Synthetic Technology of Silicon Carbide

wallpapers Industry 2020-07-07

Natural silicon carbide is moissanite, also known as moissanite.) The silicon carbide used in industry is a synthetic material, commonly known as carborundum. At present, all-silicon carbide used in production is a manufactured product. Silicon carbide powder is one of the most widely used non-oxide refractory raw materials in refractory materials. Cement-bonded silicon carbide, oxide-bonded silicon carbide, silicon nitride-bonded silicon carbide, recrystallized silicon carbide, reaction sintered silicon carbide, silicon carbide and other products produced from silicon carbide as raw materials and amorphous refractory materials are widely used in blast furnaces in the metallurgical industry, Zinc smelting furnace, kiln furniture in ceramic industry, etc.

Synthetic technology of silicon carbide:

The raw materials used for synthesizing silicon carbide powder are mainly fused quartz and quartz sand with SiO2 as the main component, and petroleum coke with C as the main component. Low-grade silicon carbide also uses anthracite with low ash content as the raw material. Auxiliary materials include sawdust and table salt.

Vein quartz is a kind of igneous rock developed in the cracks of other rocks after being differentiated by acid magma to form mineral veins. The ore veins can be up to tens of meters thick and stretch to hundreds of meters in length. Some of the milky white translucent ores are high-quality raw materials for manufacturing silicon carbide. Quartz sand includes river sand, sea sand and lake sand. It is a sedimentary sand mine. It is made of quartz-bearing ore by natural force crushing and impact. Those with suitable composition and particle size can be used as raw materials for smelting silicon carbide. This kind of raw material application is more common.

The requirement for petroleum coke is that the fixed carbon content is as high as possible. The ash content is less than 1.2%, and the volatile material is less than 12.0% so that the output and quality of silicon carbide can be improved. The particle size of petroleum coke is usually controlled below 2mm or 1.5mm. When the power of the smelting furnace is significant, the particle size can be appropriately thickened. However, if the petroleum coke particle size is too coarse, the reaction between C and SiO2 is not natural to complete. Carbon particles are easily mixed in the silicon carbide crystal layer.

(1) Acheson method

The traditional Acheson method resistance furnace's external appearance is a rectangular groove, which is a hearth made of refractory bricks. Two sets of graphite electrodes penetrate the end wall and penetrate the fireplace. The unique graphite powder furnace core is arranged between the electrodes and provides a conductive path that generates a lot of heat when energized. The furnace core is filled with a mixture of silicon raw materials, petroleum coke and wood chips, and the outside is thermal insulation.

During refining, the resistance furnace is powered, and the temperature of the core body rises to about 2600°C. The heat is transferred to the surrounding mixture through the surface of the core body to cause it to react to form silicon carbide and escape CO. CO burns into CO2 on the surface of the furnace, creating a soft, undulating blue to a yellow flame blanket; a small portion of unburned CO is dispersed in the atmosphere and pollutes the air.

After the reaction is complete and cooled, the furnace wall can be dismantled, the furnace materials can be layered and sorted, and the desired particle size can be obtained by crushing and crushing. The impurities can be removed by water washing, acid-base washing, magnetic separation, etc. to improve purity after drying and screening For the finished product. Sometimes it needs to be shaped or finely ground to make a fine powder.

This type of resistance furnace relies on the graphite furnace core body to generate heat, and the temperature near it is relatively high. It gradually decreases outward, forming an uneven temperature field. Therefore, the reaction and crystallization conditions of the materials in the furnace are also very different. Round crystals are built around the core of the stove, and the dense silicon carbide in the shape of hexagons and needles is radiated. The crystal size gradually decreases from the inside to the outside.

The furnace power and parts vary from 50 to 450 mm, and the generation temperature is 2000 to 2500°C. The second-grade SiC layer is α-SiC with crystals less than 0.5mm, in the form of hexagonal crystals, the crystal plane is bright and shiny, and it is easy to distinguish from the amorphous structure in appearance. The formation temperature of the second-grade SiC layer is about 1900 ~ 2000 ℃, and the impurity content is more, the SiC content is only 90% ~ 95%. The amorphous layer is mainly composed of β-SiC, and the naked eye can see no crystal. The SiC content accounts for about 70% to 90%, and there are more incompletely reacted carbon and vanadium-based shoes, while Fe2O3, Al2O3, The content of CaO and MgO is about 2%~5%. The amorphous material is grayish-green (black SiC furnace) or yellowish-green (green SiC furnace), which is loose and easily broken into powder. The formation temperature of the amorphous material is about 1800~1900℃.

(2) ESK smelting method

The Acheson method has simple equipment and low investment and is widely used by smelting factories.

The rapid and complete reaction of the raw materials depends on the large surface area of ​​the carbonaceous materials and the low volatility of SiO2. The raw materials' particle size must also consider the bulk density of the mixture and the exhaust of the furnace gas. Any SiC sand producer knows that the output and quality of SiC depend on the type and amount of impurities in the raw materials used. The trouble-free operation of the smelting furnace depends on the reactivity of carbonaceous materials, silica sand, coke and other natural materials. The amount of impurities in the core body affects the output of the smelting furnace, and the types of contaminants such as Al2O3 change the colour and crystalline shape, size and crystal structure of SiC. The coking process mainly determines the reactivity and the coking raw materials used.