Since acidity of the ore causes decomposition of the cyanide, an obvious method of reducing the loss is to add alkali in some form. Before doing this, the free sulphuric acid and soluble salts may be removed by leaching with water, and then a solution of caustic soda or lime is run on to the ore, and after standing for some time is drained off and followed by the cyanide solution. The insoluble basic salts are thus converted into ferric hydrate and soluble sulphates:
Fe2O3. 2SO3 + 4NaOH + OH2 = Fe2(OH)6 + 2Na2SO4
2Fe2O3. SO3 + 4NaOH + 4. OH2 = 2Fe2(OH)6 + 2Na2SO4
Leaching with water then removes the excess of alkali; but, as this cannot be done completely, except with considerable expenditure of time, it is usual to use lime instead of soda. Although the action on the iron salts is slower, an excess of lime is less detrimental than soda to the cyanide solution, and does not attack the zinc. It is found that, even after treatment of the oxidised pyrites by alkalies, the loss of cyanide is much greater than in the case of free milling ores. The reason for this may, in part at least, be attributed to the action of soda on the protosalts (such as sulphates or carbonates) of copper, zinc, by which these metals are precipitated as hydrates, readily soluble in KCy. The preliminary washing with water must always be carefully performed, until no coloration is obtained with ammonium sulphide, so as to remove the soluble salts as far as possible, but some always remain and are converted into hydrates by the alkali.
It is now usually regarded as more advantageous to add lime to the ore before it is charged into the vats, instead of as an alkaline solution. The necessary amount is added to each truck load of tailings, or is fed into the stamp battery with the ore, and is intimately mixed with it by the time it is charged into the vat.
The amount of alkali to be mixed with a charge of ore is determined by laboratory experiments, adding little by little an alkaline solution of known strength to a given weight of the ore, until the whole is neutral to litmus paper.
How to Estimate Available Cyanide
One of the most important points which the operator really wants to know is the dissolving power on gold of the solution. No method of estimation has been described which enables this to be readily determined. After discussing the matter, Clennel concludes that the best way to determine the available cyanide is to compare the rate of dissolution of freshly precipitated gold in a standard cyanide solution with that in the solution to be tested.
Estimation of Free Cyanide using Iodine and Potassium Solution