Selective separation and recovery of selenium and mercury from hazardous acid sludge obtained from the acid-making process of copper smelting plants

Acid sludge obtained from the acid-making process of copper smelting plants is a typical hazardous waste, and its mercury content is >40%. In addition to mercury, the material contains valuable metals such as copper and selenium. Thus, acid sludge treatment has both economic and environmental benefits. Unfortunately, current processes are constrained by complex procedures, severe pollution and difficulty in separating Se and Hg. There are few economically viable methods for the disposal or reuse of this waste. This study proposes a two-stage leaching process to separate and recover Se and Hg from acid sludge. In the first stage of the H2SO4 leaching selectively separated 94.5% of Cu from the toxic acid sludge, while only 0.89% of Se and 0.01% of Hg were leached. To further separate Hg and Se, a leaching system consisting of hydrogen peroxide and hydrochloric acid was adopted in the second leaching stage. During the reaction, selenide was converted into Se in the residue, while Hg was dissolved in solution as HgCl42- . >96% of Hg was leached with 146 g/L HCl and 21.8 g/L H2O2 under a liquid-to-solid (L/S) ratio of 10 at 70 C for 30 min, resulting in significant Se enrichment in the residue. The Se purity in the obtained product was over 84.8%, while the contents of impurities were all lower than 4%. Moreover, the dissolved mercury could be effectively recovered by electrolysis. These findings demonstrate that high recoveries of Cu, Hg and Se from hazardous waste can be achieved with stepwise leaching followed by electrolysis. This treatment protocol for toxic acid sludge not only avoids potential environmental harm but also significantly improves the process economics.