A process for aerating or oxygenating cyanide solutions was announced by T. K. Prentice in the Jour. C.M. and M.S.S.A., February, 1934 (see Fig. 56). It immediately attracted attention, and the article was reprinted in part by United States and Australian technical journals. The process received practical plant trials at the Nourse mine on the Rand before it was made public. These were mainly on sand which is leached at the mine. Gold extraction was slightly higher when aerated solutions were used, and consumption of cyanide was a third less than in regular treatment. The oxygen content of solutions at the Nourse mine for dissolving gold ranged from 2½ to 5½ milligrams per liter and averaged 4 milligrams per liter. At one time it fell to 1 milligram per liter, and a series of high residues resulted. Six milligrams per liter is considered desirable. The oxygen content of circuit solutions in 16 plants on the Rand averaged 4.5 milligrams per liter.
In brief, the oxygenating process is as follows, with reference to Fig. 56, which is the patented plant-scale equipment: Cyanide solution is drawn from a stock tank and pumped into the drum or cylinder shown, first passing through the pipe with ½-in. holes to form a spray. Air at 100-lb. pressure is generated by the motor-driven (3-hp.) compressor atop the cylinder. The aerated solution leaves at the rate of 2 tons per min., the balanced float valve shown, attached to the discharge pipe, regulating the level of solution in the cylinder. The solution is milky white, owing to the disseminated air, but it clears in a few minutes. At this stage it may carry 6 milligrams of oxygen per liter and retains within 2 milligrams of this amount for 22 hr., which is long enough for the solution to be effective during the first stage of leaching.
OXYGEN IN MILL SOLUTIONS
Since the presence of oxygen is an essential factor in the extraction of gold and silver by cyanide, the question of the amount of oxygen present in various parts of the circuit becomes a matter of considerable importance.
Quantitatively this may be expressed as milligrams per liter or, more frequently, as per cent of saturation under the prevailing barometric conditions.
In general, simple exposure to the atmosphere replenishes the lack of oxygen, but from a practical standpoint appreciable air must be forced into the pulp undergoing treatment to maintain a satisfactory oxygen content. Chemical oxidizers have been tried, but they are both expensive and in the long run less satisfactory than air, which is invariably introduced in the agitation step (see Chap. VIII).
Aeration or oxygenation of sand, slime, and cyanide solutions has been practiced during the several decades of cyanidation—purposely and incidentally—but not until recently has it been given the careful study that it deserves. Every time a pulp or solution is stirred or transferred, it absorbs some oxygen, but special methods or devices, have been developed to entrain air in pulps and solutions. Aeration in the Dorr agitator is obtained from the compressed air used for circulating the pulp through the revolving central lift column and also from the atmosphere when the elevated pulp is redistributed over the pulp surface in a series of small streams from the distributing launders. Additional compressed-air jets are sometimes attached to the revolving arms for greater aeration. In the Pachuca agitator compressed air used for circulating pulp through the stationary lift pipe is the only source of oxygen. The Devereux and other types of mesh-propeller agitators rely on the vortex created to entrain air.
The Turbo- and Wallace-type agitators, both of which thoroughly incorporate air in the pulp through the action of their impellers, are used in several of the Kirkland Lake mills for agitation in small tanks and are also used in the top of and near the surface of Dorr agitators to increase normal aeration.