Metal mobility and toxicity of zinc hydrometallurgical residues

Abstract About 80 % of zinc is produced by “roasting-leaching-purifying-electrodeposition”. Lead-silver and jarosite residues are generated during this process, posing a serious threat to the ecosystem and environment since it contains a lot of heavy metals such as Pb. To better understand metal mobility and toxicity of these two zinc hydrothermal residues, it was analyzed by standard leaching toxicity procedures, bioavailability test and sequential extraction procedures. The results showed that the main phases of lead-silver and jarosite residues are KFe3(SO4)2(OH)6, CaSO4·2H2O, PbSO4, Fe3O4 and ZnFe2O4. These two residues contain a high content of Fe, Pb and Zn and also a significant amount of precious metals, such as Ag and In, showing a great resource potential. Meanwhile, lead-silver and jarosite residues are classified as hazardous waste both in China and the USA since their toxicity leaching concentration of Cd, Pb and Se exceeds the threshold of 1, 5 and 1 ppm. Metals contained in these two residues could easily be released to the environment, especially Cd, Pb and Zn, though metals are mainly in the form of residual, higher than 85 %. In addition, lead-silver and jarosite residues also show a significant biohazardous potential since EDTA and DTPA extractable Zn, Pb, Mn, Cd, Cu and As of these two residues are considerably high. All these results could help the zinc manufactories, governments and also the recycling companies better understand the risk of zinc hydrometallurgical residues and urge them to hind this harm to the ecosystem, and most importantly the human being itself.