Removal and recovery of heavy metals from tannery sludge subjected to plasma pyro-gasification process

Chromium shavings are toxic waste produced by the tanning industry and include both liquid and solid materials contaminated with chromium(III). Their treatment by pyro-gasification in a plasma torch allows to convert them into an inert solid residue; unfortunately, the high temperatures involved in the process (∼1500 °C) induce the volatilization of some metals, the subsequent condensation of which leads to the formation of hazardous powders that require appropriate management. The aim of this work is to study the most convenient approach to deal with these powders, generated as a by-product when chromium-containing tannery waste is treated with plasma pyrolysis, and recover the heavy metals they contain. Initially, the best conditions were identified to extract those components (Pb, Zn and Cd) that can be leached from the powder, testing solutions of HCl and H2SO4 at various concentrations; subsequently, their recovery was studied considering different treatment strategies. Fractional precipitation by neutralization with 1M NaOH has proven effective for the selective recovery of Pb and Zn, followed by adsorption on chitosan for the extraction of Cd. Finally, an integrated process is proposed, which includes the two separation approaches mentioned above and allows a complete recovery of Cd and Pb, in addition to 96.3% of the initial content of Zn.