There are two main smelting methods of low-carbon ferrochromium (containing 0.15% ~ 0.50% carbon): high-carbon ferrochromium refining method and electro silicothermic method.
There are two methods of refining high carbon ferrochrome: refining high carbon ferrochrome with chrome ore and refining high carbon ferrochrome with oxygen. Using high-carbon ferrochrome concentrate, the refining slag has high viscosity and high melting point, and the smelting temperature must be high. Therefore, the power consumption is high, the service life of furnace lining is short, and the carbon content is not easy to reduce. Converting high carbon ferrochromium with oxygen has great advantages, such as high productivity, low cost and high recovery rate.
The traditional production method is electro silicothermic method. Electro silicothermic method is to reduce the oxides of chromium and iron with silicon in silicon chromium alloy under the condition of making alkaline slag in electric furnace, so as to prepare medium and low carbon ferrochromium.
Oxygen blowing low carbon ferrochrome
The equipment used in the production of low carbon ferrochrome by oxygen blowing is converter, so it is called converter method. According to the different way of oxygen supply, oxygen blowing can be divided into side blowing, top blowing, bottom blowing and top bottom blowing. The top blown converter method is adopted in China.
Oxygen blowing method is to blow oxygen directly into the liquid high carbon ferrochrome to decarbonize and make low carbon ferrochrome.
The main elements in high carbon ferrochrome are chromium, iron, silicon and carbon, which can be oxidized. The main task of oxidation and smelting of high carbon ferrochrome is to decarburize and preserve chromium. When oxygen is blown into liquid high carbon ferrochrome, chromium and iron make up more than 90% of the total alloy, so the chromium and iron are oxidized first, and then these oxides oxidize the silicon in the alloy. Due to the oxidation of chromium, iron and silicon, the molten pool temperature increases rapidly, the rapid development of decarbonization reaction, the higher the temperature, the more conducive to decarbonization reaction, and can inhibit the oxidation of chromium, the carbon in the alloy can drop lower.
The raw materials for oxygen top blowing refining of medium and low carbon ferrochrome are high carbon ferrochrome, chrome ore, lime and silicon chromium alloy.
The temperature of high carbon ferrochromium liquid in converter is high, usually between 1723℃ - 1873℃. The chromium content of molten iron should be higher than 60%, the silicon content should not exceed 1.5%, and the sulfur content should be less than 0.036%. Chrome ore is used as slag making material. It is required that the content of SiO3 in chrome ore should be low, the content of MgO and Al2O3 can be appropriately higher, and its viscosity should not be too large. Lime is also used as slagging material, and its requirements are the same as those of electro silicothermic method. Silicon chromium alloy is used to reduce high chromium slag in the later stage of blowing, generally using Si-Cr alloy powder under the sieve after crushing.
Smelting low carbon ferrochromium by electro silicothermic method
The smelting of medium and low carbon ferrochromium by electro silicothermic method is carried out in a fixed three-phase electric arc furnace. Self baking electrode can be used, and the furnace lining is built with magnesia brick (dry masonry). The short life of furnace lining is an important problem in the production of medium and low carbon ferrochromium. Due to the high smelting temperature (up to 1650 ℃), the service life of the furnace lining is generally short.
Raw materials for smelting medium and low carbon ferrochrome include chrome ore, silicon chromium alloy and lime. Chrome ore should be dry and pure lump ore or concentrate powder. The higher the content of Cr2O3, the better, and the lower the content of impurities. The phosphorus content in chrome ore should not be higher than 0.03%, and the particle size should be less than 60mm. Silicon chromium alloy should be broken, with particle size less than 30mm and no slag. Lime should be freshly burned and its CaO content should not be less than 85%.
Identification method: high carbon ferrochrome (4%~ 8% carbon), medium carbon ferrochrome (0.5% ~ 4% carbon), low carbon ferrochrome (0.15% ~ 0.50% carbon), micro carbon ferrochrome (0.06% carbon), ultra micro carbon ferrochrome (less than 0.03% carbon), metal chromium and silicon chromium alloy. The "loading grade ferrochromium" containing 50% chromium produced in large quantities uses ores containing Cr2O3 and low ratio of chromium to iron.
Ferrochrome is divided into carbon ferrochrome (including charging grade ferrochrome), medium carbon ferrochrome, low-carbon ferrochrome and micro carbon ferrochrome according to different carbon content. Silicon chromium alloy and ferrochromium nitride are also commonly used. Ferrochromium is mainly used as an alloy additive in steelmaking, which was added in the later stage of steelmaking refining in the past. Smelting stainless steel and other low-carbon steels must use low and micro carbon ferrochrome, so the production of refined ferrochrome has a large-scale development. Due to the improvement of steelmaking process, carbon ferrochrome (mainly charging grade ferrochrome) is used to charge the furnace when AOD method (see refining outside the furnace) is used to produce stainless steel and other steel grades. Therefore, it is only necessary to add low and micro carbon ferrochrome to adjust the composition in the later stage. Therefore, the focus of ferrochrome production is to refine carbon ferrochrome.
Carbon ferrochrome is smelted in electric reduction furnace, using coke as reducing agent and silica or bauxite as flux. The composition of slag is generally SiO2 27% ~ 33%, MgO 30% ~ 34%, Al2O3 26% ~ 30% and Cr2O3 % ~ 9.0%. Chromium carbide is formed, and the carbon content of the product is 4% ~ 9%. The capacity of modern reduction electric furnace for smelting ferrochromium is 10000 ~ 48000 KVA, which is generally closed and fixed, and the smelting power consumption is 3000 ~ 4000 kW · H / T. The electric furnace for smelting silicon chromium alloy is similar to the electric furnace for ferrochromium reduction. There are two smelting methods: one-step and two-step. One step smelting with chromite, silica and Coke Plus flux. The two-step process uses carbon ferrochrome, silica and coke as raw materials for slag free smelting. The smelting process is roughly similar to that of ferrosilicon production. The smelting power consumption is about 3000 ~ 4000 kW · h per ton.
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