The present invention relates generally to granulating apparatus and, more specifically, to such apparatus of the wet aggitative type adapted for forming powder materials, particularly advanced ceramic and similar valuable materials, into granules utilizing an agglomerating liquid.
In various and diverse technologies, raw material processing conventionally requires the initial processing of raw material in powder form into larger agglomerated granules which may be more easily handled. Preferably, such agglomerated granules should be substantially spherical in shape and as uniform as possible in composition and density for ease of flowability and handling of the granules and to enhance the quality of end products formed therefrom. The formation of such uniform granules is particularly important in the preliminary processing of materials utilized in the emerging technological field of advanced ceramics. By way of example, such materials include alumina, silicon carbide, silicon nitride, barium titanate, various metal oxides, cermets such as tungsten carbide, partially stabilized zirconia, and cubic boron nitride. Advanced ceramic materials such as the foregoing find such highly technological applications as in the ferrite cores utilized in the deflection yokes for cathode ray tubes, adiabatic diesel engines and ceramic turbine engines, ceramic gas sensing devices, certain catalyst carriers and substrates, and various engineering ceramic applications such as high speed cutting tools, grinding media and the like. Such materials are conventionally formed into end products by a dry pressing or compacting operation. However, as initially processed, such materials are in a powdered form which characteristically is light, fluffy and therefore unmanageable and unsuitable for pressing operations. Accordingly, it is conventionally necessary to first form the powdered material into uniform granules which may be more readily handled and processed. Such advanced ceramic materials also are relatively expensive and valuable and, for the specialized and highly technological applications to which they are typically put, must be of optimum purity to insure a high degree of quality in the end product. Accordingly, the aforementioned requirements of uniform granule shape, composition and density are particularly acute and important in this field.
Various forms of granulating apparatus which have been developed over past years have proved entirely unsuitable and unacceptable for formation of granules of advanced ceramic materials. So-called spray dryers are currently employed widely for the mass production of advanced ceramic granules. In spray dryers, a slurry composed of powdered raw material and a binder or other agglomerating liquid is continuously sprayed in essentially discrete droplet or globular form continuously in a downward direction into the top end of an upright hollow vessel while heated drying air is directed upwardly within the vessel to drive off the agglomerating liquid thereby transforming the globules into essentially dry granules of the powdered material. While this apparatus produces generally uniform spherical granules having acceptable flowability characteristics and providing acceptable suitability for dry pressing operations, it is difficult to produce relatively dense granules due to the heat-induced evaporation of the agglomerating liquid which tends to cause outward migration of the finer particles of the powdered material toward the granule surface and thereby often produces hollow granules. Additionally, the operation of the apparatus is highly energy intensive, requiring approximately a 50% moisture level in the slurry for acceptable granulation and accordingly requiring a considerable amount of drying energy. As a result, the ratio of granule output to energy consumed is unacceptably low. The substantial drying requirements of such apparatus generally also require that the apparatus be undesirably large and bulky. Furthermore, since the granulation process performed by such apparatus is a continuous one, the apparatus is basically unsuitable for use in forming small quantities of granules. When it is desired to change the powdered material being granulated, it is of course necessary to clean the entire apparatus which is difficult and time consuming to do and often in any event results in cross contamination from one material to the next.
Extruding machinery is also available for forming moistened powder material into generally cylindrical pellet-type form by the forced extrusion of the material through a die plate or screen. Disadvantageously, it is difficult to produce pelletized granules by this operation to smaller than approximately a 0.7 mm. diameter due to rapid resultant wearing of the screen or die plate and concomitant contamination of the pelletized granules. Moreover, the cylindrical shape of the pellets significantly restricts their flowability and, as a result, such pellets are often further processed in a spheronizing vessel wherein the pellets are repetitively beaten against the vessel walls by a rotating plate to deform and plasticize the pellets into spherical form.
Disintegration-type granulating machinery is also available for breaking large agglomerations of material into smaller pieces, but such apparatus produce generally unacceptable irregular and poorly flowable granule shapes and also suffer significant losses of relatively fine particles resulting in a low granule yield rate.
Fluidized bed granulating machines have been developed which essentially combine the functions of an extruder, a spheronizer and a dryer for granule formation. In such apparatus, powdered raw material is fed into a cylindrical vessel having a stationary screen or plate with openings therethrough, the powdered material being treated with an agglomerating liquid spray on the charge side of the screen or plate while heated drying air is forced through the screen or plate openings from the opposite side thereof to form the powder into dry granules. Disadvantageously, such granulating apparatus produces a widely varying range of granule shapes and sizes which inhibit good flowability of the granules. The apparatus is often subject to rapid component wearing with resultant granule contamination, and also is not susceptible of accurate control and reproducibility of granule size.
In another form of aggitative-type granulating apparatus, a charge of powdered material is processed in batch form in a vessel having a rotating outer annular mixing or stirring rotor and an inner impulse rotor therewithin for compatibility circulating and mixing the powdered charge material, with an agglomerating liquid spray being provided for forming the powder material into granules as circulation progresses. To enhance the powdered material distribution and granule formation, blade-like implements are typically provided on each rotor and the rotors may be rotated in opposite directions with the central impulse rotor being operated at a greater speed than the outer stirring rotor. Examples of this type of granulating apparatus are disclosed in Japanese Patent Publication Nos. 58-12050, 59-21649, 59-55338, and 59-59239, commonly owned with the present invention. While these apparatus are acceptably operative for granule formation, such apparatus produces widely ranging granule sizes with poor capability for accurate repeatable control of granule size and such apparatus also suffers rapid wearing of its internal operating components with attendant contamination of the granules formed, all of which makes this apparatus generally unacceptable for forming granules of advanced ceramic and other valuable materials.
It is accordingly an object of the present invention to provide an improved granulating apparatus of the aggitative type last above-described, which is capable of repeatably accurate production of spherical granules of uniform density and composition and high purity without significant contamination, thereby being uniquely suitable for advanced ceramic granule formation.