Study On Extraction Of Valuable Metal From High Carbon Chromium Iron Slag By Ammonium Sulfate Roasting

High carbon ferrochrome slag is produced in the process of smelting high carbon ferrochrome. Stocks at present our country has more than 20 million t, at the same time with 3 million t production increases every year.The conventional processes for dealing with high-carbon ferrochrome slag suffer from valuable resource waste issues. In order to achieve high utilization efficiency for valuable components, the technique scheme of activation roasting process by ammonium sulfate was proposed. And systematic researches on phase transformation, thermodynamics, technical parameters and kinetics for roasting treatment, phase equilibrium determination for quaternary system of MgSO4-Al2(SO4)3-(NH4)2SO4-H2O, and the separation route design for leaching solution, were carried out. Based on the studies of unit processing, a technical scheme was formed. The main results were achieved:(1) The phases of the roasted products obtained at various temperatures are different; with the aim to reach a high conversion rate for high-carbon ferrochrome slag, the roasting temperature should be no lower than 350℃. From the aspect of thermodynamics, the feasibility of decomposing high-carbon ferrochrome slag by ammonium sulfate has been confirmed.(2) The optimized parameters for decomposing high-carbon ferrochrome slag by ammonium sulfate are obtained, which are:mass ratio of (NH4)2SO4 to slag is 5:1, toasting temperature is 380℃, roasting time is 2 hours, and the particle size of slag is smaller than 74 μm. Under the combination of optimized parameters, the leaching ratios of Mg、Al、Gr and Fe are 92%、80%.85% and 92%, respectively.(3) Unreacted core shrinking model can be reasonably used to describe the roasting processing, with surface chemical reaction as the rate controlling step. The macro activation energy for decomposing Mg is 49.19 kJ/mol,for decomposing Al is 70.59 kJ/mol. During roasting processing, the formation of byproduct layer of ammonium metal sulfate hinders the mass transfer of (NH4)2SO4 into the interior of the unreacted particles.(4) The solubility for the quaternary system of MgSO4-Al2(SO4)3-(NH4)2SO4-H2O at 40℃ and 80℃ is determined. Based on the equilibrium data, and combined with the characteristic of the leaching solution of high concentration of Mg and Al, low content of Cr and Fe, a separation route of cooling crystallization of Al, selective precipitation of Cr and Fe, Mg(OH)2 precipitation by adjusting the acidity, is proposed. High-valued commercial products of NH4Al(SO4)2·12H2O and Mg(OH)2 can be gained, which finally achieves the goal of this research.