Bromine

    The application of bromine as leaching agent for gold ores has studied and practiced since 1882. The main problems found in its application are related to corrosion and high vapor pressure, which are natural characteristics of this reagent. It has been determined that a synthetic liquid bromine carrier has a considerable lower vapor pressure than liquid bromine. The dissolution of gold in a bromine solution is an electrochemical process that can be described by the following reaction,
    Au + 4Br- = AuBr4- + 3e-
    Essentially, the leaching of gold in a bromine solution is affected by bromide anodic gold concentration, pH, and cathodic and anodic potential developed during the process. The complex anion AuBr4- is stable at 1000 mV and neutral pH. The gold dissolution reaction in an acid solution of bromine is described by the following reaction,
    2Au + 3HBrO + 5Br- + 3H+ = 2AuBrS4- + 3H2O
    Bromine can be added in several forms such as a bromine salt with chlorine or hypochlorite, which convert bromide to bromine. Bromines leached gold as well as platinum at acid, neutral or alkaline pH. This aspect is advantageous when treating certain black sands concentrate difficult to clean by gravimetric devices. According to the above reaction, gold is first oxidized to Au+3 by HBrO, and then stabilized by forming the AuBr complex with bromide. In this case the best leaching conditions are obtained at ambient temperature, pH 3-5 and a potential of 700-900mV. The main advantages of bromine are related to high leaching rate, adaptability to a wide range pH values, gold from pregnant solution can be easily recovered by activated carbon, ion exchange resins or Merrill Crowe Process (zinc precipitation).
    A refractory gold concentrate containing 240 g/t Au from a Canadian operation was roasted and calcine was leached with cyanide and bromine. Next figures show the high leaching rate of bromine. Cyanidation was done at pH 10.5-11.0, 1g/L NaCN. Leaching in bromine was practiced at pH 5.0 – 6.0, 4 g/L bromine.
     
    Gold recovery using cyanide and bromine
    Pregnant solution was treated with activated carbon, zinc powder and resins. Best results were obtained with zinc powder.

     

    Method
    Au, ppm
    Recovery, %
    Feed
    Barren
    Activated carbon
    50.3
    0.5
    99.01
    Zinc powder
    50.3
    0.1
    99.80
    Resins
    50.3
    1.5
    97.02

     

    Encouraging results motivated the application of bromine in several forms. One liquid bromine carrier called Geobrom 3400 and other one known as Bio-D Leachant were offered in North America some years ago. The second product was tested in Nevada on 5,000 t test heaps and results were very optimistic.