Leaching of nickel and copper from electroplating sludges containing Ni-Fe and Ni-Cu-Cr

Abstract

Electroplating sludges are produced by wastewater and spend baths treatment of the metal finishing and electrochemical industries. Due to their high heavy metal (Fe, Cr, Ni, Cu, Co…) content this type of sludge is considered as hazardous for the environment. For many years disposal in especial landfills has been the option to deal with sludges but not the solution to the problem. Since the sludge still contains significant amounts of valuable metals a process to recover them can be an interesting option. Recovering these metals reduces the disposal problem while providing new metal resources. Hydrometallurgy offers the chance to extract almost any metal by using aqueous chemistry. Ammonia salts can form complexes with nickel and copper while their cost and toxicity is low. That allows the leaching of these target metals with ammonia as the first stage inside an hydrometallurgical rout that also engloves solvent extraction and electrowining or crystalitzation. In this paper leaching behaviour of two different sludges with ammonium sulphate is studied. Also an alternative acidic leaching will be carried on to compare both leaching methods. The sludges studied are mainly composed of Ni-Fe (sludge 1) and Ni-Cu-Cr (sludge 2). Effect on the leaching of diferent parameters like solid to liquid ratio, initial pH or number of consecutive leachings is studied with the aim to define suitablest leaching conditions. In both sludges S/L ratio and number of consecutive leachings are the parameters that seems to affect greater leaching rates. Highest leaching rates obtained for sludge 1 and sludge 2 are 13% with 5 leachings and 36% with 4 leachings respectively. High iron and chromium seems to be the main cause of the low leaching rates obtained. In sulphurical acid leaching lower initial pH benefits leaching effectivenes while too much stirring speed seems to decrease leaching rates. For sludge 2 initial pH of 0,5 and stirring speed of 1625 are the suitablest conditions achieving an extraction of 100% in 70 minutes. Sludge 1 presents leaching problems beacose of the high pH demand of iron and his leaching rates with the same conditions arent acceptable. An An hydrometallurgical process to recover nickel and copper involving acidic leaching at a constant pH of 0,5 its proposed since this conditions achieved better leaching behaviours for both sludges. Following the leaching with the precipitation of chromium and iron using ammonia allows the rute to return to the first explained ammonia leaching route where the leached obtained it’s the complex between ammonia and Ni/Cu.