Among other suggestions for rendering the presence of oxygen in cyanide solutions unnecessary, the most striking was The Sulman-Teed Process, who use cyanogen bromide, CNBr. The addition of this substance to a solution of potassium cyanide makes it much more rapid in dissolving gold. They put forward the equation:
CyBr + 3KCy + 2Au = 2K.AuCy2 + KBr
in explanation of the action of their solvent, which is inoperative except in the presence of an alkaline cyanide. Cyanogen chloride and iodide give equally good results as far as rate of solution is concerned, but are not convenient for use on a large scale. In tests on concentrates, and on various complex ores, the results obtained by Sulman, Teed, and others were remarkably good, high percentages of extraction being obtained in a few hours from ores which yielded little or no gold to ordinary cyanide solutions in the same time.
Cyanogen bromide is efficacious in the treatment of tellurides without roasting. Certain tellurides of gold appear to be dissolved by it, although but little attacked by alkaline cyanides. The Diehl process depends on this for its success. The cyanogen bromide or bromo-cyanide process was also used at Deloro, Ontario, to treat auriferous mispickel ores. The consumption of cyanide was 0.9 lb. KCy and 0.45 lb. CNBr per ton. The total cost of treatment was low for each ounces of gold extracted.
One of the peculiarities of the gold bullion produced by the use of bromo-cyanogen is that it contains tellurium which is dissolved, presumably as a bromide, and precipitated by zinc. Gold bullion produced by ordinary cyanide does not contain tellurium, which is insoluble in cyanide. It is probable that cyanides of tellurium cannot be formed.
Treatment of Concentrates
Concentrates can be treated successfully by percolation, even if they consist chiefly of sulphides of iron, lead, zinc. The time of treatment is in these cases often as much as three or four weeks, and the charges in the vats are sometimes drained and stirred up or transferred to other vats, in order to aerate them and expedite the dissolution of the gold. The use of Sulman’s bromo-cyanogen, or other accelerator, is also often desirable or even necessary. This lengthy process is now being shortened by finely crushing the concentrates in tube mills before cyaniding.
Treatment of Ore Slimes by Filter Pressing
The oxidised slimes at first encountered were either passed direct to filter presses, and cyanide forced through the cakes of slimes by compressed air, or agitated with cyanide in vats, and after the gold had been dissolved, filtered, and washed in filter-presses. The rate of solution was sometimes accelerated by aeration or by the addition of bromine (Sulman-Teed process). Bromo-cyanogen is necessary when unoxidised tellurides are present, as telluride of gold is not attacked by ordinary cyanide.
The method employed for oxidised ore is briefly as follows:
The ore is crushed dry to 25- or 30-mesh, and delivered to a dry separator where it is divided into about equal parts of slimes and sands. The Mumford and Moodie separator is shown in Fig. 75. The crushed ore is raised by the elevator, A, and falls into the hopper, C, which is always kept partly full, even distribution being aided by the rotating baffle disc, B. The disc, F, and fan, G, also rotate with the spindle. The ore falls on F and is thrown off by centrifugal force, and air currents from the fans then carry off the fine dust to the outer chamber, H, where it settles and is delivered at I. The coarse particles settle in K against the air current, and are withdrawn at L.
The slimes separated in this way were fed into the vat, A (Fig. 76), and agitated by paddles with cyanide solution. When the gold had been dissolved, which occupied about three hours, the slimes were run into the montejus or air pressure receiver, C, whence they were forced by means of air at a pressure of 60 to 80 lbs. per square inch into the filter presses, D, the filtrate from which passed through the pipe, B, into the vat, E, and thence in a constant stream to the zinc boxes. The cakes were washed in position with clear water, and then the press was opened, and the cake falling into the truck, F, was trammed to the dump. In later practice the presses were filled by a pump instead of by air pressure.
The filter presses had 20 chambers each, and formed cakes 28 inches square and 3 inches thick, the total charge being 1¼ tons of slimes. In later practice larger presses were used with 50 chambers 3 to 4 feet square. These hold 5 or 6 tons of wet slimes.
A filter press is shown in Figs. 77 and 78. A distance frame (Fig. 78 and Fig. 78a, E) lies between each two plates. Each plate and frame has 4 holes, A B C D, which correspond in position, so that when the plates are fitted together, the holes form passages through all the plates in the press. The slimes from the receiver pass through the hole, A, and so enter the spaces, K, inside the distance frames. When the spaces, K, are filled, the liquid begins to be forced through the filter-cloths and passes to the channels, B and C. When the cell is full of compacted nearly dry slimes, the inlet at A and the outlet at B are closed, and water is forced in at D and runs out at C, passing through the filter-cloths and slimes, and displacing the cyanide solution. The washing is said to be rapid and effective.
It was stated that the slimes treated in this way consisted largely of fine sand. If this were removed the leaching was made impossible.
The time of treatment in the filter presses is about one hour, of which 20 minutes is occupied in discharging the presses.
Filter-pressing has more recently been introduced into South Africa, but it has hardly yet begun to displace the decantation method.
Treatment of Sulpho-Telluride Ore
The Kalgoorlie sulphide ores contain from 0.03 to 0.15 per cent, of tellurium, 3 to 7 per cent, of iron pyrites, 6 to 17 per cent, of calcite, about 2 per cent, of soluble salts (sodium chloride, magnesium sulphate, &c.), a large percentage of silicate of alumina, and from 0.1 to 0.3 per cent, of copper. Much of the ore is rich in gold, and enormous quantities contain more than 1 oz. per ton. After numerous experiments, two processes were devised by which treatment is fairly successful. These processes are described below.
The Marriner process, in work at the Great Boulder Main Reef and the Great Boulder Proprietary Companies’ Mills, consists in (1) dry crushing in Gates’ rock breakers, followed by ball mills or roller mills, (2) roasting dead, (3) grinding wet in pans with a large quantity of mercury and hot alkaline cyanide solutions, (4) agitating by means of paddles in large vats with cyanide solutions containing from 0.01 to 0.08 per cent. KCy, (5) filter-pressing, and (6) precipitation of the gold by zinc.
In roasting, a large amount of soluble sulphates is formed, and the losses by dusting must be carefully attended to. Various furnaces are used (see pp. 224-230). The pans are used primarily to grind the ore to fine slimes, which is necessary to enable the cyanide to dissolve the gold, but any coarse gold is extracted in them by amalgamation. The agitators are from 10 to 21 feet in diameter and 5 to 16 feet deep, and are usually fitted with a vertical shaft revolving by bevel-gearing at the top and carrying two sets of radial arms at the bottom. The total cost of treatment in 1900 was put at 20s. per ton, including 4s. for roasting and 6s. to 8s. for filter-pressing. In 1904 it was given by Donald Clark as 16s. 11.44d. per ton. The tailings assay from 1 to 2 dwts., varying with the ore, and the cyanide consumption is from 1½ to 2 lbs. per ton.
The precipitated gold slimes are treated with sulphuric acid in a lead-lined cast-iron pot and filter-pressed in a press with plates of a lead-antimony alloy. The cakes are washed and dried and then smelted in a Faber du Faur zinc-distilling tilting furnace. This furnace is described in works on the metallurgy of zinc. It holds about 8 cwts. of material, and consists of a large inclined graphite pot shaped like a bottle, having a contracted mouth, and contained in a cubical tilting furnace.
In the Diehl process which was introduced at the Hannan’s Star Mine, and subsequently used at the Lake View Consols and at Hannan’s Brownhill, the ore is wet crushed, amalgamated, concentrated, separated into sands and slimes, the sands reground in tube mills, and the whole agitated with cyanide, to which bromo-cyanide is added and filter-pressed. The concentrates, which contain 30 to 40 per cent, of the values, are roasted and sent back to the wet crushing mill. The cost has been given as 28s. 3d. per ton, and with ore assaying 16 dwts. 14 grs., the final residue contained 26 grs., or 6½ per cent.
According to Knutsen, the ore at Hannan’s Brownhill is crushed in a 20-stamp battery fitted with inside and outside amalgamated plates. An average of 75 tons of ore are crushed per diem through 30-mesh screens. These are 4 Wilfley concentrators, and from 3 to 5 per cent, of concentrates are produced. These are roasted in an Edward’s mechanical furnace and sent back to the stamp mill. The tailings pass through two classifiers and the separated sands are re-ground in two slime or tube mills, each containing 2½ tons of flint balls. The product of the tube mills goes back to the classifiers. The slimes are concentrated by the removal of water in spitzkasten, and the sludge, containing 40 to 45 per cent, of solid slimes, flows to one of five agitators, each 20½- feet in diameter and 7½ feet deep. When the agitator is full, it is charged with strong KCy solution, and two hours later with strong BrCy solution. The amounts are KCy 0.20 per cent, and BrCy 0.05 per cent, of the solid material. After about 24 hours’ agitation, lime is added, the quantity being 3 to 4 lbs. per ton of solid slimes, and about two hours later the charge is filter-pressed. Each of the two presses holds 5 tons, and the time of treatment is about two hours. The gold solution is again filtered before flowing to the zinc boxes. The filter-press cakes, after being washed and then dried by air-blowing, go to the dump.
They assay from 1 to 2 dwts., whether the original ore contained 4 ozs. or 1 oz. per ton. The consumption of chemicals is 3 lbs. KCy and 1¼ lbs. BrCy per ton.