Electrical Precipitation on Mercury or Amalgamated Plates

Electrical Precipitation on Mercury or Amalgamated Plates

In the Siemens-Halske Electrical Precipitation process, the gold is deposited from solution by the passage through the liquid of a current of electricity. Moreover, as the precipitation is as readily obtained in extremely dilute cyanide solutions as in those containing 0.1 per cent, or more, very weak solutions are used in dissolving the gold from the ores when electrical precipitation is employed. Gold can be extracted from some ores as completely by a solution containing 0.03 per cent, of cyanide as by one containing 0.3 per cent., the only difference being that the time required is sometimes considerably longer. On the other hand, the advantage in using the more dilute solution is that the selective action in favour of the gold is increased, and the amount of cyanide decomposed by “ cyanicides” in the ore is diminished. In addition to this, some cyanide solution is invariably left in the ore, and if the “ weak ” solution used to finish the dissolution of the gold contains only 0.01 per cent, of cyanide, instead of 0.1 per cent., the amount of cyanide lost in this way by mechanical means is also reduced.

The process was first adopted on a large scale at the Worcester mill in the Transvaal. Here the vats are 20 feet in diameter and have their sides formed of staves 10 feet long; the five vats hold 135 tons each. The battery pulp, after passing over Frue vanners, is classified into four products by hydraulic classifiers. The first classifiers, which consist of spitzlutten, remove the coarse sand and pyrites, amounting to 15 per cent, of the pulp, and containing 15 dwts. of gold. This product is treated for nine days with solutions of 0.08 per cent, of cyanide, and, after being washed with 0.01 per cent, solutions, gives residues assaying 1½ to 2 dwts., so that from 87 to 90 per cent, of the gold is extracted. The second product yielded by the hydraulic classifiers comprises 50 per cent, of the pulp, contains 6 dwts. of gold per ton, and, after five days’ treatment with solutions ranging from 0.05 per cent, downwards, yields residues containing from 1 to 1.25 dwts., showing an extraction of 80 to 84 per cent. The finest sand is separated from the slimes by pointed boxes; the slimes amounted to 25 per cent, of the whole pulp, and assay 4½ dwts. The fine sands, constituting 10 per cent, of the pulp, contain 4½ dwts. of gold, and after treatment yield residues assaying 1 dwt. per ton. The consumption of cyanide averages ¼ lb. per ton of the tailings, of which 3,000 tons are treated per month.

The precipitation plant consists of four boxes, each 18 feet long, 7 feet wide, and 4 feet deep. Copper wires are fixed along the tops of the sides of the boxes, and convey the electric current from the dynamo to the electrodes. The anodes are made of iron, and the cathodes, on which the gold is deposited, of lead. It was stated that amalgamated copper plates had been tried but abandoned, as the mercury penetrated the copper under the influence of the current, and a dry amalgam resulted which did not adhere to the plate. Lead answers all the requirements of the cathode laid down by von Gernet, which are:

  1. that the precipitated gold must adhere to it,
  2. that it must be capable of being rolled out into very thin sheets to avoid unnecessary expense,
  3. that it must be easy to recover the gold from it, and
  4. that it must not be electro-positive to the anode, in order to prevent return currents being generated when the depositing current is stopped.
  5. the gold should be separable from the cathode without destroying the latter. This requirement is not fulfilled by lead.

At the Worcester mill the anodes are iron plates, 7 feet long, 3 feet wide, and 1/8 inch thick ; they are supported in a vertical position by wooden strips nailed to the box, and are covered with canvas to retain the small quantity of Prussian-blue produced. The sheet-lead cathodes are stretched on wires fixed in light wooden frames which are suspended between the iron plates. There are in all 3,000 square feet of cathode surface. The solution is made to circulate between the plates passing alternately over and under the edges of the anodes. The boxes are kept locked except when cleaning up is necessary, when the cathodes are removed and replaced with fresh sheets of lead. The lead, which contains from 2 to 12 per cent, of gold, is then melted into bars and cupelled. The consumption of lead is 750 lbs. per month, its cost being equal to 1½d. per ton of tailings. The gold is comparatively free from base metals. The total cost of treatment at the Worcester mill is under 3s. per ton of tailings. The electrodes are placed 1½ inches apart, and a current of 4 volts is employed, giving about 0.06 ampere per square foot. In later practice, according to Butters, the boxes are reduced in size, being now 29 feet long, 3½ feet wide, and 2 feet 9 inches deep, the iron anodes are enclosed in canvas sacking, and the space between them is filled loosely with lead turnings, which have replaced the sheet lead formerly used as cathodes. The liquid circulates through the turnings.

One of the main difficulties in electrical precipitation lies in the great resistance offered to the passage of the current by the very dilute solutions employed. In order to reduce this as far as possible, the electrodes must be of very large size, and the distance between cathodes and anodes must be small.

The ions of aurocyanide of potassium are K and AuCy2. As the result of electrolysis, therefore, potassium is set free at the lead cathode, where it attacks the water, forming potash and hydrogen; at the same time the gold in the double cyanide is displaced and precipitated, both by the potassium and the nascent hydrogen. Such hydrocyanic acid as may be formed is neutralised by the potash. Meanwhile the anion, AuCy2, is set free at the anode, but is at once split up into AuCy and Cy; the latter unites with the iron, forming cyanides, which become converted into Prussian blue, and are also oxidised in part, forming ferric oxide, and the cyanide of gold is partly precipitated in this substance, although if enough free potassium cyanide is present it may be kept in solution; even if it is precipitated, it can afterwards be recovered by aeration in an alkaline solution of potassium cyanide.

It is, however, clear, that the solutions must be circulated, and also that the more free cyanide is present, the greater is the percentage of gold precipitated on the cathode. Electrical precipitation is further considered later under the heading, “ Electrical precipitation on copper plates.”

W. Bettel gives the results of work by this process in the autumn of 1896 at two of the Transvaal mines, as follows:— The “Bonanza” mill is designed to treat about 3,000 tons of tailings and concentrates per month. The pulp is roughly concentrated in spitzlutten, about 20 per cent, of sand (containing the pyrites) being separated and treated separately from the rest. There are 12 tanks, each holding 100 tons, arranged in two rows, one discharging into the other. Each charge of tailings is treated for 2½ days in an upper vat, and 5 days in a lower one. A typical charge of tailings was treated as follows :—8 tons of alkaline wash (with 0.008 per cent. KCy) were added at 9 a.m. on October 19, 1896. Between 12 noon and 2.30 p.m., 8 tons of weak solution (0.025 to 0.03 per cent. KCy) were run on. From 19th to 21st October, 34 tons strong solution (average 0.07 per cent. KCy) were used, and the charge was then drained and transferred to the lower vat, where, between 21st and 24th October, 72 tons of 0.07 per cent. KCy solution, 22 tons of 0.025 to 0.03 per cent., and 24 tons of 0.008 per cent, KCy (“alkaline solution”) were run through, giving a total of 168 tons of solution to 100 tons of tailings.

A charge of 100 tons of concentrates was treated in the upper vat from 2nd to 7th October (5 days) with 13 tons of alkaline solution, 10 tons of weak solution, and 100 tons of strong solution. After being drained and transferred to the lower vat, the charge was treated from 7th to 19th October (12 days) with 200 tons of strong solution, 15 tons of weak solution, and 30 tons of alkaline wash, or 368 tons of solution in all. The concentrates assay about 2 ozs., and the residues 2½ dwts., the extraction being about 93 per cent. In 39 charges of tailings, on the other hand, the average gold contents was 16 1/3 dwts. per ton before treatment, and over 3 dwts. after treatment, including nearly ½ dwt. of dissolved gold, the extraction being 80.9 per cent.

The precipitation plant consists of four boxes, each 30 feet x 4½ feet and 3 feet deep, divided into two pairs in series connected with a dynamo giving 200 amperes with an E.M.F. of 8 volts. The resistance of the weak solution is considerable. Each box contains 23,960 square feet of cathode, so that the current used is 0.0041 ampere per square foot of cathode, and the rate of flow is 3 tons of strong solution per hour per box, and 1 ton of weak or alkaline solution. There are therefore 333 square feet of cathode surface per ton of strong solution, and 1,000 square feet per ton of weak solution passed through in 24 hours. The liquid after precipitation usually contains 1 or 2 dwts. of gold per ton, an average of 85 to 95 per cent, of the gold being precipitated. In cleaning up, the lead is melted in a reverberatory furnace, and sold to the Rand Central Ore Reduction Company.

In October, 1896, the total cost was 6s. 8.72d. per ton of ore, but was soon after reduced by the introduction of continuous haulage. The cost of cyanide was nearly 1s. 10d. per ton, the consumption being over 1½ lbs. per ton, the cost of lead was 1.2d. per ton, and the royalty was 1s. 6d. per ton.

At the May Consolidated Works, there were in 1896 17 vats of an average capacity of 200 tons each, and the plant is capable of treating 12,000 tons per month. The solutions used range in strength from 0.1 per cent. KCy for strong solutions, and 0.025 per cent. KCy for weak solutions, to 0.01 or 0.02 per cent. KCy for solutions for preliminary treatment. The concentrates contain from 8 to 12 dwts. per ton, and are treated for ten days; the tailings contain from 3½, to 4½ dwts. before treatment, and are treated for four days. The residues only contain from 15 grains to 1 dwt. of gold per ton. The working costs in the three months, August to October, 1896, were an average of 2s. 7.7d. per ton in treating 9,000 tons per month, exclusive of about 4d. per ton for interest and sinking fund for the cost of the vats.

The Siemens-Halske process made considerable progress on the Witwatersrand goldfield before the introduction of the Betty-Carter lead-zinc process, when its chief advantage over other methods lay in the possibility of using more dilute solutions for dissolving the gold, with a consequent reduction in the loss of cyanide, of about 1/3 lb. per ton of tailings (Bettel).

The Siemens-Halske process requires well-filtered clear solutions, which have been made alkaline, for the best work. The lead foil may be amalgamated by the use of mercuric chloride (Durant’s method) to ensure the adhesion of the gold. Continuous precipitation was carried out at the Ferreira Slimes Works in 1898, some of the lead strips being taken out and replaced by fresh ones each day. There were four boxes in use, each 36 feet long by 12 feet wide, holding 1 ton of lead each, 1 lb. of lead being consumed per ton of slimes. The extraction was 80 per cent, of the gold. The solution flowed through the boxes at the rate of 15 to 20 tons per hour, the value on entering the boxes being 29 grains per ton, and on leaving 4 grains per ton.

After the introduction of the lead-zinc couple, the Siemens- Halske process was partly displaced by it, and in the beginning of 1899, out of 16 slimes plants on the Rand, only 4 were using the Siemens-Halske and 12 the Betty-Carter process.

In the Molloy process of Electrical Precipitation on Mercury or Amalgamated Plates, precipitation was made on “sodium or potassium amalgam formed electrolytically from a solution of carbonate in contact with a bath of mercury.” In this connection it may be noted that Messrs. MacArthur and Forrest experimented on the action of sodium amalgam, using a small tower filled with pieces of sodium amalgam, through which the solution of gold in potassium cyanide was allowed to trickle slowly. They came to the conclusion that sodium was not more efficacious than zinc, but was much more expensive. The saving of expense in the production of the bullion from the precipitate was outweighed by the cost of the sodium required to form the precipitate.

Electrodeposition of gold is used in the Siemens-Halske process and the conditions necessary for success have been worked out and stated by the inventors of that method. One of the most important provisions is that the layers of liquid between the anodes and cathodes must be very thin in order that the ions may not be compelled to travel far before they reach the poles. Hence, when great quantities of liquid have to be treated, it is necessary for the electrodes to have a very large surface. Moreover, they should stand in a vertical position so that they may not become coated with slime settling from the liquids, if these are muddy. It follows that mercury is not suitable for cathodes.

According to experiments made by the author, electrical precipitation on mercury, or on amalgamated copper plates, is only effective when the current density does not exceed 0.01 ampere per square foot of cathode. With 0.01 ampere or less per square foot of cathode, gold is precipitated and amalgamated simultaneously, the plates keeping in good condition during 24 hours use, while ½ oz. of gold per square yard of plate was deposited on them. With currents of greater density, part of the gold is deposited as a non-adherent black powder easily rubbed off, but not easily detachable by a jet of water, and not easily amalgamated. As the density of the current is increased, the percentage of non-amalgamable gold thrown down rises, until at 0.30 ampere per square foot about half the precipitated gold is not amalgamated.

The dimensions of the cathodes must therefore be far greater than if they consist of lead, when currents of 0.05 ampere per square foot are used with corresponding increase in the speed of precipitation. Apart from the cost of the plates, however, the cyanide solution suffers loss by dissolving copper from the cathode. Nevertheless, copper plates have been used in several processes.

Among other suggestions for improving electrical precipitation may be mentioned that by Andreoli of using anodes made of lead peroxide, which do not waste or damage the solution, and cathodes made of iron; that by Croasdale of using amalgamated zinc plates for anodes; and that by Cowper Coles of using aluminium for cathodes.

In the Pelatan-Clerici Process, the finely crushed ore is agitated in vats (holding 25 tons each) with a solution of about 0.1 per cent. KCy. The bottom of the vat is lined with amalgamated copper plates covered with a layer of mercury. A current of electricity is passed from the sheet-iron agitators, forming the anodes, to the mercury bath, which is the cathode, so that gold dissolved by the cyanide begins to be precipitated at once. It was worked at the De Lamar Mill in Idaho in 1897, using very thick pulp. The practice there seemed to resemble pan-amalgamation with the additions of an electric current and some cyanide in the water, but subsequently plain cyaniding was resorted to.
The Riecken process, formerly used at the South Kalgoorlie Mill, was similar.