The electro silicothermal method is to use silicon in silicon chromium alloy to reduce chromium and iron oxides in chromium ore under the condition of alkaline slag production in an electric furnace, in order to produce medium to low carbon ferrochrome.
(The requirements for equipment and raw materials, as well as the smelting process, are basically the same as those for smelting low-carbon ferrochrome using the electric silicon thermal method. However, the carbon content of medium and low-carbon ferrochrome is higher than that of carbon ferrochrome, so fixed electric furnaces and self baking electrodes can be used. The carbon content of the silicon chromium alloy used as a reducing agent can also be correspondingly higher. In addition, the smelting operation is not as meticulous as the requirements for low-carbon ferrochrome.)
The raw materials for smelting medium and low-carbon ferrochrome include chromium ore, silicon chromium alloy, and lime. Chromium ore should be a dry and pure lump ore and concentrate powder, with a higher Cr2O3 content being better, and a lower impurity (Al2O3, MgO, SiO2) content being better. The phosphorus content in chromium ore should not exceed 0.03%. The particle size is less than 60mm. Silicon chromium alloy should be broken, with a particle size of less than 30mm and no slag. Lime should be freshly fired, with a CaO content of no less than 85%. The lower the CaO in lime, the higher the impurities SiO2 and Al2O3. As a result, more CaO is used to adjust alkalinity, and the true CaO is lower. If the CaO in lime is low, then the effective CaO is lower.
When silicon is used to reduce chromium and iron oxides, the SiO2 generated by the reaction aggregates in the slag, making further reduction difficult. Therefore, if no measures are taken, only 40% -50% of Cr2O3 in the ore can be reduced during reduction, and then the reduction reaction must be stopped.
If the amount of reducing agent is increased further, the silicon in the alloy will exceed the specified standard, resulting in waste, and the Cr2O3 in the slag will still be very high.
To improve the recovery rate of chromium, it is necessary to add flux lime to the slag. The CaO in lime can combine and form stable silicates: CaO · SiO2, 2CaO · SiO2 (with 2CaO · SiO2 being the most stable), in order to further reduce Cr2O3 in the slag.
The alkalinity of slag CaO/SiO2 is generally equal to 1.6-1.8.
The slag of smelting medium and low-carbon ferrochrome also contains MgO, which is brought in by chromite and furnace lining. The effect of magnesium oxide and calcium oxide is the same, so the alkalinity of the slag can also be represented by (CaO+MgO)/SiO2.
Low carbon ferrochrome (CaO+MgO)/SiO2 in smelting is generally equal to 1.8-2.0.
This maximizes the reduction of Cr2O3 from the chromium ore.
If the alkalinity is further increased, it is unreasonable. Not only can it not significantly reduce the Cr2O3 in the slag, but due to the increase in slag amount, the total amount of chromium in the slag and the electricity consumption for melting the slag also increase.
The ratio of slag to metal (slag iron ratio) is 3.0-3.5.