Metallic Copper (Cu[0]) Obtained from Cu2+-Rich Acidic Mine Waters by Two Different Reduction Methods: Crystallographic and Geochemical Aspects

Abstract

The recovery of valuable metals from different types of wastes has become of prime strategic interest given the scarcity of primary critical raw materials at international scale. Implementation of new methods or refinement of classical techniques with modern technological advances is, therefore, an active research field. Mine wastes are of special interest because their high metal concentrations make them environmentally harmful and economically profitable at the same time. In this study, we evaluated two different methods of Cu recovery from extremely acidic mine waters seeping from wastes and abandoned mines in SW Spain. Through a series of different batch experiments, we compared the method efficiency and crystallographic properties of elemental copper (Cu[0]) obtained by reduction of Cu2+ ions by (1) chemical reduction using ascorbic acid at different environmental conditions of pH (1.50–3.95), temperature (25–80 °C) and ascorbic acid concentration (10 mM to 0.1 M), and (2) classical cementation method with scrap iron at pH 1.50 and 25 °C. Our study demonstrates that the precipitation of Cu[0] can take place at pH 3.95 and low AA concentrations (0.1 M), resulting in large (µm-scale), perfectly developed crystals of copper with pseudoprismatic to acicular habit after 24 h of aging, likely through formation of a transient compound consisting in Cu2+-ascorbate and/or cuprite (Cu2O) nanocolloids. Reduction experiments at higher AA concentrations (0.1 M) showed faster precipitation kinetics and resulted in high-purity (>98%) copper suspensions formed by subrounded nanoparticles. The AA method, however, yielded very low recovery rates (15–25%) because of the low pH values considered. The cementation method, which produced tree-like aggregates formed by sub-micron crystals arranged in different directions, proved to be much more efficient (>98% recovery) and cost-effective.