Minerals Engineering, Vol.12, No.5, 545-558, 1999
Potential use of carbon felt in gold hydrometallurgy
The use of carbon felt as a three-dimensional electrode appears to be very promising for the recovery of heavy metals, and toxic compounds removal from dilute solutions, considering its favourable physico-chemical properties : high specific surface area, good fluid permeability and compressibility, chemical inertness and good electrical conductivity. This work presents the contribution of the carbon felt electrode in two different steps of the gold cyanidation recovery process : firstly as a cathode for the electrowinning of gold solution obtained after the elution of loaded carbon; secondly as an anode for the electro-oxidation of cyanide ions present at low concentration (200-300mg/l) in waste streams. In the first case, more than 10 kg of gold per m(2) of felt (2000 kg of gold per m(3) of felt) can be loaded at 400 A/m(2) from dilute gold solutions (30 mg/l Au) with classic Faradic yields (6-12 %) and high overall extraction efficiency (> 90 %). The felt homogeneously loaded with adhesive gold deposit can be smelted in an electric furnace without addition of fluxes since the carbon felt decomposes in gaseous products without formation of ashes. In the second case, the cyanide ion concentration can be lowered to as low as 10 mg/l by electro-oxidation to cyanate form (CNO-) at 400 A/m2 in the anodic compartment of a divided cell. The presence of copper ion significantly improves the current efficiency of this electro-oxidation and sharply reduces the oxidation of the carbon felt. Copper ions accelerate the oxidation rate of free cyanide ions through the formation of easily oxidizable complexes owing to a mechanism in which the Cu-I/Cu-II redox couple acts as an electron transfer mediator.