What is commonly known as the Cyanide Process has for its object the commercially profitable recovery of gold and silver from their ores, and this is accomplished by the solvent action of an alkaline cyanide solution on the precious metals.
The first step in the Cyanidation process is to mill or grind the ore to a fine state of subdivision so as to expose each particle of precious metal to the action of the solvent. The degree of comminution necessary varies for each kind of ore and can only be determined by careful experiment, but it may range from a product whose largest particles will just pass through a No. 30 sieve (aperture about 0.016 in.) to one in which the whole material will pass a No. 200 sieve (aperture about 0.0025 in.).
Whatever form of crushing and grinding machinery be employed, the resulting pulp is composed of a great variety of sizes of ore particles, ranging from the maximum screen size decided upon down to grains whose angular outline can only be seen through a microscope. Mingled with these fine particles of rock there is always more or less impalpably fine and structureless matter of a clayey type, known as colloidal material. Such a variety in the constitution of the pulp produced by the mill renders it impracticable to use a single mechanical method in the application of the cyanide solution to the ore, so the pulp is classified into two products, (a) “sand,” or that part which is sufficiently coarse to allow a solution to percolate through it by gravity, and (b) “slime,” or that part, consisting of the finer siliceous particles and the colloidal matter, which is only percolable, even in thin cakes, by the use of some form of pressure.
“All-sliming” consists in reducing the whole of the ore to such a fine state of subdivision that it is practicable to treat it by the mechanical methods used for treating slime.
Besides containing cyanide, the solution to be used for treating the ore must contain some additional or free alkali, because nearly all ores contain acid or tend to develop it during the course of treatment, and acids react with the alkaline cyanide to form hydrocyanic acid, HCN, thus involving a loss of the active reagent KCN. The alkali most commonly used is lime, both because of its cheapness, and also because some lime is needed in any case to coagulate and settle the separated slime preparatory to the slime treatment process. Caustic soda may also be used as protective alkali, though lime is usually preferable for chemical reasons, apart from other considerations.
When the precious metals have been dissolved out of the ore it still remains to recover them from the solution, or “ precipitate ” them, in such a way that the solution may be available for reuse with as little loss of cyanide as possible. This is done most commonly by the use of metallic zinc, which dissolves in the solution, while the gold and silver are left behind as a fine metallic sludge. This sludge is afterward collected, filter-pressed, and melted down with suitable fluxes into bullion bars.
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