DD296476B5 - PROCESS FOR THE PRODUCTION OF SIC 'FORM BODIES - Google Patents

Description

Field of application of the invention

The invention relates to a process for the production of porous Siliziumcarbidformkörpern for the filtration of molten metals.

Characteristic of the known technical solutions

Silicon carbide moldings having a suitable pore structure are used as filter elements for cleaning metal melts because of their high chemical and thermal resistance. Various methods are known for producing such porous SiC bodies.

After a process principle, an open-cell plastic sponge is impregnated with a slurry of very fine-grained SiC and a ceramic binder and annealed after drying at temperatures around 1275 K. The plastic is thermally decomposed and the SiC slip is present as a solid body with the pore structure of the original plastic sponge.

However, in such SiC filters is disadvantageous that they easily break due to the small wall thicknesses, which can lead to serious damage in the filtration downstream processing equipment but at least undesirable contamination of the metal. Furthermore, the filtering action of foamed ceramics is limited to coarser particles in the melt.

The service life and the throughput of liquid metal is relatively low in such filters.

To make matters worse, that the production of such filters required foam polyurethane produced in the thermal decomposition pyrolysis products harmful to the environment, which must be disposed of by means of complex downstream cleaning equipment.

Much better results are achieved with a likewise commercially available filter element, in which corundum or silicon carbide grains are bridged via oxide-ceramic or species-specific binders. The primary grain used is compact and of rounded form, with the most common spatial axis ratios in the combination range of 1: 1: 1 (Ku'gel grain) to 1: 2: 3 (oval grain). These filter elements have an increased service life and a throughput that is 3 to 10 times higher.

However, it is felt to be very disadvantageous that the binder bridges are very brittle due to material differences between the grain and the binder which, inter alia, have a different thermal expansion behavior. The structural analysis of the material cut revealed that the binder bridges of microcracks were already traversed on the unused filter. These microcracks are the cause of the brittleness and thus of the observed grain breakouts. This can have a devastating effect on the filtration downstream processing facilities and cause rejects. The inevitably occurring with the use of the filter elements chemical changes in the binder bridges by the media and temperature influences reinforce this negative effect. It comes to the crystallization of the glass phase and increased cracking of the binder and thus further framework weakening.

Although the use of tabular SiC crystals described in DE-OS 3603331 improves the strength properties gradually, but can not affect the actual weak points-which weakened by microcracks binder bridges-not in principle.

Object of the invention

The object of the invention is to provide a ceramic binder system for the production of porous SiC moldings, which ensures a high strength of the bond over a long period of time at high temperatures.

Essence of the invention

The invention has for its object to provide a solid ceramic bond between SiC grains by impregnation of the Kornhaufwerkes with a solution containing glass forming substances.

According to the invention the object is achieved in that in the aqueous solution solids such as boron carbide, silicon carbide and Silicon, with a grain size of max. 80pm are suspended, the boron carbide to the sum of silicon carbide and silicon in

a molar ratio of 1: 5 to 1:15 and the silicon carbide to silicon in a molar ratio of 8: 1 to 12: 1.

The proportion of suspended solids in the solution is 5% to 20%. It has been found that the boron carbide forms with prolonged use at application temperature boron trioxide and due Media and temperature influence onset weakening, z. B. by crystallization and cracking, by Counteracts new glass formation. The addition of silicon carbide and silicon in specified proportions related to

different reaction rate to form the oxidation product silica, stabilizes the flow of the

Aging compensation. A special SiC grain having an open microporosity of 10% by volume was also used for the production of the shaped body.

to 20% by volume.

The pore volume of this microporous special grain acts on the one hand as a reservoir for the glaze-forming binder solution,

which proved to be extremely reinforcing for the binder bridges.

A second more significant effect was probably due to the increase in surface roughness and the increased Formation of filtration-active centers on the grain surface due to the microporosity causes, which surprisingly to Deposition of solid particles resulted, the size of which was only a fraction of the diameter of the filter pores. In order to achieve a comparable effect, it has hitherto been necessary to use the filter in a complicated process

ceramic microfibers to coat.

This qualitative and quantitative filtration-improving effect of the original microporous grain was unexpected, as the Micropores are closed by the glaze formed and are primarily no longer involved in the filtration process

can and could only be observed in the stated pore volume range of 10% by volume to 20% by volume.

embodiment

4000 g of a grain size of prismatically broken SiC, taken from a sieve fraction of 3.15 mm to 6.00 mm, with an average grain density of 2.5 g / cm ³ on average and the most common spatial axis ratios of 1: 1.1: 2.2 to 1 : 3.2: 5.6 and an open microporosity of 16Vol .-%, was 1930g with an aqueous suspension containing the water-soluble salts of

18.13 g K ₂ O

7.65 g Na ₂ O 13.38 g CaO

9.75 g MgO 40.96 g Al ₂ O ₃ 292.18 g SiO ₂ (in colloidal form)

and the solids of

72.00g B ₄ C (max grain size 75pm) 12.00g Si (max grain size 40pm) 138.00g SiC (max grain size 30pm)

contained, mixed. The proportion of solids in the suspension was thus 11.5% and the B ₄ C was the summary share

Si and SiC in a molar ratio of 1: 5.4, wherein the molar ratio of SiC to Si as 8: 1 behaved. The resulting pulpy mass was molded and dried with the mold at 200 ° C. Thereafter, the resulting body became out

removed from the mold and fired at a final temperature of 1625 K in an oxidizing atmosphere.

The result was an open-macroporous molding whose SiC grains coated on all sides with a glassy layer and solid

were bridged together. Structural analysis of a material cut revealed a microcrack-free, well-formed glassy bond.

The determined macroporosity was 42% by volume.

Claims (2)

1. A process for the production of Siliziumcarbidformkörpern which are oxide-ceramic bonded and macroporous by mixing a SiC grain mixture with a glass-forming materials containing aqueous solution, then molded, dried and fired at temperatures around 1625K, characterized in that in the solution Borcabid, Silicon carbide and silicon having a maximum grain size of eOMsuspended are, wherein boron carbide to Sumje silicon carbide and silicon in a molar ratio of 1: 5 to 1:15 and the silicon carbide to silicon in a molar ratio of 8: 1 to 12: 1 and the proportion of suspended solids in the solution is 5% to 20%. 2. A process for the preparation of Siliciumcarbidformkörpern which are oxide-ceramic bound and macroporous, according to claim 1, characterized in that the SiC grain mixture primarily has an open microporosity of 10Vol .-% to 20Vol .-%.