Use of Cyanide in the Stamp Battery to treat the ore direct from the stamp milling battery instead of first passing it over amalgamated plates. Ore can be crushed with cyanide solution instead of water, and led at once into the filtering tanks. The results are stated to have shown that the coarse gold resisted the attack of the cyanide for so long a time as to render the process uneconomical.

At a mine in Bingham, Utah, a combination of the amalgamation and cyanide processes is said to have been applied successfully to ore containing a mixture of coarse free gold and fine rebellious particles. The ore is crushed and amalgamated in Huntington and Crawford mills successively, the “water” used being a solution of cyanide of potassium. After leaving the mills the solution and pulp are run into settling tanks and the liquid drawn off. It would appear likely that the decomposition of the cyanide in the mills would be excessive, but no details of the working have been published.

Elsewhere, a somewhat different method was adopted, and pyritic ores were treated with success. The oxidised ores were crushed through 1/8-inch mesh screens in Gates crushers, and rolls, and the pyritic ores through screens with 300 holes per square inch. Both kinds of ore were treated in steel vats, 8 feet deep, the depth of ore being 7 feet. There were two rows of these vats, and double treatment was adopted.

Lime was added to the ore at the rock-breaker floor, in proportions varying from 1 to 2½ lbs. per ton. By this means it was thoroughly mixed in the different machines, and any lumps were broken up. The actual cyanide treatment differed a little from the ordinary procedure, and followed lines was suggested.

As soon as a tank in the first row was filled, a strong solution, 0.25 per cent. KCy, followed by two others, the last being 0.15 per cent., was pumped on to it. The time given to this treatment would be 66 to 70 hours. The solutions were not allowed to stand, but were drained off as soon as the tank was filled. The object of this was to allow fresh air to obtain access to the mass of damp sand. Each solution would be about 27 tons to the tank of ore (165 to 170 tons).

The ore was then transferred to the second row of tanks, having lost about 67 per cent, of its original assay value in this short time. It is probable that the abundance of air entangled In the dry sand, as compared with what would be held in a tank of sand settled with water, materially assisted the solution of the gold. Also the finer grains of the free gold would be quickly taken up.

A solution of 20 to 25 tons, not exceeding 0.20 per cent. KCy, was then pumped on to the transferred sands and drained off, the mass being allowed to remain damp for about 96 hours, when weaker solutions (0.15 to 0.10 per cent.) were used in continuous washes, making up a total of 75 tons per tank (2nd treatment). A water-wash of 20 or 30 tons completed this part of the process, which would last some 275 hours.

A careful series of moisture tests and measurements of solution sumps during the treatment of four tanks (680 tons) showed that the total loss of liquid in the treatment was 96 tons, or 24 tons per tank. The moisture in the discharged residues averaged 12.3 per cent., or, say, 20 tons per tank, the remaining 4 tons would be represented by evaporation from surfaces of sumps and tanks and by leakages. As the ore contained about 3 tons of water per tank, in the form of moisture when delivered from the mill storage bin, the actual consumption of fresh liquid equals 21 tons per tank, or 1/8 ton (25 gallons) per ton of ore.

The solutions running from the first row of tanks carried from 13 to 32 dwts. per ton; those from the second row 3 to 4 dwts.; and the final wash 0.08 to 2½ dwts.

Crushing with Cyanide Solution of a chalcedonic ore with practically no “ mineral ” or slimes, a small proportion of free amalgamable gold, and a large proportion of fine cyaniding gold is treated by:

(а) Crushing with cyanide solution,
(b) Direct cyanide treatment of mixed sands and slimes by percolation,
(c) Plate amalgamation of free gold.

The ore is hard and splintery, completely free from all impurities, and unlike most of the ores from the neighbouring mines, contains little or no silver, except what is alloyed with the gold. It is crushed in the Company’s 60-stamp mill with cyanide solution in the mortars, about 2.5 tons of solution being used to 1 ton of ore. A 25-mesh screen is used, and the duty per stamp is about 2 tons per day. The slimes formed in crushing are said to amount to less than 5 per cent. The monthly output is about 2,750 tons.

The cyanide plant consists of 28 leaching-vats, each 22.5 feet in diameter and 4 feet deep. Much reticence is maintained as to the exact treatment, but the main features are understood to be as follows:

The whole of the pulp from the 60 stamps is conducted by a launder to one vat, and allowed to discharge into the centre until the vat is about half-full. The pulp is then diverted to another vat, which is allowed to fill in the same manner. The mixed sands and slimes in the first vat are allowed to settle for an hour or two, after which the fairly clean top-solution is siphoned off into a collecting-tank, whence it is pumped up to two elevated tanks, from which the solution for the stamps is supplied. The pulp is again diverted into the first vat until the charge is complete. After settlement, the clear top-solution is again drawn off. In this way three or four vats may be in course of filling at the same time.

The settlement of the slimes is effected without the aid of lime by allowing the solution to percolate from the bottom of the vat during the filling and periods of settlement. The downward currents promoted by the draining from below are said to promote the settlement of even the finest material within a reasonable time. This is a point to be noted by cyaniders troubled with slimy products.

The mixed sands and slimes are treated by percolation in the ordinary way. The depth of each charge is about 30 inches, and the weight 40 tons. The strong cyanide solution is allowed to percolate from 30 to 40 hours, while the weak cyanide and water-washes are drawn off by the aid of an air pump.

The siphon used in the sand-vats consists of a length of 2.5- inch rubber hose, to one end of which is attached a short length of wooden batten, to keep it on the surface of the solution. The other end is fixed, inside the vat, to a short iron pipe passing through the side, about 18 inches above the filter-cloth.

In the extractor-room there are five precipitation-boxes of the ordinary pattern, divided into compartments by baffle-boards; and four zinc-towers, consisting of wooden boxes about 6 feet high and 30 inches square, set on end and connected in a series, like charcoal-towers. The solutions flow upwards through the zinc-turnings, the overflow being conducted in a pipe to the bottom of the next tower, and so on to the last.

The sands are sluiced out of the vats over a wide expanse of amalgamated copper plates, which catch a certain proportion of the free gold.

The actual recovery from all sources is said to vary from 84 to 87 per cent.; but the figures of cost are not obtainable.

Crushing in Cyanide Solution on “ red ores ” are almost exclusively treated at present. The ore is crushed in stamp batteries in a weak cyanide solution containing from 1.3 to 2.2 lbs. KCy per ton (0.065 to 0.11 per cent. KCy), and having a protective alkalinity equivalent to from 1 to 1.5 lbs. of caustic soda per ton. The sands are then separated from the slimes by means of cone-classifiers, the sands cyanided by percolation, and the slimes by agitation and decantation. The battery screens range from 10 to 26 mesh, and the duty of the stamps is about 4 tons per head per diem. About 40 per cent, of the crushed ore consists of slimes, and some of the sand is left in the slimes. “ Barren solution,” free from gold, is used in cyaniding after the mill solution has been removed. The strength in cyanide of the two solutions is about the same. When the solutions are passed to the zinc boxes, fresh cyanide is added, bringing them up to about 0.15 or 0.20 per cent, in KCy.

At one mill the Moore process is used to shorten the time of decantation. In this process filter frames are lowered into the vat, and solution removed through them by vacuum leaching. At the Dakota and Horse-shoe Mills zinc barrels instead of zinc boxes are used for precipitation.
The extraction at the mills varies from 65 to 75 per cent., about half of which is obtained in the battery. The cost of treatment varies and the total consumption of cyanide being about 1 to 1½ lbs. per ton.