WO1988007400A1

WO1988007400A1 - System for centrifugation and for classification of particulate materials

Info

Publication number: WO1988007400A1
Application number: PCT/US1988/001008
Authority: WO
Inventors: Bruce E. Novich; Richard L. Pober
Original assignee: Ceramics Process Systems Corporation
Priority date: 1987-03-23
Filing date: 1988-03-23
Publication date: 1988-10-06
Also published as: KR890700383A; EP0305510A1; AU1627688A; JPH01503525A

Abstract

Colloidal particles can be separated from suspension (13) using a centrifuge which includes a static fluid volume (44) formed prior to introduction of the feed slurry (13). A more precise and accurate classification fraction is obtained thereby. A classification system, including a number of such centrifuges (233, 236, 237, 238) cascaded together, provides the ability to obtain numerous submicron classification fractions where the feed is a well-dispersed slurry of colloidal particles and the flow rate and rotation speed in and through each of the centrifuge units is controlled in real time. Such submicron classification fractions often exhibit dilatant rheology and, therefore, are very difficult to remove from the centrifuge bowl. The present invention provides a method for facilitating the removal of such dilatant cakes by the use of an excess amount of dispersant which is effective to disperse the colloids and to provide a plastic, rather than a dilatant, cake, while also having little effect on the dispersion rheology.

Description

SYSTEM FOR CENTRIFUGATION AND FOR CLASSIFICATION OF PARTICULATE MATERIALS

Field of the Invention

This invention relates to a centrifugation system for classifying particulate materials, to a system for obtaining sub-micron classification fractions from colloidal particulate feed materials, and to a method for facilitating the removal of those classified fractions from the centrifuge bowl. In a related aspect, the present invention relates to a system suitable for selective chemical reactions involving colloids during the separation process.

The State of the Art

The use of centrifuges and centrifugation systems for size classification is well known in the art. See, for example, U.S. Patent Nos. 2,085,538 and 2,097,420. Such art has been concerned primarily with the size classification of clay particles, which are used primarily for porcelain, china, and similar ceramic parts.

Advances in ceramics processing have permitted the replacement of various components of electrical and mechanical equipment with high performance ceramic parts. In addition, ceramics have found a more wide-spread use in electronic components, cutting tools, and as structural substitutes for metal components. The properties exhibited by such ceramic materials are determined primarily by the mic ostrueture of the sintered ceramic part. However, current processing systems allow for a high variability in the processing parameters, which are dependent, to a large e tent, upon the characteristics of the starting ceramic po der. Ceramic structures are typically manufactured from powders formed into a so-called "green body." This green body is subsequently sintered at high temperature to yield the final ceramic material. At present, commercially available powders have large size distributions, typically from less than 0.5 microns to about 10 microns. Because of such wide size dis ributions, orderly packing of powders into green bodies has been difficult to achieve, and relatively high sintering temperatures have therefore been required. Consequently, the sintered microstruetures and surface finish and processing properties such as shrinkage variability and surface finish have also been difficult to control. Accordingly, there is a need for a system for producing narrow size fraction powders. Classifying powders into narrow particle size ranges can be accomplished through dispersion sedimentation. The sedimentation rate is given by Stokes' Law of Settling:

Kr²g( p - _p) v= h/t =

where v=the particle's settling of velocity, h=the distance through which the particles settles, t=the time required for the particles to settle through the distance h, r is the particle radius, g=the acceleration due to gravity, _p = the particle density, _p = the density of the medium, = liquid viscosity, and K = the particle shape factor (2/9 for a sphere) , which takes into account both a particle's volume and its cross -sectional area .

The sum of a medium's buoyant force and the drag on a submicron particle makes simple gravitational settling time consuming and tedious, and therefore uneconomical. Increasing the settling forces through centrifugal sedimentation expedites the settling process. Because a particle's terminal velocity is proportional to the square of its size, large particles settle through a medium considerably faster than do smaller particles, thus allowing easy separation. For centrifical separation, the Svedberg-Nichols modification of Stokes' Law is applicable :

where t is the time required for a particle to settle through a distance x-_j_ - X , T' =^■ the rotating radius of the centrifuge to the end point of the particle's travel path, X_]_ = the rotating radius of the centrifuge to the beginning point of the particles' travel path, _^J = the angular velocity of the centrifuge in radians/second, and the other parameters are defined as in the above formula. Under traditional approaches, a specific particle size classification ("cut") is achieved by first calculating the angular velocity and residence time required to force particles larger than a largest desired size out of the dispersion to form a sediment on the centrifuge wall . In general, a slurry of particles is placed in a centrifuge bowl and centrifuged under those calculated conditions, such that the resulting overflow contains only particles finer than the upper limits of the desired increment and is decanted. The overflow is then processed in a fashion similar to that used for the original slurry, such that actually all particles larger than the lowest size desired are spun out of suspension onto the centrifuge wall. This second sediment consists of particles within the desired size range and is therefore retained. _ This "batch" centrifugation of the prior art is not well adapted for the production of narrow size distribution powders on a commercial scale. The batch process has insufficient throughput and does not produce a narrow size distribution powder. This is due, in part, because at the beginning of each centrifuga ion procedure, particle distribution in the slurry is random, whereby particles within the desired size range are forced out of suspension and smaller particles in their path are dragged along with them into the sediment. Accordingly, there is a need for a carefully controlled system for generating

TITUTE SHEET narrow size distribution powders. Such powders would find widespread use and satisfy a variety of long-felt needs in forming high performance ceramics. A further use for narrow size distribution powders is as fillers in conjunction with ferrite-ceramic compositions.

It is also important to note that one of the problems encountered in providing narrow size distribution powders by centrifugation is that the resulting powder cut is dilatant and therefore quite difficult to remove from the centrifuge bowl. In practice, removing such a dilatant sediment often requires that the powder be chiseled out in a batch mode. When clays are classified, as described in the above-mentioned U.S. patents, the resulting classification fraction is plastic, and thus an auger-type or "decanter" centrifuge can be used. However, with narrow distribution colloidal particles, the dilatent aspect of the classification cut prevents continuous classification as can be achieved with clay suspensions. There ore, there is a need for a method for facilitating the removal of narrow size classification fractions, especially those in the submicron range, such that a continuous or semi- continuous classi ication system can be provided.

Disclosure of the Invention

In one aspect, the present invention provides an improved method and system for centrifugally separating suspended colloids and for permitting selected chemical reactions involving colloids in the separation process. In general, this method comprises activating a centrifuge, introducing a fluid medium into the spinning centrifuge to form a static zone of fluid within the centrifuge bowl, and introducing a suspension of colloids into the spinning bowl at the location concentrically within the static zone .

In another aspect, the present invention provides a process for classifying commercially available broad distribution colloidal powders into precise, submicron size fractions. In general, such a system includes providing an array of centrifuge units, each unit including a static zone within a bowl-type centrifuge as described above, providing a well -dispersed feed suspension to the array of units, and controlling, in real time, both the mass flow rate through and the rotation rate of each centrifuge unit within the array.

In yet another aspect, the present invention provides a method for facilitating the removal of the submicron classification fractions provided in the foregoing aspect. In general, the method includes using a substantial excess of dispersant in the feed slurry. By this method, a plastic sedimentation cake is formed in the centrifuge bowl rather than a dilatant cake; accordingly, this plastic cake can be easily removed and thus provides for a centrifugation classification system that can be operated continuously, or at least semi-continuously, and efficiently .

Brief Description of the Drawings

Figs . 1A and IB depict vertical cross - sections of prior art centrifugation apparatus, showing a dispersion of particles at times t=tg (just after the introduction of colloids to be separated) and t=t2 (after the bowl has been spinning for some period of time after a steady state has been reached) ;

Figs. 2A and 2B depict vertical cross sections of a spinning centrifuge bowl in accordance with the present invention, showing a static zone of liquid at time t=tg

Fig. 3 depicts an expanded vertical cross - section of the spinning apparatus in Fig. 2 at time

showing the trajectories of colloids within the suspension;

Fig. 4 depicts a vertical cross-section of a spinning centrifuge bowl in accordance with the present invention, wherein the static zone includes both a separation zone and a reaction zone, the reaction zone including a reagent that reacts with the colloid, so that the larger colloids have reacted with the reagent following their separation; Fig. 5 depicts a cross-section of a two-stage system according the present invention, showing idealized particle sizes and separation of particles by size;

Figs. 6 - 8 depict process schematics of a multi¬ stage, semi-continuous system for the classification of colloidal particulates in accordance with the present invention ; Fig. 9 is a graphic representation of the particle size distribution of commercially available powder prior to classification;

Fig. 10 is a graphic depiction of the particle size distribution of alumina powder obtained after a first step in the classification process in accordance with the present invention;

Fig. 11 is a graphic depiction of the particle size distribution of alumina obtained after a second step of the classification process in accordance with the present invention;

Fig. 12 is a graphic depiction of the particle size distribution of alumina obtained after a third step of the classification process in accordance with the present invention; Figs. 13 - 15 are photo-micrographs of SiAlON classified according to the present invention;

Fig. 16 depicts a flow diagram of the centrifugal classification process in a simplified version; and

Fig. 17 is a scanning electron micrograph of 0.4 to 0.7 micron size particles classified in accordance with the present invention.

DETAIL DESCRIPTION OF SPECIFIC EMBODIMENTS Our analysis of the prior art shows that a heretofore unappreciated characteristic of the centrifugation process involves the formation of a static fluid layer in centrifuge bowl, and that particle separation is attributable to a phenomenon not properlv appreciated in gJ »33 ?? TT3C5bϊ£'^{* =}"? iT"^!"^,-.^a-?'^? '-."^" .^" "^"^ the art. The prior art, including centrifuge product literature, suggests that during centrifugation, an entire volume of liquid in a centrifuge bowl constitutes an active volume through which separation occurs. This widely held view of the centrifugation regime is incorrect, as we have discovered. In particular, we have found that a static layer of fluid, whose thickness is determined by the geometry of the bowl , forms against the bowl wall. This prior art has provided a separation technology illustrated in Figs. 1A and IB, wherein a bowl spinning about an axis A-A is provided with a colloidal suspension 13 via feed 11. At the time t=0 , when a suspension has first filled the bowl, a particulate distribution therein is substantially uniform, so that even near the outside of the diameter of the bowl and its outer wall 15, the relatively small particles intermingle among relatively larger particles. At the time t=t2 , after the centrifugation system has been running for some time, larger particles collect to form a sedimentation layer 14 against the wall 15. However, because some of the smaller particles were initially present in the vicinity of the outer wall 15, a portion of the smaller particles are also present and thereby contaminate the sedimentation layer 14. We believe that the prior art view of the centrifuga ion process, in which the entire volume of the liquid in a centrifuge bowl is viewed as an active separation volume, is incorrect. We have discovered that during centrifugation a static zone of fluid forms against the bowl wall. The thickness of the static wall is determined by the geometry of the bowl. A system and process for separating a suspended colloidal phase in accordance with our discovery is illustrated in Figs . 2A and 2B .

As a colloidal suspension is introduced via a feed 11 into a bowl spinning about an axis A-A. fine particles are forced upward in a dynamic zone 23 and over the lip of the bowl, but not in a static zone 21 of the liquid. For purposes of illustration, the thickness (in a radial

direction) of the dynamic zone 23 has been exaggerated in all the figures herein. In fact, within the static zone 21, displacement of particles is primarily radial, and there is only very limited vertical displacement. Particle separation is believed to occur primarily before the particles enter the static zone. Accordingly, in an embodiment of the present invention, a fluid medium distinct from the colloidal suspension is first introduced into the centrifuge bowl. When the bowl is spinning, the static zone 21 is established, and then the suspension is introduced via feed 11 into the bowl so as to form a dynamic layer 23 at time t=tg . After the system has been in operation for a period of ^'time , at time =t2, larger particles collect to form a layer 22 against outer wall 15. gecause initially at t=tg none of the smaller particles were present in the static zone 21, proximate to the wall 15, the smaller particles do not substantially contaminate layer 22 of collected colloids ; all of the larger particles that are collected must make the journey across the static zone 21 from the dynamic zone 23. As shown in Fig. 3, which presents an expanded vertical section of the bowl of Fig. 2B at time

vertical displacement of the fine particles, shown as solid dots, occurs substantially only in a dynamic zone 23 located concentrically inside of the static zone 21. (The trajectories of the particles are shown as depicted by the arrows . ) The discovery and appreciation of the existence of the static zone enables one skilled in the art, among other things, to more precisely calculate the residence time parameters and consequently to provide a higher quality of size fractionation than was heretofore possible using the prior art approach described above.

Additionally, the static zone can be employed as a chemical reactor, as shown in Fig. 4, wherein larger particles are brought into contact with the reagents during settling. In this embodiment, the static zone 44 includes a separation zone 42 and a reaction zone 43. To establish these zones, one may. for example, successfully α β-ϊ-'fϋT HE T place in the spinning bowl first a fluid reagent (to form zone 43) and then a second fluid (which may be distinct from the fluid reagents) to form zone 42. The suspension is then provided through a feed, as described above in connection with Fig. 2, so as to form a dynamic zone 41.

In operation, after a period of time at which

larger particles will be seen to have traversed the separation zone 42 and to be entering the reaction zone 43. In this fashion, the larger, but not the smaller particles, may be caused to react with the fluid reagents. The method of the present invention permits a wide range of reagents to be employed. The reagents may be introduced as an additional fluid component, separated from the zone by an immiscible boundry zone, or the reagent may itself form a static fluid zone used as above. Employing the static zone as a reactor provides excellent flexibility in reacting components, because of the variable residence time, particle concentration, and size fractionation.

The prior art would predict that an inaccuracy occurs as the sediment layer accumulates on the centrifuge wall. In accordance with the prior art model, the distance particles travel to reach the outer wall diminishes with the accumulation of the sediment on the centrifuge wall, and some fine particles, which would otherwise go into the overflow, should then hit the wall and remain there. The prior art would predict that this inaccuracy is unavoidable (unless the wall is continuously scraped or the operating conditions of speed and flow rate are changed) , because the calculated minimum diameter of the particles within a cut constantly decreases with sediment built up. Nevertheless, we have found that this is not the case, and the relative lack of variation in separation accuracy with accumulation of sediments on the centrifuge wall tends to confirm the validity of our model of the separation process, although we do not wish to be constrained to any particular theory.

In practice, continuous flow centrifugation may be used to classify each unit volume of dispersion as it is introduced to the system. A centrifuge basket rotating at a constant angular velocity can be filled with liquid, forming a wall of fluid. The slurry is then introduced as the calculated feed rate determined by the largest particle size desired:

Feed Rate (ml/min) = V/t_min = 60V/t_sec where V=the volume of the centrifuge basket. We have found, fortuitously, that this equation approximately describes the behavior of the system model described herein, even though it is based on the prior art model of centrifugation. An intuitive understanding of the reason for the applicability of this equation can be had by appreciating that there is a direct proportionality among residence time, settling velocity, and settling distance for the model disclosed herein and the prior art model, and the proportionality in each case is approximately the same. Particles larger than this size accelerate into the static zone, settle through this zone, and then collect against the inside wall of the basket; fine particles remain in the dynamic flow and are carried up and over the fluid wall in the overflow.

Stokes' Law can predict centrifuge cut sizes accurately if all particles and spherical, non- agglomerated, and do not collide or interact with one another. Non-spherical particles can be accounted for by the geometric factor K, but every irregularly shaped particle has a different geometric factor, causing some desired particles to be excluded from the classified colloid powder. Geometric factors can be calculated for classified material to enhance cut precision and accuracy. Agglomerated particles are also irregularly shaped and usually much larger than the primary particles of which they consist. De-agglomeration of powders by ultrasonic dispersion or other means is very important since undesired fine particles would otherwise be contained in agglomerates, which would then be separated or classified with the actual particles having the same size as those agglomerates. Concentrated suspensions vield powder cuts very different from theoretical due to the numerous particle interactions; these interactions disturb the particle's velocity, sometimes causing larger particles to be carried over with fine particles in the supernatant. The following examples are illustrative of centrifugation using the aforedescribed static zone:

Example 1 Into a spinning centrifuge of the bowl type having apacity were added approximate amounts of the following: 100 ml 1/2 weight percent Corcat P600 (a cationic amino based polymeric flocculent obtained from Cordova Chemical Company, Michigan) ; and 240 ml deionized water. This liquid mixture was used to form a static zone. A feed suspension was then introduced, comprising: 10 vol . % A16 alumina powder (available from Aluminum Company of America, Pittsburgh, Pa) 89 vol . % deionized water; and

1 vol . % Narlex LD-45 (available from National Starch Company, Bridgewater , New Jersey) , a anionic polymeric polyelectrolyte dispersant.

If the above procedure if followed but the Corcat flocculent is omitted, the result is a well-packed, high density, stiff, low moisture (86% solids by weight) sediment cake which is difficult to remove from the bowl. If the above procedure is followed verbatim, including the flocculent, the result is a poorly packed, creamy (40 weight % solids) sediment cake which allows easy removal of classified product from the bowl. The flocculent neutralizes the dispersion effects of the LD-45. Example 2

The procedure of Example 1 was followed as above, but with the substitution of a halogenated hydrocarbon (such as Freon TF , available from E. I. DuPont De Nemours and Company, Wilmington, Delaware) used to form the static zone. The halogenated hydro-carbon is immiscible with water and has a higher density than does water. so the system is stable both thermodynamical1y and mechanically. The particles are stable in the aqueous zone, but not in the hydrocarbon zone, so the particles flocculate when they enter the hydrocarbon zone.

Example 3 The procedure as described in the above examples was followed, but with an acid (pH=3, nitric acid/water solution), in lieu of the Corcat flocculent. Similar results are achieved. The system works with acid because as the LD-45 is neutralized, its dispersing powder is also reduced. The foregoing description and examples have generally described a single centrifuge including a static zone. Generally, a preferred embodiment of the present invention comprises two or more separate centrifugation steps to produce particles of decreasing sizes. The overflow product from one centrifugation stage is used as the feed input for the succeeding centrifugation stage. Residence time and centrifugal acceleration are the fundamental parameters determinative of particle size separation. These parameters are varied by controlling the feed rate and centrifuge speed for each centrifugation stage; these parameters are calculated with reference to the minimum particle size desired in the basket of each stage. Particles smaller than the desired size are driven into the overflow, drained through the drainage holes, and pumped through to the succeeding stage. The centrifugation process described herein is useful for any type of fluid or material, including, but not limited to ceramics, including oxides such as alumina, glasses such as cordierite , SiAlONS, and the like. Depending on the particles type chosen, the skilled artisan can chose an appropriate liquid and dispersant. Typical liquid media include water (either deionized or tap) , alcohols (such as ethanol or isopropanol ) , and other organic solvents such •as methylethyl ketone . Typical dispersants include inorganic acids (such as nitric acid) . inorganic bases (such as ammonia) , organic acids (such as para- hydroxynebenzoic acid) , organic bases (such as triethanolamine) , and polymeric dispersants, which can be

Si msrnm £. anionic (such as NARLEX LD-45, available from National Starch and Chemical Corp. , Bridgewater, New Jersey), cationic (such as EMCOL-55, available from Witco Chemical Company, Organic Division, New York, New York) , and nonionic (such as CLINDROL/101 , available from Clintwood Chemical Company, Chicago, Illinois) . It is also important to note that dispersants are typically chosen empirically depending on the specific colloid to be dispersed and subsequently classified. Fig. 5 depicts a cross-sectional view of a two-stage system in accordance with the present invention. This figure shows a cross - sectional view of particle distributions in the two-stage classification system. For simplicity, only three particle sizes are shown. The first stage feed 101 contains particles of all three sizes. Two smaller size fractions are removed from suspension as overflow 102 from stage one, forming a second stage feed 103, and leaving the larger size particles in the first stage sediments 104. The smallest size particles are removed from suspension as stage two overflow 105, leaving the intermediate size particles in stage two sediments 106.

Figs. 6-8 are schematic diagrams of a multi-stage con¬ tinuous system for the classification of particulates in accordance with the present invention. In Fig. 6, commercial, "raw" powder 201 is passed through screen oven 203, and weighing device 204, and finally feed to dispersion device 205, preferably a Sweco mill (available from Sweco, Inc. , Los Angeles, California) with media. Liquid 206 is passed through filter 207 and driven by pump

208 into totalizer 209 and, in turn, to dispersion device 205. The preferred liquid for use with alumina is isopropryl alcohol or water, and a preferred dispersant such as Narlex LD-45 210 is fed to filter 211, pump 212, and totalizer 213, and subsequently into mixing device

205. Setpoint controller 214 communicates with totalizer

209 and controls pump 208 while setpoint controller 215 communicates with totalizer 213 and controls pump 212.

The dispersion of liquid, dispersant, and colloidal particulates is then feed through filter 216 by pump 217 and past feedback devices 218 and 219; these control devices monitor the mass flow rates, temperature, specific gravity, and viscosity of the suspension stream. The dispersion is admitted to classifier stock holding tank 220 through valve 221 when appropriate conditions are indicated by monitoring devices 218 and 219.

Providing a well-dispersed feed slurry, including the liquid medium dispersant and colloidal powder, is a critical step in the classification process. The particles are not comminuted so as to avoid increasing irregularities in particle shape. Powder, liquid, and dispersant are added to the dispersion device in a selected order at appropriate stages. It is preferred that the powder be added in staged order of addition to liquid and dispersant already in the dispersion device; however, although more difficult ^'to obtain, a well- dispersed slurry can be obtained by mixing all components at once and agitating for a prolonged period of time. Dispersing is conducted for a pre-determined time, typically for about 6 hours , under controlled conditions (generally relating to speed and temperature conditions). Monitoring devices 218 and 219 periodically measure parameters indicative of the dispersion quality of the mixture, and samples of the mixture are returned to the dispersion device 205 from the particle size test stage 222 via line 223.

The stock suspension in tank 220 is moved by pump 224 to classifier feed tank 225. The output of pump 224 is monitored by monitoring device 226 and by controller 227. The suspension input to the classifier feed tank 225 is stirred and diluted with dilution fluid from tank 229 via pump 228. The output of pump 228 is monitored by monitoring device 230 and regulated by controller 231. A preferred dilution fluid for alumina is water, which is typically added at a ratio of about 3 parts of dilution liquid to one part of stock. The diluted suspension is referred to as classifier feed suspension. This classifier feed suspension is then pumped by pump 232 to first stage classifier 233, shown in Fig. 7. The output of pump 232 is monitored by device 234 and regulated by controller 235. Monitoring device 234 is a commercially available unit which measures density and temperature and mass flow rate. Typical flow rates into the first stage are approximately 0.4 kilograms per minute. Classifiers 223, 236, 237, and 238 are commercially available centrifuge devices modified with a tachometer and commercially available set point contollers 239, 240, 241, and 242. Classifiers are typically operated at a maximum speed of approximately 6000 rpra . An exemplary centrifuge device is available from International Equipment Company, Needham Heights, Mass, as model K, in which a solid basket rotor #1357A can be used. However, it will be evident to the skilled artisan that other components may be substituted and adjustments, made accordingly.

It is particularly preferred that each centrifuge is activated and filled with liquid (for example, solely water when water is used as the liquid medium for alumina) prior to the introduction of a feed dispersion, as discussed above regarding the static zone. For example, classifier 233 is activated, then "primed" with water before feed is pumped from tank 225. After feed is introduced, classifier 233 is operated at a speed and feed rate conditions calculated to yield the desired particle size in the centrifuge basket. The centrifuge is operated for a pre -determined time, or until such sufficient separation is observed. The classifier is then shut down, and bowl waste is removed by pump 243 along line 244 to holding tank 248; classifiers 236, 237, and 238 likewise have bowl waste removed via lines 245 to 247 to holding tank 248. The bowl waste can contain significant amounts of material concentrated in a particular size fraction.

Wet powder 249 is then removed from the centrifuge basket. During centrifugation, particles smaller than the minimum size desired in the first stage basket ("fines") are

drained from overflow into tank 250 from which they are pumped by pump 251 to second stage classifier 236. This flow rate is controlled by a feed regulation system com¬ posed of pump 251(a), which is monitored by device 252(a) and regulated by setpoint controller 253(a); the second stage 236, third stage 237, and fourth stage 238 classifiers operate in a similar fashion. Likewise, fines from overflow tanks 254 and 255 are pumped to classifiers 237 and 238 by pumps 251(b) and 251(c), respectively, monitored by devices 252(b") and 252(c) , and regulated by setpoint controllers 253(b) and 253(c) . However, it will be appreciated that each succeeding classifier stage separates out a range of particles having a smaller mean diameter than the range of the preceding stage. Each succeeding stage must therefore operate at a lower mass flow rate and higher rpm range than the previous stage. The last stage classifier is fitted with an in-line static mixer 267, in which the fines suspension is mixed with a flocculent to facilitate the fines recovery. Multiple classifier stages thus operate in a cascaded fashion, with each succeeding stage classifying the fines of the previous stage, and with holding tanks 250, 254, and 255 functioning as accumulators to buffer output rates to feed rates. Although in this particular embodiment four classification stages are being envisioned, the actual number of stages may vary depending on the application. Optionally, the particles which are so small as to remain in suspension after passing through a par¬ ticular classifier can be flocculated out and the liquid remaining can be recycled through the process. One method for accomplishing this is illustrated in Fig. 8. Floc¬ culent, for example water and nitric acid (or water and Corcat P600, as mentioned above) from a tank 268 is filtered through device 256, then pumped via pump 257 through the in-line static mixer 267 into the last stage classifier 238. The flow rate is monitored by device 258 and regulated by set point controller 259. The liquid passing out of the classifier, now particle free, is collected in overflow tank 260 and sent by pump 261 to holding tank 262. Thereafter, the liquid passes through filter 263 and is sent by pump 264 through the molecular seives 265 and 266 where water is removed, thereby leaving liquid which is ready to be pumped back to reuse or recycle as liquid 206, as shown in Fig. 6. Example 4 A four-stage classifying system was first primed with one residence volume of water. Alumina (60 weight per cent) A-16 Superground available from Aluminum Company of America, Pittsburgh, Pa. , was mixed with water (39.25 weight percent) and a dispersant (Narlex LD-45, 0.75 weight percent suspension basis) . This mixture was milled, filtered, and pumped into the classifier feed tank. The starting size distribution range is shown in

Fig. 9. The feed suspension was then pumped at 1.0 kg/min into the first stage classifier, which was operated at 973 rpm , which thereby resulted in the removal of particles larger than 1.2 microns from the feed stream. The particles were collected in a bowl.

The suspension containing particles smaller than 1.2 microns was pumped at 0.85 kg/min into the second stage classifier operating at 1268 rpm. In the second stage classifier, particles having a size of about 0.8 to about 1.2 microns with a specific surface area of about 7.6 m^ g± or - 0.5m^ per gram, calculated for over one hundred and fifty production runs, was achieved. The actual size distribution achieved is shown in Fig. 10.

Liquid containing particles smaller than 0.8 microns was pumped at 0.760 kg/min into the third stage classifier, which was operated at 1942 rpm. Here, a separation of particles within the size range of about 0.5 to 0.8 microns, and a specific surface area of 9.56 _m /g ±. 0.5 m^/g, calculated over 150 production runs, was achieved. The actual size dis ribution of these particles is shown in Fig. 11. Particles smaller than 0.5 microns were pumped at a rate of 0.506 kg/min into the fourth stage classifier, which was operated at 2700 rpm. Here, a separation of particles of 0.3 to 0.5 microns and having a specific surface area of 11.5 m /g ± 0.5 m^/g , calculated over 150 production runs, was achieved. Fig. 12 shows the actual size range of these particles. Particles smaller than 0.3 microns which remained in suspension were pumped through a static T-mixer with a dilute nitric acid solution to bring the suspension screen pH to 3 +. 1. The resulting flocculated suspension was introduced into a fifth stage classifier at 0.45 kg/min which was operated at 2700 rpm, wherein all remaining particles were removed. The type of system just described could be scaled up to classify a large volume of material rapidly, for commercial production, i.e. , up to about one metric ton per hour. Example 5

A classification system similar to the one described in Example 4 was used, except that SiAlON was used instead of alumina. The system configuration and milling procedures were also the same. The SiAlON was classified in water using Narlex LD-45 dispersant. Fig. 9 is a photomicrograph of the particles from the 2.0 to 3.0 micron fraction, demonstrating the relatively narrow size ranges. Fig. 10 depicts the classified SiAlON particles from the 0.1 to 3.0 micron cut. The very fine particles of Fig. 11 are less than 0.1 micron.

As previously described, the present process provides the ability to classify colloidal particles into submicron classification fractions. Also as previously described, such classification fractions are dilatant rather than plastic, and therefore are not easily removed from the centrifuge bowl. The addition of a plasticizer or lubricant, such as a humectin, is not desireable because such agents may tend to cause flocculation , thereby ruining the dispersion of the colloids, and such agents may also segregate non-uniformly during the classification process .

It has been found that the presence of an excess of a polymeric dispersant not only allows for and facilitates the dispersion of the particles prior to classification, this excess dispersant also provides the lubrication needed to allow the classified particles to flow after classification. Exemplary polymeric dispersant include those that are commercially available, such as that designated Narlex LD-45, and as discussed above ,(preferably used in combination with alumina powders classified in water), and those designated Darvan C and Darvan 821 A, both available from R. T. Vanderbilt Company, Norfolk, Conn. The amount of the dispersant added will vary with the type of dispersant and the particular powder composition. In any case, the amount of dispersant should be effective to impart plasticity to the classified cake, but not at such a high concentration as to interfere with flow or dispersion characteristics of the slurry as it is classified, especially regarding viscosity .

Example 6 A two stage classifying system was first primed with one residence volume of water. Alumina (60 weight %) A-16 Superground (available from Aluminum Company of America, Pittsburgh, Pa) was mixed with water (39.25 weight %) and a dispersant (0.75 weight % of Narlex LD-45, weight % suspension basis) . This mixture was milled, filtered, and then pumped into the classifier feed tank. The feed suspension was then pumped at 0.85 kg/min into the first stage classifier, which was operated at 1268 rpm and which resulted thereby in the removal of particles larger than 0.8 microns from the feed stream. These particles were collected in a bowl.

The suspension containing particles smaller than 0.8 microns was pumped at 0.760 kg/min into the second stage classifier, which was operated at 1942 rpm. Here, a separation of particles having an average size of 0.5 to 0.8 microns and a specific surface area of 9.6 _m /g ±_. 0.5 m^/g, calculated for 150 production runs, was achieved.

In all of these narrow sized classified cakes, the powder formed a dilatant sediment which was verv difficult to remove from the centrifuge bowl. A hammer and chisel proved most effective for powder removal after drying the cake and bowl together at 200°C for 30 minutes .

Example 7 A stock suspension of the apprximate amount of the following materials was made and milled for twenty-four hours on a roller mill:

4.0 kg A-16 SG alumina (available from Aluminum Company of America, Pittsburgh, Pa.) 3.6 kg deionized water; and 50 g Narlex LD-45

A feed suspension was made, starting with 1.0 kg of the stock suspension and diluted with 2.2 kg of distilled water and 800 grams of the LD-45 dispersant. This feed suspension was milled for about 30 minutes, filtered, and then pumped into a classifier feed tank similar to that in the above-described classification system, which is depicted in a simplified form in Fig. 16. The feed suspension was pumped at 0.3 kg/min into a first classifier, which was operated at 3120 rpm. The fines overflow from the first classifier was pumped at a rate of about 0.17 kg/min into a second classifier which was operated at 4000 rpm. The fines overflowing from this classifier were discarded. Adding the additional dispersant causes an increase in solution viscosity from 1.1 to 2.4 cps, measured at 100 sec

The equation (1) described above predicts the particle size cut in the second bowl. This equation predicts an average equivalent spherical diameter particle size fraction of about 0.4 to 0.8 microns, which was then verified by scanning electron microscopy, as depicted in Fig. 17.

The narrow size range powder collected in the bowls was not dilatant. Rather, it was plastic and could easily be scooped out of the bowl. These results verify that the amount of dispersant used was effective to provide a well- dispersed slurry while also creating a centrifuge cake that plastically deforms rather than exhibiting dilalant behavior. The cake had a viscosity of 670 cps at 100 sec

Claims

What is claimed is:

1. A system for centrifugal separation of a colloidal phase in fluid suspension, the system comprising: a spinning centrifuge bowl ; a fluid medium distinct from the suspension, introduced into the spinning bowl in sufficent quantity toform a static zone; and

Feed location concentrically inside of the static zone, so that as the suspension is introduced into the bowl, the colloids settle through the static zone.

2. A system according to claim 1, wherein the fluid medium includes at least one reagent so that the colloids react with at east one reagent as such matter settles through the static zone.

3. A system according to claim 1, further comprising a plurality of layers of fluid media disposed concentrically with respect to one another and all being disposed concentrically outside of the feed location.

4. A system according to claim 1, wherein the colloidsare selected from the group consisting of ceramics, metals, and suspended liquid phases.

5. A system according to claim 8, wherein the colloids are ceramics .

6. An improved method of centrifugally separating a col¬ loidal phase, of the type comprising the steps of adding a fluid suspension of the colloidal phase to be classified into a centrifuge bowl and spinning the bowl so that larger colloids are separated from smaller colloids, wherein the improvement comprises: filling a spinning centrifuge bowl with a fluid medium distinct from the suspension, so that a static zone of liquid forms in the bowl; and subsequently introducing the suspension into the bowl concentrically inside of the static zone.

7. A method of centrifugally separating a colloidal phase in a fluid suspension, comprising:

(a) activating a centrifuge including a bowl : (b) introducing a fluid medium distinct from the suspension into the spinning centrifuge bowl so that a static zone of fluid forms in the bowl ; and then

(c) introducing the suspension into the spinning bowl at a location concentrically inside of the static zone.

8. A method according to claim 7, wherein step (b) includes the step of introducing a fluid medium including at least one reagent that reacts with the colloidal phase subsequent to step (c) .

9. A method according to claim 7, wherein step (c) includes the step of introducing a suspension selected from the group consisting of ceramics, metals, and suspended liquid phases.

10. A method according to claim 7, wherein step (c) further includes introducing the suspension into the bowl at a controlled mass flow rate.

11. A method according to claim 8, wherein step (b) includes the step of forming a plurality of distinct layers concentrically disposed with respect to each other in the centrifuge bowl, each layer including at least one reagent .

12. A multi - component semi -continuous system for the classification of colloids in a liquid suspension, comprising : (i) an array of units, including a first unit and a second unit for classifying suspended colloids, each unitincluding a centrifuge device having a colloid input and a fines output and also including a static fluid layer in the bowl of each centrifuge device; (ϋ) transport means for transporting the fines outputof the centrifuge device in the first unit to the colloid input of the centrifuge device in the second unit, so thatcolloidal material can be transported therebetween; (iii) dispersing means for providing a well -dispersed suspension of the colloids in the liquid;

(iv) first control means for regulating in realtime a mass flow rate through the colloidal input of each unit: and (v) second control means for regulating in realtime a rotation rate of each centrifuge device, the first control means and the second control means effective to provide a sub-micron classification fraction of the colloids in at least one of the units.

13. A system according to claim 12, wherein the transport means includes bowl waste means for recycling liquid remaining in the centrifuge devices after classification.

14. A system according to claim 12, wherein the system includes last and next-to-last units in the array, the system further comprising:

(i) flocculation means for flocculating particulate matter present in the fines output of the next-to- last unit so as to produce flocculated particulate matter; and

(ii) flocculation transport means for transporting the flocculated particulate matter to the centrifuge device in the last unit.

15. A system according to claim 14, further comprising: (i) molecular seive means for removing waterfrom non- aqueous liquid separated from the flocculated particulate matter; and

(ii) third transport means for connecting the molecularseive means to the last centrifuge device, and for transporting the liquid separated from the flocculated particulate matter to the molecular seive.

16. A method for the classification of colloids comprisin :

(i) providing a well-dispersed suspension of the colloids in a liquid medium;

(ii) providing an array of units, including a first unit and a second unit for classifying the dispersion of suspended colloids, each unit including a centrifuge devicehaving a colloid input and a fines output; (iii) providing a static liquid medium layer in the bowl of each centrifuge device, including activating the centrifuge devices so that each of the centrifuge devices rotates at a desired rate; (iv) introducing the dispersion of suspended colloids into the colloid input of the rotating centrifuge device of of the first unit;

(v) transporting an effluent from the fines output of the centrifuge device of the first unit to the colloid input of the centrifuge device of the second unit;

(vi) controlling in realtime a mass flow rate through each colloid input; and

(vii) controlling in realtime the rotation rate of each of the centrifuge devices, the controlling of each mass flow rate and each rotation rate effective to provide a sub-micron classification fraction of the colloids in at least one of the units.

17. The method defined by claim 16, wherein the controlling includes monitoring the mass flow rate through each colloid input .

18. The method 'defined by claim 16, further including controlling the rate of rotation of a centrifuge device in response to a monitored mass flow rate.

19. The method defined by claim 16, wherein the array includes ultimate and penultimate units, further comprising :

(1) flocculating colloids in an effluent of the fines output of the penultimate unit;

(2) transporting the flocculated effluent to the colloid input of the ultimate unit; and

(3) recovering the flocculated colloids in the ultimate unit.

20. A process for classifying colloidal materials, comprising: (a) preparing a slurry comprising colloidal particles and dispersant, employing a quantity of dispersant in substantial excess of that amount necessary for effectively dispersing of the par¬ ticles; (b) introducing the slurry into a centrifuge means for classifying the particles, the centrifuge means including a bowl means for receiving classified particles: and (c) removing the classified particles from the bowlmeans .

21. A process according to claim 20, wherein step (a) includes the step of using a dispersant that is polymeric.

22. A process according to claim 20, wherein the slurry of step (a) includes colloidal particles selected from the group consisting of ceramics, metals, and mixtures thereof .

23. A process according to claim 21, wherein step (a) includes employing an acrylic acid based polymeric polyelectrolyte in an amount ranging from approximately 20-30 weight percent.

24. A slurry of well-dispersed particles of different sizes and suitable for classification, comprising: particles and dispersent, the dispersant present in an amount in substantial excess of the amount necessary to effectively desperse the particles.

25. Sub-micron classified particles produced by the systems of claim 12.

26. Sub-micron classified particles produced by the method of claim 16.

27. Classified colloidal particles produced by the process of claim 20.

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