CYANIDE PRACTICE, a term used to cover generally the diverse applications of the process of securing the precious metals from their ores through the use of potassium cyanide. While the process follows certain basic lines, the applications of it vary almost with every mine in which it has been put into practice. This article can give only the general outline of the process, and the interested reader must be referred to the voluminous mass of periodical literature to be found in the current mining magazines and the books suggested below. It should be noted in passing that cyanidation in the extraction of gold from ores has practically superseded the former processes of smelting, pan-amalgamation, bromination, chlorination and in large measure plate-amalgamation, as the most thorough as well as economical method. In a number of plants, however, cvanidation has been abandoned for the newer method of flota tion (q.v.).
The cyanide process is based on the fact that water containing only a very small percentage of potassium cyanide dissolves gold and silver, and that these metals may be recovered from this solution by precipitation with metallic zinc. In the treatment of gold and silver ores by this process the first operation is the crushing of the ore to such a degree of fineness that the cyanide solution may reach every particle of the precious metal present. Until very recently this crushing was done by stamp mills, and this is still the practice at many mines. The crushed material from the stamps was then separated into (fslimen (the finer) and °sand)) (the coarser). The slime was treated by agitation with the cyanide solution in a stirring apparatus, and the sand in a percolator. In the percola tion process a much stronger solution of cyan ide was required, and a much longer time was consumed. The approved practice in the more progressive mines at present is to use the stamps only for the coarser crushing and to reduce this crushed material to the utmost fine ness by further grinding in a tube-mill with quartz pebbles. It is required in some plants that the powder shall all pass a 150-mesh screen. In many plants a solution of cyanide is fed into the material during the crushing. In this case it is usual to pass the crushed material over amalgamating plates of copper as it comes from the stamps: the mercury on the plates seizes the coarser particles of gold at a considerable sav ing of time and expense.
In the case of ordinary stamp-milled material the separation of the fine and the coarse is made by water jets playing upward from the bottom of the V-shaped separator box. These jets are
not strong enough to prevent the coarse material from sinking to the point of the V, but produce sufficient current to carry away the slime over the top of the box and into the collecting vat. This preparation is preliminary to the process ing with cyanide, but without such preparation the cyanide process is only a partial success.
The cyanide process proper consists of three operations: (1) Dissolving the precious metal; (2) precipitating the metal from the solution; (3) smelting the precipitates. In the first opera tion, if "sand" is to be treated by percolation, the material is filled into a tank and some lime or other alkali is scattered over the top and washed into the mass with water, which is then drained away. Then a cyanide solution, carrying one-fourth of 1 per cent of potassium cyanide, is run into the tank from the bottom and this is allowed to stand for a sufficient period — varying from 6 to 12 hours, according to the kind of ore and its degree of fineness. This solution is then drawn off and a weaker solution run in: and this is repeated several times, the proportion of cyanide being less each time, down to a proportion of one tenth of 1 per cent, the last ((washing') being with clear water. The entire operation requires from four to eight days.
In the treatment of slime, after it is carried over into the collecting tank it is allowed to settle partially, lithe being added, after which it is allowed to settle completely. It is then mechanically agitated with more or less of the cyanide solution, according to the proposed after treatment. In what is known as the 'decantation)" process about four tons of the cyanide solution are used to one ton of slime. After agitation (in one of a dozen different ways) the slime is left to settle, and then three tons of the clear cyanide solution containing probably three-fourths of the gold and silver present in the slime are decanted. Three tons of new or "barren" cyanide solution are then added and agitated with the slime, which is again settled and again three tons of solution are drawn off. This procedure is repeated until 97 or 98 per cent of the gold and silver has been recovered. The alternative method of extracting the gold-laden solution is the filtra tion process. There are numerous variations of the filter, the ultimate object sought being to get the last drop possible of the gold-bearing cyanide solution out of the slime. Each type has its advocates, and none seems superior to the others.