DD296476A5 - PROCESS FOR THE PRODUCTION OF SIC'FORMKOERPERN - Google Patents

Abstract

The invention relates to a process for the preparation of porous silicon carbide moldings for the filtration of molten metals. According to the invention, they are made with a binder containing boron carbide, silicon carbide and silicon. manufacture; SiC Formkoerper; Binder; SiC; silicon; boron carbide; Grain shape; Suspension; liquid solution}

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 state of the art Silicon carbide moldings with a suitable pore structure are due to their high chemical and thermal Resistance used as filter elements for cleaning molten metals. Various methods are known, according to

where such porous SiC bodies are produced.

After a process principle, an open-cell plastic sponge with a slurry of fine-grained SiC and a Iceramic binder soaked and annealed after drying at temperatures around 1275 K. The plastic will be there

thermally decomposed and the SiC slip is as a solid body with the pore structure of the original plastic sponge

In such SiC filters, however, is disadvantageous in that they easily break out due to the small wall thickness, what to

serious damage in the filtration downstream processing equipment but can at least lead to unwanted 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 required for the production of such filter foam polyurethane in the thermal Decomposition produces environmentally harmful pyrolysis products by means of complex downstream purification systems

must be disposed of.

Much better results are achieved with a likewise commercially available filter element, in the corundum or Silicon carbide grains are bridged on oxide ceramic or species-specific binder. The primary grain used is compact

and of rounded form, with the most common spatial axis ratios in the combination range from 1: 1: 1 (spherical grain) to 1: 2: 3 (oval grain). These filter elements have an increased life and from 3 to 10f higher throughput. 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 different thermal expansion behavior

The structural analysis of the material cut revealed that the binder bridges of microcracks already existed on the unused filter

were crossed. 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

the media and temperature influences increase this negative effect. It comes to the crystallization of the glass phase and increased

Cracking of the binder and thus further degradation of the framework. 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. 80 pm 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. For the production of the shaped body, a special SiC grain was also used. Contrary to the usual opinion, to use rounded grain for the production of moldings became

prismatically broken SiC grain with spatial spacings in the combination range of 1: 1: 2 to 1: 3.5: 6 used. The

Grain also had an open microporosity of 10% by volume to 20% by volume. This special SiC grain caused an increase in strength due to partially flat contacts in the composite. essential

However, with regard to the application as a filter, it seems that the roughness and the irregularity in the pore space by the special grain increase, which produce a better filtration effect.

embodiment

4000g of a grain size of prismatically broken SiC, taken from a sieve fraction of 3.15mm to 6.00mm, with a single grain density of an average of 2.5 g / cm ³ 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 .-%, with 1930g of 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.

12,00g Si (max grain size 40 цт)

138.00g SiC (max grain size 30um)

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 mold at 200 ⁰ C. Thereafter, the body obtained from the Form 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 vitreous bond.

The determined macroporosity was 42% by volume.

Claims (2)

A process for the preparation of Siliziumcarbidformkörpern which are oxide-ceramic bound and macroporous by mixing a SiC grain mixture with an aqueous solution containing glass forming, then molded, dried and fired at temperatures around 1 625K, characterized in that in the solution boron carbide , Silicon carbide and silicon having a maximum grain size of 80 μητι are suspended, wherein 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 and the proportion of suspended solids in the solution is 5% to 20%. 2. A process for producing Siliziumcarbidformkörpern which are oxide-ceramic bound and macroporous, according to claim 1, characterized in that the SiC grain mixture is pronounced edged and prismatic broken, the most common Raumachsverhältnisse in the combination range of 1: 1: 2 to 1: 3, 5: 6, and that the grain material has an open microporosity of 10Vol .-% to 20Vol .-%.