How to form silicon carbide ceramic foam filter

Impurities and contaminants such as oxides, borides, carbides and unwanted intermetallics in a melt can lead to objectionable surface defects and undesirable inclusions in the final cast product. These surface defects and/or inclusions may render the cast product commercially unacceptable and/or interfere with commercial machining and fabrication practices. To prevent such difficulties, most melts are filtered to remove any unwanted impurities and contaminants. Generally, melt filtration is accomplished by passing the melt through a filter medium having a desired permeability. Typically, the filter medium takes the form of a porous body such as a bed filter or an open cell porous foam structure; however, it is also known to use powdered materials such as alumina, silica or clay powders to filter some types of melts.

Bed filters typically consist of layers of different sized bed media placed within a molten metal conduit such as a transfer trough connecting a furnace and a casting mold. The type of bed media and the size of the bed media particles generally depend upon the type of impurities and contaminants to be removed from the melt. For example, it is known in the art to use materials such as alumina, silica, silicon carbide, chromite, fosterite, magnesia spinel, periclase, zirconia and coke as bed media.

The use of such bed filters, however, is not without problems. For example, the melt may have to be passed through a relatively long length of bed media to obtain effective filtration. If sufficiently long, the bed filter length could interfere with the ability to rapidly filter a given volume of molten metal. Other problems include the passage of too many solids, the strong tendency towards channeling which reduces filter efficiency, the changing of the pore size during use and the consequent reduction in molten metal flow caused thereby, the need to use a fluxing gas in conjunction with some bed filters to remove certain impurities, and the need to use special molten metal conduits to generate sufficient head pressure to obtain the desired molten metal flow through the filter.

In an attempt to avoid these problems, open cell ceramic foam filters have been used in lieu of bed filters. This type of filter generally comprises an open cell, hydrophillic, flexible organic foam impregnated with an aqueous ceramic slurry. The organic foam may be a cellulosic foam or a polymeric foam such as polyurethane foam. The filter is typically formed by immersing a pliant foam material in a ceramic slurry and repeatedly subjecting the foam material to compression and expansion to drain off the excess slurry material. After draining, the coated foam material is subjected to a drying operation followed by exposure to elevated temperatures to burn out or volatilize the flexible organic foam and sinter the ceramic coating. In forming these filters, it is important that a pliant material be used so that draining of the slurry can be effected by compression and expansion of the foam material. It is also important for the organic foam material to be of a type that burns out or volatilizes at a temperature below the firing temperature of the ceramic material.

InU.S.Pat. No. 4,007,923 toChina, a multi-stage technique for treating molten metals to remove solid and gaseous impurities is illustrated. In this technique, the molten metal flows through a series of successively arranged purification stages including a deslagging stage where the molten metal is filtered through a woven refractory filter, a fluxing stage, an adsorption stage where the molten metal is passed over a plurality of impurity-adsorbing refractory plates and a final filtration stage where the molten metal is filtered through a rigid porous refractory filter medium.

In addition to the foregoing filtration devices, it is known in the art to form filtering devices from foamed or porous silicon carbide materials. Such a filter may be formed by first preparing a mix containing silicon carbide grit, a resin binder and a pore forming material. After being formed into a desired shape, the mix is subjected to a heat treatment for setting the binder and carbonizing the resinous material.

While open pore structures have been used to remove impurities from molten metals, they too have problems. The effectiveness of these filters is related to the min. filter pore size which can be used and the operating conditions needed to obtain passage of the melt through the porous structure. As pore size decreases, the head of metal required to prime and sustain metal flow through these porous structures increases. For this type of filter to have commercial applicability, the head and other operating conditions must be reasonable. In the past, the use of some of these porous structures for filtering copper melts has been limited by relatively high priming head requirements and relatively low metal flow rates. Still other porous structures have had limited applicability because the smallest impurity which can be removed and still have reasonable operating conditions is about 200 µm. Impurities this size can cause significant defects in the final product. Accordingly, it is an object of the present invention to provide an improved molten metal filter and a method for forming the same.

It is a further object of the present invention to provide a filter as above which obtains high filtration efficiency.

It is a further object of the present invention to provide a filter as above which has particular utility in filtering copper, iron and related alloy melts.

These and other objects and advantages will become more apparent from the following description.

In accordance with the present invention, it has been found that the foregoing objects and advantages may be readily achieved. The present invention provides a highly efficient method of filtering molten metal, particularly copper and copper alloy melts, through a disposable high temperature resistant filter characterized by a rigid porous material having an open cell structure with a plurality of interconnected voids and a relatively thin, substantially uniform silicon carbide coating throughout. It has been discovered that materials having exposed silicon carbide particles have particular utility in the filtration of copper, iron and related alloy melts. While the mechanism by which the silicon carbide acts to remove the unwanted impurities and contaminants from the melt is not completely understood, it is known that the unwanted impurities and contaminants can be removed by forming members through which the melt flows or with which the melt comes into contact from the silicon carbide containing materials. The filters of the present invention take full advantage of this discovery.

The filters of the present invention exhibit improved strength properties such as reduced friability and improved thermal conductivity through the formation of the relatively thin, substantially uniform silicon carbide coating throughout the porous substrate material. The filters of the present invention are preferably formed by impregnating the rigid porous substrate material with a slurry having a viscosity in the range of about 1 to about 50 centipoise and containing silicon carbide having a maximum settling rate of about 0.1 mm/min. It has been found that a silicon carbide containing slurry having these characteristics drains well from the substrate material, thereby reducing the number of clogged pores, and forms the desired silicon carbide coating. In a preferred embodiment of the present invention, the silicon carbide slurry has a viscosity in the range of about 5 to about 30 centipoise. In addition to the silicon carbide, the slurry preferably contains an inactive carrying medium, a binding agent and a wetting agent. The impregnating step may comprise either immersion of the rigid substrate material in the slurry, pouring of the slurry through the rigid substrate material or another suitable slurry application technique. After draining of any excess slurry, the impregnated rigid substrate material is furnace dried to substantially prevent foaming of the slurry coating material during firing and fired to bond the silicon carbide particles to the substrate material. Unlike other filter fabrication techniques, the filter forming process of the present invention does not require the use of pliant porous substrate materials and the burning out or volatilization of the underlying substrate material.

In accordance with the present invention, it has been found that it is possible to prepare low cost porous filtration media for molten metal using underlying porous substrate materials having larger pore sizes than can ordinarily be used. The ability to use larger pore size porous materials is due to the interactive nature of the silicon carbide coating applied to the substrate material. As a result, the filters of the present invention are less susceptible to clogging by non-metallic particulate and can be used with practical priming heads. Without the silicon carbon coating, the pore size required for effective filtration would be too small due to impractical priming heads. Since the filters of the present invention are extremely inexpensive to prepare, it is quite feasible to use these filters on a throw away basis.