Recovery of heavy metals from metal industry waste waters by chemical precipitation and nanofiltration

Abstract

Heavy metals are one of the most important contaminants in water and soil. Heavy metals are discharged to the environment by several industries, such as mining, metallurgical, electronic, electroplating and metal finishing. The removal of heavy metals from wastewaters is of critical importance due to their high toxicity and tendency to accumulate in living organisms. Moreover, heavy metals can not be degraded or destroyed. This study is carried out in collaboration with Met-Mex Penoles, the world’s fourth largest metallurgical complex, and the largest producer of refined silver and metallic bismuth. The complex is located in the city of Torreon Coahuila, Mexico, in a region in which water is scarce. Among the main environmental objectives of Met-Mex Penoles is the reduction of waste water and solid wastes. In the plant under study, a number of waste waters have been targeted. All of them have in common a high concentration of heavy metals (the most abundant ones are As, Cd, Pb, Se, Fe and Zn) and a low pH (below 2 units) as a result of a variety of hydroand pirometallurgical processes that take place in the production processes (i.e. electrolytic separation of zinc, acid scrubber towers, lead refinery, chemical separation of selenium, ammonic sulphate production, etc.). All these metals found in the waste waters are still valuable products that can be recovered or removed from the residual waters allowing the reuse of a clean liquid stream into the production process. The objective of this work is to carry out a comparison between two different waste water treatments in order to optimize the reuse of water and the recovery of heavy metal ions (Zn, Se, As, Cd and Pb). First of all, metal precipitation using sulfides has been evaluated in terms of metal removal, amount of sludge produced, and water reuse. Then, nanofiltration is studied as an alternative separation processes as well as a second step in an integrated process with precipitation, in order to obtain a water stream that can be directly reused in the production process. *Corresponding author.