Development of a Process for Copper Recovering from Galvanic Sludges

Galvanic coating processes are based on metal plating baths and are responsible for the production of large amounts of wastewaters. Subsequent physical-chemical treatment of the wastewaters generates solid wastes called galvanic sludges. These sludges have a hazardous character and are often disposed, mainly on landfills, without any economical or environmental benefits. The development of alternatives and viable ways to reduce the environmental impact and recover the valuable metals contained in those sludges such as copper, chromium, nickel or zinc, which content might reach 30% (wt.%, dry weight) are of utmost importance. The present work has been developed in the aim of the project VALMETAIS and proposes a hydrometallurgical process for copper recovery from galvanic sludges produced by Ni/Cr plating plants. This procedure has been developed on laboratory scale and is based on leaching of sludges in sulphuric acid solution followed by copper cementation step, using iron scrap as a precipitating agent. The sludge has been characterized for its chemical and physical properties. Chemical analysis showed a copper concentration of more than 10% (dry base). Preliminary leaching tests in both sulphuric acid and ammoniacal media were performed in order to determine the best operating conditions for this step of the process and to assure the best metal recovery conditions in subsequent separation methods. Sulphuric acid yielded much higher metal ion dissolution when compared with ammoniacal leaching. Optimal experimental leaching parameters were defined as follows: sulphuric acid solution 100 g/l, a solid to liquid ratio of 1:10, stirring speed of 400 rpm at room temperature and under atmospheric pressure. It was found that metals dissolution was almost complete in 30 minutes of reaction time. Extraction rates of 99% for Cu and Ni were obtained under the leaching conditions above mentioned. The solid residue separated from the leaching solution is mostly constituted by gypsum (CaSO4), and presents a metal content below 1%. The subsequent extraction of cooper from the obtained solution is achieved by a cementation step with iron scrap. Copper precipitation was performed at a pH of 2 which was achieved through adding new sludge to the filtered leaching solution. Such pH level led to insignificant precipitation of other metals present in the leaching solution, namely chromium. The recovery rate of copper is about 90% and the purity grade of the resulting copper cement enables its application as a commercial product.