The principle of dissolving gold and silver values in solutions of potassium or sodium cyanide is old and has been thoroughly carried out in practice for many years. There have been only a few changes in the chemical procedure and also few changes in the type of equipment used.
The gold and silver are dissolved by the chemical solution, usually with varying amounts of oxygen or air added to complete the chemical reaction. The amount of air or oxygen required for a particular ore varies and depends not only upon the size of the particles of the mineral to be dissolved, but also upon the chemical composition of the ore. In most instances the value of the ore and the size of the mineral particles are the deciding factors.
In the past the usual practice to secure complete dissolution of the gold and silver values was to reduce all of the ore to a very finely divided state, practically all through 200 mesh. Recently, however, through the introduction of novel concentrating devices, such as the Mineral Jig and the Unit Flotation Cell, it becomes possible to take out a very large percentage of the values by recovering the valuable metallic minerals (as well as most of the sulphide minerals), which are treated separately, and their removal often eliminates the necessity of fine grinding the remaining relatively barren coarse , particles of the ore. This also results in materially reducing the time and equipment required for the dissolution of the remaining gold and silver values, if their recovery warrants the treatment, and a material saving in equipment cost for this purpose can thereby be effected.
The basic principle of adding certain chemicals to counteract unfavorable chemical constituents in the ore is still necessary on each particular ore and, therefore, complete test work is essential to determine the mesh of grind; the time required for agitation; the settling ratio of the ore and the amounts of chemicals required, together with the correct flowsheet.
The basic method of treatment used for many years in cyaniding gold and silver ores is still fundamentally sound. The four flowsheets shown here are distinct, and show the advancements made in cyanide equipment and methods, described below: