Bromo salts are a mixture of 57 per cent sodium bromide, NaBr, and 43 per cent sodium bromate, NaBrO3, in the form of light-gray, light- yellow, or reddish-brown crystals or powder.
The use of bromo salts for treating a telluride concentrate in the Wright- Hargreaves plant at Kirkland Lake is described by J. T. Willey in E. and M.J., July 7, 1928.
Two 12- by 10-ft. tanks were used as collectors. They had mechanical agitators and a capacity of 15 tons solids and 15 tons solution. When thoroughly mixed, samples were taken for specific gravity, moisture, alkalinity, and cyanide content. If necessary, sulphuric acid was added to reduce alkalinity to 0.1 per cent, and cyanide was added to increase the strength to 1 lb. per ton.
Bromocyanide was next made by mixing bromo salts, cyanide, and sulphuric acid. To make 1 lb. bromocyanide it requires 521 grams bromo salts in 1500 cc water, 207 grams potassium cyanide in 1500 cc water, and 486 grams 66°Be. acid in 4500 cc water. Twenty pounds bromocyanide was made for each charge of 15 tons concentrate. The procedure was as follows:
Three 40-gal barrels were used. Into the first, which was immediately above the agitating tank, was put the requisite quantity of sulphuric acid. For 20 lb. bromocyanide this barrel held twenty times the quantity of water and acid given in the formula. The other two barrels were immediately above the acid barrel. From the bottom of each of these barrels was a lead pipe leading to the acid barrel, which itself had a lead pipe leading to the agitator. Plugs served as valves. The acid barrel was covered to prevent escape of bromine fumes and also had a hand-worked paddle mixer. Into one of the upper barrels was placed twenty times the quantity of bromine salts and water given in the recipe, and in the other barrel an equivalent amount of cyanide and water. The salts were then dissolved. Next, the plugs in the upper barrels were pulled out simultaneously, letting the solutions into the acid barrel, the contents of which were stirred. The 20 lb. bromocyanide was then run into the agitator. Stirring- proceeded for 24 hr. when the pulp was sampled and assayed. If high in gold, more bromocyanide was run in, and the agitation repeated until extraction was complete. During July, 1927, 70 tons concentrate averaged $76 per ton before treatment and $1.50 after treatment, equivalent to 98 per cent extraction.
The bromocyanide process has been successfully applied to some mispickel-gold ores, for the reason that this process requires no oxygen for gold dissolution. Some of the reactions involved embrace the following:
(FeAsS)2 + 11CNBr + 11H2O = As2O3 + FeSO4 + 11HBr + 11HCN
and, 2Au + 3NaCN + CNBr = 2NaAu(CN)2 + NaBr
The HCN is absorbed by the addition of alkali during the process.
It is the opinion of Julian and Smart that the activity of the bromo salt is not due to the liberation of cyanogen, though that probably occurs, but to a liberation of oxygen according to the equation
2 BrCN + NaCN + 4NaOH = 2NaBr + 2NaCN + NaCNO + 2H2O + O
Since BrCN is decomposed by alkali, it is important that free alkali should be kept very low during the treatment, and any lime necessary for settlement being added at its conclusion.
BrCN + 2NaOH = NaBr + NaCNO + H2O
The process was once used at Deloro mine, Ontario, Canada, for the treatment of an arsenopyrite ore and was developed in Australia by Dr. Diehl for treatment of the sulphotelluride ores of the Kalgurli district. It is practically obsolete today but should be kept in mind as a possible method for handling refractory gold-bearing materials.