Migration Of Lead And Its Influence On Reduction Characteristics Of Molten High-lead Slag For Direct

Direct reduction process of molten high-lead slag is a clean energy saving advanced technology for lead smelting, which involves a series of complex physicochemical changes.In our preliminary work on five-component system lead slag,it was observed that actual measured values of melting pointand phase composition presented a great deviation with that of theoretical value. That can be attributed to the constant change of slag composition, especially Pb O and Zn O content in the actual process of smelting.Migration of Pb O and Zn O plays a critical role to slag composition variation, physical and chemical properties, and subsequently guarantee anterograde of reduction process.Therefore,this paper investigated the migration and variation mechanismof lead and zinc in the high-lead slag, and the influence of which on the physicochemical properties and reduction characteristics for direct reduction process.It is expected that this work can provide certain theoretical reference with great practical and applied value for direct lead smelting process.Physicochemical properties of the lead slag were calculated using FACTSage software and verified by slag experimental analysis.Effect of composition variation on phase equilibria, distribution and physicochemical properties of the lead slag for direct reduction process at particular temperature were investigated also. The influence of different process parameters on Pb and Zn distribution among phases in reduction process were inspected with static reduction experiments using coal as the reducing agent.The main results are as follows:(1) TG-DSC analysis showed that rate of lead can reached 27.17% between 800℃to 1200℃ when Pb O content of high lead slag are 43%. Whether or not adding carbon, the reaction kinetics model of lead and zinc volatilization in the high lead slag follows the line equation:ln[G(α)/T1.894661]=-10.0453-6.59939/T, with an apparent activation energy E at 54.7880KJ/mol. Volatile mechanism can be attributed to the chemical reaction controlled by diffusion and mass transfer speed.(2) The melting point of high-lead slag increase as the Fe O/Si O2 ratio rise up, and as the Pb O content decrease with reduction process when the ratio of Fe O/Si O2 is between 1.2and 2.0, Ca O/Si O2 is between 0.4 and 1.0.Experimental value of slag melting temperature and viscosity had a large deviation with that of the theoretical calculation value, about 3℃ to+150℃, which may be mainly caused by the volatile of Pb O.(3) Phase equilibria of slag system with Fe O/Si O2 ratio at 1.54, Ca O/Si O2 at 0.4 show that the main precipitated phases are Melitite, Olivine and Spinel forslag with low Pb O content; the main precipitated phases are Pb2(Fe,Zn)Si2O7 and Pb3(Ca,Pb)2Si3O11 for high Pb O content slag. At temperature from 900℃ to 1100℃, crystals of calcium silicates, Fe2O3, Fe3O4 and feldspar emerge with less Pb-containing phases in the system.(4) The static reduction experiments show that recovery rate of lead increase with the Fe O/Si O2 and Ca O/Si O2 ratio rise up when Fe O/Si O2 ratio is between 1.4 and 1.6, Ca O/Si O2 between 0.4and 0.8. Zinc mainly form silicate that enters reduction slag. The most satisfying process parameters for valuable metals recovery are that Fe O/Si O2 at 1.54,Ca O/Si O2 at 0.4,with a coal reductant ratio of 3.5%, reduction time of 60 min at 1250℃.On the basis of migration mechanism research of Pb, Zn and oxides in slag, prediction model between physicochemical properties and reduction characteristics for direct reduction process can be established, as a result,which may predict variation trend of physicochemical properties.It is believed that these works can provide theoretical reference to increase recovery rate of lead fordirect lead smelting process.