Thermodynamic Analysis And Mechanism Of The Carbothermic Reduction Of Tungsten Slag

A significant amount of tungsten slag is produced during the treatment of tungsten concentrate using the alkali leaching techniques and has been listed in the National Catalogue of Hazardous Wastes.Tungsten slag was difficult to detoxify and recycle through conventional treatment methods.The pyrometallurgical process has the characteristics of a high additional value alloy,a shorter procedure,and reduction,but existed the drawback of difficulties of control on the composition of the alloy.Using Fact Sage thermochemical software,the reduction sequence of oxides has been analyzed and the effect of alloying,CO concentration,gangue content on the reduction of the principal component of tungsten slag was systematically investigated.Besides,the CaO-SiO₂-Al₂O₃-MnO system also was studied.The effects of equilibrium condition,temperature,and reductant addition on the carbothermal reduction process were investigated taking black tungsten slag as the research objective.The calculations were compared to experimental results.The reduction sequence of oxides is Bi₂O₃>PbO>WO₃>Fe O>Na₂O>Nb₂O₅>MnO during the reduction of tungsten slag.WO₃,Fe₂O₃,Nb₂O₅,and MnO₂ are reduced to metals in this process and collected in the alloy.CaO,SiO₂,and Al₂O₃ cannot react with carbon and are the major constituents of the slag,whereas Bi₂O₃,PbO and Na₂O are reduced to Bi,Pb and Na,which volatilize and are collected in fumes.The formation of alloy promotes the reduction of MnO₂,but less affects the reduction of Fe₂O₃.MnO₂ and Fe₂O₃ are entirely reduced at a temperature of 1300°C,at which point the alloy transforms into a molten Fe-Mn-C alloy.But the formed alloy is invariably a high-carbon alloy on sufficient reduction addition.The decrease of CO concentration increases the efficiency of reduction of MnO and facilitates the formation of molten alloy.However,It is difficult to form the molten alloy as the share of CO in the air under 30%.Increasing the share of SiO₂ inhibits the reduction of MnO as the share of SiO₂ under 50%,but as the share of SiO₂ exceeds 50%,the effect of reduction of MnO on SiO₂ remains largely the same.Al₂O₃ facilitates the reaction of MnO with slag and inhibits the reduction of MnO.MnO is dissociated from the slag with the facilitation of CaO.Increasing the share of CaO increases the reduction of MnO.but,with the excessive addition of CaO,the reduction of MnO is inhibited as MnO reacts CaO to a solid solution.The melting temperature of slag remains under 1400?as the ratio of Al₂O₃/SiO₂ is 0.3.MnO preferentially reacts with reductant to form carbide.The carbon content in the alloy first increases and then decreases.An increased temperature and reduced reductant addition are conducive to decreasing the carbon content of the alloy.The recovery of W and Nb is affected by the alloy's carbon content.This is beneficial to the recovery of Nb,which results in a decreased carbon content in the alloy.An increased temperature and decreased reductant addition are further conducive to increasing the recovery of Nb,but do not affect the recovery of W,which remains constant.Temperatures in the range of 1500–1600°C and reductant addition range of 14–16%are the optimal carbothermic reduction conditions in tungsten slag.Under these conditions,the recovery percentage of W,Fe,and Nb are over 90%,and that of Mn is more than 50%.Furthermore,Bi and Pb can be collected in fumes.