WO1998020181A1

WO1998020181A1 - Process for plasma spraying ceramic residues

Info

Publication number: WO1998020181A1
Application number: PCT/US1997/020043
Authority: WO
Inventors: Steven John Dekanich; Johnny Page Forester
Original assignee: Molten Metal Technology, Inc.
Priority date: 1996-11-06
Filing date: 1997-10-31
Publication date: 1998-05-14

Abstract

A method for preparing useful ceramic-containing articles from ceramic phase residues formed as a by-product of a treatment process producing a molten ceramic slag. The method is particularly useful in converting ceramic phase residues containing hazardous or radioactive substances into useful articles. The ceramic phase residue is converted to particulate form of appropriate size for feeding to a plasma arc sprayer, injected into the hot plasma jet. The resulting molten ceramic spray is caused to impinge on a solid surface to form a solid or porous ceramic article or coating, or is quenched so as to form a particulate ceramic material. Alternatively, a contaminated particulate material such as soil, may be sized and fed directly to the sprayer. A method for making ceramic filters or filter membranes using plasma arc spraying is also disclosed.

Description

PROCESS FOR PLASMA SPRAYING CERAMIC RESIDUES

Background of the Invention Field of the Invention

The present invention relates to methods for the preparation of useful ceramic products by application of plasma arc spraying techniques. In particular, the invention provides a process for making useful ceramic products from stabilized ceramic-phase residue or slag resulting from the treatment of hazardous or radioactive waste materials, or other slag producing processes such as smelting metallic-containing ores or refining metals. The process of the invention is especially useful to make ceramic filters, shaped objects and particulate materials, and to apply ceramic coatings to metals, refractories and aggregates. The invention additionally provides a unique method for making ceramic filters and filter membranes using plasma arc spray techniques. In a further embodiment, the invention provides for the direct treatment of contaminated soil or other particulate material of high inorganic content containing hazardous or radioactive material, by means of a non-transferred arc plasma sprayer.

Description of Related Art

Slag is the impure ceramic-phase material that is produced, for example, as a by-product in making pig iron, and in smelting copper, lead, and other metals from their ores. The composition of slag can vary widely depending on its origin. For example, limestone is added to steel blast furnaces in order to remove impurities, and the resulting slag contains silicate of calcium, magnesium, and aluminum, which floats to the top of the melted iron. Slag from copper and lead-smelting furnaces contains iron silicate, and other metals in small amounts. Slag from open-hearth steel furnaces contains lime and some iron. This slag is generally considered a waste product and finds a use, at best, as fill and under pavement in building roads.

Slag or other ceramic-phase products are also produced as a waste product of various processes now under development for the treatment of hazardous waste. The treatment of hazardous waste has become a major problem throughout the developed world, and there is mounting governmental pressure for reductions in the use and disposal of hazardous materials, and minimization of disposed waste in general. Suitable dumps or storage facilities for toxic waste and low level radioactive materials have become filled, and heightened public concern and tightening government regulation have severely restricted land disposal or long term storage as a viable option.

Incineration is becoming less of an acceptable alternative to land disposal due to increasing government regulation and escalating public fear about local air and water quality. Moreover, the incomplete combustion of incineration leaves a relatively large volume of leachible residues which, together with the non-combustible materials, must still be disposed of. In addition to dealing with currently produced waste materials, hazardous and radioactive waste materials have been stockpiled for many years waiting the development of suitable disposal processes, and hundreds of military, industrial, and toxic waste disposal cites already have severely contaminated soil which must be processed and cleaned before these areas may be put back into active use. In recent years, several methods have been under development for the treatment of, in particular, hazardous and/or low level nuclear waste materials of the type described above, which produce a ceramic phase residue or slag. These methods rely on high temperatures, or in one method on the catalytic action of a molten metal bath, to decompose or vaporize the organic constituents, and convert the inorganic constituents into a stable ceramic-phase residue or slag in which remaining hazardous or radioactive materials are immobilized. A metallic phase may also be formed if metals are present. The objective of immobilizing the toxic or radioactive components is to produce a delisted vitreous or ceramic phase residue which can be safely disposed of in a land fill. Such residues must be able to meet the Universal Treatment Standards (UTS) limits set by the EPA for minimum amount of hazardous materials in the leachate under specified test conditions, such as the Toxicity Characteristics Leaching Procedure (TCLP). Nevertheless, the normal disposition of the stable ceramic product resulting from these processes has heretofore been burial.

One such process producing a ceramic-phase residue, developed by Molten Metal Technology, Inc. is called Catalytic Extraction Processing (CEP). Solid iron or other metal is loaded into a reactor and melted, typically by induction heating, and brought to a temperature of about 1500°C. Fluxes, such as lime and silica, are added to form an initial ceramic layer. The waste stream is fed into the reactor where the catalytic effect of the molten metal dissociates organic compounds and reducible materials (including metals and certain radionuclides) into their respective elements which dissolve in the metal bath. The composition of the resulting off-gas and solid products depends on the feed and conditions maintained. By adding proper co- reactants and/or controlling operating conditions, the dissolved intermediates can react to form desired off-gasses, such as synthesis gas, and useful metal or metal alloys can be recovered from the molten metallic phase. Residual inorganic hazardous materials or low level radioactive wastes are encapsulated in the ceramic phase in a non-leachable form acceptable for burial.

A cyclone vitrification process disclosed by Babcock & Wilcox treats contaminated inorganic hazardous wastes, sludges and soils that contain heavy metals and organic constiuents in the form of solids, slurries or liquids. The contaminated sludge or soil is introduced into a cyclone furnace barrel together with a fuel (such as natural gas, oil or coal) and combustion air, where the material is heated to 2400 to 3000°F. The inorganics are melted, and the organic materials are destroyed in the gas phase or in the molten slag layer. The slag is dropped into a water-filled slag tank where it solidifies. A two-stage fluidized-bed cylonic agglomerating incinerator has been proposed by The Institute of Gas Technology. Contaminated soils and sludges are fed into a fluidized-bed maintained at 1500 to 2000°F where the combustible fraction of the waste undergoes gasification and combustion. The solid fraction, containing inorganic and metallic contaminants, undergoes a chemical transformation at 2000 to 3000°F in the "hot zone" of the incinerator, where it is agglomerated into glassy pellets that are said to be essentially nonleachable and suitable for disposal in a landfill. The gaseous product is further treated in a second stage. The Plasma Arc Centrifugal Treatment (PACT) process, developed by

Retech Inc., thermally decomposes organic waste inside a sealed chamber while simultaneously vitrifying inorganic components on a rotary hearth. Waste material is fed into a sealed centrifuge, where it is heated by a transferred-arc plasma torch. Organic material is evaporated and destroyed upon entering the primary chamber, and the resulting off-gas travels through a secondary combustion chamber, and then to an air pollution control system. Inorganic material is reduced to a molten phase that is uniformly heated and mixed by the motion of the centrifuge and the effect of the plasma arc. Material can be added during the process to control slag quality. When the centrifuge is slowed, the molten material discharges from the throat as a homogeneous, non-leachable slag into a mold where it is solidified by cooling into a form suitable for long-term storage. It is possible to operate the PACT system in a manner such that most of the metal remains unoxidized, forming a separate metal layer underneath the slag.

A transferred arc plasma torch is also used as the heat source in the method of decomposing hazardous wastes into vitrified solids and non- hazardous gasses in U.S. Pat. Nos. 5,541,386, 5,451,738 and 5,319,176. In this method, the waste material, after being passed through a dehydrator and a high-temperature dryer to vaporize hydrocarbon liquids, is fed into the focus point of a plasma reactor where plasma arc jets are focused on the surface of a pool of the vitrifiable components. The vitrifiable components of the waste feed are melted, and the volatile components are volatized. The melted vitrifiable components are solidified on a quench roller and broken into chips. The volatile components are said to break down into elemental constituents which recombine into simple, non-hazardous molecular forms, and the vitrifiable components are vitrified into a stable non-hazardous material.

The above treatment processes have the objective of breaking down volatile or decomposable waste materials, and isolating inorganic hazardous or radioactive materials in non-leachable metallic or ceramic phase. However, this solid waste residue still contains contaminants and needs to be stored or disposed of in a safe manner. As the desirability and availability of safe storage or burial options for this contaminated residue diminish, there is a growing need for alternatives. One possible alternative is the conversion of this contaminated solid residue into useful products.

The conversion of such metallic or ceramic phase residues into useful products is generally addressed in U.S. Pat. No. 5,545,796, which focuses on the use of contaminated metals or other radioactive or hazardous waste materials for the construction of containment systems for waste or "fresh" radioactive or hazardous chemicals or other materials. The containment systems can be made from, e.g., slabs cast from melted contaminated metals such as metal tubing from nuclear power plants. It is proposed that other radioactive or hazardous waste materials can be processed into small pieces, particles or fibers and mixed with suitable binders and fillers, and used to make slab, brick, block, wall or other materials useful for such containment systems. The reference lists a vast variety of articles in addition to containment systems, which could conceivably be made from contaminated materials, without further elucidation. An objective of the present invention is to make useful ceramic articles by means of a plasma arc sprayer utilizing ceramic phase residues formed in slag producing treatment processes such as those noted above.

Another objective of the present invention is to provide a further alternative to long term storage or burial of stabilized contaminated ceramic phase residues, through the use of a plasma arc sprayer to convert such materials into useful ceramic shaped, particulate or coated articles.

A further objective of this invention is to utilize plasma arc techniques to convert ceramic phase residues into solid or porous shaped articles useful as construction materials, structural members, insulation, refractories, filters or containers using plasma arc spraying techniques.

Yet a further objective of this invention is to utilize plasma arc techniques to convert such ceramic phase residues into ceramic coated articles, such as coated metals, coated refractories, or coated grids or screens to form porous membranes for use as, e.g., filters.

Yet a further objective of this invention is to utilize plasma arc techniques to convert ceramic phase residues into ceramic particulates or beads for use as, e.g., filter beds, fillers, or as feed to plasma arc sprayers or the like.

A further object of this invention is to provide a unique method for making ceramic filters and ceramic filter membranes using a plasma arc sprayer. It is also an objective of this invention to reduce the amount of hazardous and/or toxic inorganic substances, and to encapsulate radioactive substances, contained in such a ceramic phase residue by directly subjecting it, in particulate form, to the extremely high temperatures of a non-transferred plasma arc sprayer, thereby rendering it more acceptable for recycle as useful ceramic articles.

A further embodiment of this invention has the objective of providing for the direct treatment of contaminated soil or other particulate material of high inorganic content containing hazardous and/or radioactive substances, by means of a non-transferred arc plasma sprayer, so as to both reduce its content of hazardous substances, and to encapsulate the radioactive substances and/or other hazardous substances which may remain in a substantially non-leachable form.

Other objectives will also be apparent hereinafter.

Summary of the Invention

Broadly stated, ceramic-phase residue resulting as a by-product of processes for the treatment of hazardous and/or radioactive waste, or from other slαg-producing processes such as smelting or metal refining, is converted into useful products by a process which comprises converting the residue into a particulate form suitable for feeding to a plasma arc sprayer, and feeding the particulate material through the sprayer in a manner such that the particles are entrained, and partially or wholly melted in the plasma jet exiting the sprayer.

When forming a shaped article, the molten particles are caused to impinge upon a removable or destructible mold wherein or on which they solidify to form the shaped article. When forming a coated article, the molten particles are caused to impinge upon the substrate to be coated, whereupon they solidify to form a ceramic coating.

When forming ceramic particles or beads, the molten particles are permitted to cool and solidify in a quenching gas or other fluid, rather than aggregating or impacting on a solid surface while in the molten state.

More specifically, the invention provides a method for preparing a ceramic-containing article from a ceramic phase residue formed as a byproduct of a treatment process producing a molten ceramic slag, said method comprising the steps of: a) processing said ceramic phase residue into a solid particulate material having a size and uniformity suitable for feeding to a plasma arc sprayer; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle; c) injecting said solid particulate material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon a surface axially spaced from said nozzle while said ceramic spray is still at least partially molten so as to deposit at least one layer of ceramic material on said surface, which layer thereupon solidifies to form said article. In a further embodiment, the invention provides a method for the preparation of a filter element or membrane comprising a ceramic coating on a metal screen substrate, and the filter element or membrane thereby produced. This method comprises the steps of: a) securing said metal screen substrate in a frame, said screen having a pore size of between about 50 and 850 microns; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle axially spaced from said screen; c) injecting a solid particulate ceramic material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon said metal screen while said ceramic spray is still at least partially molten so as to deposit at least one layer of ceramic material on said screen, which layer thereupon solidifies to form said filter element. In yet a further embodiment, the invention provides a method for the preparation of a porous ceramic filter element from a particulate ceramic material, and the porous ceramic filter element thereby formed. This method comprises the steps of: a) providing a removable mold having a surface defining the desired shape of said filter element; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle axially spaced from the surface of said mold; c) injecting a solid particulate ceramic material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon said surface while said ceramic spray is still at least partially molten so as to deposit multiple layers of ceramic material in said mold, which layers thereupon solidify to form said filter element. In a further embodiment, the invention provides a method for the preparation of a filter element comprising a ceramic coating on a porous substrate, such as a foamed ceramic or refractory material or a porous aggregate, and the filter element so formed. This method comprises the steps of: a) placing said porous substrate in spaced relationship with the nozzle exit of a non-transferred plasma arc sprayer; b) forming a hot plasma jet exiting the nozzle of said plasma arc sprayer and directed towards said porous substrate; c) injecting a solid particulate ceramic material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon said porous substrate while said ceramic spray is still at least partially molten so as to deposit at least one porous layer of ceramic material on said substrate, which layer thereupon solidifies to form said filter element. In carrying out these methods, in a preferred embodiment the particulate material is heated sufficiently in the hot plasma jet so as to substantially fully melt and to vaporize at least a portion of the hazardous substances, such as heavy metals, and steps c) and d) are carried out in an enclosed environment to confine and collect the vaporized materials so that they can be transported to an off-gas treatment facility.

In yet a further embodiment, the invention provides a method for the treatment of soil or other particulate inorganic-containing material contaminated with at least one hazardous or radioactive substance, so as to reduce the content of said hazardous substance, and to encapsulate any radioactive or hazardous substance remaining in said particulate material in a substantially non-leachable form, said process comprising: a) obtaining said contaminated particulate material in a form having a nominal particle size range suitable for feed to a non-transferred arc plasma sprayer; b) forming a hot non-transferred plasma jet exiting a plasma arc sprayer through a nozzle; c) injecting said solid particulate material into said hot plasma jet under conditions such that the injected particulate material is heated to a molten state in said plasma jet and to a temperature sufficient to at least partially reduce the content of said substance in said particulate material; and d) causing said molten ceramic spray to solidify. It will be appreciated that the invention provides a number of particularly useful features. It provides a means for recycling waste ceramic phase residues into useful products. The invention further provides a means for reducing the amount of toxic or hazardous inorganic materials, such as heavy metals, contained in contaminated waste ceramic phase residues from thermal treatment processes so as to permit the recycling of such residues into a variety of useful products.

The invention also provides a novel ceramic-containing filter element that can itself be recycled by the method of the invention. This filter element can take the form of a filter membrane comprising a ceramic coating on a metal screen substrate, a porous ceramic filter element built up from a particulate ceramic material, or a filter element comprising a porous ceramic coating on a porous substrate. This is particularly advantageous when the filter element is used in a toxic or hazardous environment, such as a HEPA filter or pre-filter in nuclear processing or generating facilities.

The invention also provides protective ceramic coatings on metals, refractory or aggregate materials which can be used, e.g., for the containment of radioactive or hazardous waste materials, and which may be recycled in accordance with the invention when the waste material is treated in one of the ceramic-phase producing treatment processes described above.

The invention also provides an advantageous method for reducing the hazardous or toxic substance content of a ceramic phase material by means of the high temperature to which it is heated when drawn or injected into the plasma jet of a non-transferred plasma arc sprayer. The invention can also be used for the direct treatment of contaminated particulate materials such as contaminated soils by drawing or injecting such particulate material directly into the plasma jet. Radioactive and/or other remaining substances are encapsulated in a substantially non-leachable form.

Other features and advantages of this invention will become apparent from the detailed description of preferred embodiments.

Description of Preferred Embodiments

Currently, methods are being developed to produce stable ceramic- phase residues or slags that will immobilize radioactive or other hazardous components in a stable vitreous phase. Such processes have the objective of producing a delisted product phase, the normal disposition of which is burial. The present invention provides an alternative to burial by converting the ceramic phase residue to a particulate form, and passing the particles directly into the plasma jet of a plasma arc sprayer so as to not only reduce their content of hazardous components and encapsulate the radioactive components, but to simultaneously convert the ceramic phase residue into useful ceramic products.

Most metal/slag producing processes, such as those noted above, can be used to produce particles of appropriate ceramic content and size for use in plasma arc sprayers to carry out the process of this invention. Preferably the particles will be of a uniform size, and have a nominal particle size within the range of between about 0.1 to 250 microns. More preferably the particles for feeding to the plasma arc sprayer will have a size range of between about 0.5 and 150 microns. Uniformity of particle size, i.e., the range of particle size within a given feed, is not critical in many coating applications, provided the material can be reliably fed to the sprayer. However, the uniformity of the particle size becomes more important when the method of this invention is used to make filters. Here it becomes preferred for proper filter operation that the particulate material be sieved to a relatively narrow or "sharp" cut of particle size such as, for example, shown in the examples.

Adequately uniformly sized particles can be produced by a various known means. For example, appropriate uniformly sized particles can be made by crushing and classifying the ceramic residue after it has been cast and cooled. Alternatively, the ceramic phase residue, in its molten state either directly or after remelting, can be injected or aspirated into a quenching gas. The particle size would be controlled by the temperature and velocity of the aspirating gas, the temperature and viscosity of the ceramic material, in addition to the configuration of the equipment. Suitable apparatus that can be used to create such uniformly sized particulates is sold, for example under the name JET CASTER by Retech, Inc. of Ukiah, California.

In carrying out the examples, the sample materials sprayed through the plasma arc sprayer were (1) a crushed and classified surrogate ceramic phase residue and (2) a fine clay-containing soil.

The surrogate ceramic phase residue used in the examples is termed WETF (West End Treatment Facility), which is representative of a ceramic phase residue resulting from treatment of a sludge from a DOE facility at Oak Ridge, Tennessee by the Catalytic Extraction Process. It is intended to be representative of a ceramic phase residue obtained from the treatment of actual mixed hazardous and radioactive waste. While this surrogate lacked the specific hazardous and radioactive components, it is deemed entirely sufficient for testing the preparation and spraying of the particulate ceramic material in accordance with the present invention. The composition of this surrogate material is given in Example 1.

The fine clay-containing soil used in the examples was east Tennessee Red Clay (TRC), which was dried by heating it up to 1,000°F, and then sized to obtain a -60 mesh +100 mesh material, and a finer -100 mesh material. It was found that the -100+150 mesh material, which is about 150 micron in size, fed better than the larger material in the particular plasma arc sprayer being used. The preparation and composition of this Tennessee Red Clay material is provided in Example 2.

For the examples, a Bay State/Sterling PG-Series Model PG120-4 plasma spray gun and a Bay State Sterling 40 KW Dual Voltage Power Supply Model PS-1005 was used for this study, as described in Example 4. Argon was chosen for this series of tests, although Argon/H₂, Argon/He, Nitrogen or Nitrogen/H₂, could have been used. A Bay State/Sterling Fluidized Bed Powder Feeder Model PF-750 and a Bay State/Sterling powder blender/feeder (screw feeder) Model PF-500 provided good results for all powder sizes tested. This plasma arc sprayer gave a swath per pass of approximately one-eighth to one-quarter inch in width. To build up the ceramic article or coating, it was therefore necessary to go back and forth in parallel, slightly overlapping swaths. Of course, larger and/or more powerful plasma arc sprayers may be used in this invention, depending on the amount, size and nature of the particulate material being sprayed, and the article being made. The spraying procedure may also be automated by use of robotics and/or appropriate control under e.g., a computer program. When building up multiple layers, it is desirable to apply the coating in a different direction, such as 45 to 90 degrees to the previously layer.

Preferably a non-transferred arc plasma arc sprayer is used in this invention. In this type of plasma arc sprayer, a gas is introduced between two electrodes which are a part of the sprayer, and ionized to form a high temperature plasma jet which is propelled at a high velocity out of the nozzle of the sprayer. The plasma jet may reach temperatures of between about 5000 and 10,000°F. The particulate feed may be introduced or injected into the plasma jet either within the sprayer itself, such as between the electrodes or within the nozzle, or outside but adjacent to the nozzle exit. Argon is the preferred plasma forming gas, but nitrogen, hydrogen, or other gases or mixtures thereof, may be used as well. Gas mixtures containing hydrogen may be used if higher temperatures are needed. The amount of feed gas depends upon the requirements of the particular plasma arc sprayer being used, and the determination of the appropriate gas and feed rate is well within the skill of the art.

The non-transferred plasma arc sprayers preferred for this invention differ from the transferred-arc plasma torches used as a heat source in some of the known processes discussed above. The non-transferred arc plasma sprayer used in the examples was a Bay-State Sterling Inc. PG-Series PLASMAGUN. This plasma jet sprayer is designed to produce high-velocity, high-temperature plasma for the application of ceramic, metal, and refractory materials to a variety of substrates. This unit is typical in basic design of non-transferred arc sprayers, and consists of a rear electrode (here a cathode), a gas injection ring, a nozzle terminated at its exit by the other electrode (here an anode), and a powder feed port. An arc drawn between the cathode and the anode ionizes the working gas to create a high- temperature plasma jet which exits the nozzle at very high velocities. In one embodiment, the particulate ceramic material is injected into the plasma jet as it forms within the nozzle. In an alternative embodiment, the particulate material is fed into the hot plasma jet just as it exits the nozzle, from a point adjacent to the exit of the nozzle. In either case, the particulate material is fed into the hottest part of the plasma jet. This high temperature not only melts the ceramic particles, but vaporizes at least part of the hazardous or toxic materials which may be encapsulated therein.

With a transferred-arc plasma torch, the arc is drawn between one electrode in the torch and a second electrode outside of the torch. Thus, in the known processes discussed above, the transferred plasma arc produced by the torch is aimed at the mass of molten material in order to transfer heat. By contrast, with the non-transferred plasma arc sprayers preferred in the present invention, the finely divided particulate material is introduced into the plasma jet itself, with the result that the particles are heated to a far higher temperature than in the known thermal processing methods using transferred plasma arcs as the heat source.

The exceedingly high temperature of the plasma jet will, in addition to decomposing any organic materials present, vaporize at least a part of the inorganic hazardous materials present in the particulate material being sprayed. This would include, e.g., trace metals such as mercury, lead and other heavy metals, thereby reducing the content of these hazardous materials in the product being made. Any remaining hazardous material will be encapsulated within the ceramic material with the effect of greatly reducing its availability, for example, by leaching. Because of the vaporization of these trace inorganic materials, it is desirable to carry out the spraying of contaminated particulates in an enclosed environment to contain and permit the recovery of these vaporized materials. Such vaporized materials would be recondensed and filtered out of the off-gas, for instance in a graphite bed. The resulting ceramic-containing article will be of reduced hazardous or toxic content, and should be more acceptable for use in recycled articles than the starting ceramic phase residue.

The coated metal mesh membrane filters of the examples were made with stainless steel mesh. Although higher temperature metals are available, such as molybdenum and tungsten, care must be taken to prevent oxidation of the metal/ceramic interface when the ceramic layer is applied. This oxidation may significantly decrease the adherence of the ceramic layer to the mesh screen. If such an oxidizable material is used, the spraying should be carried out in a controlled inert gas environment. Depending upon the intended application, other metals may be used as well for the metal mesh screen. The preparation and coating of metal mesh screens to form membrane filters may be carried out, e.g., by the procedures described in Example 4.

Membrane filters made on a stainless steel screen should be able to withstand temperatures in excess of 500°C, as high as 800°C and up to 1000°C. If the intended use is expected to exceed those temperatures, then fully ceramic filters may be preferable. Such fully ceramic filters can be prepared, in accordance with this invention, by building up a porous filter element from multiple layers of plasma sprayed material in a removable mold (such as graphite), or by plasma spraying porous membrane layers on a more porous refractory substrate, such as a foamed ceramic or refractory material, or a porous aggregate such as a porous cement or concrete substrate. Such a self-supporting, all ceramic filter element may also be built up from multiple layers of the plasma sprayed material on a mesh or screen substrate made of a material which may be removed from the final product by heat or chemical action, such as a polymeric material or a metal having a relatively low melting temperature. A filter test fixture of a type known to those of skill in the art can be used to test the filter efficiency and cleanability. This fixture is used to carry out the standard Dioctyl Phthalate (DOP) smoke test, such as ASTM D 2986-71 used, e.g., for testing High Efficiency Particulate Air (HEPA) filters for integrity of the porosity of the filter, pressure drop across the filter, and cleaning efficiency. A fan located at the end of the fixture provides an induced draft for DOP or powder testing. The DOP oil is aspirated to form droplets of about 0.3 microns, which has the visual appearance of a smoke. If the filter has pores larger than about 0.3 microns, this can be detected by the appearance of smoke (oil) coming through the filter, or by a detector capable of detecting minute quantities of oil coming through the filter. After performing the DOP tests, sized powder ranging from 15 microns to sub- micron are introduced into the fixture via a DOP test port. The coarser particles are collected on a pre-filter and finer particles (sub-micron) are collected on a HEPA filter. While the powders are being introduced into the system, differential pressures across the pre-filter and the HEPA filter are continuously monitored. When the filters are loaded with powder, the fan is stopped. Hopper slide gates are opened and the air jets activated. The jet of air impinging on the powder knocks the powder from the surface of the filters. The powder is allowed to settle in the hopper, and the slide gate is shut.

Shaped articles, including filters, can be made in a removable mold by spraying multiple layers of ceramic material and then removing the mold. For example, a graphite mold or form can be used to make a filter of a particular configuration. The graphite can be machined to the shape required. Then multiple layers of ceramic material are sprayed onto the graphite form to a thickness of, for example, from about 1 /32^nd to h inch in thickness, sufficient to be self supporting once the graphite form is removed. Preferably the filter will be comprised of multiple layers having a differing particle size and porosity from one side to the other. Either the finer and less porous membrane coating can be applied first to the mold, followed by the coarser and more porous layers, or the membrane coating can be applied last, depending on the intended direction of fluid flow during filtering.

Membrane filters can also be built up out of ceramic grids to, for example, 1 /32^nd of an inch in thickness or more. The self-supporting strength could be enhanced by shaping it, such as making it in corrugated form. Such self-supporting ceramic filters would be suitable for relatively small spans, but for larger spans, a backing support material is required.

In building a filter, whether on a removable mold such as graphite, or on a metal screen or grid, it is desirable to first lay down one or more layers of relatively course spray, giving a relatively high porosity followed by one or more layers of a finer spray to give the desired porosity of the final filter. The relative coarseness of the layer can be controlled by the size of the molten particles in the spray, which in turn can be controlled by the size of the solid particulate material fed into the hot plasma jet. When using the method of the invention, it was found that no subsequent thermal treatment or sintering of the filter elements was needed.

Examples Example 1

Preparation of Surrogate Particulate Ceramic Phase Material

The particulate surrogate ceramic phase residue used in the examples is termed WETF (West End Treatment Facility), which is representative of a ceramic phase residue resulting from treatment of a sludge from a DOE facility at Oak Ridge, Tennessee by the Catalytic Extraction Process. It is intended to be representative of a ceramic phase residue obtained from the high temperature treatment of an actual mixed hazardous and radioactive waste. While this surrogate lacked the specific hazardous and radioactive components, it is deemed entirely sufficient for testing the preparation and spraying of the particulate ceramic material in accordance with the present invention. The ceramic phase surrogate material was crushed to the following micron sizes:

Table 1

-106 + 90 Nominal size (100 microns

-75 + 63 Nominal size (70 microns)

-63 + 53 Nominal size (57 microns)

-53 + 45 Nominal size (49 microns)

-45 + 38 Nominal size (39 microns)

-38 + 32 Nominal size (35)

-32 + 25 Nominal size (28)

-15 +8 Nominal size (10)

-5 +.7 Nominal size ( 1)

-3 +.3 Nominal size ( .5)

The material analyzed with the following composition:

Table 2

Component Perce

SiO, 33.33

NaCl 0.27

NaCO, 0.54

CaO 37.55

CaC0₃ 20.13

A O, 1 .49

MgC0₃ 2.72

Fe₂0₃ 3.97

Example 2.

Preparation of Soil Sample The fine clay-containing soil used in the examples is east Tennessee red clay. Analysis showed that the clay had the following composition:

Table 3

One inch of clay rich soil was spread on a 2 ft. x 3 ft. x 3 in. metal pan and heated at a rate of 100°F per hour to 1000°F to dry it out, held for three hours and cooled at a rate of 100°F per hour to room temperature. After drying, the clay was crushed and sized to -250 micron + 150 micron and -150 micron.

Example 3

Preparation of Metal Screen Mesh Substrate

Standard stainless steel screen mesh having openings 51 microns, 150 microns (100 mesh), and 250 microns (60 mesh) were prepared for coating to make filters. Additionally, a commercial spatter screens sold for kitchen use, which was estimated to be about 20 mesh with openings of approximately 850 microns, was also prepared for coating. It is expected that finer mesh screens could be coated, although finer screens are not necessary to obtain an effective membrane filter. At the coarser end of the spectrum, the spatter screen having about 850 micron openings was successfully coated. However, it was found preferable to apply portions of the coatings at an oblique angle of about 45° to the plane of the screen surface in order to fill in the relatively large openings with the porous ceramic material. Although it would be possible to form membrane filters from screens with openings of up to 1 millimeter or larger, it becomes more difficult to make a filter of uniform porosity.

To carry out the spraying of the mesh screen, the screens were assembled in 6 inch x 6 inch steel frame supports fabricated from one inch wide by 1 /8 inch-thick steel plate. The screens were sandwiched between the steel plates, which were then tack welded and the weld surfaces ground.

The fabricated frame/steel mesh assemblies were degreased by soaking in acetone and double rinsing them with alcohol. Prior to plasma spraying, the 60 and 100 mesh screens were grit blasted with 60 grit Al₂0₃. ^Tne 51 micron screen was grit blasted with 120 grit Al₂0_3' The screens were then blown with compressed air to remove residual Al₂0₃.

Example 4

Preparation of Membrane Filters on Metal Mesh Substrate

A Bay State/Sterling PG-Series Model PG120-4 plasma spray gun and a Bay State Sterling 40 KW Dual Voltage Power Supply Model PS-1005 was used for this study. Argon was chosen for this series of tests, although Argon/H₂, Argon/He, Nitrogen or Nitrogen/ H₂, could have been used. A Bay State/Sterling Fluidized Bed Powder Feeder Model PF-750 and a Bay State/Sterling powder blender/feeder (screw feeder) Model PF-500 provided good results for all powder sizes tested. Various parameters were tested to achieve optimum, adequate and less preferable results, as tabulated on Table 4 below.

TABLE 4

The appropriate stand-off distance, that is the distance between the nozzle exit of the plasma arc sprayer and the surface being sprayed, depends on the equipment being used, the nature of the substrate being sprayed, and the desired effect. Using the plasma jet sprayer described above, it was found that suitable results were achieved with a stand-off distance of from 1 /2 to 1 inch when applying ceramic coatings to metal, refractory or aggregate surfaces. When applying a ceramic coating to a metal mesh substrate, larger stand-off distances were preferable, up to 2 to 3 inches, in order to avoid unduly distorting or blowing holes through the metal mesh with the high temperature plasma jet. An appropriate stand-off distance for a given set of circumstances and sprayer equipment can be determined by trial and error.

Thus, a shorter stand-off distance would be appropriate for coating high- temperature resistant materials such as refractory materials, aggregates or metal plates. Even with these types of materials, a shorter distance would provide more melting or softening of the substrate to enhance the bond of the ceramic coating on the substrate. Shorter stand-off distances also tend to provide a denser, more compact coating, where the particulate material is fully molten when impacting the surface of the substrate.

On the other hand, a longer stand-off distance would be called for with lower melting temperature or more fragile substrates, such as a metal mesh material. Porosity of the resulting ceramic coated or solid material can be enhanced by increasing the stand-off distance, whereby the molten particulate material may be partially solidified before impacting the surface of the substrate. The following powders were used to prepare the filters in the following examples:

1 : 1 ratio of -106 +90 micron ceramic and -150 micron clay -75 +63 micron ceramic 10 micron ceramic • 1 micron ceramic 0.5 micron ceramic

The 60 and 100 mesh screens were placed on a tubular fixture adapted to draw an induced draft through the mesh screens while being sprayed. The screens were fixed to one end, and an electric fan to the opposite end of the fixture. Means were provided to inject cooling air into the fixture just before the fan to cool the hot gas and avoid damage to the fan. In those instances where an induced draft was not used, the filter frame assemblies were simply held in a V-clip at an appropriate distance from the sprayer.

The screens were sprayed in the following sequence:

A screen was placed into the screen holder. The fan and air to the fan cooling system was turned on prior to spraying. 1 :1 ratio of -106 +90 micron ceramic and -150 micron clay was sprayed first. From one to four coats were sprayed onto one side, the screen was turned and from one to four coats were applied on the opposite side.

10 micron ceramic was next applied. From one to three coats were applied to one side, the screen was turned and from one to three coats were applied to the opposite side. 1 micron ceramic was next applied. From one to four coats were sprayed on one side only.

0.5 micron ceramic was next applied. From one to four coats were sprayed on one side only, on top of the 1 micron ceramic layer(s).

The 51 micron screens were placed in the same fixture and an induced draft was applied to the screen prior to and during spraying. The screens were sprayed with the following sequence:

10 micron ceramic was first applied. From one to three coats were applied to one side, the screen was turned and from one to three coats were applied to the opposite side. 1 micron ceramic was next applied. From one to four coats were sprayed on one side only. 0.5 micron ceramic was next applied. From one to four coats were sprayed on one side only, on top of the 1 micron ceramic layer(s).

The 60 and 100 mesh screens were fixed to a stand and sprayed

(without induced draft) in the following sequence:

1 :1 ratio of -106 +90 micron ceramic and -150 micron clay was sprayed first. From one to four coats were sprayed onto one side, the screen was turned and from one to four coats were applied on the opposite side. 10 micron ceramic was first applied. From one to three coats were applied to one side, the screen was turned and from one to three coats were applied to the opposite side. 1 micron ceramic was next applied. From one to four coats were sprayed on one side only. 0.5 micron ceramic was next applied. From one to four coats were sprayed on one side only, on top of the 1 micron ceramic layer(s).

The 51 micron screens were fixed to a stand and sprayed (without induced draft) with the following sequence:

10 micron ceramic was first applied. From one to three coats were applied to one side, the screen was turned and from one to three coats were applied to the opposite side.

1 micron ceramic was next applied. From one to four coats were sprayed on one side only. 0,5 micron ceramic was next applied. From one to four coats were sprayed on one side only, on top of the 1 micron ceramic layer(s).

Example 5

The 60 mesh (250 micron) stainless steel screen (Part No. 34) was placed in the induced draft fixture and each side sprayed with one coat of 1 :1 by volume ratio of -106+90 ceramic powder and -150 micron clay. One coat of 10 micron ceramic powder was sprayed on top of the 1 : 1 layer. The filtering membrane was made by applying four coats of one micron ceramic powder on one side and 4 coats of 0.5 micron ceramic powder to the 1 micron surface. When both sides were examined at 40 x magnification, uniform coverage and little difference in structure on the membrane side and the opposite side was observed. Example 6

The 100 mesh (150 micron) stainless steel screen (Part No. 8) was placed in the induced draft fixture and each side sprayed with one coat of 1 :1 by volume ratio of -106 +90 ceramic powder and -150 micron clay. One coat of 10 micron ceramic powder was sprayed on top of the 1 :1 layer. The filtering membrane was made by applying three coats of one micron ceramic powder on one side and two coats of 0.5 micron ceramic powder to the 1 micron surface.

When both sides were examined at 40 x magnification, uniform coverage and little difference in structure on the membrane side and the opposite side was observed.

Example 7

The 51 micron stainless steel screen (Part No. 44) was placed in the induced draft fixture and each side sprayed with one coat of 10 micron ceramic powder. The filtering membrane was made by applying two coats of 0.5 micron ceramic powder to the one micron surface. When both sides were examined at 30 x magnification, uniform coverage and little difference in structure on the membrane side and the opposite side was observed.

At 40 x magnification, deposited particles on the screen surface and imbedded finer particles between the screen openings were observed on the membrane side, while the opposite side showed uniform complete coverage without evidence of the screen.

The membrane side of part No. 44 at 100 x magnification showed deposited particles on the screen surface and imbedded finer particles between the screen openings. Some voids, black holes on the photomicrograph, gave the appearance that these spaces between the wire mesh were not completely filled with finer powder. However, using back lighting techniques, no points of light were noted, indicating there is not a clear path from one side of the filter to the opposite side.

Example 8

The 60 mesh (250 micron opening) stainless steel screen (Part #33) was placed in a "V" holder and each side was sprayed with one coat of 1 :1 by volume ratio of -106+90 ceramic powder and -150 micron clay. Both sides were sprayed with one coat of 10-micron ceramic powder. The filtering membrane was made by applying four coats of one micron ceramic powder on one side of the screen and four coats of 0.5 micron ceramic powder to the one micron surface. At 30 x magnification, the membrane side and the opposite side showed uniform coverage and little difference in structure. The coatings were not as dense as part No. 34. Part No. 33 at 40 x magnification also showed uniform coverage and little difference in structure on the membrane side and the opposite side. The coatings were not as dense as part No. 34.

Example 9

The 100 mesh (150 micron opening) stainless steel screen (Part No. 6) was placed in a "V" holder and each side sprayed with one coat of 1 :1 by volume ratio of -106+90 ceramic powder and -150 micron clay. One coat of 10 micron ceramic powder was sprayed on top of the 1 :1 layer. The filtering membrane was made by applying three coats of one micron ceramic powder on one side and two coats of 0.5 micron ceramic powder to the one micron surface. At 30 x magnification, the membrane side and the opposite side showed uniform coverage and little difference in structure. The coatings were not as dense as part No. 8.

Part No.6 at 40 x magnification also showed uniform coverage and little difference in structure on the membrane side. The coatings were not as dense as part No. 8.

Example 10

The 51 micron stainless steel screen (Part No. 43) was placed in a "V" holder and each side sprayed with one coat of 10 micron ceramic powder. The filtering membrane was made by applying one coat of one micron ceramic powder to one side and two coats of 0.5 micron ceramic powder to the 1 -micron surface. At 30 x magnification, the membrane side and the opposite side showed uniform coverage and little difference in structure. A considerable amount of mesh was left exposed. Part No. 43 was not as dense as part No.44.

Part No.43 at 40 x magnification also showed uniform coverage and little difference in structure on the membrane side and the opposite side. The coatings were not as dense as part No. 44.

Example 11

The splash screen (approximately 20 mesh (850 micron)) was placed in a vice and each side sprayed with three coats of 1 :1 by volume ratio of - 106+90 micron ceramic powder and -150 micron clay. The first coat was applied at a 45° angle from left to right. The gun was rotated, and the second coat was applied at a 45° angle from right to left. The final coat was applied at 90° to the surface. Three coats of -75+63 micron ceramic powder were sprayed on each side using the 45⁰-45°-90° technique. Using back lighting techniques, the screen showed several points of light, Two additional coats of -75+63 micron ceramic powder were applied at 90° on both surfaces. Three coats of 10 micron ceramic powder were applied at 90° to each side of the screen, The filtering membrane was made by applying one coat of one-micron ceramic powder at 90° on one side and three coats of 0.5 micron ceramic powder to the one-micron surface. The first coat of 0.5 micron ceramic was applied at 90° to the screen surface. The screen was rotated 90° and two additional coats were applied at 90° to the screen surface. After the final spraying, the only points of light noted were near the area where the screen was secured for spraying. At 30 x magnification, the membrane side showed uniform coverage. The opposite side of the splash screen showed a coarser surface structure than the membrane side. However, at 100 x magnification, the surface showed that the major holes had been filled with fine particles.

Example 12

Testing Results (General macro and microscopic examination results)

Table 5

In order to test the uniformity of the coating, a drop of water applied to the ceramic membrane side of the 51 micron opening, 250 micron opening and 150 micron opening screens. This produced a nearly identical water mark on each side of the respective screens. This test reveals that there was uniform wicking from the membrane side of the filter to the opposite side, indicating uniform porosity. The splash screen produced a smaller water mark on the opposite side.

Example 13

Preparation of Ceramic Coated Refractory Materials This example demonstrates that refractory coatings can be made by plasma spraying sized particles onto a refractory surface. Using radioactive ceramic phase or slag material for refractory lining in nuclear applications will eliminate the need and expense for the disposal of radioactive slags. Since spent or damaged linings can be recycled, this concept creates a closed- loop system in which radioactive feeds are converted to useful end products. The following powders were used to spray the refractory bricks:

• 1 :1 ratio of -106 +90 micron ceramic and -150 micron Clay

• -150 micron clay

The study involved the use of two Al A refractory bricks. The two refractory bricks were grit blasted using a 60 grit Al A abrasive. The refractory bricks were placed on a stand and sprayed in the following sequence: A 1 : 1 ratio of -106 +90 micron ceramic and -150 micron clay was sprayed using a back and forth, top to bottom motion. One side of each brick was sprayed until a substantial layer was achieved (up to 1 /2 inch).

The 150 micron clay was next sprayed, using a back and forth, top to bottom motion. One side of each brick was sprayed until a substantial layer was achieved. Visual examination of the bricks shows excellent coverage and adherence on both surfaces. After applying the 1 :1 ratio of -106+90 micron ceramic powder and -150 micron clay coating, the bricks were allowed to cool. No spalling was noted during cool down. The opposite side was plasma sprayed with -150 micron clay powder until a substantial layer was produced. During the second spraying process, both bricks retained a considerable amount of heat (temperature profiles were not taken), During the heat up and cool down, no spalling was noted.

Macroscopic examination of the 1 :1 ratio material showed uniform coverage and a porous structure. Macroscopic examination of the -150 micron clay showed uniform coverage and a denser structure than the 1 :1 ratio coating. A vitrified layer appears to have formed at the interface of the brick and sprayed material.

The test data demonstrates that crushed and sized ceramic-phase material and crushed and sized raw clay can be successfully plasma sprayed. Visual, macroscopic and microscopic examination shows that the coatings are uniformly applied and have good adherence.

Example 14

Preparation of Ceramic Coated Metallic Materials

This example demonstrates that refractory coatings can be made by plasma spraying sized particles onto a metal surface. Using radioactive ceramic phase or slag material for metal linings in nuclear applications will eliminate the need and expense for the disposal of radioactive slags. Since spent or damaged linings can be recycled, this concept creates a closed- loop system in which radioactive feeds are converted to useful end products.

The following powders were used to spray the metal plates. • 1 :1 ratio of -106 +90 micron ceramic and -150 micron clay

• -150 micron clay

The study involved spraying carbon and stainless steel test coupons. The two test coupons were grit blasted using a 60 grit Al₂0₃ abrasive, placed on a stand and sprayed in the following sequence:

A 1 :1 ratio of -106 +90 micron ceramic and -150 micron clay was first applied using a back and forth, top to bottom motion. One side of a steel plate was sprayed until a substantial layer was achieved. Next the -150 micron clay was sprayed, using a back and forth, top to bottom motion. One side of steel plate was sprayed until a substantial layer was achieved.

Visual examination of the steel plates showed excellent coverage and adherence. After applying the 1 :1 ratio of -106+90 micron ceramic powder and -150 micron clay coating, or the -150 micron clay powder, the steel plates were allowed to cool. No spalling was noted during cool down. Although back cooling was introduced during the spraying process, the steel plates retained a considerable amount of heat (temperature profiles were not taken). During the heat up and cool down, no spalling was noted.

Macroscopic examination of the 1 :1 ratio material showed uniform coverage and a porous structure. Macroscopic examination of the -150 micron clay shows uniform coverage and a denser structure than the 1 :1 ratio coating. A mechanical and metallurgical bond appears to have formed at the interface of the brick and sprayed material. Additional characterization will be performed at a later date.

The test data demonstrates that crushed and sized ceramic-phase material and crushed and sized raw clay can be successfully plasma sprayed. Visual, macroscopic and microscopic examination shows that the coatings are uniformly applied and have good adherence. The steel plates showed excellent coverage and adherence for both the 1 :1 ratio of -106+90 micron ceramic powder and -150 micron clay coating, The 1 :1 ratio material showed uniform coverage and a porous structure and the -150 micron clay showed uniform coverage and a denser structure than the 1 :1 ratio coating,

Example 15

Plasma Spraying of Spiked Sample of Tennessee Red Clay

Dry Tennessee Red Clay (TRC) was crushed in a ball mill and classified using a sieve shaker into two cuts, a coarse cut with a particle size of -60+100 mesh (-250+150 micron) and a fine cut with a particle size of -100+140 mesh(-150+106 micron), Four samples were spiked with pesticides and trace metals as follows:

Table ό

Material Amount Spiked ug/Kg

Ag 1250

As 1250

Ba 25000

Cd 250

Cr 1250

Hg 50

Pb 1250

Endrin 2350

Lindane 2370

Heptachlor 2370

Heptachlor 2380

Epoxide

Methoxychlor 2340

The spiked TRC samples were plasma sprayed onto four Al₂0₃ refractory brick samples. Each sample after coating with the spiked material was tested in accordance with the Toxicity Characteristics Leaching Procedure (TCLP) by extraction procedure SW-846-1311. In carrying out the extraction procedure, each sample was placed in an aqueous sodium hydroxide/acetic acid buffer, having a pH of about 5.5, in an amount of 2 liters per 100 grams of coating on the sample. This buffer solution is intended to simulate rainwater in a landfill. The sample and buffer were subjected to agitation for 24 hours and the aqueous solution was analyzed for the concentration of the pesticide and trace metals which leached out of the sample. The resulting concentrations were compared against the Universal Treatment Standard (UTS) limits published at 40 CFR 268, as reported in the following table.

Table 7

This table demonstrates that the process of the invention does result in a significant reduction and/or encapsulation of the spiked material. All metal constituents analyzed were below UTS limits. The organic constituents analyzed were also found to be below characteristic limits. This material therefore would not be characterized as hazardous by the EPA.

Example 16

Preparation of Filter on Porous Aggregate Support

A full size cleanable production filter was prepared and tested. A substrate comprising a 17.5" x 17.5" x 0.5" porous concrete slab was plasma sprayed in accordance with the invention. A layer of -60+100 mesh Tennessee Red Clay material (see Example 15 prior to spiking) was first applied by plasma spraying to the substrate, followed by an overlay layer application by plasma spraying of a -100+140 mesh TRC material. Because of the large and uneven porosity of the particular concrete slab used, both sides of the substrate were coated. With other substrates, preferably only the membrane side of the filter is coated with the porous plasma sprayed layers. The filter was tested prior to an actual run and its flow characteristics were appropriate, and no points of light could be seen upon visual inspection, indicating that there were no open pores, i.e., all air would have to follow a tortuous path. The filter was used in a cleanable filter production test in RPU-3 DOE Mixed Waste Demonstration Phase I Campaign, Run No RPU-3-96009, which involved injecting a chlorinated hydrocarbon into a molten metal bath, resulting is a gas stream with carbon dust and other particulates. The dust iadened gas was passed through a gas handling train (GHT) which comprised, from the reactor, a knockout pot, baghouse, the test filter, coarse filter, scrubber, HEPA's and finally the stack

In carrying out the test, the test filter was inserted into the housing and loaded with dust during operation. The filter was removed and cleaned using a HEPA vacuum. The filter was reinstalled and the operation repeated.

The test filter was installed in place of a disposable prefilter between the baghouse and the coarse filter. Projecting from the prior run No. 96008 using the prefilter (which the test filter replaced), it was expected that 21 disposable prefilters would have been required. Instead, only a single cleanable test filter was needed, which was still suitable for continuing use. Also projecting from prior run No. 96008 using the prefilter, it was projected that 80 coarse filters (following the prefilter) would be required. However, because of the high efficiency of the test cleanable filter, only 47 coarse filters were required.

A second run (run No. 96010) using the same test prefilter was carried out under the same conditions and procedures as run No. 96009. Again, only the same single cleanable test filter was needed, and it was still suitable for continuing use after the two runs. Again, projecting from prior run No. 96008 using the prefilter, it was projected that 80 coarse filters (following the prefilter) would be required. However, because of the high efficiency of the test cleanable filter, only 42 coarse filters were required for this second run.

The cost savings in replacement prefilters and coarse filters resulting from use of the cleanable test filter in these two runs is substantial. The single cleanable test filter took the place of 42 disposable prefilters projected from prior run No. 96008, and the need for coarse filters was reduced from a projected 160 filters down to a total of 89 filters, a savings of 71 coarse filters over run Nos. 96009 and 96010. Further significant cost savings are realized in reduced hazardous material transportation and disposal costs for the used disposable prefilters and coarse filters. Moreover, if the cleanable test filter is damaged, it can be crushed, sized and resprayed in accordance with to invention to make, e.g., further cleanable filters.

Claims

What is claimed is:

1. A method for preparing a ceramic-containing article from a ceramic phase residue formed as a by-product of a treatment process producing a molten ceramic slag, said method comprising the steps of: a) processing said ceramic phase residue into a solid particulate material having a size and uniformity suitable for feeding to a plasma arc sprayer; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle; c) injecting said solid particulate material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon a surface axially spaced from said nozzle while said ceramic spray is still at least partially molten so as to deposit at least one layer of ceramic material on said surface, which thereupon solidifies to form said article.

2. The method of claim 1 wherein said particulate material is substantially fully melted in step b).

3. The method of claim 1 or 2 wherein at least steps c) and d) are carried out in an enclosed environment adapted for the confinement and collection of hazardous or toxic materials vaporized from said solid particulate material when subjected to the high temperatures of said hot plasma jet.

4. The method of claim 1 wherein said treatment process is selected from the group consisting of a ) α high temperature thermal treatment of waste materials containing toxic, radioactive or other hazardous substances wherein inorganic components form a molten ceramic phase; b) a molten metal catalytic process for the treatment of waste materials containing toxic, radioactive or other hazardous substances wherein inorganic components form a molten ceramic phase; and c) a metal refining process producing a molten ceramic phase slag.

5. The method of claim 1 wherein said solid particulate material is formed by a process selected from the group consisting of a) processing said ceramic phase residue in solid form by crushing and classification, and b) processing said ceramic phase residue in molten form to produce molten droplets which are thereafter solidified.

6. The method of claim 5 wherein said particulate material is formed by processing said ceramic phase residue in molten form by injecting or aspirating said molten ceramic phase into a quenching gas.

7. The method of claim 1 wherein said solid particulate material has a maximum nominal particle size of approximately 250 microns.

8. The method of claim 7 wherein said solid particulate material is uniformly sized.

9. The method of claim 7 wherein said solid particulate material has a nominal particle size range of between about 0.1 and 250 microns.

10. The method of claim 1 wherein said hot plasma jet is formed by an arc drawn between a cathode and an anode of a nontransferred arc plasma jet sprayer and exits said sprayer through a nozzle.

11. The method of claim 10 wherein said solid particulate material is injected into said hot plasma jet before exiting said nozzle.

12. The method of claim 10 wherein said nozzle provides at least a portion of said anode.

13. The method of claim 10 wherein said solid particulate material is injected into said hot plasma jet after said jet exits the nozzle at a point adjacent to said nozzle.

14. The method of claim 10 wherein the solid particulate material is substantially fully melted in said plasma arc, and said nozzle is axially spaced from said surface such that the molten ceramic spray is still molten when impinging upon said surface.

15. The method of claim 10 wherein said nozzle is axially spaced from said surface such that the molten ceramic spray is only partially molten when impinging upon said surface.

16. The method of claim 1 wherein said surface comprises a removable mold upon or into which said ceramic-containing article is formed by deposition of multiple layers of said ceramic material.

17. The method of claim 16 wherein said surface is selected from the group consisting of graphite, an organic polymeric material, and a metallic material.

18. The method of claim 16 wherein said ceramic-containing article is a filter.

19. The method of claim 18 wherein the porosity of said filter is controlled, at least in part, by the fineness of said molten spray and the axial distance of the nozzle from the said surface.

20. The method of claim 19 wherein the fineness of said molten spray is controlled, at least in part, by the particle size of said solid particulate material injected into said plasma jet.

21. The method of claim 19 wherein the axial distance of said nozzle from said surface is such that the molten ceramic spray is only partially molten when impinging upon said surface.

22. The method of claim 21 wherein the porosity of the filter is made to vary between a first layer and a subsequent layer of said multiple layers by changing the fineness of said molten spray.

23. The method of claim 1 wherein said surface is a metallic material, and said metallic material is coated with at least one layer of said ceramic material.

24. The method of claim 23 wherein the exit of nozzle is axially spaced from said metallic material such that the heat of the molten spray partially melts the metallic material when impinging upon said surface.

25. The method of claim 23 wherein said metallic material is selected from the group consisting of iron, zinc, nickel, copper, tungsten, aluminum, lead, magnesium, molybdenum, platinum, tin, titanium and uranium, and any mixture or alloy thereof.

26. The method of claim 23 wherein said metallic material forms a drum container.

27. The method of claim 1 wherein said surface is a refractory material, whereby said refractory material is coated with at least one layer of said ceramic material.

28. The method of claim 27 wherein the exit of said nozzle is axially spaced from said refractory material such that the heat of the molten spray partially melts the refractory material when impinging upon said surface.

29. A method for the preparation of a filter element comprising a ceramic coating on a metal screen substrate, said method comprising the steps of: a) securing said metal screen substrate in a frame, said screen having α pore size of between about 50 and 850 microns; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle axially spaced from said screen; c) injecting a solid particulate ceramic material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon said metal screen while said ceramic spray is still at least partially molten so as to deposit at least one layer of ceramic material on said screen, which thereupon solidifies to form said filter element.

30. The method of claim 29 wherein the metal of said screen is selected from the group consisting of stainless steel, molybdenum and tungsten.

31. The method of claim 29 wherein said solid particulate material is uniformly sized and has a maximum nominal particle size of between about 0.1 and 250 microns.

32. The method of claim 31 wherein multiple layers of said ceramic material are deposited on said screen, at least one layer being a base coat sprayed from a particulate material having a nominal size of between about 10 and 250 microns, and at least one subsequently sprayed layer being a membrane coat sprayed from a particulate material having a smaller nominal size of between about 0.1 and 20 microns.

33. The method of claim 32 wherein at least one of said multiple layers is at least one intermediate coat deposited after said base coat and before said membrane coat sprayed from a particulate material having a nominal size of between about 5 and 70 microns and intermediate in size to said base coat and said membrane coat.

34. The method of claim 31, 32 or 33 wherein at least one layer is applied to both sides of said screen.

35. The method of claim 29 wherein said particulate material is obtained from a ceramic phase residue formed as a by-product of a treatment process producing a molten ceramic slag.

36. The method of claim 35 wherein said treatment process is selected from the group consisting of ) α high temperature thermal treatment of waste materials containing toxic, radioactive or other hazardous substances wherein inorganic components form a molten ceramic phase; b) a molten metal catalytic process for the treatment of waste materials containing toxic, radioactive or other hazardous substances wherein inorganic components form a molten ceramic phase; and c) a metal refining process producing a molten ceramic phase slag.

37. The method of claim 35 wherein said solid particulate material is formed by a process selected from the group consisting of a) processing said ceramic phase residue in solid form by crushing and classification, and b) processing said ceramic phase residue in molten form to produce molten droplets which are thereafter solidified.

38 . The method of claim 37 wherein said particulate material is formed by processing said ceramic phase residue in molten form by injecting or aspirating said molten ceramic phase into a quenching gas.

39. A method for the preparation of a porous ceramic filter element from a particulate ceramic material, said method comprising the steps of: α) providing α removable mold having a surface defining the desired shape of said filter element; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle axially spaced from the surface of said moid; c) injecting a solid particulate ceramic material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon said surface while said ceramic spray is still at least partially molten so as to deposit multiple layers of ceramic material in said mold, which thereupon solidifies to form said filter element.

40. The method of claim 39 wherein said solid particulate material is uniformly sized and has a nominal particle size of between about 0.1 and 250 microns.

41. The method of claim 39 wherein at least one of said multiple layers is a base coat sprayed from a particulate material having a nominal size of between about 10 and 250 microns, and at least one of said multiple layers is a membrane coat sprayed from a particulate material having a smaller nominal size of between about 0.1 and 20 microns.

42. The method of claim 41 wherein at least one of said multiple layers is at least one intermediate coat between said base coat and said membrane coat sprayed from a particulate material having a nominal size of between about 5 and 70 microns and intermediate in size to said base coat and said membrane coat.

43. The method of claim 39 wherein said particulate material is obtained from a ceramic phase residue formed as a by-product of a treatment process producing a molten ceramic slag.

44. The method of claim 43 wherein treatment process is selected from the group consisting of a ) α high temperature thermal treatment of waste materials containing toxic, radioactive or other hazardous substances wherein inorganic components form a molten ceramic phase; b) a molten metal catalytic process for the treatment of waste materials containing toxic, radioactive or other hazardous substances wherein inorganic components form a molten ceramic phase; and c) a metal refining process producing a molten ceramic phase slag.

45. The method of claim 43 wherein said solid particulate material is formed by a process selected from the group consisting of a) processing said ceramic phase residue in solid form by crushing and classification, and b) processing said ceramic phase residue in molten form to produce molten droplets which are thereafter solidified.

46. The method of claim 45 wherein said particulate material is formed by processing said ceramic phase residue in molten form by injecting or aspirating said molten ceramic phase into a quenching gas.

47. A method for preparing a particulate ceramic product from a ceramic phase residue formed as a by-product of a treatment process producing a molten ceramic slag, said method comprising the steps of: a) processing said ceramic phase residue into a solid particulate material having a size and uniformity suitable for feeding to a plasma arc sprayer; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle; c) injecting said solid particulate material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a spray of molten ceramic particles; and d) quenching said molten ceramic spray in a quenching gas so as to individually solidify said molten ceramic particles without aggregation thereof in a molten state.

48. The method of claim 47 wherein at least steps c) and d) are carried out in an enclosed environment adapted for the confinement and collection of hazardous or toxic materials vaporized from said solid particulate material when subjected to the high temperatures of said hot plasma jet.

49. A filter element comprising a porous ceramic coating on a metal screen substrate, said screen having a pore size of between about 50 and 850 microns, and said coating having been applied by the steps of: a) securing said metal screen substrate in a frame; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle axially spaced from said screen; c) injecting a solid particulate ceramic material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon said metal screen while said ceramic spray is still at least partially molten so as to deposit at least one porous layer of ceramic material on said screen.

50. The filter element of claim 49 wherein the metal of said screen is selected from the group consisting of stainless steel, molybdenum and tungsten.

51. The filter element of claim 49 wherein said porous ceramic coating is comprised of multiple porous layers of ceramic material formed from spraying uniformly sized solid particulate material having a nominal particle size of between about 0.1 and 250 microns.

52. The filter element of claim 51 wherein said coating is comprised of multiple porous layers of ceramic material, at least one layer being a base coat sprayed from a particulate material having a nominal size of between about 10 and 250 microns, and at least one subsequently sprayed layer being a membrane coat sprayed from a particulate material having a smaller nominal size of between about 0.1 and 20 microns.

53. The filter element of claim 52 wherein at least one of said multiple layers is at least one intermediate coat deposited after said base coat and before said membrane coat, sprayed from a particulate material having a nominal size of between about 5 and 70 microns and intermediate in size to said base coat and said membrane coat.

54. A porous ceramic filter element comprising multiple layers of fused ceramic particles, said filter element having been made by the steps of: a) providing a removable mold having a surface defining the desired shape of said filter element; b) forming a hot plasma jet exiting a plasma arc sprayer through a nozzle axially spaced from the surface of said mold; c) injecting a solid particulate ceramic material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon said surface while said ceramic spray is still at least partially molten so as to deposit multiple porous layers of ceramic material in said mold, which thereupon solidifies to form said filter element.

55. The filter element of claim 54 wherein said porous layers of ceramic material have been formed by spraying a uniformly sized solid particulate material having a nominal particle size of between about 0.1 and 250 microns.

56. The filter element of claim 54 wherein at least one of said multiple porous ceramic layers is a base coat sprayed from a particulate material having a nominal size of between about 10 and 250 microns, and at least one of said multiple layers is a membrane coat sprayed from a particulate material having a smaller nominal size of between about 0.1 and 20 microns.

57. The filter element of claim 52 wherein at least one of said multiple porous ceramic layers is at least one intermediate coat between said base coat and said membrane coat, sprayed from a particulate material having a nominal size of between about 5 and 70 microns and intermediate in size to said base coat and said membrane coat.

58. A method for the preparation of a filter element comprising a ceramic coating on a porous substrate, said method comprising the steps of: a) placing said porous substrate in spaced relationship with the nozzle exit of a non-transferred plasma arc sprayer; b) forming a hot plasma jet exiting the nozzle of said plasma arc sprayer and directed toward said porous substrate; c) injecting a solid particulate ceramic material into said hot plasma jet under conditions such that the injected particulate material is at least partially melted in said plasma jet to form a molten ceramic spray; and d) causing said molten ceramic spray to impinge upon said porous substrate while said ceramic spray is still at least partially molten so as to deposit at least one porous layer of ceramic material on said substrate, which thereupon solidifies to form said filter element.

59. The method of claim 58 wherein said porous material is selected from the group consisting of a foamed ceramic material and a porous aggregate.

60. The method of claim 58 wherein said solid particulate material is uniformly sized and has a maximum nominal particle size of between about

0.1 and 250 microns.

61. The method of claim 60 wherein multiple layers of said ceramic material are deposited on said porous substrate, at least one layer being a base coat sprayed from a particulate material having a nominal size of between about 10 and 250 microns, and at least one subsequently sprayed layer being a membrane coat sprayed from a particulate material having a smaller nominal size of between about 0.1 and 20 microns.

62. The method of claim 61 wherein at least one of said multiple layers is at least one intermediate coat deposited after said base coat and before said membrane coat sprayed from a particulate material having a nominal size of between about 5 and 70 microns and intermediate in size to said base coat and said membrane coat.

63. The method of claim 60, 61 or 62 wherein at least one layer is applied to both sides of said porous substrate.

64. The method of claim 58 wherein said particulate material is obtained from a ceramic phase residue formed as a by-product of a treatment process producing a molten ceramic slag.

65. The method of claim 64 wherein said treatment process is selected from the group consisting of a ) α high temperature thermal treatment of waste materials containing toxic, radioactive or other hazardous substances wherein inorganic components form a molten ceramic phase; b) a molten metal catalytic process for the treatment of waste materials containing toxic, radioactive or other hazardous substances wherein inorganic components form a molten ceramic phase; and c) a metal refining process producing a molten ceramic phase slag.

66. The method of claim 64 wherein said solid particulate material is formed by a process selected from the group consisting of a) processing said ceramic phase residue in solid form by crushing and classification, and b) processing said ceramic phase residue in molten form to produce molten droplets which are thereafter solidified.

67. The method of claim 66 wherein said particulate material is formed by processing said ceramic phase residue in molten form by injecting or aspirating said molten ceramic phase into a quenching gas.

68. A method for the treatment of soil or other particulate inorganic- containing material contaminated with at least one toxic or hazardous substance, so as to reduce the content of said substance and to encapsulate any such substance remaining in said particulate material in a substantially non-leachable form, said process comprising: a) obtaining said contaminated particulate material in a form having a nominal particle size range suitable for feed to a non-transferred arc plasma sprayer; b) forming α hot non-trαnsferred plasma jet exiting a plasma arc sprayer through a nozzle; c) injecting said solid particulate material into said hot plasma jet under conditions such that the injected particulate material is heated to a molten state in said plasma jet and to a temperature sufficient to at least partially reduce the content of said substance in said particulate material; and d) causing said molten ceramic spray to solidify.

69. The method of claim 68 wherein said particulate material has a nominal particle size range of between about 0.1 and 250 microns.

70. The method of claim 68 wherein at least step c) is carried out in an enclosed environment adapted for the confinement and collection of hazardous or toxic materials vaporized from said solid particulate material when subjected to the high temperatures of said hot plasma jet.

71. The method of claim 68 wherein said molten ceramic spray is solidified by causing it to impinge upon a surface axially spaced from said nozzle while said ceramic spray is still at least partially molten so as to deposit at least one layer of ceramic material on said surface.

72. The method of claim 68 wherein said molten ceramic spray is directed into a molten bath which molten bath is thereafter caused to solidify.

73. The method of claim 68 wherein said molten ceramic spray is solidified by contacting said spray with a quenching fluid.