Title of Invention

"A PROCESS FOR THE PRODUCTION OF CERAMIC FILTER USEFUL FOR MOLTEN METAL FILTRATION"

Abstract

in the present invention, a process has been developed for the production of high temperature ceramic filter used for molten metal filtration. In this process, a polyurathene foam is used as a preform material to get the shape with desired pores per inch. Aluminosilicate minarals and high alumina based ceramic compositions have been developed which were used as ceramic slurry to coat the foam walls by dipping process. A novel technique has been applied to remove the excess slurry by blowing of cold air followed by instant drying by hot air. The ceramic coated polyeurathene foam having tortuous path gives a rigid ceramic part on sintering at high tcmperatures. The product produced by the present process is a highly porous (72 - 75% Apparent porosity) continuous open-cell material with high permeability to fluids, possesses very low density (0.4 - 0.7 gins/cc), high thermal shock resistance and high temperature stability. The ceramic filter produced by the present invention can be used in metallurgical industries both in ferrous & non-ferrous sectors in the filtration of wide range of molten metal to remove foreign inclusions caused by oxides, dross, gas & slag.

Full Text

The present invention relates to a process for the production of ceramic filter useful for molten metal filtration. The high temperature ceramic filter produced by the present invention can be used in metallurgical industries both in ferrous and non-ferrous sectors in the filtration of wide range of molten metals. The modern technology both in ferrous and non-ferrous sectors demands for more cleaner metals and alloys which are free from any foreign particles caused by oxides, dross, gas and slag. For preventing inclusions of such kind, high temperature ceramic filters are used in the filtration of low temperature alloys such as aluminium, copper, bronze and the high temperature metals such as low carbon steel, stainless steel and super alloys. The present day method for the production of ceramic filter is the well known foaming method. Reference may be made to the work of W.P. Minnear, Ceramic Transactions, Vol. 26, 1992 of USA wherein a chomical mixture containing the desired ceramic component plus a variety of organic constituents is treated to evolve a gas which creates bubbles in the material and causes it to foam. The material after drying and firing resuh.s porous ceramic material. This f?aming technique is also described in 1973 patent by Sunderman et al. (U.S. Patent No. 3745201). In this approach preswelled ball-shape granules of clay are pressed in a mould which has the shape of the final pro(luct. the mould is then passed through a furnace at around 1000 0C in an oxidising environment. The heat and high pressure air first cause the clay balls to bind one another when sufficient heat is transferred throughout the clay particles, growth occurs and the mould is filled which finally resultant in ceramic foam. In this process clay used are mostly common clay. Propellants such as calcium carbide, calcium hydroxide, aluminium sulphate, hydrogen peroxide are used to presswell the clay granules. The above processes are cumbersome, lengthy and also environmentally hazard due to many steps i~d the use of harzardous propellants like hydrogen peroxide, calcium carbide etc. In another work of Wood, et. al, U.S. patent No. 3833386, 1974, a sponge polymer network is produced simultaneously with the foaming of a ceramic filled slurry. Reference may also be made to the. work of Lielferich, et. al. U.S. Patent No. 4871485, 1989 wherein a technique is described for producing a ceramic foam by reacting metal with alkali to produce hydrogen gas. This process utilised an alumino silicate hydrogel, a refractory ceramic material, a particulate metal, a surfactant, and a gel strengthening agent. The hydrogel used in this process is the sodium salt of silicate and aluminate which have detrimental effect for high temperature application. This process has also a disadvantage of producing hydrogen gas which requires special attention for handling. Besides above references, many other literatures have been cited which describes the sponge, foaming techniques utilising various types of ceramic ii~iterials and agents. The existing sponge and foaming method have the following disadvantages. (a) involvement of many cumbersome steps, (b) non-unifomi distribution of pores, (c) non-continuous open cell structure, (d) improper reticulated path, (d) inadequate permeability of fluids, (0 foaming technique is not suitable for varied high temperature ceramic composition, (g) yield moderate quality of cast parts, the) incapable of filtering large volume of high temperature alloys, (i) use of hazardous raw materials. Another reference may be made to the work of Alfred University College of Ceramic Research Foundation in the year 1989 where a open cell material of fully sintered ceramic is developed utilising polyurathene foam as a preformed material. In this method a ceramic slurry is prepared. The foam is dipped into the solution, drying after removing the excess slurry and final sintering of the product. This existing method of producing open cell structure by polyurethane foam has also the following disadvantages: a) clogging of pore due to presence of excess ceramic slurry in the structure, b) the above clogging of pores results inadequate permeability to fluids, c) non-uniformity in pore wall thickness, d) low productivity due to more processing time particularly during drying. The main object of the present invention is to provide a process for the production of high temperature ceramic filter useful for molten metal filtration which obviates the drawbacks as detailed above. Accordingly, the present invention provides a process for the production of ceramic filter useful for molten metal filtration characterized by (a) wet grinding and intimate mixing for a period of 8 to l0hrs of 70~85O/o wt0/o calcined alumina, 10-25 wt0/o alumino-silicate minerals, 5-10 wt0/o dopants, 0.05-0.1 wt0/o deflocculant with 40-60 ~O/o water of total mix to obtain a ceramic slurry (b) sieving the ceramic slurry through 325 mesh BS sieve, ( c) adjusting rheological properties such as specific gravity, viscosity of the slurry such as herein described; (d) dipping a substrate consisting of open-cell reticulated foam structure in the slurry to obtain a coated substrate; (e) removing excess slurry by blowing cold air;( f) drying the coated foam substrate such as herein described, and finally (g) sintering the product in a furnace at a peak temperature in the range of 1400-16000C for a soaking period of 2 to 3 hrs. In an embodiment of the present invention, calcined alumina used may contain minimum 99.9% A1203 with alpha content more than 95% and the average particle size of calcined alumina used is 6-8 p.m. In another embodiment of the present invention the alumino silicate minerals used may be selectedlraw materials such as mullite, kaolin, sillimanite, singly or in combination. In still another embodiment of the present invention, the dopants used may be such as Talc, MnO2, MgO, TiO2, Cr2O3 of chemical grade and may be added singly or in combination. In yet another embodiment of the present invention, defflocculants used may be selected from materials such as sodium-alginate, sodium hexa metaphosphate, sodium tripoly phosphate, sodium polyacrylate or mixture thereof in still another embodiment of the present invention the foam used as preform substrate may be selected from materials such as nylon, polyeurethene, polyethylene having 4 to 20 pores per inch. In still another embodiment of the present invention drying the coated foam substrate is done first with hot air, 20- 24 hrs in air followed by oven drying in the temperature range of 90 - I l0"C for 20-24 hrs. In another embodiment of the present invention the rate of heating was controlled by the schedule of room temperature to 5000C @ 10C per minute 'holding at 5000C for 1 hr 7 5000C - 12000C @ 30C per minute, 12000C - peak temperature @ 50C per minute. In the present invention novel ceramic slurries based on aluminosilicate minerals and high alumina have been developed which can withstand wide range of hot metals and alloys. These ceramic slurries with controlled rheological properties (specific gravity and viscosity of the slurry are coated on a sacrificial catalytic substrate (polyurathene foam) which have reticulated tort vs path and burns off during sintering process. After proper sintering, a rigid ceramic part is obtained that provides rigorous paths to remove impurities when molten metal is poured through it during the casting process. The product produced by the present process is a highly porous continuous open-cell material with high permeability to fluids, possesses very low density, high thermal shock resistance and high temperature stability. The novelty of the present invention resides in the novel ceramic slurry which is used to coat the open cell reticulated foam structure to obtain a novel high temperature ceramic filter useful for filtration of molten metal. The inventive step lies in coating the novel ceramic slurry on an open cell reticulated foam and removing the excess slurry by blowing cold air followed by instant drying by blowing hot air on to the slurry coated substrate resulting in a ceramic filter having no clogged pores and uniform pore wall thickness. In the present invention (i) the ceramic slurry is made up of finely divided ceramic particles, water and additives. The ceramic particles chosen depends upon the particular application and desired properties of the final product. For example for a niolten metal filter, the ceramic material chosen must be able to withstand the chemical attack of the metal and must have high temperature strength retaining capacity. (ii) Dipping of a properly selected polyurathane foam into the ceramic slurry.(iii) Removal of excess slurry by blowing of cold air. (iv) Instant drying of the coating by blowing hot air. The following examples are given by way of illustration and should not be construed to limit the scope of the present invention. C Example - I 850 gins of calcined Alumina, 100 gm of sillimanite sand , 50 gins of a mixture of MnO2 & TiO2 were wet ground and mixed in a pot mill with 550 cc of water for a duration of 20 hrs. The slurry thus obtained was passed through 325 mesh BS sieve and its specific gravity was adjusted to 1.75 gm/cc after addition of 0.1% of Na-hexa metaphosphate. The sluriy was coated on a polyurethene foam having 5 pores per inch by dipping method. The excess slurry was removed by blowing of cold air. The coated foam thus obtained was subjected to hot air drying followed by air drying for a duration of 24 hrs. The air dried foam was then dried slowly in oven at 9O'~C for a duration of 24 hrs. The dried sample was finally sintered in an electric furnace following the schedule of room temperature to 5000C ~ 10C per minute (holding at 5000C for I hr) 5000C - I 2000C ~ 30C per minute, 1 2000C - 1 6OO~'C @ 5~C per minute and then soaked at I 6004'C for a duration of 2 hrs. The product obtained had the following characteristics: Bulk density 0.70 gm/cc Porosity 75% Cold crushing strength :18 Kg/cm2 Modulus of rupture 23 Kg/cm2 Example - 2 750 gins of calcined alumina, 150 gnis of sillimanite sand, 100 gins of a mixture of Talc & MnO2 were wet ground and mixed in a pot mill with 600 cc of water for a duration of 18 hrs, The slurry thus obtained was passed through 325 mesh BS sieve & its specific gravity was adjusted to 1.65 gm/cc after addition of 0.1% of Na-hexa metaphosphate. The slurry was coated on a polyeurethene foam having 10 pores per inch by dipping method. The excess slurry was removed by blowing of cold air. The coated foam thus obtained was subjected to hot air drying followed by air drying for a duration of 24 hrs.. The air dried foam was then dried slowly in oven at 90'C for a duration of 24 hrs. The dried sample was finally sintered in an electric furnace following the schedule of room temperature to 50Oo'C @ 10C per minute (holding at 5000C for 1 hr) 5000C - 12000C @ 30C per minute, 12000C - 14750C @ 50C per minute and then soaked at 14750C for a duration of 2 hrs. The product obtained had the following characteristics: Bulk density . 0.40 gmlcc Porosity : 72% Cold crushing strength :10 KgIcm2 Modulus of rupture : 12 Kg/cm2 The main advantages of the present process are: 1. Adequate permeability of fluids due to non clogging of pores. 2. Increase in productivity due to faster drying by hot air blowing. 3. The present product when used for filtration reduces rework, improves yield and upgrades the quality of cast part. We Claim: 1. A process for the production of ceramic filter useful for molten metal filtration characterized by (a) wet grinding and intimate mixing for a period of 8 to l0hrs of 70~85O/o wt0/o calcined alumina, 10-25 wt0/o alumino-silicate minerals, 5-10 wt0/o dopants, 0.05-0.1 wt0/o deflocculant with 40-60 wt0/o water of total mix to obtain a ceramic slurry (b) sieving the ceramic slurry through 325 mesh BS sieve, ( c) adjusting rheological properties such as specific gravity, viscosity of the slurry such as herein described; (d) dipping a substrate consisting of open-cell reticulated foam structure in the slurry to obtain a coated substrate; (e) removing excess slurry by blowing cold air;( f) drying the coated foam substrate such as herein described, and finally (g) sintering the product in a furnace at a peak temperature in the range of 1400-16000C for a soaking period of 2 to 3 h rs. 2. A process as claimed in claim 1 wherein calcined alumina used contains minimum 99.00/a A1203 with alpha content more than 950/a and average particle size is in the range of 6 — 8 pm. 3. A process as claimed in claims 1 & 2 wherein alumino silicate minerals used are selected from raw materials mullite, kaolin, sillimanite, singly or in combination. 4. A process as claimed in claims 1 to 3 wherein the dopants used is Talc, MnO2, MgO, TiO2, Cr2O3 and added singly or in combination. 5. A process as claimed in claims 1 to 4 wherein the deflocculates used are selected from materials sodium alginate, sodium hexa meta phosphate, sodium polyacrylate, sodium tripoly phosphate or mixture thereof. 6. A process as claiemd in calims 1 to 5 wherine the foam used as preform substrate is selected from materials nylon, polyeurethane, polyethylene having 4 to 20 pores per inch. 7. A process as claimed in claims 1 to 6 wherein drying in step (f) is done first with hot air, 20-24 hrs followed by oven drying in the temperature range of 90-1100C for 20-24 hrs. 8. A process as claimed in claims 1 to 7 wherein the rate of heating for sintering is controlled by the schedule of room temperature to 5000C at the rate of 10C per minute, holding at 5000C for 1 hr, 5000C - 12000C at the rate of 30C per minute, 12000C — peak temperature at the rate of 50C perminute. 9. A process for the production of high temperature ceramic filter useful for molten metal filtration substantially as herein described with reference to examples.

Documents:

608-del-2000-abstract.pdf

608-del-2000-claims.pdf

608-del-2000-correspondence-other.pdf

608-del-2000-correspondence-po.pdf

608-del-2000-description (complete).pdf

608-del-2000-form-1.pdf

608-del-2000-form-19.pdf

608-del-2000-form-2.pdf

608-del-2000-form-3.pdf