Bromine leaching as an alternative method for gold dissolution

It is known since the end of the XIX century that bromine is an excellent leaching reagent for gold but the research concerning its potential application as an alternative reagent has been relatively despised, mainly because elemental bromine is hazardous and extremely difficult to store or transport in safe conditions. We performed experimental tests in order to assess the feasibility of bromine produced in-situ, in the leaching reactor, to dissolve gold using ore samples from the unexploited Portuguese mine of Castromil. The reagents used in the production of bromine (NaBr, NaOCl and HCl) can be more safely stored than bromine. The ore sample was crushed and grinded in order to produce a particle size distribution (D-80 = 0.48 mm) that would guarantee that most of the gold and electrum would be liberated in accordance with previous mineralogical studies. The following operating conditions were assessed: temperature, solid/liquid ratio, composition of the leaching reagent, stirring speed, residence time, effect of pretreatments and the addition of oxidants to the bromine solution. The highest gold dissolution (89%) was achieved with a previous roasting, although the simultaneous addition of H2O2 and FeCls allows for a similar recovery (80%) with much lower environmental impact. These results were achieved with the following leaching solution: 4.4 g NaBr, 30 mL NaOCl, 42 mL HCl and 28 mL H2O and with the following operating conditions: a solid/liquid ratio of 1, temperature around 95 degrees C, stirring speed of 450 rpm, natural reaction pH and a leaching time of 4 h. The comparison between bromine, cyanide and thiosulfate leaching for the same ore indicates that bromine is a good technical alternative for leaching this gold ore. The following advantages may be stressed: fast dissolution rate, non-toxic reagents, it does not require the use of pH regulator and can be performed with low quantities of leaching solution.