Dissolution of gold in oxidized bromide solutions

Bromide stabilizes the auric ion by forming a stable complex in acidic aqueous solution. The oxidizing agent normally used in this process is the bromine. This is, however, a corrosive liquid with high vapour pressure and this is the main reason why it has not found acceptance yet as an industrial oxidant. In the present work, ferric ion, hydrogen peroxide and sodium hypochlorite were tested as alternative oxidants, alone or in combination, to evaluate the dissolution of gold in a bromide-containing aqueous solution. The advantage is that this is a cleaner technology and thus environmental protection costs may be minimized. Using the rotating disc technique, the effects of rotating speed, leaching time, temperature, oxidant concentrations, used together or separately, in an aqueous bromide solution with different concentrations of this complexant were evaluated. Ferric ion alone is able to dissolve gold, but kinetic rates are not elevated. Ferric ion and hydrogen peroxide can dissolve gold at higher rates. Hydrogen peroxide alone will not oxidize the gold and therefore the complex cannot be formed. Sodium hypochlorite alone, however, is able to promote the dissolution of gold in a sodium bromide solution. One of the main conclusions is that the gold dissolution obtained with the bromide solution as used in this work may, in some cases, be as good as or even superior to those obtained with the cyanidation process.