Fundamental Study On The Efficient Selective Extraction Of Zinc And Comprehensive Utilization Of Valuable Metallic Elements From Zinc-containing Electric Arc Furnace Dust

As an important non-ferrous metal,zinc(Zn)has been widely used in many fields of social life.With the increase of Zn production and consumption,Zn mineral resources have become increasingly scarce.An important way to alleviate this problem is to increase the proportion of recycled Zn in the total output.Therefore,in this thesis,by using Zn-containing electric arc furnace dust(EAFD)as a research object,the efficient and selective extraction of Zn,and comprehensive utilization of valuable metallic elements for green and high value-added material were systematically studied.To explore the migration mechanism of elements during Zn extraction process and lay the foundation for the comprehensive utilization of valuable elements in the Zn-containing EAFD,the existed forms of metallic elements,such as Zn,Fe,Mn,Ca and Pb,Cr,etc.were investigated in detail.The crystal phases and their corresponding contents were as follows:(Zn,Mn)Fe2O4(34%),ZnO(1.5%),Fe3O4(34%),PbO(PbSO4)(1%),Cr2O3(CrO3/CrO42-)(0.3%),SiO2(5%),CaCO3(8%),CaO(1%)and water soluble substances(such as KCl etc.)(10%).Crystalline water contained in ferric chloride hexahydrate(FeCl3·6H2O)was the only source of water molecules,and the efficient extraction of Zn from Zn ferrite(ZnFe2O4)and Zn-containing EAFD was achieved by using a solid FeCl3·6H2O under mild hydrothermal conditions,and meantime,the amount of secondary waste generated can be controlled.The mechanism of Zn efficient extraction was revealed through the analysis of FeC13·6H2O and ZnFe2O4 phases transformation during hydrothermal process.In addition,the migration and transformation of Fe,Pb,Cr,Mn,Ca etc.during the Zn extraction process from Zn-containing EAFD were studied in detail.It is shown that the extraction efficiencies of Zn,Pb,Ca,Mn were 97.4%,98.3%,93.4%,and 97.5%,respectively,under the optimal experimental conditions(mass ratio of FeC13·6H2O to Zn-containing EAFD after water washing(RF/TE,g/g)7:7,reaction temperature 220 0C,reaction time 10 h).Moreover,Fe2O3,ZnO and ZnFe2Oo4 powder with higher purity were obtained by adjusting the pH values of the leaching solution.The main phase of leaching residue was Fe2O3 which can be returned to sinter and make use of Fe.A novel bifunctional reagent of solid ammonium ferric sulfate dodecahydrate(NH4Fe(SO4)2· 12H2O)was introduced and the crystalline water contained in the extractant was the only source of water molecules,realizing the efficient and selective extraction of Zn from Zn-containing EAFD by using a hydrothermal process.The bifunctional mechanism(extraction of Zn and removal of Fe in one step)was elucidated in detail by analyzing NH4Fe(SO4)2· 12H2O and Zn-containing EAFD phases transformation during hydrothermal process.The effects of solid NH4Fe(SO4)2·12H2O to the dust after water washing mass ratio(RN/Z,g/g),hydrothermal temperature and time on Zn and Fe extraction were systematically investigated.It is indicated that when hydrothermal process was controlled at 220 ? for 10 h with RN/Z of 7:3,93.2%Zn entered into the solution.While 98.4%Fe in the forms of NH4Fe3(SO4)2(OH)6 and Fe2O3 existed in the residue,which can be returned to sintering process.The leachate(0.1 mol/L Zn2+)without purification could be directly used to obtain nanoflower-like metallic Zn through two-electrode electrodeposition method.The multi-metal codoped Ni-Zn spinel ferrite with high purity and good magnetic property was successfully synthesized by a solid reaction method using Zn-containing EAFD and nickel-containing reagent as raw materials.The synthesis mechanism of metal codoped spinel ferrite was revealed.The effects of the mass ratio of Zn-containing EAFD to NiCl2·6H2O(RZE/N,g/g),calcination temperature and time on the chemical compositions,microstructure and magnetic properties of as-prepared samples were systematically investigated.Under the optimal conditions(calcination conditions:RZE/N=2:1.2,1000 ?,2 h;diluted HAc washing conditions:25 ?,flip oscillation 30 r/min,24 h,pH=2.88,20:1 mL/g),the synthesized metal codoped Ni-Zn spinel ferrite has a saturation magnetization(Ms)of 56.8 emu/g and a coercive force(He)of 58.5 Oe.In addition,the toxicity of the synthesized pure spinel ferrite was conducted using the U.S.(TCLP)and European(EN 12457)toxicity testing standards.It is shown that the obtained spinel ferrite was a non-toxic product,realizing the transformation of the dust from hazardous solid waste to green and high value-added magnetic material.And the overall utilization ratio of the Zn-containing EAFD reached about 70%.