The production of gold bullion from gravity or flotation concentrates is often an important economic consideration for an isolated gold mining operation. It is assumed in this case that the coarse free gold has been recovered by the Mineral Jig in the grinding circuit and that the jig concentrate has been amalgamated. This treatment produces a portion of the gold (25 to 40 percent of the total) as amalgam, but the balance of the recoverable gold is present as non-amalgamable flotation concentrate together with the amalgamation residue. The problem is therefore, to develop a batch cyanidation procedure that will convert the gold contained in these products into bullion. A batch procedure is desired because of the small quantity of total concentrate (5 to 6 tons) produced daily and the greater flexibility and simplicity afforded by a batch type process.
The Gold Leaching Process Flowsheet
Gravity and flotation concentrates that contain a portion of gold closely associated with sulphides may require fine grinding (—200 mesh) followed by pre-aeration and/or pre-agitation with lime prior to cyanidation in order to minimize the chemical consumption. Refractory gold concentrates may require roasting, but this step is avoided if possible due to the greater cost involved.
BATCH CYANIDATION CYCLES
As shown in the flowsheet, the reground concentrates are alternately pumped to one of three 10’x 10′ Super Agitators equipped with turbine-type propellers. These agitators produce intense mixing and aeration for optimum dissolution of the gold. The Super Agitator with its hood protected propeller permits the solids to settle in the bottom of the tank and then be repulped by simply starting the motor. The agitators are operated as batch units, that is, one agitator is filling while the next agitator is mixing and the third agitator is settling and decanting.
This can best be understood by following the functions of one agitator through all its operating cycles during a 72-hour period. Referring to Tables I and II, Agitator “A” is filled with reground concentrates and given two washes, one with water during pre-aeration followed by one with pre-agitation in the presence of lime. These wash waters are decanted to waste.
Following pre-agitation with lime and decanting the solution to waste, agitator “A” is filled with Number 1 mill solution. After agitation for dissolution of the gold and settling, the pregnant solution is decanted to the pregnant solution tank.
Agitator “A” is then filled with Number 2 mill solution and after agitation and settling, the solution is decanted to Number 1 mill solution tank. The agitator is then filled with barren solution or water and after agitation and settling, the solution is decanted to the Number 2 mill solution tank. The agitator is filled with water and the tailings washed to waste preparatory to filling with a new batch. This completes the entire cycle of operations for agitator “A”. The simultaneous cycles of the other agitators are shown in Table II. The time intervals shown for each cycle can be varied depending on the physical and metallurgical characteristics of the feed.
ZINC PRECIPITATION
The pregnant solution passes from the storage tank to a Zinc Dust Precipitation Unit for recovery of the gold precipitate. The barren solution, as needed, is returned to the three agitators for washing. A portion of the barren solution is also bled off and discarded to prevent the build-up of sulpho-cyanates.
CHEMICAL CONSUMPTION
Inasmuch as the concentrates from any ore will usually represent an eentrenchmentof the base metals and other sulphides contained in the crude ore, a relatively high chemical consumption can be expected during cyanidation. However, the chemical consumption in terms of pounds per ton of concentrate when related back to the crude ore basis will usually not be excessive. The exception would be a concentrate containing appreciable copper which, of course, would preclude the use of cyanidation.
Remarks
The gold extraction will be in the order of 85% depending on the operating conditions. The batch cycle cited in this study include the pre-aeration and pre-agitation (with lime) steps to demonstrate their necessity in some instances.
Source: This article is a reproduction of an excerpt of “In the Public Domain” documents held in 911Metallurgy Corp’s private library.