A Novel Process To Recover Zinc,Lead And Silver From Residue Of Zinc Pressure-leaching By Using Bioleaching Combined With Chloride Leaching

There are many valuable metals in the residues from zinc pressure-leaching,such as zinc,lead and silver,etc.,which have a high value of comprehensive recovery.At present,three types of methods were mainly used for treating these residues,including pyrometallurgy method,chemical leaching and mineral processing.However,these methods have disadvantages of high energy consumption,large reagent cost and low recovery rate.Aiming at solving these problems,a novel process to recover zinc,lead and silver from residue of zinc pressure-leaching by using bioleaching combined with chloride leaching was proposed and experimentally studied in this work.The technological process contains three key sections,such as the breeding and domestication of highly efficient leaching microorganisms,bioleaching of zinc pressure-leaching residue and chloride leaching of bioleaching residue.For this process,the relevant fundamental studies were carried out in this paper,which can provide theoretical and technical guidance for recovering zinc,lead and silver from zinc pressure-leaching residue with the purpose of environmental-friendly,high efficiency and comprehensive utilization.The process mineralogy of zinc pressure-leaching residue obtained from a zinc smelter in Guangdong Province was studied.The contents of zinc,lead and silver were 3.16%,17.00%and 261.20 g/t,respectively.The phase of lead sulfate and lead sulfide accounted for 97.56%of the total lead-bearing phase;the phase of zinc sulfate and zinc sulfide dominated 86.52%of the total zinc-bearing phase;the phase of silver and silver sulfide occupied 77.18%of the total silver-bearing phase.The particle size less than 30?m accounted for 86.54%of the residue,and the intergrowth relationship of minerals in the residue was relatively complex.Sulfate-bearing phases were often embedded with gangue minerals,such as orthoclase and quartz,etc.The different gangue minerals were disseminated with each other,and a few of monomer dissociated lead sulfate and other minerals were detected.A high efficient leaching strain was bred and domesticated,and its suitable growth temperature was 45?.The conditions for the growth of bacteria were optimized and the optimum conditions were initial pH 1.5,pulp density 10%,inoculation size 10%and leaching time 15 days.The zinc content in the bioleaching residue was 0.4070%,and its leaching extent was 88.07%.The contents of lead and silver were 17.94%and 271.44 g/t,respectively.The lead and silver were enriched in the bioleaching process,and the oxidation rate of lead sulfide and silver sulfide were 71.51%and 62.60%,respectively.The leaching behavior and mechanism of bioleaching process of zinc pressure-leaching residue were revealed.In the early stage,the dominant bacteria were iron-oxidizing bacteria and sulfur-oxidizing bacteria,causing relevant reactions exists in the bioleaching system,including iron oxidation,sulfur oxidation and sulfide oxidation reactions.According to the Eh-pH diagram,when pH value was in a stable stage,the increasing of redox potential of the leaching system can promote the oxidation reaction of zinc sulfide,lead sulfide and silver sulfide.In the middle and later stage of bioleaching,the pH value of the leaching system gradually decreased to a stable range.The relative occupation of sulfur oxidizing bacteria decreased due to the lack of energy source.Nevertheless,the iron oxidizing bacteria gradually grew into dominant strains due to the energy source was sufficient for iron oxidizing bacteria.The oxidation of Fe2+to Fe3+by iron oxidizing bacteria leaded the increasing of the redox potential of the leaching system and caused a further oxidation of zinc sulfide,lead sulfide and silver sulfide.The chloride leaching experiments by using of NaCl-CaCl2-HCl-H2O was systematically studied and the optimized conditions were obtained.In conditions of NaCl 300 g/L,CaCl2 20 g/L,pH 0.5,H2O2 2%(V/V),liquid-solid ratio 10:1,leaching temperature 90?,stirring speed 350 r/min,leaching time 120 min,the leaching extents of Pb and Ag can reach to 95.47%and 83.86%,respectively.Compared with the chloride leaching of un-biotreated residues,the leaching extents of Pb and Ag increased 4.59%and 15.33%after bioleaching process,respectively.Additionally,94.40%Zn was leached by bioleaching combined with chloride leaching process.The chloride leaching kinetics of the bioleaching residue in NaCl-CaCl2-HCl-H2O system by using unreacted shrinking core model under the studied conditions were successfully modeled and the semi-empirical kinetic equations were established.The results showed that the activation energy of chloride leaching of Pb and Ag were 22.66 and 10.88 kJ/mol,respectively.The chloride leaching of Pb was controlled by both inner diffusion control step chemical reaction step,and the Ag chloride leaching was controlled by inner diffusion control step.