High-temperature ceramic filters, their manufacture and use

Abstract: A high-temperature ceramic filter having a three- dimensional reticulated skeleton structure is prepared by applying a ceramic slurry to a synthetic resin foam having an open cell three-dimensional reticulated skeleton structure, followed by drying and firing. The ceramic slurry contains 20-40 parts by weight of aluminum titanate, 40-60 parts by weight of mullite, 2-20 parts by weight of alumina, and agalmatolite. The filter is useful for the filtration of molten metals.

Description: This invention relates to ceramic filters for use in the filtration of molten metals, typically molten stainless steel alloy, molten steel and molten cast steel. Molten metals generally contain impurities in the form of inclusions which have not been removed during refinement. In order to obtain high-grade metal stocks intended for thin plate manufacture, thin wire drawing, and precision casting, it is necessary to remove such inclusions, typically by passing the molten metals through filters. Known filters for the filtration of molten metals are ceramic foams having internal open cells. Such conventional ceramic filters, however, have many problems in use for the filtration of molten stainless steel alloy, molten steel and molten cast steel and few filters heretofore available are satisfactory for practical use. This is because the filters are subject to very rigorous conditions in terms of temperature in that the molten metal to be filtered is as hot as about 1,500°C or higher, which requires a strict choice of the material and structure capable of withstanding thermal shocks generated at the instant when the hot molten metal reaches the filter as well as damages and wear due to collision with the hot molten metal. There are known filters formed of oxide and non-oxide system ceramics. One known example of non-oxide system ceramics is a filter based on silicon carbide as disclosed in Japanese Patent Application Kokai (JP-A) No. 197829/1985. This filter is resistant against thermal shocks, but its use in the ambient atmosphere is limited to 1,450°C above which silicon carbide decomposes. The filter is thus compatible with molten cast steel, but not with the casting of stainless steel alloy which requires the filter to withstand a temperature of at least 1,500°C, sometimes at least 1,600°C during casting. In addition, long-term operation is not recommended since silicon carbide can react with iron at such elevated temperatures. As the oxide system ceramics, JP-A 137073/1991 discloses a ceramic composition based on zirconia. This ceramic undergoes crystallographic transformation with a temperature change due to the presence of monoclinic system zirconia, generating cracks. The ceramic also has a high coefficient of thermal expansion due to the co-presence of isometric system crystals so that its spalling resistance is not good. To prohibit cracking due to crystallographic transformation, it was proposed to add alumina to zirconia as disclosed in JP-A 141666/1986. This composition has a high coefficient of thermal expansion due to the presence of alumina, and cannot accommodate the filtration of molten metal at 1,500°C or higher as required for stainless steel alloy casting. Also, JP-A 278235/1987 discloses a silica/alumina/ zirconia system, which will fail upon instantaneous casting of molten metal at 1,500°C or higher since it has a high coefficient of thermal expansion and cannot tolerate thermal shocks. Thermal shock resistance or the ability to withstand heat of molten metal is correlated with not only heat resistance, but also a coefficient of thermal expansion as expressed by the following formula: R = S(1 - ν)λ/Eα wherein R is the coefficient of thermal shock resistance, S is strength, α is the coefficient of thermal expansion, ν is Poisson's ratio, E is Young's modulus, and λ is heat transfer rate. Therefore, materials having a low coefficient of thermal expansion must be used where thermal shock resistance is required. When hot molten metal is cast on the above-mentioned exemplary materials at room temperature, they cannot tolerate such a great temperature difference and fail because they have a great coefficient of thermal expansion irrespective of sufficient heat resistance. So, there were proposed systems using cordierite or aluminum titanate having a low coefficient of thermal expansion. Although cordierite itself has a coefficient of thermal expansion of 2x10 -6 /°C at 1,000°C which is as low as about 1/4 or 1/5 of those of the above-mentioned zirconia and alumina and is thus useful as a thermal shock resistant material, the ceramic filter based on cordierite is practically limited to molten metals at temperatures below 1,300°C because cordierite has a relatively low melting point of 1,450°C. The aluminum titanate base ceramic is disclosed in JP-A 74981/1988. According to this disclosure, a powder raw material and a blowing agent are concurrently blended and expanded and fired to form a porous body. Since the blowing agent in the form of an organic material is blended in an amount as large as about 40% by weight, the ceramic forming the skeleton after firing has an inhomogeneous structure rather thana dense structure. Since it is difficult to control the cells or network formed by blowing, there is obtained a porous body across which a considerable pressure loss occurs. We previously proposed in JP-A 56553/1994 a ceramic filter having a three-dimensional reticulated structure of aluminum titanate/mullite/alumina. Although this ceramic filter is advantageously used at high temperatures of 1,500°C or higher, it is limited in use because aluminum titanate which is one of filter-forming components is expensive. There is a need for ceramic filters of higher strength enough to accommodate for large scale casting. A general aim herein is to provide new and useful ceramic filters, as well as methods for their manufacture and use. Preferred aims include the provision of an inexpensive high-temperature ceramic filter which has heat resistance, low thermal expansion, thermal shock resistance against molten metals above 1,500°C such as molten stainless steel alloy, strength, minimized clogging, and a low pressure loss, and is thus effective for the filtration of molten metals. Making investigations on a three-dimensional reticulated ceramic foam having open cells which is prepared by applying a ceramic slurry to a substrate in the form of a synthetic resin foam; removing excess slurry, drying and firing the coated substrate, the inventors have found that the above object can be attained by using a ceramic slurry comprising 20 to 40 parts by weight of aluminum titanate, 40 to 60 parts by weight of mullite and 2 to 20 parts by weight of alumina, and agalmatolite. As is well known, a three-dimensional reticulated ceramic foam having open cells is preparable by applying a ceramic slurry to a substrate in the form of a synthetic resin foam, removing excess slurry, drying and firing the coated substrate. In order that the ceramic foam have a sufficient strength to withstand filtration of molten metal, a large quantity of the ceramic slurry must be applied to the synthetic resin foam. Thick coating often leads to clogging of open cells. Quite unexpectedly, when the ceramic slurry is of a system comprising aluminum titanate, mullite, alumina, and agalmatolite in a blending proportion as defined above, and preferably when agalmatolite is contained in an amount of 0.13 to 1.0 mol per mol of alumina, this ceramic slurry has good slurry properties so that there may be obtained a clogging-free ceramic filter e.g. having a bulk specific gravity of 0.5 to 1 and 4 to 30 pores per 2.5cm. We were able to make filters with low pressure loss and good filtering performance. As agalmatolite thermally decomposes during firing, silica resulting from agalmatolite will react with alumina to form mullite. This leads to quite unexpected results. First, the fired product has a lower coefficient of thermal expansion than expected from simple blending of these ceramic components in the same blending proportion. Secondly, since the proportion of mullite is increased, heat resistance is enhanced. Since the mullite created during firing is acicular grains, strength is enhanced due to their tanglement in the structure. The low coefficient of thermal expansion, combined with enhanced heat resistance and strength, permits the ceramic material to withstand thermal shocks by molten metals at about 1,500°C or higher, inter alia molten stainless steel alloy at about 1,600°C. In addition, the ceramic material is resistant against attack by molten metals. The amount of expensive aluminum titanate used is reduced. Then this ceramic filter is advantageously used for the filtration of molten metals. Accordingly, the present invention provides ceramic filter having a three-dimensional reticulated skeleton structure, preparable by applying a ceramic slurry to a foam, e.g. synthetic resin, having an open cell three-dimensional reticulated skeleton structure, drying and firing the coated substrate, characterized in that the ceramic slurry is comprised of 20 to 40 parts by weight of aluminum titanate, 40 to 60 parts by weight of mullite, 2 to 20 parts by weight of alumina, and agalmatolite. Other aspects are the preparation method, and use of the filters. DETAILED DESCRIPTION The high-temperature ceramic filter of the invention is formed from a ceramic composition of an aluminium titanate-mullite-alumina-agalmatolite system. Aluminum titanate has higher heat resistance and a lower thermal expansion than cordierite. Aluminum titanate is blended in an amount of about 20 to 40 parts, preferably about 25 to 35 parts by weight per 100 parts by weight of the ceramic composition. Less than 20 parts by weight of aluminum titanate provides less contribution to the lowering of a coefficient of thermal expansion. More than 40 parts by weight of aluminum titanate will add to the cost and can invite a lowering of strength due to the crystallographic transformation of aluminum titanate during firing. The aluminum titanate is preferably in powder form having a mean particle size of about 1 to 10 µm. Preferably it contains iron, e.g. as an iron compound such as iron oxide, in an amount of 1 to 4% by weight calculated as iron element. Without an iron compound added, aluminum titanate has the crystallographic behavior that intragranular cracks may develop on cooling after heating for firing. Such cracks may cause a substantial lowering of mechanical strength or a failure. Addition of an iron compound is effective for preventing this behavior. Although the exact mechanism is unknown, the iron compound is effective for preventing cracks from generating due to crystallographic transformation. Mullite is relatively low in thermal expansion and is used for the purpose of stabilizing a ceramic slurry and the heat resistance thereof. Mullite is blended in an amount of about 40 to 60 parts, preferably about 45 to 55 parts by weight per 100 parts by weight of the ceramic composition. On this basis, less than 40 parts of mullite will detract from heat resistance stability whereas more than 60 parts of mullite will render a ceramic composition difficult to fire, resulting in low strength. Alumina is added as a firing binder and for synthesizing mullite and effective for improving the heat resistance and strength of the resulting ceramic. Alumina is blended in an amount of about 2 to 20 parts, preferably about 4 to 12 parts by weight per 100 parts by weight of the ceramic composition. Less than 2 parts of alumina is less effective as a firing binder, resulting in a fired ceramic material having low strength. More than 20 parts of alumina contributes to higher strength, but provides a too higher coefficient of thermal expansion, detracting from thermal shock resistance. According to the present invention, agalmatolite is blended with aluminum titanate, mullite, and alumina. Agalmatolite is an essential component for forming mullite by bonding with alumina during firing. It is composed mainly of Al 2 O 3 ·4SiO 2 ·H 2 O. Agalmatolite may be synthesized from alumina and silica or commercially available agalmatolite may be used. Agalmatolite pyrolyzes into silica and alumina midway of firing, and the alumina is entirely consumed to form mullite (3Al 2 O 3 ·2SiO 2 ). The ceramic lowers its coefficient of thermal expansion in accordance with formation of mullite. The excess silica vitrifies to exert a binder function, contributing to strength. The agalmatolite is added in a quantity determinable to produce such an effect, and preferably in an amount of 0.13 to 1.0 mol per mol of alumina. On this basis, less than 0.13 mol of agalmatolite means that alumina is excessive and a fully low coefficient of thermal expansion would not sometimes be expected. More than 1.0 mol of agalmatolite means that excess silica is formed in more amounts so that the ceramic would be vulnerable to attack by molten metals. The ceramic components in the slurry may consist essentially of said components. Mullite, alumina, and agalmatolite mentioned above preferably have a mean particle size of about 1 to 30 µm. The resulting ceramic filter typically has a bulk specific gravity of 0.4 to 1.0, preferably 0.6 to 0.8, and contains 4 to 30 pores per linear 2.5 cm, preferably 6 to 13 pores per linear 2.5 cm, giving suitable properties for the filtration of molten metal. A bulk specific gravity of less than 0.4 usually gives too thin a skeleton to tolerate the pressure of molten metal. A bulk specific gravity of more than 1.0 may lead to frequent clogging and hence a higher pressure loss, which requires a longer filtration time or invites less smooth outflow of molten metal. A filter containing less than 4 pores per 2.5 cm is often insufficient to remove inclusions from molten metal whereas a filter containing more than 30 pores per 2.5 cm tends to give a higher pressure loss, which requires a longer filtration time or invites less smooth outflow of molten metal. The ceramic filter generally has a coefficient of thermal expansion of up to 3.5x10 -6 /°C. A coefficient of thermal expansion above this limit usually leads to lower thermal shock resistance. The high-temperature ceramic filter may be prepared by a conventional method involving the steps of starting with a synthetic resin foam as a substrate, applying a slurry of the above-defined ceramic composition to the substrate, removing excess slurry, drying and firing the coated substrate. The preferred synthetic resin foam is a flexible reticulated (cell membrane-free) polyurethane foam. The slurry is prepared by dispersing the ceramic components in water together with a binder such as polyvinyl alcohol and carboxymethylcellulose (CMC) and a deflocculant such as quebrachio. Since the ceramic composition contains aluminum titanate, mullite, alumina, and agalmatolite with the alumina serving as a firing binder, the firing temperature is as high as about 1,650°C to about 1,750°C. In general, firing at 1,600°C or higher requires a furnace whose structure is significantly different from ordinary firing furnaces designed for lower than 1,600°C and thus leads to cost increases including both installation investment and maintenance cost. It is thus recommended to use a firing aid, for example, oxides of alkali and alkaline earth metals. The metal oxide firing aid is preferably added in an amount of 0.05 to 3 parts by weight per 100 parts by weight of the ceramic composition whereby the firing temperature can be lowered by about 30 to 100°C. Less than 0.05 parts of the firing aid may be ineffective. More than 3 parts of the firing aid can further lower the firing temperature, but may promote alumina grain growth or act as a flux component to vitrify alumina, adversely affecting a coefficient of thermal expansion and strength. The slurry has a viscosity which may be adjusted by the amount of water in accordance with the desired cell size and bulk specific gravity of a ceramic porous body although the viscosity is generally in the range of 150 to 250 poise, especially 170 to 200 poise. In the slurry is immersed a synthetic resin foam, for example, a polyurethane foam of a three-dimensional reticulated (cell membrane-free network) skeleton structure having 4 to 30 cells per linear 2.5 cm. After application of the slurry, excess slurry is removed from the foam by any suitable method such as roll pressing, centrifugal separation and air blowing. If desired, the slurry is diluted and applied to the once slurry applied substrate e.g. by spraying. The slurry-loaded foam is then dried and fired at a temperature of about 1,600 to 1,750°C. Firing can be done at about 1,500 to 1,720°C when a firing aid as mentioned above is additionally blended. To protect the ceramic filter from damage , the filter should have a thick and rounded skeleton from a structural aspect. To this end, the bulk specific gravity should be increased. In preparing a ceramic filter from a system using a synthetic resin foam as a substrate, there is a tendency for an increase in bulk specific gravity to be accompanied by clogging. As proposed in our JP-A 56553/1994, this problem is overcome by replacing mullite by electrically fused mullite; clogging can then be prevented so that a pressure loss is reduced without detracting from other physical properties. However, use of electrically fused mullite would add to the cost. We have now found that for providing thick skeletons, it is effective to apply, e.g. by spray coating, further slurry which is as defined herein, and desirably the same as for the first application, but more dilute than the first application. More particularly, the slurry is applied to a foam such that the slurry applied foam after removal of excess slurry may have a bulk specific gravity which is about 20% lower than the bulk specific gravity as fired. Thereafter the diluted slurry is spray coated to the slurry applied foam to make up the shortage of weight. This enables a thick-skeleton ceramic filter to be obtained after firing. EXAMPLE Examples of the present invention are given below by way of illustration. All parts are by weight. Example 1 To 30 parts of aluminum titanate powder, 51 parts of mullite, 6 parts of alumina, and 13 parts (0.16 mol per mol of alumina) of agalmatolite were added 4 parts of polyvinyl alcohol, 0.1 part of quebrachio, 0.05 part of calcium carbonate and 19 parts of water. There was obtained a ceramic slurry having a viscosity of 160 poise. In the slurry was immersed a polyurethane foam having a three-dimensional reticulated (cell membrane-free network) skeleton structure dimensioned 7.5 x 7.5 x 2 cm and containing 6 cells per 2.5 cm. Excess slurry was removed off by means of a roll. The slurry squeezed out was diluted to 30 poise and spray coated to the foam such that the coverage weight by spraying might be 10% of the coverage weight by immersion. The foam was dried for 6 hours at 60°C. The foam was then fired at 1,600°C, obtaining a ceramic filter of 6.7 x 6.7 x 1.9 cm. The ceramic filter was measured for bulk specific gravity, a coefficient of thermal expansion (µ) over 25 to 1,000°C, and a pressure loss by suction at 20 m/s. The filter was also tested for resistance against thermal shocks on water immersion. The thermal shock resistance was tested by maintaining filter samples at predetermined temperatures of 600°C, 800°C and 1,000°C for 20 minutes, dropping the samples into water at room temperature, taking out and drying the samples, cutting the samples to a width of 35 mm, and bending the specimens at a span of 60 mm to measure flexural strength. The results are shown in Table 1. Examples 2-4 & Comparative Example 1 Ceramic filters were prepared by the same procedure as in Example 1 except that the amounts of aluminum titanate, mullite, alumina, and agalmatolite were changed as shown in Table 1. Physical properties were similarly measured. The results are also shown in Table 1. Example Comparative Example 1 2 3 1 Composition (pbw) Aluminum titanate 30 30 20 30 Mullite 51 50 60 50 Alumina 6 13.7 4.8 20 Agalmatolite 13 6.3 15.2 - Agalmatolite/alumina (molar ratio) 0.61 0.13 0.90 - Bulk specific gravity 0.62 0.61 0.58 0.61 µ (10 -6 /°C) 2.8 3.1 2.9 3.9 Pressure loss (mmAq) 38 40 37 43 Thermal shock resistance (kg/cm 2 ) ΔT°C 0 20 18 16 17 600 17 17 15 18 800 20 19 13 16 1000 16 18 15 7 Softening temp. (°C) 1560 1580 1520 1540 The high-temperature ceramic filters embodied here showed improved heat resistance, a low coefficient of thermal expansion, improved thermal shock resistance and a low pressure loss. Such filters are thus advantageously used for the filtration of molten metal, especially molten stainless steel alloy, molten steel and molten cast steel. Although some specific embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that the invention may be practised otherwise than as specifically described in the Examples.

Claims: A ceramic filter having a three-dimensional reticulated skeleton structure, obtainable by applying a ceramic slurry to a foam having an open cell three-dimensional reticulated skeleton structure, drying and firing; the ceramic slurry comprising 20 to 40 parts by weight of aluminium titanate, 40 to 60 parts by weight of mullite, 2 to 20 parts by weight of alumina, and agalmatolite. A ceramic filter according to claim 1 wherein the agalmatolite is mineral or synthetic agalmatolite composed mainly of Al 2 O 3 .4SiO 2 .H 2 O. A filter according to claim 1 or claim 2, for which the ceramic slurry contains 0.13 to 1.0 mol of agalmatolite per mol of alumina. A filter according to any one of the preceding claims wherein the aluminium titanate contains from 1 to 4 wt% of iron, determined as the element. A filter according to any one of the any preceding claims having a coefficient of thermal expansion of up to 3.5x10 -6 /°C. A filter according to any one of the preceding claims having a bulk specific gravity in the range from 0.40 to 1.0 and containing from 4 to 30 pores per linear 2.5 cm. A filter according to any one of the preceding claims in the preparation of which a first said ceramic slurry is applied to the foam, any excess of the first slurry is removed, and a second, more dilute said ceramic slurry is then applied to the foam. A method of making a ceramic filter comprising applying a slurry containing the following ceramic components: 20 to 40 parts by weight aluminium titanate; 40 to 60 parts by weight mullite; 2 to 20 parts by weight aluminium; and agalmatolite, to a foam having a three-dimensional open-cell reticulated structure, drying and firing. A method according to claim 8 in which a first said slurry is applied to the foam, any excess first slurry is removed and a second said slurry, more dilute than the first, is applied to the foam. Use of a filter as defined in any one of claims 1 to 7 for the filtration of molten metal. Use according to claim 10 in which the molten metal is stainless steel alloy, steel or cast steel.

Inventors: ODAKA FUMIO, TANUMA EIGO, ODAKA, FUMIO, TANUMA, EIGO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: C04B35
IPC codes: B22C9, C04B38, C04B35, B01D39