The manufacturing process of silicon carbide

wallpapers Technology 2021-03-31
Due to its low natural content, silicon carbide is mostly man-made. The common method is to mix quartz sand and coke, use the silica and petroleum coke in it, add salt and wood chips, put it in an electric furnace, heat it to a high temperature of about 2000°C, and obtain silicon carbide powder after various chemical processes.
Preparation of SiC smelting block
Silicon carbide (SiC) has become an important abrasive due to its great hardness, but its application range exceeds that of ordinary abrasives. For example, its high-temperature resistance and thermal conductivity make it one of the first-choice kiln furniture materials for tunnel kilns or shuttle kilns, and its electrical conductivity makes it an important electric heating element. To prepare SiC products, firstly, SiC smelting block [or: SiC pellets] must be prepared. Because it contains C and is super hard, SiC pellets were once called: emery. But be careful: it has a different composition from natural emery (garnet). In industrial production, SiC smelting block usually uses quartz, petroleum coke, etc. as raw materials, auxiliary recovery materials, spent materials, and after grinding and other processes, they are blended into a charge with a reasonable ratio and a suitable particle size (in order to adjust the gas permeability of the charge, an appropriate amount must be added. When preparing green silicon carbide, it is also necessary to add the appropriate amount of salt) prepared at high temperature. The thermal equipment for preparing SiC smelting block at high temperature is a special silicon carbide electric furnace. Its structure consists of a furnace bottom, an end wall with electrodes on the inner surface, a detachable sidewall, and a furnace core body (full name: energized heating element in the center of the electric furnace. Use graphite powder or petroleum coke to install in the center of the charge according to a certain shape and size, generally round or rectangular. Its two ends are connected with electrodes). The firing method used in this electric furnace is commonly known as buried powder firing. It starts heating as soon as it is energized. The temperature of the furnace core is about 2500°C or even higher (2600-2700°C). When the furnace charge reaches 1450°C, SiC is synthesized (but SiC is mainly formed at ≥1800°C) and CO is released. However, SiC will decompose when ≥2600℃, but the decomposed Si will form SiC with C in the charge. Each electric furnace is equipped with a set of transformers, but only a single electric furnace is powered during production, so as to adjust the voltage according to the electrical load characteristics to basically maintain constant power. The high-power electric furnace needs to be heated for about 24 hours. After a power failure, the reaction of generating SiC is basically over. After a period of cooling, the sidewall can be removed, and then the charge can be taken out gradually.
Charge after high-temperature calcination
The furnace charge after high-temperature calcination is unreacted material (for heat preservation in the furnace), silicon oxycarbide (semi-reactive material, the main components are C and SiO), and the binder layer (it is very tightly bonded). The main component of the material layer is C, SiO2, 40% to 60% SiC, and Fe, Al, Ca, Mg carbonate), amorphous material layer (the main component is 70% to 90% SiC, and it is cubic SiC That is β-sic, the rest are C, SiO2 and carbonates of Fe, Al, Ca, Mg), second grade SiC layer (the main component is 90% to 95% SiC, the layer has formed hexagonal SiC, but the crystal is relatively It is small, very fragile and cannot be used as an abrasive), first-class SiC ((SiC content <96%, and it is hexagonal SiC that is a coarse crystal of SiC), furnace core graphite. Among the above-mentioned layers, there is usually no reaction material and a part of the oxycarbide layer material are collected as spent material, and the other part of the oxycarbide layer material is collected together with the amorphous material, the secondary product, and part of the bonded material as the reheat material and some of the material is tightly bonded and lumpy. The binders with a high degree of concentration and many impurities are discarded. The first-grade products are classified, coarsely crushed, finely crushed, chemically treated, dried and sieved, and magnetically separated into black or green SiC particles of various sizes. To make silicon carbide powder, it must go through a water selection process; to make silicon carbide products, it must go through a process of forming and sintering.