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Leaching Gold with Bromine

Bromine has been used in the first years of the last century and can be considered an option when there is an auriferous ore difficult to treat by usual methods. Bromine can be added in several forms. One possibility is to add a bromide salt with chlorine or hypochlorine which converted bromide to bromine. The latter leaches gold as well as platinum at any pH and this is really advantageous. Other possibility is to employ bromocyanide.

By using bromine there some advantages such as easy extraction, and adaptability to several pH values. There disadvantages such as high consumption, interference with assays by atomic absorption.

Au + 2Br- + Br₂ = AuBr₄^- + e

Years ago, bromocyanide was used at the Decloro Mine, Ontario (Canada) on mispickel ore. The process consisted of grinding exceedingly fine, and then agitation with cyanide solution to which bromide of cyanogen was added to intervals. During the treatment protective alkalinity is kept at the lowest possible point due to the instability of the reagent, lime sufficient for settlement being added after the treatment is finished. The quantities of free cyanide and bromocyanide are varied according to the assay value of the ore; any additional bromocyanide needed being added at intervals of several hours.

KCN + Br₂ = KBr + BrCN

BrCN + 2KOH = KBr + KCNO + H₂O

BrCN + 3KCN + 2Au = 2KAu(CN)₂ + KBr

The quantity of BrCN may be determined in a cyanide solution by acidifying with hydrochloric acid, adding excess of iodide and titrating with thiosulphate. BrCN is decomposed by alkali, then is very important observe the free alkalinity. Once finalized the process, can be added enough amount of lime for settling.

Other option to be considered in this leaching type is to employ chlorine to generate bromine from its salts. The free bromine is converted to bromide ions by reaction with native gold as well as by other reactions with gangue species.

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