Gold Leaching

Gold Leaching

Gold Leaching Process Circuit NO. CY-1

This leach plant shows the continuous counter-current decantation system, in which all the ore is first reduced to a very fine state in the grinding mill-classifier circuit, in a Gold Leaching solution. The slime overflow of the classifier, usually 70%—200 mesh, or finer, is sent to the first thickener, known as the primary thickener.

Here the pregnant solution, containing a large part of the dissolved gold and silver values, overflows from the top of the thickener and is sent to a clarifier, then to the precipitating system where the gold and silver values are precipitated, the precipitate periodically being reduced to bullion in a furnace.

The underflow pulp of the primary thickener, containing the remaining values and usually at a pulp ratio of 1:1, is elevated by a diaphragm pump into a series of agitators. The agitators are used to aid in the dissolution of the remaining values, augmented by the introduction of air. The time of treatment required depends upon the values still remaining in the slimes or fine sands in the mill pulps.

From the agitators, the pulp at a density of approximately 1:1 overflows directly into the first of a series of secondary thickeners. This feed pulp is diluted with the overflow solution from the second of the secondary thickeners, which solution has been enriched by the removal of values from the second and third secondary thickeners. After the solution is intimately mixed with the pulp it enters the thickener feed well, rises to the top of the thickener, is overflowed and is pumped to either the mill solution tank or introduced into the precipitation system. The thickened pulp from the first of the secondary thickeners settles to the bottom, is raked to the center discharge, and is then pumped to the second secondary thickener where it is mixed with the overflow solution from the third of the secondary thickeners. Thus the pulp flows in one direction with gradually decreasing values, while the solution is pumped in the opposite direction with gradually increasing values, from a barren wash water to a sufficiently enriched solution suitable for precipitation or mill solution storage.

Gold Leaching Process Circuit NO. CY-2

This next gold leaching circuit is practically the same as Flowsheet No. CY-1, only in place of the last two secondary thickeners a rotary filter is used. On some ores two or more thickeners are used between the agitators and the filter. There is no question about the lower soluble loss when using a filter but the cost of the filter operation, amounting to from 8c to 10c per ton, is an important item as compared to the low cost of operating thickeners. Therefore, the question as to whether a filter or a thickener is to be used at the tail end of the Gold Leaching mill is a matter of economics, depending upon the housing facilities, initial investment in the plant, and operating cost. A tailing filter is necessary where water for milling is scarce, or where pollution of streams is prohibited.

Gold Leaching Process Circuit  NO. CY-3

That Leaching Process Circuit is shown one of the outstanding improvements made in cyanidation whereby the coarse metallic minerals are removed from the grinding circuit by means of the Mineral Jig. The hutch product from this jig is amalgamated with a Amalgamator or Clean¬Up Pan and thus the coarse mineral values, such as metallic gold, are removed from the circuit before complete fine grinding takes place. This jig hutch product containing most of the fine as well as the coarse metallics, can also be treated by a separate grinding and Leaching treatment plant.

By the early removal of the coarse gold and other minerals, it is possible to materially reduce the chemical consumption in the Gold Leaching plant and also the time required for agitation, as well as offering the possibility of grinding at a coarser mesh. In other words, the removal of a certain portion of the minerals at a coarse mesh, and their separate treatment, either by amalgamation or by a small Gold Leaching plant, eliminates the necessity of fine grinding the entire tonnage to recover the values from this small portion of the ore that often contains the majority of values.

The balance of the circuit is then similar to that shown on Flowsheet No. CY-1, with the advantages, however, of lower chemical consumption and a lower agitating time, with less equipment needed for final thickening and filtering. This is due to the high efficiency of the Mineral Jig in removing not only the coarse values that are free and that would require additional time and equipment, but also in removing most of the middling values that would require finer grinding of ore and higher costs.

Gold Leaching Process Circuit NO. CY-4

This is similar to Leaching Process Circuit Flowsheet No. CY-3, excepting that a Unit Flotation Cell is used between the ball mill and the classifier to remove cyanicides or harmful elements from the Gold Leaching circuit, thus preventing an excessive chemical consumption. This concentrate produced on the Unit Flotation Cell can be sent to a smelter or treated separately from the balance of the Gold Leaching circuit. This method of treatment has been proved distinctly advantageous and should have an important application on ores where copper and other cyanicides are present. Flotation chemicals or reagents used in the Unit Flotation Cell do not affect the subsequent treatment by gold cyanidation.

In many instances this Unit Flotation Cell, due to the additional aeration secured, plays an important part in the chemical reaction between the cyanide and the gold and silver values. In the cyaniding of silver ores it is essential to have ample air and therefore the air that is introduced in the Unit Flotation Cell and in the Super-Agitators, is very important.


Source: This article is a reproduction of an excerpt of “In the Public Domain” documents held in 911Metallurgy Corp’s private library.