Related Art Conventional milling machines use mechanical crushing or crushing and attriting to break mineral particles into smaller particles. Conventional milling machines have low efficiency due to the frequent application of stress where there are no particles. The result is that much of the energy input is wasted in non- productive contact between, for example, crushing mechanisms or between a crushing mechanism and the mill wall, both of which lower the overall energy efficiency of the process. A more efficient method of inducing simple tensile failure uses a high pressure fluid jets or different fluid jets in comminution processes.

Size reduction involves rupturing the chemical bonds within the material in order to generate new surfaces. Thus, the chemical processes associated with fracture will significantly affect the energy required to induce this fracture. This influence extends beyond the bonds themselves to include the surrounding environment : For example, the presence of fluid at the crack tip will lower the

forces required to expand the crack and improve efficiency. One explanation for this effect is that the fluid penetrates into microcracks ahead of the major crack front and thus take part in the highly reactive events that occur during fracture.

Because the capillary flow of these fluids into the material ahead of the main front runs at the velocity of crack propagation it provides a means of transmitting energy more easily within the crack tip zone. Thus, fluid energy mills, which provide the fluid for the capillary flows are commonly used in comminution of materials.

Fluid energy mills output a slurry of fluid and particles. Normally, the particles must be dried prior to further processing, as is the case for ceramic powders. Due to the fine size of powders, i. e. large surface area to volume, drying can be a lengthy, burdensome process. Furthermore, if the slurry contains amulti-component mixture ofparticles, segregation of materials can occur during drying, which results in undesirable non-homogeneity. Additionally, powders can agglomerate during the drying process, requiring an additional process to de- agglomerate.

Spray dryers have been developed to dry powder particles while they are suspended in the air. Spray drying involves atomizing a slurry of particles and a fluid, into a spray and evaporating the fluid from the spray using warm air. The dry particles are then separated from the fluid vapor. Spray drying is used in industries such as chemical, dairy, and food.

Resulting particulate from fluid energy mills is commonly used for powder processing, important in the ceramic, structural ceramic, and electrical industries. In one example, comminuted powder is used to create ceramic components for the ceramic industry. A binder, which acts like glue, is spread through the powder to create a powder-binder mixture. The mixture is injection molded or precision cast to create solid, formed compacts of the mixture. The formed compacts are sintered at high temperatures where the binder is vaporized or decomposed in a"burn out"process, creating a final monolithic product.

Standard, known requirements for selecting a suitable binder include: -clean bum-out with no residual residue; -non-reactive with the powders, so as to not alter the surface properties; -thermally debinds within a relatively short time period; -appropriate rheological properties such as low viscosity to aid mixing; -appropriate wetting properties for proper adhesion to the powder surfaces.

Ideally, binder covers nearly every individual grain of the ceramic powder. The coating thickness is controlled by the viscosity and the wetting properties of the binder. Unless the viscosity is very low, a thin coating is difficult to achieve. However, low viscosity binders typically require addition of solvents or increased temperature during mixing of the binder material with the powder. Both of these are undesirable as solvent needs to be removed prior to further processing of powders, and elevating temperature during mixing requires precise temperature control within the bulk of the mixture, requiring special equipment and increasing cost. Furthermore, large volume fraction of binder is undesirable as it can leave large voids in the ceramic as the binder is vaporized and removed from the ceramic. Such voids compromise the integrity and mechanical properties of any finished product.

Prior to introducing binder to a powder, the powder must be thoroughly processed. Processing of powders often includes drying the powder, if it is wet.

Particles from a spray dryer have typically required further treating before being useful, such as adding binder for ceramics, as described above.

What is needed is a method and apparatus to introduce an additive, such as a solid material or a fluid binder, during the comminution stages of powder processing, wherein the solid additive sufficiently blends and the binder coats the solid blend during comminution and drying, and wherein a thin coating of the binder onto the particles can be achieved.

Summary of the Invention The present invention relates to a system and method for comminution and processing of solid particles of one or a multitude of materials. The method includes introducing the materials into a fluid energy mill for comminuting the materials into particles. The fluid energy mill outputs a slurry comprised of the particles and an energy transfer fluid. The slurry is introduced into a spray dryer.

The spray dryer atomizes the slurry, which falls through a heat zone, vaporizing the fluid. A collector catches the dry, falling particles.

In another embodiment, the fluid energy mill outputs a slurry and introduces the slurry into a hydrocyclone before the slurry is introduced into the spray dryer. They hydrocyclone sorts the milled particles by size, returning oversized particles to the fluid energy mill.

In another embodiment, the fluid energy mill outputs a slurry, introduces the slurry into a spray dryer and introduces the dried particles into a cyclone, where the particles are sorted by size. Any oversized particles are returned to the fluid energy mill.

In another embodiment, an additive is introduced into the fluid energy mill to be processed with the material. The additive can be either a solid material or a fluid, such as a binder or solvent. The slurry, output from the mill, includes the additive, the particles of the material, and an energy transfer fluid. The slurry is introduced into an hydrocyclone and spray dryer. The hydrocyclone sorts particles according to size, and the spray dryer atomizes the slurry, which falls through a heat zone, vaporizing the fluid. A collector catches the dry, falling particles which have been coated, and/or blended, and/or dissolved with the additive.

Brief Description of the Figures The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIG. 1 shows a first embodiment of a system of the present invention for the comminution and processing of solid particles of a material.

FIG. 2 shows an embodiment of a spray dryer equipped with a collector and condenser.

FIG. 3 shows another embodiment of FIG. 1, including a hydrocyclone and a recycling line.

Detailed Description of the Preferred Embodiments A preferred embodiment of the present invention is now described with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the left most digit of each reference number corresponds to the figure in which the reference number is first used. While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the invention.

FIG. 1 shows an embodiment of a system 100 for comminution, blending and processing materials into particles. System 100 includes a high pressure pump 102, connected to a fluid energy mill 104. Mill 104 has attached a feed pump 106 for introducing particles into a spray dryer 108. Connected to spray dryer 108 is a condenser 110 and a collector 112. A recycling circuit 114 connects condenser 110 to high pressure pump 102. However, it would be apparent to one skilled in the relevant art that various configurations of these elements could be used to implement system 100 of the present invention.

Fluid energy mill 104 could be any standard fluid energy mill capable of comminuting a material and can contain feeders to introduce solid material, additives, and/or binder into the mill, as would be apparent to one skilled in the relevant art. In one embodiment, the fluid energy mill is the mill described in copending U. S. Application No. 09/413,489, filed October 6,1999, pending, which is incorporated herein, in its entirety, by reference hereto.

System 100 is useful for comminuting and processing more than a single material. For instance, an additive could be introduced to system 100 for blending with the particles in mill 104 for powder processing. The additive could be a solid material, added to the mill to be comminuted with the primary material, or the additive could be a fluid, such as a fluid binder for powder processing or a solvent.

In one embodiment, mill 104 is designed to achieve ultra-fine particles having a resultant size, also referred to as a product size, of less than 15 microns.

Preferably, the ultra-fine particles have a product size within a range of 1-5 microns. Still more preferably, the product size of the ultra-fine particles resulting from the use of mill 104 is below 1 micron.

It would be apparent to one skilled in the relevant art that mill 104 could be used to process a variety of other materials, both organic and inorganic, having various feed sizes. For example, mill 104 could be used to process any of the following: ceramic materials such as ceramics for electronic applications, such as, for example, barium and strontium titanates, lead zirconates, and other titanate and ziconates, and high temperature ceramic superconductors; ceramics for structural applications, such as, for example, alumina, zirconia, magnesia and other oxide systems ; non-oxide ceramics, such as, for example, nitrides, borides, silicides and carbides used for toughness and abrasive applications; carbon and carbon by products (including coke and coke byproducts), minerals such as for example, anthracite, magnetite, alumina, mica, silica, and zircon, metals such as, for example, chromium, nickel, zinc, copper and brass, used for powder metal sintering, alloying and other applications, other oxides used for abrasive and

cutting uses such as garnets, and rare earth oxides, and any other material that needs to be finely ground. Further, mill 104 could be used to process a variety of organic materials, including, for example, wood, food products and products for use as pharmaceuticals.

Typically, the average size of a material added to mill 104 for comminution is less than % 2 inch in diameter, and preferably less than 1/4 inch, however, as would be apparent to one skilled in the relevant art, larger particles could be introduced into mill 104. Larger particles, however, may take a greater amount of time to comminute to very small particle sizes. A grinder or crusher could be used to reduce larger particles to a smaller size, prior to introducing the particles into mill 104.

In one embodiment, the particles to be comminuted in mill 104 are dry as they are fed into the mill. In another embodiment, the particles could be fed into mill 104 as part of a slurry, e. g., a mixture of material particles and a fluid. The fluid could be aqueous or non-aqueous, such as, for example, water or organic fluids such as alcohols and oils.

Introduction of a solid additive to fluid energy mill 104 produces a mixture of comminuted material and additive. The material and the additive are simultaneously comminuted and aid in the breakdown of each other. The additive can be introduced at different stages of the mill to achieve different comminution ratios. As such, the resultant mixture can contain particles of additives that are larger, or less comminuted than the particles of material. For example, if the additive is introduced at the beginning stage of the mill, along with the material, the additive and the material may have roughly the same size reduction ratio, as both the material and the additive are subjected to the same milling process. Likewise, if the additive is introduced at a midstage, or toward the latter stages of the mill, the additive will be subject to less comminution, and therefore, may have a different size reduction ratio. This is, of course, also dependent on the initial sizes of the material, the material properties and the type

of comminution stages used in the mill, as would be apparent to one skilled in the relevant art.

Simultaneous comminution of the material and the additive in the fluid energy mill provides intimate and intense mixing. As the material particles collide and contact the additive particles, the particles comminute into an evenly- blended mixture of material and additive particles.

The additive and the material each can be fed into the mill at a desired rate to achieve desired proportions and to create a properly proportioned mix. Such mixing eliminates the need for additional mixing or adding materials after the comminution process to obtain a desired proportion.

As stated, the additive could also be a fluid, such as a solvent or a binder.

When the additive is a fluid, it can be introduced as a secondary stream into the fluid energy mill, in addition to the stream of the energy transfer fluid. The additive can be introduced as a fluid stream at the front end of the fluid energy mill, such as at high pressure pump 102, or introduced further along in mill 104 in one of the succeeding chambers. When such an additive is introduced with the material into the main chamber of a high pressure fluid energy mill, the fluid additive evenly coats the fractured material surfaces as they are formed.

In manufacturing processes requiring binders, it is important that nearly every individual grain of the particle is coated so that a resulting product, formed from the particles and binder, will exhibit consistent material properties. For ceramics, it is advantageous to compress coated particles into a dense compact, with the density approaching the theoretical density of a solid ceramic. The compact is sintered, resulting in a finished ceramic virtually devoid of holes and gaps. Thus, it is important that the coating on the particles be thin, as a thick coating, or excess binder, will leave gaps and holes in the resulting product when the binder is removed from the product during sintering.

In order to obtain a thin coating of binder, the binder must be spread evenly over the individual solid particles with a minimum possible thickness.

This is achieved by either lowering the viscosity of the binder or forcing the

binder, under pressure, to penetrate and spread evenly over the particles.

However, the lower viscosity range of a binder is limited. Exceeding the lower limits of the range results in binder that segregates from the particles, dripping down the product, and resulting in degradation of its lubricating properties required during subsequent powder compaction. It is advantageous to use as little binder as possible, while obtaining a sufficient coat on each particle. This reduces the occurrence of voids and gaps when the binder is removed during sintering.

In a fluid energy mill, the binder material is subject to all the pressures and energy of the energy transfer fluid. Two suitable binders for use in a fluid energy mill comminuting aluminapowder are methyl-methacrylate ("MMA") and Cyanoacrylate. These binders are not water soluble, and therefore, are not subject to being dissolved when water is the energy transfer fluid used in the mill.

Furthermore, these binders exhibit the desired properties of having a fast polymerization with a controllable reaction. MMA polymerizes under standard atmospheric conditions. Therefore, it is necessary to add an inhibitor. For example, adding up to 0.1% of an inhibitor such as Topanol or Hydroquinone to the MMA solution will prevent polymerization at atmospheric pressure.

It is generally preferable that the binder material not be dissolved by the fluid used for energy transfer in the mill. However, dissolution of the binder material is permissible if the binder has a lower vapor pressure than the vapor pressure of the fluid used for energy transfer in the mill. This allows the fluid to vaporize in the spray dryer and leaves behind the binder material to form a thin coat over the dried solid particles. Examples of binders that could be used in the present invention include but are not limited to: CARBOWAXe Polyethylene Glycols, manufactured by Union Carbide Chemicals and Plastics Co., Inc. of Danbury, CT and Methocelcellulose ethers, manufactured by DOW Chemical of Midland, MI. Polyvinyl alcohols can also be used. Examples of polyvinyl alcohols that could be used in the present invention include: Elvanol 75-15 manufactured by E. I. du Pont de Nemours & Co., Inc. of Williamton, DE; and

Airvol 205, manufactured by Air Products and Chemicals, Inc. ofAllentown, PA.

The energy transfer fluid could be, for example, water or an appropriate alcohol such as isopropanol, which have higher vapor pressures than the binders, and readily evaporate in the spray dryer leaving behind the dry powder mixture. It would be apparent to one skilled in the relevant art that a variety of fluids could be used as the energy transfer fluid in the mill.

In most cases, chemical reactions or surface property alterations are not permitted to occur in the mill or the spray dryer. For example, in the case of barium or strontium titanates, water can alter the surface properties of the particles by introducing hydroxyl ions which are undesirable in high performance electronic parts fabricated from these materials, such as, miniaturized multi-layer capacitors, and various transducers. To avoid chemical alteration, isopropanol alcohol is used as the energy transfer fluid according to the present invention which is totally inert with respect to the above solids. An alcohol soluble binder such as CARBOWAX can be added to the mill during comminution. The process described above can be also be used for lead zirconates, which is also water sensitive. Lead zirconates is used extensively to fabricate active electronic devices such as solid state pressure sensors.

In some cases, chemical reactions and surface property alterations are desirable. In these cases, a solvent can be used as the additive, rather than a binder, to initiate reaction of the materials in the fluid energy mill. The solvent is preferably miscible with the fluid used for energy transfer. Furthermore, the solvent could be used as the energy transfer fluid if high concentrations of solvent are required for complete dissolution.

It is advantageous to initiate reactions in the fluid energy mill because of the thorough blending achieved in the mill. Further, as the particles are comminuted, they have a greater surface area/volume ratio, increasing the rate of reaction.

One example of such a reaction produces coal tar pitch. Coal tar pitch is used to manufacture numerous carbon products, such as, graphite electrodes for

the metal industry, specialty graphite and refractories for the nuclear industry, and graphite fibers for high performance composites. A solvent, N- methylpyrrolidone ("NMP"), forms coal tar pitch from anthracite. The reaction rate ofNMP and anthracite coal particles increases dramatically by applying high pressure and mechanical impact in a fluid energy mill. The extracted carbon in the resulting coal tar pitch is used to produce many carbon products including those mentioned above. In this case, a spray dryer may be used to reclaim the unused solvent and energy transfer fluid through evaporation, condensation and recycling.

As shown in FIG. 1, system 100 includes a spray dryer 108 directly attached to the output of fluid energy mill 104 via an optional feed pump 106.

Fluid energy mill 104 outputs a slurry containing comminuted particles of a material and the energy transfer fluid. If an additive was introduced into the fluid energy mill, the output will include the comminuted material, the energy transfer fluid and the additive. As would be apparent to one skilled in the relevant art, the material and the additive could be comprised of more than one material or additive.

As shown in FIG. 2, spray dryer 108 is attached to feed pump 106, and is comprised of atomizing components, such as a nozzle 204 and a heating chamber 206. Typically, a spray dryer mixes a spray and a drying medium, such as air, to efficiently separate the particles from the fluid as the particles fall through the air.

There are four general stages to spray drying: atomizing, mixing, drying, and separation. First, the feed or slurry is atomized into a spray. This is accomplished by introducing the slurry to feed pump 106, which forces the slurry through atomizing nozzle 204. The energy required to overcome the pressure drop across the nozzle orifice is supplied by feed pump 106.

Second, the spray is mixed with a drying medium, such as air. Air can be added through a blower via nozzle 204, via an additional nozzle, or can be merely present in chamber 206. As would be apparent to one skilled in the relevant art, other drying mediums could be introduced in spray dryer 108. For instance,

when the fluid, additive, or material is oxygen sensitive, inert gases such as nitrogen can be introduced as the drying medium. If a gas is added through a blower, the gas can be injected into chamber 206 simultaneously with the atomized slurry. A conventional method of introducing gas and slurry simultaneously uses concentric nozzles, where one nozzle introduces gas and the other nozzle introduces slurry.

Third, the spray is dried. Drying occurs as the atomized spray is subjected to a heat zone in chamber 206 or, alternatively, a hot gas, such as air or an inert gas as described above, is injected into chamber 206. Flash drying quickly evaporates the fluid from the slurry, leaving only the dry particles. The small size of droplets allows quick drying, requiring a residence time in the heat zone ranging from 1-60 seconds, depending on the application. This short residence time permits drying without thermal degradation of the solid material.

Fourth, the product is separated from the gas. As the particles continue to fall, they exit chamber 206, accumulating in particle collector 112, located at the bottom of chamber 206. The now vaporized fluid is exhausted, or alternatively, collected in condenser 110. The spray dryer by-products are vaporized fluid and dry particles.

Using a spray dryer in connection with a fluid energy mill provides several advantages over conventional drying techniques. For instance, spray drying produces an extremely homogeneous product from multi-component solids/slurries. A spray dryer can evaporate the energy transfer fluid from the slurry, leaving the additive and material. If the additive is a fluid, drying temperatures are held below the degradation temperature of the binder. As the energy transfer fluid evaporates, a very thin coating of binder polymerizes on each particle. After being dried in the spray dryer, the particles are sufficiently coated for molding into compacts for sintering. Additional processing is not necessary.

Furthermore, the resulting collected particles are fine, dry and fluffy.

Conventional techniques, such as boiling the vapor offthe particles, leave clumpy

conglomerates of particles and result in less thorough blending of additives. The spray dryer also dries particles much faster than drying by conventional techniques. A spray dryer quickly dries a product because atomization exposes all sides of the particles to drying heat. The particles are subjected to a flash dry, and depending on the application, can be dried anywhere between 3 and 40 seconds. Thus, heat sensitive particles can be quickly dried without overheating the particles. As drying begins, the vaporized fluid forms around the particle.

This"protective envelope"keeps the solid particle at or below the boiling temperature of the fluid being evaporated. As long as the evaporation process is occurring, the temperature of the solids will not approach the dryer temperature, even though the dryer temperature is greater than the fluid evaporation temperature.

An additional advantage is that the spray dryer can operate as part of a continuous process providing dry particles as they are collected, rather than having to collect particles and then dry them. This also allows for fast turn- around times and product changes because there is no product hold up in the drying equipment.

The volume of an acceptable chamber 206 can be determined by the equation, (residence time) * (volume flow rate) = volume of chamber, where volume flow rate is the throughput. Additionally, because of a larger surface area per unit mass, finer particles normally require longer residence time to dry than larger particles. Therefore, residence time may be longer for the finer materials.

Also, materials having hydroscopic properties will require a longer residence time in chamber 206. Increased temperature may also be used to accelerate drying of such materials.

The spray dryer can be used for drying any slurry, whether the slurry is comprised of particles of a material, an additive, and an energy transfer fluid or comprised of only particles of a material and an energy transfer fluid. Further, the spray dryer can be a standard spray dryer, known in the art of spray drying.

Spray dryer manufacturers and vendors include companies such as U. S. Dryer

Ltd. of Migdal Ha'emek, Israel, Niro, Inc. of Columbia, MD, APV of Rosemont IL, and Spray Drying Systems, Inc. of Randallstown, MD.

A conventional spray dryer can be outfitted with condenser 110. Because all drying takes place in an enclosed chamber 206, capture and condensation of the vapors is easily performed. Condenser 110 collects the vaporized fluid from chamber 206 and allows the spent fluid to be recovered. Thus, spray drying offers a simple way to contain the vapors from the evaporated fluid. Fluid recycling circuit 114, as shown in FIG. 1, can connect condenser 110 to high pressure pump 102 located at the first chamber of the fluid energy mill. This allows condensed fluid to be recycled by returning the used fluid from the spray dryer to the fluid energy mill. This reduces waste and contains the fluid, which is especially important when the fluid is a regulated product, such as isopropanol.

Isopropanol can be used as the fluid in the fluid energy mill, introduced into the spray dryer where it is vaporized, recondensed in the condenser and returned to the fluid energy mill for reuse. In this way, the fluid vapors are contained without risk of releasing harmful vapors into the atmosphere.

If the fluid is water, the water can be released from the spray dryer as vapor, can be condensed to be discarded, or can be recycled through the fluid recycling circuit. It would be apparent to one skilled in the relevant art that a variety of fluids could be used as the energy transfer fluid in the mill.

In another embodiment, the slurry is introduced from the fluid energy mill directly into the spray dryer. This embodiment does not use a feed pump connected to the nozzle for atomizing. Instead, fluid restrictors are used at the energy mill outlet port to maintain the high pressures in mill 104. The slurry bypasses feed pump 106 and is injected directly from the outlet of mill 104 into spray dryer 108. In order to achieve proper separation of particles and fluid in spray dryer 108, the fluid pressures at the outlet of mill 104 must be sufficiently high to achieve complete atomization of the slurry. By eliminating the need for a feed pump to introduce the slurry to the spray dryer, the system operates more economically.

FIG. 3 shows another embodiment of system 100 for comminution, blending and processing materials into particles. This embodiment includes a hydrocyclone 310 located between mill 104 and feed pump 106. Hydrocyclone 310 can be located either before or after feed pump 106, but is preferably located before it. A second feed pump (not shown) can be used to introduce slurry from mill 104 to hydrocyclone 106, or, the slurry can be introduced into hydrocyclone 310 directly from mill 104, as shown in FIG. 3.

Hydrocyclone 310 aids in classifying solid particles exiting fluid energy mill 104 by separating very fine particles from coarser particles. The coarser particles are fed through a recycling line 312 back into high pressure pump 102, to be reintroduced into mill 104 for further comminution and processing.

Alternatively, recycling line 312 can feed the particles directly back into mill 104, or can remove the particles completely from system 100. As the particles are still under pressure from hydrocyclone 310, recycling line 312 is a tube or enclosed circuit, which transfers the particles to mill 104 or high pressure pump 102.

The slurry from mill 104 enters the hydrocyclone at high velocity through an inlet opening and flows into a conical separation chamber. As the slurry swirls downward in the chamber, its velocity increases. Larger particles are forced against the walls, dropped to the bottom, and discharged through a restricted discharge nozzle into recycle line 312. The spinning forms an inner vortex which lifts and carries the finer particles up from the bottom of hydrocyclone 310, before they exit the discharge nozzle, and propel them through a forward outlet to feed pump 106 or, alternatively, directly to spray dryer 108.

In another embodiment, hydrocyclone 310 is a dry type cyclone, located after spray dryer 108. In this embodiment, particles are dried in spray dryer 108 and gathered in collector 112. The dry particles are introduced from collector 112 into cyclone 310, where the particles are sorted according to size. Cyclone 310 operates substantially similar to the hydrocyclone described above, using a gas as the fluid in place of a fluid. Again, oversized particles are reintroduced into fluid energy mill 104 or high pressure pump 102 via recycling line 312. Because

gases normally have less surface tension than fluids, dry separation normally results in finer and more accurate size distribution.

Hydrocyclone 310 can be a commercially available hydrocyclone used for classification, clarification, counter-current washing, concentration, etc., of particles. Examples of hydrocyclone and cyclone manufactures are Warman International, Inc. of Madison, WI (CAVEX Hydrocyclone Technology), Polytech Filtration Systems, Inc., of Sudbury, MA (POLYCLON Hydrocyclone Technology), and Dorr-Oliver, Inc., of Milord, CT (DORRCLONEW HYDROCLONES).

Because hydrocyclone 310 recycles the larger or more coarse fraction of material back to mill 104 for further size reduction, hydrocyclone 310 assists in achieving a narrow size distribution of finished particles. Furthermore, hydrocyclone 310 offers more intimate mixing of the particles and additives.

Residence time in hydrocyclone 310 is typically short, and is a function of the processing rate, and the equipment size (volume). Thus, residence time = equipment volume/processing rate (volume/time). Typically, the residence time in hydrocyclone 310 is less than 60 seconds, and is preferably from 2-50 seconds.

Thus, use of hydrocyclone 310 does not restrict the processing rate achievable in mill 104 and subsequent spray dryer 108.

Depending on the size and capability of the hydrocyclone, residence time will vary for a given processing rate. Therefore, a properly sized hydrocyclone must be used to efficiently comminute, blend and process particles. An improperly sized hydrocyclone could impose limits on the residence times in other components of system 100.

The following examples using system 100 will further assist in understanding the present invention.

EXAMPLE 1 System 100 can be used to obtain an intimate mixture of strontium titanate, SrTi03, magnesium oxide, MgO and barium titanate, BaTi03, with the finished particle size in the range of 0.5-2 microns, useful in waveguides, delay

lines, filters, and other radar applications. The nominal composition is, 60 v/o MgO, 40 v/o (0. 55BaO-0. 45SrTiO3), where the latter material is 55 mole percent BaTi03, 45 mole percent SrTi03. The density of the BSTO is about 5.62-5.63 g/cc.

It's preferable if the MgO particle size is somewhat smaller than the other phases. Thus, MgO is fed as the primary feed into a first stage of fluid energy mill 104, and the mixture of barium and strontium titanate (preferably as calcined clinkers) is added in a second stage of mill 104. As a result of comminuting MgO in the first stage, the particle size of MgO is smaller than the particle size of the titanate mixture, introduced into the second stage. Since these materials are very reactive with both water and carbon dioxide, isopropanol is used as the fluid energy medium. An appropriate binder additive such as CARBOWAX'can be added to either the first or second stage of the mill.

CARBOWAXs is soluble in isopropanol and provides an intimate mix with the slurry. The slurry can be introduced from mill 104 directly to hydrocyclone 310, or, can be sent to a feed pump located between mill 104 and hydrocyclone 310, depending on the outlet pressure of mill 104.

Hydrocyclone 310 removes particles from the slurry mixture having a diameter greater than approximately two (2) microns. Efficiency of removal of the oversized particles in hydrocyclone 310 can be increased by adjusting the pressure inlet to the hydrocyclone for the desired size fraction of the slurry under 2 microns. Oversized particles are sent back to high pressure pump 102 or reintroduced directly into mill 104 via recycle line 312.

At the product outlet of the hydrocyclone, a feed pump 106 may be employed to direct the slurry to spray dryer 108. Hot nitrogen gas approximately at about 200-600°C is used as the drying medium to flash evaporate the isopropanol. The dried powder mixture, coated with binder deposited onto the surface of the particles is collected in collector 112, at the bottom of the spray dryer for the further processing such as pressing and shaping, etc.

The resulting vapors and gases in spray dryer 108 are directed into condenser 110. Condenser 110 condenses isopropanol through either pressure changes, temperature change, or both, as would be apparent to one skilled in the relevant art. The condensed isopropanol is recycled through recycling circuit 114 back to high pressure pump 102. Nitrogen is reclaimed from condenser 110 and is either released or reintroduced into spray dryer 108 for further drying.

EXAMPLE 2 System 100 can also be used to obtain an intimate powder mixture composed of lead-zirconium titanate and barium titanate, useful in high voltage and high current capacitors for power supply filters. The targeted nominal composition includes equal amounts (molar) of unhoped PZT, PbO: 0.51Zr: 0.49Ti03 (the normal starting composition for PZT materials) and barium titanate (BaTi03). The resulting product should have a narrow particle size distribution of about 1-2 micron, with no large agglomerates, and well mixed.

Because there is no requirement for size differentiation between the two materials, both PZT and BaTi03, with the above proportions, can be introduced to the first stage of mill 104. Again, isopropanol is used because both PZT and BaTi03 are water sensitive. An appropriate binder, such as CARBOWAX@, is used in the mill in a similar manner as described with reference to Example 1.

System 100, using hydrocyclone 310, sprayer dryer 108, and condenser 110 will be employed as described with reference to Example 1. The final product is a mixture that is agglomerate free, highly homogeneous and has a narrow particle size distribution.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.