Heat treatment-enhanced bioleaching of new electroplating sludge containing high concentration of CuS and its mechanisms

Electroplating sludge by CaO/Ca(OH)₂ precipitation craft represents a typical toxic heavy metals-bearing hazardous waste worldwide. Recently, sulfide electroplating sludge is growingly produced as a substitute for Ca(OH)₂-based hydroxides sludge since sulfide precipitation process sharply reduces the production of sludge, posing an urgent challenge to the traditional treatment ways suitable for hydroxides sludge. In this work, bioleaching was firstly utilized to extract Cu from the new sulfide electroplating sludge containing high concentration of CuS, and a thermal pre-treatment was used to promote the bioleaching performance. The results showed that a low Cu dissolution of 53% from the original sulfide sludge by bioleaching was gained because of the refractory nature of sulfides. However, a thermal treatment evidently enhanced the bioleaching efficiency of sulfide electroplating sludge. The Cu release efficiency sharply rose from 59% to 100% when the roast temperature lifted from 200 °C to 600 °C. However, a higher temperature of 800 °C strongly injured the bioleaching, gaining the lowest Cu release of 46%. A full conversion of refractory CuS into highly water soluble CuSO₄ occurred with the 600℃-roast, concomitant with the formation of a finer particle which witnessed a great rise in the specific surface area from 3.64 to 12.06 m²/g, accounting for 100% Cu liberation by bioleaching. In contrast, the calcination at 800 °C prompted the decomposition of CuSO₄ to form CaSO₄ which not only covered the sludge as coating layer but also induced sludge aggregation to form a larger particle with a smaller specific surface area of 1.22 m²/g, inhibiting the bioleaching. This present works revealed the internal mechanisms for a perfect combined process of the thermal treatment and bioleaching for tackling the sulfide electroplating sludge.