), paper mill sludge, and combinations thereof. As used herein, "paper mill sludge" is a general term, including primary tissue mill sludge and other types of slude resulting from papermaking processes, which describes a dewatered waste product from paper mills primarily containing short cellulose fibres which have passed through the forming wire of a wet forming papermaking process. It will be appreciated that the feed preparation steps illustrated in Figure 1 are not necessary for all rawfibrous cellulosic materials, such as paper mill sludge, which can be fed directly into the process of this invention if its water content is not too high to prevent agglomeration.However, paper mill sludge can also be subjected to the process of Figure 1 in combination with other raw materials to the extent that its water content does not adversely affect the operations, particularly the fiberization step.

Specifically shown in Figure 1 are bales of waste paper 5 being conveyed on a belt conveyor 6 to a suitable bale breaker 7 which initially breaks up the bales into a more manageable form. The broken bale material from the bale breaker is deposited on another conveyor 8 and is metered by rake-back rolls 9 onto a third conveyor belt 11 which carries the material past a metal detector 12. The metal detector signals an operator in the event any metals are present which may damage subsequent apparatus and must be removed. Undesirable materials can be deposited into a tote box 13 with the aid of a suitable means such as a pivotable deflecting plate 14.

The fibrous cellulosic material is then suitably conveyed, as by conveyor belt 15, to one or more fiberization or attrition devices such as hammermills 16 and 17, which are designed and operated in a manner familiar to those skilled in the art of fiberization. The purpose of the hammermills or any other attrition device used herein is to shred or fiberize the raw material into small fibre-sized pieces, including individual fibres and aggregates of fibres. (For purposes herein, "fibre-sized" means a size on the order of about i inch x 41 inch or less. As the pieces become larger, it becomes increasingly difficult to process the material into the smooth- surfaced particles described herein.) Forced air is continuously directed into the hammermills, as through line 18, to keep the material moving through the hammermills.A particular fibrous cellulosic feed material which has been successfully processed through the hammermills, for purposes of illustration, contained (by weight) 25% newsprint, 60% clay-coated glue-grade waste paper, and 15% primary tissue mill sludge (containing about 75 weight percent water). Paper mill sludge typically contains from about 60 to about 90 weight percent water and therefore could cause plugging in the hammermills if present in too large an amount. Hammermills may plug up if the total moisture content of the fibrous cellulosic material approaches about 20 weight percent or so. Hence, when paper mill sludge is used, it must be blended with dry feed materials to "dilute" the moisture content. It is preferable that the paper mill sludge be added to the system after the hammermills to avoid any such problems.Of course, if paper mill sludge is to be used as the sole feedstock, the feed preparation steps illustrated in Figure 1 are unnecessary.

The resulting fibres, aggregates of fibres, andlor fibre-sized pieces of the fibrous cellulosic material are conveyed, with the aid of additional forced air through line 19, from the hammermill 17 through a conduit 21 to a fibre condenser drum 22 wherein the material is allowed to settle. Fines and dust present in the airspace above the settling fibres are drawn out of the fibre condenser drum 22 via conduit 23 and directed to a baghouse 24 where they are filtered out of the air and recycled or disposed.

A suitable fan 26 provides the necessary means for drawing the air and dust into the bag house. Additional dust conveyed through line 28 from a dust hood located at the hopper of the screw-feeder of Figure 2 is also directed to the baghouse.

Figure 2 illustrates a method embodying the steps of this invention, i.e. the steps of agglomerating fibres; aggregates of fibres, and/or fibre-sized pieces of the fibrous cellulosic feed material to form individual agglomerated particles, compacting the surfaces of the agglomerated particles, and drying the particles. As will become apparent from this specification, the steps of agglomeration, surface compaction, and drying are not necessarily accomplished in three separate pieces of apparatus. More typically, one apparatus will simutaneously agglomerate the the particles and compact the surfaces of the particles or alternatively compact the surfaces of the particles and simutaneously dry the particles. In fact, as discussed in connection with Figure 4, one apparatus can accomplish all three process steps.Directing attention back to Figure 2, fibres, aggregates of fibres, and/or fibre-sized pieces of a fibrous cellulosic feed material from a suitable source, such as the fibre condenser drum 22 of Figure 1 and/or a source of paper mill sludge, are fed into a feeding device such as the hopper 31 of screw conveyor 32 which serves to controllably meter the feed material to the blender 33. Optionally, additional ingredients such as odor absorbants (e.g. sodium bicarbonate, activated carbon, borax, etc.), anti-static agents, flame retardants, etc., can also be added at this point in the process to the extent that such additives are desired as processing aids or to impart certain characteristics to the final product. Antistatic agents are particularly useful for reducing the tendency of the final product particles to cling to an animals's fur, as when used used as a cat litter.Quaterrary ammonium salts, such as di-cocodimethyl--ammonium chloride or methylbis (2-hydroxyethyl) cocoammonium nitrate, are suitable anti-static agents for this purpose and can be added at levels from about 0.5 to about 5 dry weight percent based on the air dry weight of the fibrous cellulosic material. Any dust generated in the hopper can be removed by a suitable vented hood system which directs the dust to the baghouse.

In the blender 33, the fibres, aggregates of fibres, and/or fibre- sized pieces of the fibrous cellulosic feed material are thoroughly mixed or blended with an amount of water which is sufficient or almost sufficient for agglomeration. Agglomeration is a process wherein particle size is increased around a nucleation site by continual rolling of the particles. This motion increases the exposure of the growing particles to the fibres, aggregates or fibres, and/or fibre-sized pieces of the fibrous cellulosic feed material, thereby affording an opportunity for intimate contact and growth by mechanical intertwining of fibres and adhesion. It has been found that when the proper moisture level is achieved for any given blend of such fibrous cellulosic material, agglomeration occurs very readily.

As the moisture level of the blend is increased still further, larger agglomerated particles can be formed. Of course, if too much moisture is present, a slurry will be formed and agglomeration will not occur at all. Therefore the blender primarily serves to condition the fibrous cellulosic feed material for the subsequent agglomeration step. The agglomerated particle size can thus be somewhat controlled by the moisture level. The blender suitably consists of a tubular vessel with an inlet 34 and a series of internally axially rotating tynes or paddles 35 (paddles are illustrated) which mix and move the feed material toward the exit 39. If tynes are used, virtually no agglomeration takes place in the blender.

However, if paddles are used it has been found that some agglomeration will occur in the blender, which is advantageous because there is accordingly a lesser burden on the subsequent apparatus to effect agglomeration. The blender also preferably contains multiple water inlet ports, such as 36, 37, and 38, which are desirable to achieve proper moisture control. Warm water, if available, may be preferable because it more reality softens cellulosic fibres. A specific apparatus which has been used successfully is manufactured by Ferro Tech (Model 12T35). It has been found advantageous to add the water in stages as shown to provide better distribution.For example, about 80 percent of the water to be added to the blender can be added through the first port 36, about 10-20 percent can be added through the second port 37, and the remainder, if any, can be added through port 38. The moisture content of the blended material leaving the blender should be in the range of from about 50 to about 80 weight percent based on the air dry weight of the cellulosic fibres, but the precise level must be optimized for the specific feed material and the specific agglomeration device(s) being used. At this stage of the process, the moist blended material can exhibit a degree of agglomeration, although on the whole, the material can be described as a very loose, crumbly mass with little individual particle integrity.

The moist blend of fibres, aggregates of fibres, and fibre-sized pieces of fibrous cellulosic feed material leaving the blender is then deposited into a rotating mechanical agglomerating device 41 which primarily serves to roll the moist material into agglomerated particles and to complete any agglomeration which may have occurred in the blender. Agglomeration can be accomplished in any suitable rotating device, such as a rotating disc-type agglomerator or a horizontal rotating drum. As an example, a disc agglomerator manufactured by Ferro Tech having a 3 foot diameter disc pitched at an angle of 47" off horizontal rotating at about 27 r.p.m. has been found suitable.

However, a substantially horizontal rotating drum is preferred because it is less expensive and provides improved agglomeration while simultaneously conpacting the surface of the particles to form a densified skin as will hereinafter be described. A 23 inch inside diameter drum which is 5; feet long and rotating at 22 r.p.m. has been found sufficient for purposes of agglomeration at production rate of about 50 Ibs. of air dry cellulosic fibre per hour. Although the substantially horizontal rotating drum is preferably level, it can be slightly inclined or deciined to increase or decrease the average particle residence time as desired. The ends of the drum are open, except for a circumferential wall at the inlet end to prevent material from falling out. Movement of the particles from one end of the rotating drum to the other occurs naturally as the particles seek the lowest level and fall out of the outlet end of the drum. The space occupied by the particles within the rotating drum is relatively small based on the internal volume of the drum. Typically, the volume percentage occupied by the particles is only about 2 to about 5 percent. This permits the particles to ride up the inside of the drum during rotation and, upon reaching a certain point, fall or roll back down to the bottom of the drum to create a mixing or rolling action. Additional water 42 can be added to the agglomerating device to further enhance the agglomeration process as necessary. It can be advantageous, for example, to add some of the water at this point if the material in the blender would otherwise be too wet to leave the blender properly.It has been found that fromout 0 to about 30 weight percent additional water is advantageous. This can be easily accomplished with suitably positioned spray nozzles inside the drum which are preferably positioned near the inlet of the drum to provide the necessary agglomeration moisture as soon as possible to the material to be agglomerated. The moisture content of the material at this point should not exceed about 85 weight percent and preferably not exceed about 80 weight percent if cat litter is the desired end use of the particles.

Higher moisture levels can cause formation of particles which are too large for this purpose.

The agglomerated particles 43 leaving the agglomerating device typically have a moisture content of from about 75 to about 80 weight percent.

The particle size typically ranges from about 1/32 inch to about 5/8 inch in diameter or larger. The particles are classified in a vibratory expanded metal screen 46 to remove the particles larger than 5/8 inch, which are recycled through conduit 47 to the hammermills. These particles are preferably rejected because they are too large for cat litter and are difficult to dry relative to the smaller particles.

However, the particles retained for further processing will eventually shrink as much as about 30 percent in size by the time they are dried. Hence, particles at this stage of the process which are larger than the final desired size can be retained.

The agglomerated particles leaving the agglomerating device also may contain large numbers of protruding fibre ends and'or fibrils. These exposed fibre ends and fibrils are undesirable if the particles are to be ultimately used as a cat litter because they cause the particles to cling to the cat's fur and therefore can be tracked away from the litter box. Therefore it is necessary that the surfaces of the agglomerated particles be substantially smooth (see Figure 5). One way of accomplishing this is to compact the surface of the particles by rolling or bouncing the moist particles, as in a vibrating fluidized bed dryer, wherein the surfaces of the particles are compacted while being only partially dried. The same surface compaction can be also accomplished, for example, in a substantially horizontal rotating drum.In fact, it is preferred to use a substantially horizontal rotating drum to effect agglomeration followed by second substantially horizontal rotating drum to effect surface compaction. The surface of the second drum is preferably teflon-coated to improve the surface compaction by preventing the particles from sticking to the inner walls. The surface compaction step must be performed while the particles have at least about 15 weight percent moisture and serves not only to reduce the numbers of protruding fibre ends and fibrils, but also enhances the strength of the hydrogen bonding in the surface of the particles by increasing fibre-to-fibre contact in the surface. Compaction imparts a smoother surface to the particles and also gives the particles a more dense skin which resists dust formation and preserves particle integrity when heavily moistened.

Directing attention back to Figure 2, the particles of acceptable size passing through the screen 46 are directed to a hot air fluidized bed dryer 48 to compact or further compact the surfaces of the particles and to reduce the moisture content to preferably about 30 weight percent. A particular dryer which has been found to be suitable has a vibrating bed plate surface of 32 square feet with a 3% open area and is 16 feet long. The agglomerated particles enter the dryer at the inlet 49 and leave the dryer through outlet 51. Hot air at about 380 F. (6700 standard cubic feet per minute) enters at inlet 52 and is upwardly directed through a large number of orifices in the bed plate. The air exhausts at outlet 53.A particle residence time of about 30-60 seconds has been found to be suitable, which, for the particular apparatus used, corresponds to a particle bed height (at rest) of about 2 inch. In operation the fluidized particle bed has a very low density. In fact, the smaller particles may rise as high as about one foot above the bed plate.

The larger particles tend to merely roll along on the surface of the bed plate, urged toward the outlet by the vibratory motion imparted by a suitable vibrating means connected to the bed plate. Dust contained in the exhaust from the dryer can be directed to the baghouse for disposal or recycle.

The surface-compacted agglomerated particles leaving the first fluidized bed dryer 48 typically have a moisture content of about 30 weight percent. A moisture content of from about 15 to about 50 weight percent is considered suitable to permit good surface compaction and still retain particle integrity during subsequent processing. The shape of the agglomerated particles at this stage of the process is generally spherical, although bumpy and irregular. The surfaces of the bumps, however, are smooth. If desired, the surface- compacted agglomerated particles 55 can be screened in a second vibratory screen 56 to remove any particles 57 of unwanted size, either large and or small.

The acceptable particles 58 are then directed to a final drying means, such as a second vibrating fluidized bed dryer 60, in which the particles are dried to a moisture content of about 10 weight percent or less, preferably about 5 weight percent, to inhibit bacterial growth which is enhanced by high moisture levels. Because the sole purpose of this fluidized bed dryer is to dry the particles, for economy it is preferably operated in a manner wherein the fluidized particle bed is much more dense than in the first dryer. Typically, the bed height will be about one or two inches during operation and accordingly less air through conduit 61 is supplied to the bed than for the first dryer. By way of example, about 3,000 standard cubic feet per minute of hot air (475"F.) is supplied to the final dryer, which can be physically identical to the first.Particle residence times can also be about the same as in the first dryer, but can vary greatly as desired. Obviously, the design of both dryers can be altered by anyone skilled in the art to meet the demands of a particular system.

The dried, surface-compacted, agglomerated particles 70 leaving the second dryer are preferably deposited onto a third vibratory screening apparatus 71 to remove any particles which are either too large or too small for the desired end use. These particles can be recycled to the hammermills. The preferred screening apparatus shown uses a double screen which recycles particles 72 which are less than 1/16 inch in diameter and particles 73 which are greater than 3/8 inch in diameter. (As used herein, "diameter" refers to the largest linear dimension of the particle and is not intended to imply that the particle has a true circular or spherical shape). This size classification has been found to be preferred for animal litters. As shown, the acceptable particles 75 leaving the centre portion of the screen are recovered as a final product for bagging and packaging as desired.During packaging, fragrances such as encapsulated fragrances or oils can be mixed with the dried particles to improve product acceptance.

Figure 3 illustrates a preferred optional step to the process of Figure 2, wherein after the surfacecompacted particles have been partially dried in the fluidized bed dryer 48 and screened, the particles 58 are formed into platelets in a suitable device. This can be accomplished, for example, by flattening the particles 58 by conveying them through a nip between a press roll 80 and a conveyor belt surface 81. The conveyor belt can be driven and supported by rolls 82 and 83. The shape of the particles is thereby changed from generally spherical to what is referred to herein as "platelet-like" (See Figure 6.) This is advantageous because the platelet-like particles have less tendency to roll if they are scattered on the floor during use, thereby confining the particles to a smaller area for clean-up.It is important that this step be carried out with the particles in a moist condition (from about 15 to about 50 weight percent moisture) to avoid breaking the densified surface skin and causing the particles to crumble. On the other hand, in a moist condition the particles are susceptible to adhering to each other if pressed together under sufficient pressure. Therefore it is preferable to deposit the particles onto the conveyor belt sub stantiaily in a monolayer to avoid combining several individual particles into a larger single mass. If the particle size or height varies greatly among the particles present at this point in the process, it can be advantageous to classify and separately flatten small, medium, and large particles.Otherwise, for example, if the nip clearance is set for the larger particles, the smaller particles may pass through without being sufficiently flattened.

Figure 4 represents another alternative embodiment of the process of this invention in which some of the equipment illustrated in Figures 2 and 3 is eliminated. This is accompanied by using a single substantially horizontal rotating drum to effect several steps, namely agglomeration, surface compaction, and partial drying. This can be done by extending the length of the rotating drum sufficiently to accommodate these process functions.

As shown, as with the other illustrated embodiments, a suitable fibrous cellulosic material is deposited into the hopper 31, which supplies the material to a screw feeder 32. The material is metered into the inlet 34 of the blender 33, which is preferably of the rotating paddle-type which can cause some degree of preliminary agglomeration.

As previously illustrated, the paddles 35 are mounted on an axially rotating shaft which mixes and moves the material to the outlet 39. Water addition is provided through inlet ports 36, 37, and 38 as before.

The moist blended material is then deposited into a substantially horizontal rotating drum 41, which is similar to the rotating drums previously described, except for its length. For a production rate of about 175 pounds of air dry cellulosic fibre per hour, a drum length of about 12 feet, an inside diameter of about 23 inches, and a rotational speed of about 25 r.p.m. is believed suitable. Inside the drum, near the inlet, a series of optional water spray nozzles 90 are suitably positioned to apply additional agglomeration water as needed as previously described. The nozzles are suitably connected to a water source 91. At a point nearer the outlet of the drum, hot air from a suitable source 93 can be directed at the particles by suitable nozzles 96. Alternatively or additionally, the surface of the drum or a portion thereof can be externally heated.In this manner the drum causes agglomeration in a first zone as previously described in connection with Figure 2. In a second zone, after the water addition and agglomeration, the particles ar continuously rolled over each other to compact and/or further compact their surfaces. In the third zone, the particles are partially dried, preferably to about 30-50% moisture, by the hot air and still further surface-compacted. The partially dried particles 98 leaving the rotating drum are then screened in a vibrating screen 46 as previously described, dried in a fluidized bed dryer 60 as previously described, and screened again in a vibrating screen 71 if necessary as previously described. Optionally, the particles leaving the screen 46 can be flattened into platelets prior to final drying'as described in connection with Figure 3.

Figure 5 is a magnified photograph (approximately 4.5 X) of representative particles of this invention which, for purposes herein, are deemed to have an irregular, generally spherical shape. As is readily apparent from the photographs, the surfaces of the particles may contain large bumps, but the surfaces of the bumps are relatively smooth and there are substantially no visible protruding free fibre ends or fibrils.

Figure 6 is a photograph similar to Figure 5, illustrating representative particles of this invention which, for purposes herein, are deemed to have a platelet-like shape. These particles are simply particles similar to those shown in Figure 5 which have been slightly crushed or flattened to reduce their spherical nature as previously described. To illustrate the extent of flattening, the platelets in the photograph have been glued on edge to show the flattened profile. These same particles, when viewed from above as they lie on a flat surface, appear generally round and look very much like the particles of Figure 5.

Figure 7 is a magnified (8X) photograph of crosssections of representative particles of this invention which have been formed from a feedstock consisting of 25% newsprint, 60% clay-coated gluegrade waste paper, and 15% primary tissue making slude as previously described. As can be seen from the photograph, the internal structure, which is somewhat layered or lamellar in nature, is generally less dense in the centre of the particle relative to the compacted outer surface or skin, which completely surrounds the particle. The presence of the densified skin is very important for minimizing dust formation and maintaining particle integrity when heavily moistened. Formation of the skin is due to surface compaction caused by the rolling action during and after agglomeration.

Figure 8 is a magnified (8X) photograph of crosssections of representative particles of this invention which have been formed from a feedstock consisting of tissue mill sludge as previously discussed. As with the particles of Figure 7, these particles also have densified, surface-compacted skins which surround the less dense interior of the particle. Although the internal structure is not as layered as the particles of Figure 7, there nevertheless is some degree of orientation to the structure, which appears to run generally parallel to the surfaces of the particles.

Figure 9 is a magnified (approximately 4.5X) photograph of the fibrous cellulosic feed material leaving the blender, which is prior to the primary agglomeration step. As is clearly shown, although a degree of agglomeration has already taken place at this point in the process, the surfaces of what might be best described as large aggregates of material are not compacted and are dominated by free fibre ends and fibrils. These types of structures are to be avoided. It will be appreciated, however, that during large scale production some particles having protruding fibres and fibrils may be present in the product in small numbers. This situation however, is a matter of quality control and is not outside the scope of this invention.

As previously mentioned, agglomerated particles of this invention are light in weight and highly absorbant. Typically, they have an aggregate density of from about 0.2 to about 0.3 grams per cubic centimetre and an absorbant capacity of from about 2 to about 2.5 grams of water per gram of particle. These properties can vary, of course, depending upon the nature of the raw feed material, the processing conditions, the presence of additives, etc. In addition, for use as a cat litter, the particles of this invention are relatively dust-free when compared to commercially available claybased cat litters.

To illustrate this last point, a test was designed to measure the amount of dust generated by pouring a bag of cat litter particles into a litter box.

(The measured weight, in grams, of dust collected pursuant to this test can be referred to as the "Dust Level Index".) Specifically, a 22 inches x 15 inches x 5 inches high plastic cat litter box was fitted with a plexiglass cover measuring 22 inches x 15 inches x 7 inches high. The cover was provided with a hole at one end measuring 6 inches in diameter. A high volume air sampler (a Fixt Flo air sampler manufactured by Mine Safety Appliances, Inc.) containing a tared glass fibre filter was fitted to the hole. The opposite end of the cover was provided with a series of small holes to permit air flow across the interior volume of the assembly from one end to the other. The top of the cover was also provided with an 8 inch diameter hole for pouring in the test sample. When conducting a test, the air sampler was first turned on.The volumetric flow rate of air through the sampler was 21.6 cubic feet per minute. A bag of sample particles to be tested (wherein the particles occupy about 368 cubic inches) was poured into the litter box through the 8 inch hole in the top of the cover and the litter was spread evenly over the bottom of the litter box by hand. The top hole was covered.

The air sampler was run for 5 minutes for each sample and all airborne dust was collected. The filters in the sample were desicated overnight and weighed to determine the amount of dust collected (background room air dust level was taken into account). For a 10 pound bag (368 cubic inches of iitter) of Tidy-Cat 3 brand cat litter, a net of about 276 grams of dust was collected. For the same volume of particles of this invention only about 4 grams of dust was collected. Hence the Dust Level Index for the particles of this invention was 60-fold less than the Dust Level Index for the commercially available clay litter. In general, the particles of this invention will exhibit a Dust Level Index of about 10 or less, preferably less than about 5 as illustrated.

Claims (21)

1. A particle comprising agglomerated fibres, aggregates of fibres, andxor fibre-sized pieces of a fibrous cellulosic material, said particle having a compacted outer surface substantially free of protruding fibrils.

2. A particle as claimed in Claim 1 having an irregular, generally spherical shape.

3. A particle as claimed in Claim 1 having a platelet-like shape.

4. A particle as claimed in any of the preceding claims held together substantially solely by hydrogen bonding.

5. A particle as claimed in any of the preceding claims having an aggregate density of from about 0.2 to about 0.3 grams per cubic centimetre.

6. A particle as claimed in any of the preceding claims having a water absorbant capacity of about 2 grames of water per gram of particle.

7. A particle as claimed in Claim 1 wherein the fibrous cellulosic material comprises paper mill sludge and/or waste paper.

8. A particle as claimed in any of the preceding claims having a Dust Level Index of about 10 or less.

9. A particle as claimed in any of the preceding claims having a diameter of from about 1/16 inch to about 3/8 inch.

10. A particle as claimed in Claim 9 having an aggregate density of from about 0.2 to about 0.3 grams per cubic centimetre and a water absorbant capacity of about 2 grams of water per gram of particle.

11. A method for making cellulosic particles comprising: a) agglomerating a moist blend of fibres, aggregates of fibres, and/or fibre-sized pieces of a fibrous cellulosic feed material to form individual agglomerated particles; b) compacting the surface of the agglomerated particles to form a densified skin substantially free of protruding fibrils; and c) drying the agglomerated particles.

12. A method as claimed in Claim 11 wherein the particles are agglomerated in a rotating disctype agglomerator.

13. A method as claimed in Claim 11 or 12 wherein the surfaces of the agglomerated particles are compacted by bouncing or rolling the agglomerated particles in a fluidised bed supported by upwardly flowing air.

14. A method for making cellulosic particles as claimed in any of claims 11 to 13 in which the moist blend used to form agglomerated particles has from about 75 to about 85 weight percent moisture; the particles then being partially dried to from about 30 to about 50 weight percent moisture and formed into platelets; the particles then being dried to a moisture content of about 10 weight percent or less.

15. A method as claimed in claim 14 wherein the particles are surface-compacted and partially dried in a fluidised bed dryer.

16. A method as claimed in claim 14 or 15 wherein the partially dried particles are deposited onto a moving belt and flattened in a nip between the moving belt and a press roll.

17. A method as claimed in claim 16 wherein the flattened particles are dried in a fluidised bed dryer to a moisture content of about 5 weight percent.

18. A bed of particles as claimed in any of claims 1 to 13 and/or as made by a method as claimed in any of claims 14 to 17 within a container.

19. A bed of particles as claimed in Claim 18 wherein the particles further comprise an amount of antistatic agent sufficient to lessen the tendency of the particles to cling to fur.

20. A particle substantially as hereinbefore described.

21. A method of making particles substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.