Comprehensive Recycling Of Valuable Metals From Lead Smelting Slag Based On Carbothermic Reduction

Lead producing process generates considerable amounts of lead smelting slag which contains high content of heavy metals.Lead slag is harmful to the environment since they typically contain many toxic heavy metals and complex components.Piles of lead slag have been rising year after year due to lack of effective handling methods.The rising stockpile of lead slag has already become a critical bottleneck for the eco-friendly and sustainable development in lead smelting industry.Therefore,this paper takes lead slag from blast furnace as the research object,the carbothermic reduction and volatilization were applied to recover iron and some heavy metals from lead slag.Phase transformation of heavy metals and iron was investigated,especially the volatilization mechanisms and kinetic characteristics of heavy metals.On this basis,pilot scale test was also carried out to validate the feasibility of this process,and it can provide the research basis for comprehensive utilization of lead smelting slag.(1)Firstly,the technological mineralogical characteristics of lead slag were analyzed.Iron mainly exists in silicate phases;Zinc exists in forms of zinc ferrite(ZnFe2O4)and sphalerite(ZnS).The Pb-containing mineral was lead-iron-bearing silicate,PbO and PbS.Tin exists in forms of SnO2 and stannous sulfide.Thermodynamic analysis during carbothermic reduction shows that iron silicate was reduced to metallic iron.During the carbothermic reduction process,volatile phase of Zn is metallic zinc vapor.Pb element can volatile in forms of Pb,PbO,and PbS,nonvolatile phase was Fe-Pb-S.Tin element can evaporate out of the pellets through SnO and SnS,nonvolatile phase was Fe-Sn-S sulfide and Fe-Sn alloy.(2)Separation and enrichment behaviors of polymetallic components during carbothermic reduction of lead smelting slag were studied.The effects of different technological conditions on volatilization ratio of heavy metals and metallization degree of the pellets were investigated.Results show that the metalliztion ratio was 88.72%,the volatilization ratio for Zn,Pb and Sn were 96.27%,93.77%and 70.26%at 1200℃ for reducing 30 min with C/O mole ratio of 2.0.The heavy metals volatile from pellets are concentrated in the secondary flue dust.The content of Zn,Pb,and Sn in the dust was 41.10%,28.35%,3.91%,respectively,and main phases were ZnO,PbSO4 and Zn2SnO4.For the metallized pellets,molten iron of 93.74%can be obtained via smelting separation,and the iron recovery is 93.27%.For the secondary flue dust,Zn and Pb element was processed in stages via sulfuric acid leaching,carbonation desulfurized conversion and roasting decomposition,zinc sulfate and lead-rich residue can be obtained.The recovery rate of Zn is 91.26%through sulfuric acid leaching,ZnSO4 product produced from evaporation crystallization meet the national standards.The main phase in lead-rich residue was PbO,in which Pb and Sn content is 69.2%and 9.31%.(3)The phase transition and microscopic structures evolution of metal components were explored.Results showed that iron silicate was reduced to metallic iron.ZnFe2O4 and ZnS was reduced to Zn(g)for volatilization,and then re-oxidized and formed ZnO(s).In the earlier stage,lead-iron-bearing silicate and SnO2 were reduced to volatile SnO,SnS,PbO,and Pb for volatilization,then PbSO4 and SnO2 was generated in the flue gas and gathered into the secondary dust.In the later stage,however,the volatile SnO and PbO which did not diffuse out of the pellets timely were further reduced to nonvolatile metallic Sn and less volatile Pb,and the nonvolatile alloy phase(Fe-Pb-Sn、Fe-Sn)and polymetallic sulfide phase(Fe-Pb-Sn-S)generated by the reactions between Sn/Pb and Fe/FeS.(4)On the basis of the researches above,the relationship between the kinetics behaviors of heavy metal volatilization and the phase transformation was investigated.Results showed that the removal of Zn,Pb and Sn were controlled by the three-dimensional internal diffusion,and the kinetics behaviors of heavy metal volatilization were associated to the formation and aggregation morphology of metallic iron.In the whole reaction process,Zn element did not react with Fecontaining phases.Consequently,the volatilization of Zn presented a consistent kinetics behavior,while the volatilization of Sn and Pb conformed to a two-staged kinetics model.In Stage Ⅰ,Sn and Pb elements was volatilized in the form of PbS(g),PbO(g),SnS(g)and SnO(g)at a lower metallization degree,the apparent rate constant is larger.In Stage Ⅱ,the nonvolatile Fe-Pb-Sn and Fe-Pb-Sn-S formed and wrapped in Fe/FeS phases.The volatilization of Sn and Pb was confined,as a result,the apparent kinetics rate constant in Stage Ⅱ was lower than that in Stage Ⅰ.(5)Finally,a pilot scale carbothermic reduction test of 70 t lead slag in the rotary hearth furnace(RHF:8 m inner diameter,with 2 m belt width)followed by EAF smelting separation was carried out.During the reduction process,metallization ratio of iron is 74%,and the volatilization ratio of Zn,Pb,and Sn are 88.67%,87.68%and 70.83%respectively.The heavy metals were recovered by dust collected from the flue,and the contents of Zn,Pb,and Sn were 45.48%,25.62%,and 4.78%,respectively.The content and recovery of Fe in molten iron can reach 91.76%and 88.31%after EAF smelting.Base on the pilot-scale data,preliminary economic calculations for engineering program 300,000 tons of per year were conducted.which verifies the feasibility of carbothermic reduction-smelting separation process for lead smelting slag.