Extraction of Gold and Silver from Geothermal Fluid

Abstract

This paper describes five separate experiments conducted at KA35 well at the Kawerau geothermal field. The experiments were designed to try to recover the gold and silver which was initially dissolved in the geothermal water. The fluid from the well was depressurised and fed through a mild steel collection vessel. The vessel contained a series of collection devices which were analysed after each experiment for gold and silver recovery. A wide variety of materials were tested in a number of difference physical environments. The gold recoveries varied from 0.007 to 0.24 mg per tonne of geothermal fluid. Silver recoveries were 0.096 to 0.84 mg per tonne of geothermal fluid. The maximum recoveries were obtained in the first experiments using mild steel collection plates, and subsequent changes to physical parameters and collection materials only reduced the gold and silver recovery. A surprising discovery was that activated charcoal is useless as a collection medium and actually seems to redissolve depositing gold. The experiments concluded when the well became blocked by calcite deposition. It is thought that the characteristics and chemical composition of the fluid discharged by the well was changing towards the end of the experiments.