Filtration assembly having integral heating means for maintaining the metallic material being filtered in the molten state

The filtration of molten metal is obtained by providing the filtration medium, which may, for example, be formed of a granulated material, with a conductive coating or conductive particles. Electrical energy is coupled across the filter to create a current flow through the conductive material. The magnitude of the current generates heat of a level sufficient to maintain the metallic material being filtered in the molten state as it passes through the filter medium. The granules may be covered with a conductive coating or conductive particles. In an alternative embodiment, the granules may be formed of a material, such as silicon carbide, which is capable of being inductively heating to maintain the metallic material being filtered in the molten state as it is passed through the filter medium.

FIELD OF THE INVENTION 
The present invention relates to filters for filtering metallic material 
and more particularly to a novel filter assembly having combined heating 
and filtering means which is especially advantageous for use in filtering 
impurities from metallic materials. 
BACKGROUND OF THE INVENTION 
The present invention is directed to a new method and apparatus for 
pre-heating the media employed for filtration of molten metals. It is 
necessary to pre-heat such media in order to prevent freezing of the 
molten metal filtrate upon initiation of the filtration process. A 
considerable amount of sensible heat is required when employing the 
present day filtration apparatus since most commercially available media 
have a high heat capacity. It is also particularly important to obtain 
maximum temperature uniformity during the pre-heating operation since 
local over-heating of the media significantly degrades the filtration 
operation and may also damage the media. 
The traditional method employed for pre-heating molten metal filter media 
is by direct use of a gas flame which directly impinges upon the media. In 
one arrangement, a burner utilizing a mixture of air/natural gas is 
temporarily and removably placed near the bottom of a fixture employed to 
contain the reticulated or extruded monolithic ceramic media. The air/gas 
mixture emitted by the burner is ignited to produce a flame which impinges 
upon the underside of the media. Pre-heating continues for a prescribed 
period of time, or until some minimum media surface temperature is 
reached. At that time, the flame is extinguished, the burner is removed 
and metal flow through the media is initiated. The flow through the filter 
media must begin quickly after removal of the burner from the fixture to 
avoid need for repeating the pre-heat process due to cooling of the media. 
Various gas-fired and electrical resistance heated hot air blowers have 
been used as an alternative to pre-heating the media by direct flame 
impingement. The expected benefits of the latter approach is to provide 
greater temperature uniformity and avoid local over-heating of the filter 
media. However, the limitations of excessive pre-heat time and low 
attainable temperature severely restricts the use of the forced hot air 
techniques. 
In addition, all of the aforementioned preheat methods require direct 
access to the filter media and are not amenable to isolated filtration 
processes such as those which may be employed in a vacuum furnace. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention overcomes the disadvantages of the prior art 
techniques by providing method and apparatus which is characterized by 
providing heat generation by direct resistance (Joule), electrical heating 
or induction heating of the filter media to obtain the desired 
pre-heating. In order to practice the direct resistance technique, the 
filter media is rendered electrically conductive by either: 
1. depositing or coating the media surface with a suitable conductive 
material; 
2. including an electrically conductive material in the ceramic slurry 
comprising the media. 
In order to practice the induction heating method, the filter media is 
rendered capable of being inductively heated by including either: 
1. a magnetically susceptible material in the ceramic slurry comprising the 
media; or 
2. inductively heating a silicon carbide media. 
In the direct resistance method, the media surface ma preferably be coated 
with a fugitive electrically conductive material. Suitable coating methods 
include electroless or electrolytic plating, vapor deposition, and vacuum 
impregnation. The resulting coating is preferably uniform and sufficient 
in thickness to provide the appropriate level of heat generation. 
The second case under the direct resistance method involves the inclusion 
of electrical conductive or susceptible particles in the filter 
construction itself. Acceptable materials comprising the particles 
included iron, tungsten, nickel, silicon carbide as well as other suitable 
materials. 
Finally, the entire filter media may be constituted from a susceptible 
material such as silicon carbide. 
Once a suitable media has been produced, heating can be accomplished by 
either direct resistance or induction. In the former case, the filter 
media holding fixture is provided with electrodes that contact the 
perimeter of the media. A current of suitable magnitude is then passed 
directly through the filter media to produce heating due to the I.sup.2 R 
losses. 
Employing the induction pre-heating method, an inductor coil is 
incorporated in the media holding fixture and is activated by a suitable 
power supply to inductively pre-heat the media. 
The pre-heat apparatus and method of the present invention provides 
distinct advantages over the state-of-the-art methods which include, among 
others, highly uniform pre-heating of the media; the capability of 
maintaining pre-heat temperatures for extended periods without 
over-heating; predictable and reproducible media pre-heat levels and the 
capability of providing pre-heating in an isolated media, such as a vacuum 
furnace. 
OBJECTS OF THE INVENTION AND BRIEF DESCRIPTION OF THE FIGURES 
It is therefore one object of the present invention to provide a media for 
filtering molten metallic materials in which the heating means forms an 
integral part of the filtration media. 
Still another object of the present invention is to provide a novel media 
for filtering molten metallic materials in which the pre-heating of the 
media is accomplished by direct resistance electrical heating. 
Still another object of the present invention is to provide a novel media 
for filtering molten metallic materials in which the pre-heating of the 
media is accomplished by inductive heating of the filter media. 
Still another object of the present invention is to provide a novel filter 
media of the type described in which suitable materials are incorporated 
into the filter media to achieve either direct resistance electrical 
heating or heating by induction. 
Still another object of the present invention is to provide a novel filter 
media of the character described herein in which the filter media is 
capable of being heated by induction.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF 
FIG. 1 shows the conventional apparatus 10 employed in the prior art for 
pre-heating a filter media. 
Apparatus 10 is comprised of a tundish basin 12 which may, for example, be 
of a substantially cylindrical shape and having an inwardly directed 
annular flange 12a for positioning and supporting disk-shaped filter media 
14. A conduit 16 coupled to a source of natural gas extends through the 
base 12b of basin 12 for coupling to burner 18. An air/natural gas mixture 
is emitted by burner 18. This mixture is ignited causing the flame 
developed thereby to heat filter 14. In order to be assured that the 
entire volume of the filter is heated, two or more burners 18 may be 
provided. Nevertheless, and whether one or more burners are employed, it 
is extremely difficult to obtain uniform heating of filter 14. In 
addition, when using a plurality of burners, the time required to remove 
all of the burners subjects the media to undue cooling, thus degrading the 
filtration operation. 
When filter 14 is elevated to the desired temperature, the flow of natural 
gas to conduit 16 is terminated, filter 14 is temporarily removed, burner 
18 is removed from basin 12 and filter 14 is replaced to initiate the 
filtration process. All of the above operations must be performed as 
quickly as possible in order to be assured that filter 14 is maintained at 
the desired pre-heat level and thereby to avoid the necessity for 
repeating the pre-heat operation, thus making the conventional technique a 
rather tedious and cumbersome undertaking. 
FIG. 2 shows one preferred embodiment of the present invention in which 
tundish basin 12 is fitted with a filter media 20 designed in accordance 
with the principles of the present invention. In addition, the basin is 
provided with a pair of electrodes 22 and 24 which are respectively 
connected to positive and negative terminals of a power source (not shown) 
by conductive leads 26 and 28, respectively. 
Pre-heating of filter 20 takes place by coupling electrical energy to 
electrodes 22 and 24 causing a current to flow through the electrical 
material which forms an integral part of filter media 24. The I.sup.2 R 
losses developed by the conductive material serves to heat the filter 
media in a substantially uniform manner. 
In order to provide more uniform heat through the filter media, the filter 
media may be placed in a tundish dish having a substantially rectangular 
shaped supporting flange and wherein the filter media 20' shown in FIG. 2b 
is provided with elongated electrodes 22', 24' extending the entire length 
of the short sides of filter 20'. Other techniques may be employed, such 
as providing a heavier concentration of conductive material in those 
regions displaced from the center region of the circular shaped filter 
media 20 shown in FIG. 2a to render the current flow and hence the heating 
of the entire filter media more uniform. 
In the embodiment shown in FIG. 3, the filter 20 is either formed of a 
ceramic slurry containing a magnetically susceptible material or the media 
may be formed of silicon carbide. An induction coil 30 may be embedded in 
or surround the external wall 12c of basin 12. Opposite ends of the 
inductive coil are connected to a suitable A/C current source 32 by 
conductive leads 34, 36. In the embodiment shown in FIG. 3, filter 20 is 
pre-heated by applying electrical energy in the form of an alternating 
current to induction coil 30. The induction coil sets up an alternating 
magnetic field which induces a current in the magnetically susceptible 
material in filter 20 to heat the material (or the silicon carbide) to the 
desired pre-heating level. The techniques employed in the arrangement 
shown in FIGS. 2 and 3 provide for uniform heating of the media and 
further do not require removal of the source of the pre-heat, as is the 
case with the prior art techniques shown in FIG. 1. 
The preferred mode of the present invention is believed to be the 
pre-heating of the filter media by direct electrical resistance heating. 
FIG. 4 shows a detailed view of a portion of the filter which may be 
employed in the embodiment of FIG. 2 which is comprised of reticulated 
foam substrate granules 38 each having an electrically conductive coating 
40. The reticulated or extruded monolithic ceramic media may, for example, 
be coated with nickel by electroless or vapor deposition means to achieve 
a coating thickness of the order of 0.22 inches. Electrodes are 
incorporated into the sealing surfaces of the media holding fixture 12a in 
order to establish electrical contact between the electrodes 22, 24 and 
the media. 
An electrical current in the range of from 5 to 100 amperes per cubic inch 
of media at a potential voltage of from 6 to 70 volts is passed through 
the media. When the desired pre-heat temperature is attained, the current 
flow is terminated and metal flow through the media is initiated. 
In the embodiment of FIG. 5, electrically conductive or magnetically 
susceptible material, in particulate form is introduced into the ceramic 
slurry comprising the media to provide conductive particles 42 upon the 
surfaces of the reticulated foam substrate granules 38. The resulting 
filter media is electrically coupled to the electrodes in a manner similar 
to that described hereinabove in connection with the embodiments of FIGS. 
1 and 4. 
Alternatively, in the case where the particles 42 introduced into the 
slurry are magnetically susceptible, the filter media is introduced into 
the apparatus of FIG. 3 to provide heating of the filter media by 
induction through coil 30. 
FIG. 6 shows a detailed view of a filter media comprised of silicon carbide 
granules 44. 
A filter media of the type shown in FIG. 6 is arranged within the apparatus 
of FIG. 3 and the filter is pre-heated by induction through the use of an 
induction coil 30 as shown in FIG. 3 and 6. The particles of the filter 
media are retained in a bonded state, through the employment of any 
suitable bonding material. 
The filter method and apparatus of the present invention provides a vastly 
improved filter structure for the reasons that media pre-heating is highly 
uniform, the media can be maintained at pre-heating temperatures for 
extended periods without over-heating; the media pre-heating level is 
predictable reproducible and pre-heating can be accomplished for isolated 
media for example, of the type employed in a vacuum furnace. 
A latitude of modification, change and substitution is intended in the 
foregoing disclosure, and in some instances, some features of the 
invention will be employed without a corresponding use of other features. 
Accordingly, it is appropriate that the appended claims be construed 
broadly and in a manner consistent with the spirit and scope of the 
invention here.