New Slag for Nickel Matte Smelting Process and Subsequent Fe Extraction

Large quantities of residual slag with high Fe content are left behind in pyrometallurgical nickel processing. How to separate and recover iron from the high fayalite residue is still a problem because iron mainly exists as iron-magnesium silicate in slag. This article suggests that the slag for nickel smelting is adjusted by increasing the amount of lime, partly instead of quartz, to form liable reduction compounds for subsequent Fe extraction. Based on the thermodynamic analysis and phase diagram of the FeO-SiO₂-CaO-MgO slag system, the new slag composition was chosen, the suitable melting point of relevant slags was measured, and the phase change was observed. The new slag with a composition of 7 to 9 pct MgO, 10 to 15 pct CaO and 1.5 to 1.8 Fe/SiO₂ was found to work well for nickel matte smelting. With the increase of CaO and decrease of SiO₂ content in slag, Ca(Fe, Mg)Si₂O₆ and Mg(Ca)Fe₂O₄ are the main existing phases instead of (Fe, Mg)₂SiO₄ in the residual smelting slag. More than 50 pct of Fe-containing phases are in the form of MgFe₂O₄ when 15 pct CaO is added, which is what we want because the MgFe₂O₄ is much better for magnetic separation and reduction than the original (Fe, Mg)₂SiO₄. In this way, Fe can be much more easily extracted from residual slag and have multipurpose benefits.