WO1988001532A1

WO1988001532A1 - Ceramic filter for filtering molten metal

Info

Publication number: WO1988001532A1
Application number: PCT/CH1987/000100
Authority: WO
Inventors: Jerry W. Brockmeyer; James E. Dore; Leonard S. Aubrey
Original assignee: Schweizerische Aluminium Ag
Priority date: 1986-08-28
Filing date: 1987-08-14
Publication date: 1988-03-10
Also published as: ZA876106B; CN87106034A; DK227988A; JPS63503289A; ES2005272A6; AU7784487A; DK227988D0; EP0280695A1; CN1011486B; AU598025B2; BR8707442A

Abstract

The ceramic filter has a plurality of passageways for the liquid metal and is coated with a thin layer of an inorganic material which can be easily wetted by the molten metal.

Description

Ceramic filter for the filtration of molten metal

The invention relates to ceramic filters for the filtration of molten metal from a bonded or sintered ceramic body containing a multiplicity of flow paths for the flow through the molten metal.

In general, molten metal contains penetrated solids which have a detrimental effect in the finished cast product. These penetrated solids appear as inclusions in the finished cast product after the molten metal has solidified and cause defects in the finished product. The inclusions can come from various sources, for example from superficial oxide films and insoluble impurities such as carbides, borides and others. Ceramic filters, as described in US Pat. Nos. 3,893,917, 4,081,371 and 3,962,081, are particularly suitable for the filtration of molten metal. These ceramic foam filters with an open cell structure with a large number of interconnected cavities, which are surrounded by a network of ceramic in order to ensure winding paths through them, are particularly suitable. These filters are particularly advantageous for various reasons, for example their excellent filter efficiency, low cost, ease of use and the possibility of using them on an exchange and disposable basis. The fact that the ceramic foam filters are particularly practical and inexpensive to manufacture and can be used on a disposable and disposable basis has led to widespread use in the field of molten metal treatment in recent years. However, particular problems arose with the filtration of molten metals such as steel, which have poor flow behavior. When filtering such materials, it is often difficult to start the flow of metal through the filter. This leads to the build-up of a high level of metal over the filter before the filter is wetted, ie before the metal begins to flow through the filter. Furthermore, it was found that often the molten metal does not penetrate the entire filter or that the filter is not flowed through in all areas. Furthermore, again in other cases, the filter does not wet at all and an excessively high metal protrusion forms over the filter. This problem is particularly important for steel, but it also exists with other metals, depending on the alloy and casting temperature.

One method to solve this problem was to overheat the alloy and thus achieve better flow behavior. It has been found that overheating the melt improves the fluidity, which in turn promotes wetting. However, this is not a completely satisfactory solution, since an excess of overheating often causes other problems, depending on the alloy in question. Overheating above normal levels does not always lead to a successful solution to the problem.

Ceramic foam filters coated with US Pat. No. 4,302,502 were also known, in which a proportionally large activation layer is provided for the removal of specific impurities from the molten metal. However, the teaching about these filters does not deal with the above-mentioned problem of the metal overhang. The object of the present invention is to avoid the disadvantage of the metal supernatant and, accordingly, to propose an improved ceramic filter for the filtration of molten metal which promotes the filtration of poorly flowable metal without building up an excessive metal supernatant. It is a further feature of the present invention to provide an improved filter of the type mentioned above, based on a bonded ceramic body which is cheap and quick to produce and which does not interfere with the filtration process or the properties of the finished casting ¬ rend influenced. Another object of the present invention is to provide an improved filter which avoids the need for excessive overheating in order to make the filter wettable.

According to the invention, this is done by means of a ceramic filter, characterized by a thin coating which extends completely through its expanses, made of an inorganic material which can be easily wetted by the molten metal and which has a thickness of 5 Å up to 1 micron adheres to this ceramic body, and the coating is removed on contact by the molten metal.

The present invention accordingly describes a highly effective ceramic filter for the filtration of molten metal, which overcomes the problem of the poor flow behavior of metals, such as steel, and shows favorable wetting properties, without the excessive build-up of a metal protrusion and without the need for a strong one Overheating the metal.

The ceramic filter of the present invention comprises a bonded or sintered ceramic filter with a plurality of flow paths for the flow through the molten metal with a thin coating, bonded to the ceramic body and extending essentially over the entire extent of the inorganic material which is wetted by the molten metal. The coating is removed by the flowing molten metal.

The ceramic filter is preferably a ceramic foam filter with an open-cell foam structure and a multiplicity of interconnected cavities which are surrounded by a network of this ceramic. The ceramic body preferably contains aluminum oxide as the ceramic material. The preferred thickness of the coating is 10 to 100 Å. The coating is uniform in its layer thickness and contains, preferably for the filtration of steel, silicon dioxide.

As stated above, it has been a problem in technology that metals with poor flow behavior, e.g. Steel, do not flow cleanly through ceramic filters. The problem is caused by the material of the ceramic filter that is not fully wetted by the molten metal, i.e. the molten metal does not flow or is not distributed on the surface of the filter material. For example, in the case of a ceramic filter containing aluminum oxide as the filter material, this material is not completely wetted by steel.

The ideal situation, a high filter efficiency, is achieved if the material of the base metal to be filtered does not properly wet the filter, but the inclusions that have to be removed are able to completely wet the filter material. For example, an aluminum oxide inclusion in steel completely wets a ceramic filter based on aluminum oxide, while steel only partially wets the filter. The inclusions therefore have the tendency to leave the metal stream and adhere to the filter. ten, while the base metal tends not to stay in contact with the filter or inclusions. It is also advantageous that the base metal does not wet the filter and thus causes less attack on the filter material. This behavior represents the desirable situation as soon as the flow through the filter begins, ie when the filter is already cast on. It is therefore desirable to be able to provide a filter that has these characteristics while being easily cast on. It is also desirable to be able to provide a filter that can be easily cast on by the molten metal without interfering with the advantageous properties of the filter or the filter effectiveness.

The ceramic body of the present invention is a bonded or sintered ceramic body with a large number of flow paths for the flow through the molten metal. All known ceramic bodies can be used, e.g. extruded ceramic bodies, bodies made of sintered ceramic particles or preferably ceramic foam filters with an open cell structure, characterized by a large number of interconnected cavities, which are connected by a network of ceramic, and produce tortuous flow paths. A particularly suitable ceramic foam filter is described in DE-OS 34 24 504.

According to the invention, a thin coating is applied to this ceramic body, this coating essentially extending through the entire body and the layer adhering to the body. The coating is an inorganic material which is completely wetted by the molten metal and has a thickness of 5 Å to 1 micron. In an expedient embodiment, the thickness of the coating is 10 to 100 Ä. This shows that the coating is very thin. During use of the filter, the coating is removed by contact with the molten metal, and after the start of the metal flow, the entire filter surface necessary for filter effectiveness is available. The coating is preferably completely removed in less than 5 seconds, either by dissolution by the molten metal or by mechanical action of the molten metal on the ceramic network. A coating is therefore preferred which is soluble or partially soluble in the molten metal. In order to achieve a suitable and rapid removal of the coating at the beginning of the metal flow, a coating of less than 100 Å is preferably used, although it is also possible. To use coatings of up to 1 micron, in particular if the mechanical action is used to remove the particular coating.

As described above, the coating should extend essentially through the entire ceramic body in a uniform layer thickness, although fluctuations in the layer thickness and uncoated sections are still tolerable. In order to promote wetting, however, the coating should extend over the entire network and should preferably be uniform.

The coating material should be a material that is not detrimental to the molten metal to be filtered or in the final cast product. Therefore, mechanical separation of the coating from the network is less desirable than solubility or partial solubility in the metal, since the mechanically separated coating parts can appear as inclusions in the final cast product. In addition, the coating material should not have any on the molten metal have a disadvantageous effect, which leads, for example, to reduced mechanical properties, increased grain size or undesirable second phases.

In a preferred embodiment, a colloidal silicon dioxide coating is used, particularly since it was found that this is particularly advantageous for steel filtration and that a suitable silicon dioxide coating can thus be achieved on the ceramic body. In view of the fact that thin coatings are used on the ceramic body, the silicon dioxide coating in the amounts used has no adverse effects on the steel to be filtered. Silicon dioxide could have a disadvantageous effect at most in large quantities, which are not the subject of the invention. The thin silicon dioxide coating according to the invention is also completely removed on contact with the molten steel.

In a preferred embodiment, an aqueous dispersion of a colloidal silicon dioxide is used, starting from a 0.5 to 10% concentrated colloidal silicon dioxide in water, preferably in a concentration of 1%. Additives can of course be used in the aqueous dispersion, such as wetting agents, dispersants or organic materials which support the tendency to coat, etc.

In accordance with the preferred embodiment, the aforementioned dispersion is produced and the ceramic body is immersed in the dispersion in order to achieve a thin coating thereon, essentially through the entire ceramic body. Excess material is removed, for example shaken off, the moist ceramic body is dried, for example after-fired at 93 to 149 ° C., preferably at 121 ° C., and at 982 to 1038 ° C. and preferably at 1000 ° C. The te The temperature treatment should be high enough that the silicon dioxide does not hydrate again and adheres firmly to the ceramic body. The drying and re-firing can take place in separate steps or continuously. It is not necessary to keep the temperature. The material coated in this way was found to be particularly advanced for steel filtration.

Of course, no coating will be used which requires such a high firing temperature from which damage to the respective ceramic body can occur.

It is also possible to use materials other than silicon dioxide, provided that this material is an inorganic material that is easily wetted by the molten material and removed in contact with the molten metal. Silicon dioxide is particularly preferred for steel filtration because it is extremely well wetted by steel and is easily detached from the molten metal. Other suitable additional inorganic materials that can be used include, for example, an aqueous silicate dispersion, such as sodium silicate, and salts, such as sodium chloride or calcium chloride, the latter salts simply being dried on the network instead of being fired again. It is also possible to use oxides, such as those of calcium or magnesium, which can then be dried and burned as salts, for example as carbonates. Metals such as nickel, iron and chromium can be used as organometallic solutions or dispersions for coating, followed by a drying process and a heat treatment to convert them to metal. Given the very small amount that is used for the coating, these metals are not troublesome. The effectiveness of the coating composition according to the invention is the use of an inorganic material in a thin coating which is easily wettable by the molten metal and easily removable under the action of the molten metal. Because the coating is slightly wetted by the metal, the metal easily passes through the ceramic body and also the tortuous flow paths in the ceramic body when the preferred ceramic foam material is used, and the casting time and the metal excess can be considerably reduced.

In addition, it was found that the coated composition does not merely reduce the casting time. Rather, it was found that the coated filters surprisingly bring about a faster flow of the filtering process, for example the total material flow through the ceramic body according to the present invention is generally faster than through an uncoated ceramic body. Surprisingly, filters according to the present invention retain a higher flow rate after casting than uncoated filters. This is despite the fact that the coating is removed on contact with the molten metal. This may be caused by the faster casting of the entire ceramic filter and the faster flow through all pores in the ceramic filter body. This also represents a progress worth considering.

The present invention is explained in more detail using the examples.

example 1

Bound ceramic filter body, 18.7 mm thick, with an open cell structure, characterized by a large number of interconnected cavities, surrounded by a A network of ceramics, the filter body being based on an aluminum oxide ceramic, is predetermined. These filters used had 10 pores per cm. Samples of the filters were coated in accordance with the present invention by immersing the filter bodies in an aqueous dispersion containing 1% colloidal silicon dioxide for a short period of time, which ensured complete impregnation, in order to obtain a thin coating, dried at 121 ° C and baked at 1000 ° C. The thickness of the coating after firing was around 10 Å and the coating was present in aggregates of silicon dioxide particles essentially distributed throughout the entire ceramic body. Various samples of the coated filters and an uncoated filter were fitted into hollow sprue tube pieces. Each of these sprue tube pieces was slowly immersed in a bath of steel deoxidized with aluminum and kept at a temperature of 1621 ° C. The start of the pouring is determined by visual observation through the pipe center. The result showed that the coated filter wetted to a depth of 5.08 cm immediately after immersion in the bath. The uncoated filter did not wet up to the immersion depth of 5.08 cm. The tube was further immersed to a depth of approximately 10.16 cm, the tube breaking due to the thermal shock. Filter wetting was never observed with uncoated filters.

Example 2

Coated and uncoated filters corresponding to essentially the composition according to Example 1 and with the coatings essentially produced according to Example 1 were provided.

1. Coated and uncoated filters 1.86 cm thick, were placed side by side in a pouring pan. Molten steel was directly from an induction oven poured into the casting crucible at a temperature of 1621 ° C. When the crucible is filled, the metallostatic pressure on the filter increases until the metal overhang reaches the height of the overflow. The time of filter wetting was determined by visual observation of the outflow side of the filter. The results showed that the coated filter wets 4 to 5 seconds earlier than an uncoated filter. The flow of the molten metal through the coated filter appeared to be very fluid, while the flow through the uncoated filter was very tough and slow in comparison. At the end of the racking, a total of 76.6 kg had passed the coated filter, while only 11.3 kg had passed through the uncoated filter. In addition, the resulting cast was of high quality. The coating on the filter had no disruptive influence on it. Furthermore, the silicon oxide coating was essentially removed by dissolving in the molten metal during contact with the molten metal.

Example 3

Coated and uncoated filters essentially in the composition according to Example 1 were produced according to Example 1, except that the filter body was characterized by 6 pores per cm. Four sand molds were prepared, three of which had a filter device. All four molds were cast directly from an induction furnace with 204 kg of a middle carbon steel deoxidized with aluminum. The results are shown in Table 1 below. 12 Table 1

Cast-Test-Cast No. conditions temperature results

coated 1621 ^β C filter wetted, filter clean casting surface no filter 1599 ° C Numerous slag inclusions on the casting surface uncoated. 1621 ° C filter failed

Filter when wetting coated 1595 ° C filter partially

Filter wetted

The above results clearly show that cast 1 has a totally inclusion-free cast surface when a coated filter is used, compared to cast 2, in which no filter was used and in which there was a considerable number of inclusions in the material Ren. Guss 3, which contained an uncoated filter, failed completely when the filter was wetted. Although the casting process was carried out at the same temperature as casting 1. Cast 4 used a coated filter that was partially wetted, even though the casting temperature was 26 ° C lower.

Claims

Expectations

1. Ceramic filter for the filtration of molten metal, comprising a bonded or sintered ceramic body containing a multiplicity of flow paths for the flow through the molten metal, characterized in that a thin coating completely penetrates through the extends over the entire length of the filter, made of an inorganic material that can be easily wetted by the molten metal, adheres to the ceramic body in a thickness of 5 Å to 1 micron and the coating is removed on contact by the molten metal.

2. Ceramic filter according to claim 1, characterized gekenn¬ characterized in that the ceramic body is a ceramic foam filter with an open-cell foam structure and a plurality of interconnected cavities, which are surrounded by a network of this ceramic.

3. Ceramic filter according to claim 1 and 2, characterized in that the coating is 10 to 100 Å thick.

4. Ceramic filter according to claims 1 to 3, characterized in that the coating is uniform in its layer thickness.

5. Ceramic filter according to claim 1 to 4, characterized ge indicates that the coating is a silicon dioxide coating.

6. Ceramic filter according to claim 1 to 5, characterized in that the ceramic body contains aluminum oxide.