Thermodynamics Research Of Silicon Alloys During Oxidation Refining Process

Both alloys of silicon and FeSi75 are important raw materials in metallurgy, chemistry and electrics industries. The metallurgical grade silicon alloys are produced by reduction of silicon dioxide in sub-arc furnace through carbothermal process. The impurities, such as aluminum, calcium and phosphor, existing in materials of silica and reductant will also be reduced into the products at high temperature and under strong reduction conditions. If the alloys are used in metallurgy, chemistry and electrics industries, the impurities must be removed. The methods of removal such impurities as Al and Ca include chlorination and oxidation refining process industrially. While the chlorination refining process is prohibited for its pollution aspects, the oxidation refining method is paid more attention to.When the impurities are oxidized, the oxides produced are removed by absorbing into slag phase, which is the principle of the oxidation refining process. The thermodynamics equilibrium between slag and metal will be researched in this thesis. The main reactions between slag and metal during refining process are as following,The equilibrium compositions of the slag and metal system are depended on the activities of components and thermodynamicsconditions. The activities or the activity coefficients of the components in Si-Al-Ca(-Fe) alloy and SiO2-CaO-Al2O3 slag system are calculated based on the multi-component sub-regular melts model(SELF-SReM). With the results calculated above, the concentration of slag equilibrated with silicon and 75SiFe alloys at 1550 C are given. Thus the equilibration between Si-Al-Ca(-Fe) alloys and SiO2-CaO-Al2O3 slags during the oxidation refining process can be predicted. The slag compositions equilibrated with the same aluminum and calcium are affected a little if the temperature varies finitely. Though the element of iron doesn't take part in the reactions of the refining process, it will influence the activities of other components. The relationship of aluminum and calcium equilibrated at same conditions between the silicon and the FeSi75 alloys have been calculated by the SELF-SReM model, which showed that the compositions of impurities in FeSi75 are lower than that of hi silicon alloys.In practice, the refining process is related to the chemical reaction stoichiometry relations of the system also. The stoichiometry reaction model of the refining process was given in this paper. The initial alloy and slag compositions and the slag/metal ratio will affect the whole refining process from the stoichiometry reaction model. The initial slag composition and the slag/metal ratio determined the path to reach equilibrium of the oxidation refining process. Thus the refining process will lie on both the thermodynamics and the stoichiometry reaction relationship of the system.With the analysis of thermodynamics and chemistry stoichiometryreaction model above, the thermodynamics equilibrium experiments were carried out between Si-Al-Ca(-Fe) alloys and SiO2-CaO-Al2O3 slag system at 1550 掳C . Based on the experimental data, therelationship between the slags and alloys with different impurities of Al and Ca were acquired in silicon and FeSi75 alloys. The experimental relationship between the activities and compositions of the alloys were given, which showed the inluence of the compositions to the activities. The methods that the activities of Al and Ca in silicon or FeSi75 alloys are constant are inaccurate. The impurities of Al and Ca in silicon alloys is removed easily than those in FeSi75 alloys when equilibrated under the same conditions, which is consistent with the results calculated by sub-regular melts model. When equilibrated with the same slag and temperature, the contents of Al and Ca between the silicon metal and FeSi75 alloy showed certain quantitative relationship, but not the linear relationship given by other authors.The oxidation refining experiments of FeSi75 alloys were further more done in both induced furnace of 100 KVA scale and industry scale. The induced furnace...