Patent Application
20090025425
The invention relates to a method and apparatus for preparing particulates from melts of glass or glass-ceramic compositions by spray-forming. In particular, the invention relates to a method and apparatus for preparing glass compositions of small particles, with narrow size distribution, in bulk quantities (>1 kg/min).
There are numerous known methods for spraying molten material (mostly metals and polymers) to obtain particulate powders, such as spherical powders, having a defined size.
U.S. Pat. No. 2,997,245 discloses an apparatus for pulverizing and/or decomposing solid as well as molten materials. One embodiment of the apparatus comprises a crucible, which contains molten material, and is positioned above a system of annular resonator chambers resembling a Hartmann-Sprenger chamber. Compressed gas streams through an annulus against an edge belonging to a first annular resonator chamber, and then into a second annular resonator chamber. The arrangement of both resonator chambers generates supersonic, non-stationary shockwaves having ultrasonic frequency, which are then directed towards and focused on a small area of the material to be pulverized.
U.S. Pat. No. 4,485,834 discloses an apparatus for atomizing a molten material such as complex metal alloys, i.e., materials which are solid at room temperature. The apparatus utilizes an atomization die assembly which comprises a set of opposed linearly configured die elements which define a longitudinally configured die space or gap through which a sheet of molten material is discharged. The die elements have oppositely disposed orifice-bearing surfaces which form a nozzle assembly. An ultrasonic atomizing gas exits the linear array of orifices and impinges against a predetermined focal region of a sheet of molten material passing through the die space or gap. Each of the orifices have having resonating cavities associated therewith for generating ultrasonic high frequency shock waves for disintegrating the sheet of molten material at the focal region into fine particles. The ultrasonic shock waves generated exhibit frequencies of about 20 kHz to 30 kHz (primary), and at about 100 kHz (secondary).
While the known processes and systems provide for formation of solid particles through the use of ultrasonic shockwaves, the resultant particles can vary widely in size distribution. The formation of glass and glass ceramic particles having a controlled particle size distribution is important for processes which utilize such materials as precursors, such as in the preparation of opto-ceramic elements, filter media, and catalysts supports. Thus, the present invention is directed to a method and an apparatus for providing solids particles, especially glass and glass ceramic particles, by subjecting a stream of molten material to ultrasonic shock waves, whereby the size distribution of the resultant particles can be easily controlled.
Furthermore, previously disclosed processes and systems often require complex arrangements to heat the melt delivery tube during operation, such as by injecting pressurized preheated gas, in order to prevent freezing of the melt discharge outlet. Additionally, many known processes require the pressurization of the melting crucible in order to force the melt into an atomizer. Thus, the present invention is directed to a method and an apparatus for providing solids particles which does not require the pressurization of the melting crucible, and does not required the supply of preheated gas at pressures in excess of 2 bar.
According to a process aspect of the invention, there is provided a process for the production of particulate compositions, especially particulate glass compositions, the process comprising:
According to an apparatus aspect of the invention, there is provided an apparatus for the production of particulate compositions, especially particulate glass compositions, the apparatus comprising:
The invention uses modified sonic/supersonic nozzle geometries in a novel arrangement. These modifications of nozzle geometry can be achieved using by using methods known to those skilled in the art of jet engine design and fuel injection.
Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:
As shown in
Additionally, the system is provided with a tube (referred to as the Refractory Metal Delivery Tube) for introducing the molten material into the nozzle. The tube is connected to a source of molten material, preferably molten glass. The tube is positioned so that the outlet of the tube, from which the molten material is dispensed, is within the nozzle and a peripheral opening (preferably, an annular opening) is formed between the inlet of the nozzle and the delivery tube. This peripheral opening provides an inlet for the introduction of a heated gas such air, CO, CO2, He, N2, Argon, etc., or mixtures thereof. The preheated gas is generally at a pressure of 1 to 2 bar, which is always lower than the pressure of the gas supplied through Gas Inlet 1, and a temperature of generally 750° C. to 2000° C. In general, the temperature and the pressure of the preheated gas depend on the material to be sprayed.
The Shockwave Generator is preferably made from an Adjustable Ringset comprising 4 rings which define the shape of the resonating cavities, each of which resembles a modified Hartmann-Sprenger chamber (similar in shape to the modified Hartmann-Sprenger chamber of U.S. Pat. No. 4,485,834). See
As illustrated in
The resultant mixture of cold/hot gas and particles enters the diffuser part of a super-sonic nozzle. As described in more detail below, a standing shockwave is formed at the outlet of the diffuser. Ultrasonic shockwaves, generated by the introduction of compressed gas (e.g., air) through Gas Inlet 2 into a modified Hartmann-Sprenger chamber, are transmitted into the gas/particle stream, to further breakup larger particles, thus narrowing the particle size distribution.
In operation, referring to the arrows in
Molten material is then supplied through the Refractory Metal Delivery Tube. The rapidly moving gas stream severs droplets from the end of the tube; the mixture of particles, cold and hot gas passes through the supersonic nozzle. See
Once the shockwave-cone is stable, compressed air at a pressure of, for example, 5 to 200 bar is supplied through Gas Inlet 2 and enters a distribution manifold. From the manifold, the air enters through an annular gap into a modified Hartmann-Sprenger chamber wherein a shockwave is generated. In a typical arrangement, pressure pulses at frequencies of 20 kHz, 30 kHz and 100 kHz are generated in the shockwave-generator. These shock waves exit the chamber into the nozzle through an annular gap and are transferred through the shockwave-cone into the particle stream. The shockwaves interact with the droplets in the gas stream breaking up larger particles. After exiting the nozzle, the particles are quickly quenched by the surrounding atmosphere.
The resultant particulate products form the process according to the invention can be used in a variety of applications that utilize glass or glass ceramic particles as precursor materials. For example, glass or glass ceramic particles can be used to produce catalyst supports, such as supports for high temperature catalysts, and filtration media, i.e., both liquid and gas filtration media.
In addition, the process according to the invention can be used as a starting material for making nano-porous microparticles, as described in copending and commonly-assigned patent application, entitled “Nano-Porous Precursors for Opto-Ceramics via Novel Reactive Dissolution,” [Attorney Docket No. RDD-13], filed even date herewith, the entire disclosure of which is hereby incorporated by reference. These nano-porous microparticles can be used to prepare catalyst supports, filtration media, and opto-ceramic elements, particularly opto-ceramic element for use in high energy laser applications.
The process and apparatus according to the invention can used for processing materials that melt at temperatures ranging from, for example, room temperature to in excess of 2500° C., where the upper temperature is only limited by the refractoriness of the containment (melting crucible).
Further, the process and apparatus according to the invention are particularly advantageous for processing materials that are highly unstable, such as compositions that rapidly crystallize and/or undergo phase separate upon cooling, owing to the rapid cooling rates of the inventive process.
The particulates formed by the process can be directly sprayed to form a coating, a three dimensional object (with or without the use of a mold), or can be collected as loose solid particles. The loose solid particulates can exhibit a number of physical characteristics depending on the operational parameters and/r composition of the melt. For example, the formed particles can be: spherical (normal shape, due to surface tension), hollow (due to gas evolution during cooling), flattened spheres (molten glass hitting a surface), fibers (using high speed laminar flow), or irregular smooth surfaced particulates (teardrops, donuts, etc).
The melt spraying process according to the invention has various advantages. The process can be used to prepare particulates with uniform chemistry and uniform thermal history. The particles are formed directly from the melt thereby reducing the amount post-particle formation processing. Further, the process parameters can be adjusted to control the particle size distribution range and can be used to provide particulate corn positions with a narrow size distribution. The process is applicable to glasses with a wide range of melt temperatures and does not require the use of binders. The process permits the redox characteristics of glass to be controlled by the melt/spraying conditions. Further, the particulates can be produced in an inert environment, thus eliminating/reducing the chance for surface contamination, hydrolysis, adsorption, etc.
The laserglass LG-810 cullet was melted at a temperature of 1000° C. in a gas-heated 100 ml-crucible made out of high-temperature steel, and supplied to the spray-forming apparatus through a stainless-steel pipe with 5 mm diameter. Compressed air provided by a diesel compressor (AirMan 185CFM) at a sustained pressure of 8 bar and a peak flow rate of 180 SCFM was supplied to Gas Inlet 1 and Gas Inlet 2. The composition of the Nd-doped fluorophosphate laser glass LG-810 developed for high-energy laser applications at 1060 nm is as follows (see: Lee Cook and Karl-Heinz Mader, “Ultraviolet Transmission Characteristics of a Fluorophosphate Laser Glass”, Journal of The American Ceramic Society, Vol. 65, No. 5, December 1982):
After establishing flow, the spray-forming of the molten material was completed in less than 5 seconds. The resultant product was a mixture of fibers and spherical particles. See
The borosilicate glass was molten from cullet at a temperature of 1550° C. in an inductively heated 100 ml-crucible made out of a platinum-iridium alloy, and supplied to the spray-forming apparatus through a platinum-iridium alloy pipe with 5 mm diameter. Compressed air provided by a diesel compressor (AirMan 185 CFM) at a peak pressure of 8 bar and a peak flow rate of 180 SCFM was supplied to Gas Inlet 1 and Gas Inlet 2. The composition of the borosilicate glass was as follows:
After establishing flow, the spray-forming of the molten material was completed in less than 5 seconds. The resultant product was a mixture of fibers and spherical particles. See
The entire disclosures of all applications, patents and publications, cited herein are incorporated by reference herein.
The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.
Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
