Ferric and cupric chloride leaching of valuable metals from process residues

The object of the current study was to compare the extraction yields of various valuable metals from process residues by ferric and cupric chloride leaching as an alternative to cyanidation. Flotation tailings, with low amount of gold, copper, nickel, cobalt and zinc were used as a raw material. In the chloride leaching experiments, the effect of the oxidant type and concentration ([Fe³⁺] and [Cu²⁺] from 10 to 50 g/L), was investigated on the metals extraction. At the same time, pH (1 and 1.8), temperature (95 °C), solid/liquid ratio (25 %), oxygen feed rate (1200 mL/min), leaching time (24 h), and stirring rate (950 rpm) were kept constant. Additionally, the reference leaching experiment with sodium cyanide (NaCN) at 22 °C, pH = 11, solid/liquid ratio of 25 %, air feed rate 500 ml/min, stirring rate 400 rpm and 24 hours of leaching time was run to benchmark leaching efficiencies of cyanide- free processes. The results showed that the maximum gold extraction was observed with cyanidation (78 %) and followed by ferric and cupric chloride leaching. For copper, the extractions were higher in chloride leaching (75 - 100 %) whereas cyanide could dissolve 50 % of copper. Extraction of nickel and zinc were slightly higher in ferric chloride leaching compared to cupric chloride leaching, but cobalt extraction was similar in both chloride leaching methods. However, extraction of these metals was insignificant in cyanide leaching. With these results, it seems that chloride leaching with the presence of cupric and ferric oxidants can be used as an alternative cyanide-free method for extraction of metals from process residues.