Basic Research On The Removal Of Tin From Tin-iron Concentrates By Gas-phase Sulfidation

Tin-bearing iron ore is recognized as one of the typically complex iron ore resources,with more than 0.5 billion tons of reserve in China.The tin impurity reduces the quality of this iron,and this ore cannot be used for ironmaking when the tin content is in excess of 0.08 wt%.The ideal separation effect of tin and iron can be achieved via Reduction-sulfurization roasting process.However,few researches paid attentions to the thermodynamic reaction pathway and the tin and iron phase transfer of the Sn-bearing iron concentrates under highly purified sulfur gases(H₂S(g),SO₂(g)and COS(g)).In this study,the Sn-bearing iron concentrates from a tin smelting plant in Yunnan,China were used as raw materials.A relevant thermodynamic model was established,using the thermodynamic software Fact Sage 7.2.The thermodynamic reaction pathways were determined between sulfur-containing gases,like hydrogen sulfide(H₂S(g))or sulfur dioxide(SO₂(g)),and the Sn phase in the raw material during the sulfurization process,using two different treatments.One treatment had(H₂S(g))as the reducing and curing agent and the other used carbon monoxide(CO(g))as the reducing agent with sulfur dioxide(SO₂(g))as the curing agent.Also,the effects of the roasting temperature,roasting time,gas flow rate,particle size,and the composition of the gas mix on the Sn removal rate were systematically studied.In the sulfurization experiment with the hydrogen sulfide(H₂S(g))as the reducing and curing agent,once the hydrogen sulfide(H₂S(g))produced hydrogen(H₂(g))and gaseous sulfur(S₂(g))through autogenous thermal decomposition,S₂(g)was more likely to react with iron oxide(Fe₃O₄)in the raw material,forming ferric sulfide(Fe₇S₈).H₂(g)preferentially reacted with tin dioxide(SnO₂)in the raw material to form elemental Sn,which was then sulfurized with S₂(g)to form volatile stannous sulfide(Sn S).When carbon monoxide(CO(g))was added to the roasting system,the reduction and sulfurization of tin dioxide(SnO₂)are promoted,and the utilization rate of H₂S(g)was improved.Under the optimal experimental parameters,the removal rate of Sn reached as high as 95.34%.Iron in the roasted residue was found as Fe₇S₈,and tin was found as stannous oxide(SnO),which was wrapped in the Fe₇S₈.In the sulfurization process with carbon monoxide(CO(g))as the reducing agent and sulfur dioxide(SO₂(g))as the curing agent,the Sn phase in the raw material(SnO₂)was reduced to Sn and SnO by carbon monoxide(CO(g)),and then the highly volatile stannous sulfide(Sn S)was formed by sulfurization with S₂(g)from the reaction product of CO(g)and SO₂(g).Under the optimal experimental parameters,the removal rate of Sn from the raw material reached as high as 93.18%.The iron phase in the roasted residue was mainly ferrous oxide(FeO)or iron(Fe),and the tin phase is mainly stannous oxide(SnO)or a Fe-Sn alloy.Moreover,stannous oxide(SnO)was wrapped with ferrous oxide(FeO),and the Fe-Sn alloy is embedded in or distributed along the edge of FeO.When either the CO(g)concentration or the roasting temperature increased,the formation of the Fe-Sn alloy was promoted.This research provided theoretical guidance for the removal of the Sn impurities in the Sn-bearing iron concentrates using reducing-curing method.Furthermore,it gave some ideas for the use of hydrogen sulfide gas and sulfur dioxide gas produced in industry.