| There are plenty of low-grade sulfur-oxygen mixed zinc ores in the southwest of China,which are characterized by the low grade of zinc,the symbiosis of zinc sulfide ore and zinc oxide ore,the structure of complex,and the high contents of alkaline gangue and SiO2,so it had been not suitable to use acidic leaching.Ammonia leaching method can avoid the dissolution of alkaline gangue and SiO2,which leads to the reducing of reagent consumption,and it can handle such complex zinc ore.However,the insoluble sphalerite or marmatite in the sulphur-oxygen mixed zinc ore can't be leached by this method,resulting in low leaching rate of zinc.In view of the shortcomings of ammonia leaching method and the high content of CaCO3 in sulphur-oxygen mixed zinc ore,the oxidizing roasting of low-grade sulfur-oxygen mixed zinc ore containing CaCO3 was performed in this paper,aiming to transform zinc sulfide minerals in ore to soluble zinc oxide and improve the leaching rate of zinc in ammonia leaching process.The major research results are drawn as follows:?1?the zinc grade of low grade sulfur-oxygen mixed zinc ore is only 8.73%,and the content of Si,Fe and Ca are relatively high.The mineral constituents made up of zinc-bearing mineral heteropolar ore?Zn4Si2O7?OH?2·H2O?,zinc sulfide ore?ZnS?,quartz?SiO2?,pyrite?FeS2?and calcite?CaCO3?,etc.The leaching rate of zinc was only 59.68%with the ore leached by ammonia solution,and this is due to the difficult leaching of sphalerite.?2?The thermodynamic analysis shows that CaCO3 can promote the ZnS oxidation to ZnO,and then the formation SO2 by ZnS oxidation will be further translated into CaSO4.The dominant regions of the Zn-Ca-S-O system,Fe-Ca-S-O system and Zn-Fe-S-O system indicate the final stable phases are ZnO,Fe2O3,CaSO4 and CaO in the calcination process under conditions of temperature of 700?,pO2/p?=10-0.67 and 10-13?pSO2/p??10-3.Meanwhile,CaO generation by CaCO3 self-decomposition will first react with Fe2O3 to form Ca2Fe2O5.?3?TG-DSC analysis shows that when CaCO3 was added to the mixture of FeS2 and ZnS,the 4FeS2+11O2=8SO2+2Fe2O3 reaction would occur in the temperature range of 420?525?,which was controlled by the interfacial chemical reaction corresponding the apparent activation energy of 79.88 kJ·mol-1;In the range of 600?680?,the reaction are 2ZnS+2O2=3ZnO+3SO2 and 2CaCO3+2SO2+O2=2CaSO4+2CO2,which were controlled by the interfacial chemical reaction,and the apparent activation energy was 215.13 kJ·mol-1;The mixture of FeS2 and ZnS without CaCO3 were also controlled by interfacial chemical reaction in these two stages,and corresponding apparent activation energies were 88.62 kJ·mol-1 and 244.95 kJ·mol-1,respectively.Apparently,the addition of CaCO3 decreased the apparent activation energies of FeS2 and ZnS reactions.?4?The phase transformation between ZnS and FeS2 in the oxidation process would be promoted with addition of CaCO3 according to Ca:S of 1:1,and the sulfur of the material can be converted into CaSO4.In addition,the formation of ZnFe2O4 can be restricted effectively by CaCO3.?5?the hard-soluble ZnS in the high-iron sphalerite can be converted to a readily soluble ZnO with CaCO3.Under conditions of the Ca:S of 1:1,the roasting temperature of 700?and the roasting time of 1 h,The products of oxidation roasting of high iron sphalerite mainly contain CaSO4,ZnO,Fe2O3,CaO and Ca2Fe2O5.The formation of CaSO4 indicates that CaCO3 had a positive effect on sulfur fixation,and meanwhile the generation of Ca2Fe2O5 showed that the insoluble ZnFe2O4 formation would be inhibited by CaCO3.?6?During the oxidation roasting of low grade sulfur-oxygen mixed zinc ore?8.73%Zn?,both heteropolarite and sphalerite can be converted into easily soluble ZnO with CaCO3.Under conditions of the Ca:S of 1:1,the roasted temperature of 700?and the roasted time of 1 h,the phases in the roasted product were CaSiO3,CaSO4,ZnO,Fe2O3,SiO2,Ca2Fe2O5 and PbSO4,and the zinc leaching rate from the roasting products can reached to 90.65%using ammonia solution,Compared with the direct leaching of low-grade sulfur-oxygen mixed zinc ore,it increased by 30.97 percentage points. |
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