Cyanide recovery from a gold mill barren solution containing high levels of copper

The recovery of cyanide from a gold mill solution containing high levels of copper (400 mg/L to 900 mg/L) using the A VR process was evaluated. It was determined that acidification to pH 2 to 3 provides a suitable balance bet ween HCN generation, copper precipitation and acid consumption. A short pil ot plant campaign showed that similar to 75% of the feed WAD cyanide was re covered while the remainder precipitated along with copper upon acidificati on. The acid consumption averaged 3.5 g H2SO4 per litre of solution. The co pper precipitate, consisting mostly of CuCN and CuSCN, was readily separate d from the solution by gravity. The volatilization of HCN in countercurrent packed columns averaged 98.7% efficiency Reabsorption of HCN was practical ly 100% in NaOH solutions and slightly lower in a lime slurry. Basic volati lizer design parameters applicable to a large-scale operation were determin ed, including the Henry's law constant and the mass transfer coefficient of HCN gas.