Purification of Gold by Electrolysis

Purification of Gold by Electrolysis

The Moebius Process of Purification of Gold by Electrolysis is now in successful operation and is said to be specially suitable for refining copper bullion containing large proportions of silver and gold with small quantities of lead, platinum, and other metals, but is chiefly used in parting dore silver containing not more than 20 per 1,000 of base metals.

The apparatus required consists of a number of wooden vats coated inside with graphite paint, and filled with a solution containing 1 per cent, of nitric acid, which constitutes the electrolyte. The anodes consist of plates of bullion of about ½ inch thick, 18 inches long, and 10 inches wide, which are hung in muslin bags destined to catch the insoluble impurities after the silver, copper, &c., have been dissolved. The cathodes consist of plates of pure silver, slightly oiled to prevent adhesion of the deposited metal. These plates are continually scrubbed by a mechanical arrangement of wooden brushes by which their surfaces are kept free from loose crystals of electrodeposited silver. The loose silver falls on to trays placed below, which are removed at intervals, and the silver collected from them.

The current should have an electromotive force of from one to three volts for each vat. The copper is not deposited unless the solution becomes too weak in silver or too rich in copper, and even if some happens to be deposited, it falls into the cathode trays with the silver and does little harm, since it will be gradually redissolved if the conditions are corrected. When too much copper has accumulated in the solution the latter must be removed, the method being as follows: The bullion anodes are replaced by carbon ones, and a weak current passed until all the silver is deposited. The silver cathodes are then replaced by copper ones, and a strong current passed so as to deposit the copper as rapidly as possible as a loose powder, which falls into a copper box. This box is connected with the cathode to prevent corrosion by the acid which is set free. The liquid thus regenerated is used again as the new electrolyte.

The process is stated to be the cheapest parting process known. If no copper were present in the bullion it is clear that there would be no consumption of acid, and it would never be necessary to change the electrolyte. Under the most favourable conditions, therefore, with water power available and the amount of copper in the bullion very small, the cost of parting dore silver would be merely nominal.

The following description of the Moebius plant which is in successful operation at the Pinos Altos Mine, Chihuahua, Mexico, is an abstract of that given by Mr. George Maynard. The plant is capable of treating from 3,500 to 4,000 ozs. of dore silver in 24 hours, and consists of a tank 12 feet long, 2 feet wide, and 20 inches deep, divided into seven compartments. Pour silver plates (the cathodes) and six dore plates (the anodes) are placed in each compartment in such a way that an anode is opposite to each face of a cathode. The trays to catch the deposited silver have perforated bottoms and are covered with asbestos cloth, and in order to facilitate cleaning-up, the electrodes, frames, anode bags, trays and brushes can all be lifted up simultaneously by a hoisting arrangement worked with a crank and handle, so that the exciting liquid alone remains in the cells.

The bullion is from 800 to 900 fine in silver, and 25 to 50 in gold, the rest being chiefly copper. The exciting liquid which at first contains 1 per cent, of nitric acid, is gradually converted into a solution of silver and copper nitrates. The silver nitrate is continually decomposed by deposition of the metal, but the copper nitrate accumulates, fresh acid being added at intervals to prevent the copper from being deposited. The silver dissolves from the anodes and is precipitated on the cathodes in the form of heavy tree-like crystals. The action of the brushes or scrapers is of vital importance to the success of the process, the advantages derived from their action being summarised as follows:

  1. The liquid is agitated and so kept homogeneous.
  2. Polarisation is prevented.
  3. The electrodes can be brought very near to one another, and the resistance thus reduced without any fear that short circuiting will ensue by the bridging over of the space between them by crystals of deposited silver.

The lead (as peroxide), platinum metals, antimony and other impurities remain with the gold in the bag surrounding the anodes. When the exciting liquid becomes too highly charged with copper, the solution is used instead of bluestone in the amalgamating pans of the mill, but the copper could of course be recovered as usual if it were desirable. The manual labour is all performed by the assayer and his assistant; the cathode trays are hinged, and by letting them turn on their hinges the silver is let fall into a movable tank on castors furnished with a false bottom, on which it is washed and dried, and it is then ready to be melted into bars of about 999 fine. The gold slimes are similarly washed and filtered; when they are fused, the lead is almost all slagged off.

The electric current employed is 170 amperes, the electromotive force being 8 volts ; this requires an expenditure of 2½ H.P. to generate the current to refine from 3,500 to 4,000 ozs. per day. The cost of parting is said to be less than 1/3 cent per gross oz. of bullion, and the royalty is 1/3 cent per oz.

The original cost of the plant is said to have been about $6,000, including $1,000 for the silver in the cathode plates. This sum included the cost of conveying the plant to the mine, which was very high, as a journey lasting several weeks on mule-back had to be performed. The amount of silver contained at any one time in solution in the bath is about 300 ozs. The weight of the forty-two anode plates is about 4,200 ozs. when they are fresh, and this silver can be melted up and recovered about forty-eight hours after the operation has been started. The clean-up takes place once a month.

The Moebius process was also in successful operation at the works of the Pennsylvania Lead Company at Pittsburg from 1886 to 1897 ; here it is said that from 30,000 to 40,000 ozs. of dore bullion were refined daily. The dore bullion did not contain more than 2 per cent, of impurities (lead, copper, bismuth, &c.), and was cast into large anode plates, 1.3 cm. thick and weighing 13 to 15 kilogrammes, which are less advantageous than the light thin anodes used at Frankfort. Each anode was dissolved in about 2½ days. When the copper rose to more than 4 or 5 per cent, in the solution, part of the liquid was withdrawn, and the silver precipitated as chloride. The silver produced was from 999 to 999.5 fine, and could be raised by repeated washing to 999.85, but it is better to reduce it to 998 by adding copper, as no excess over that degree of fineness is paid for in the English market.

At the Frankfort refinery, where this process is in use, the general arrangements are similar to those noted above. The operation is begun with a very dilute solution of nitric acid and with a strong current of as much as 300 amperes per square metre (0.2 ampere per square inch). As the operation proceeds, copper accumulates in the liquid (the silver anodes being only about 950 fine), and reaches about 4 per cent., the silver being about 0.5 per cent., and the free nitric acid from 0.1 to 1.0 per cent. As the copper accumulates, fresh acid is added, and the current reduced to about 200 amperes per square metre. The E.M.F. required for each cell is 1.4 to 1.5 volts. The anodes are from 6 to 10 mm. thick, and each weighs about 1.5 kilogrammes; they are completely dissolved in about 36 hours, and another plate is at once substituted, the gold slimes being cleared out of the anode bag either once or twice a week. The total capacity of the Frankfort refinery (3,200 ozs. per vat in 24 hours), is about 30,000 ozs. per day, the area of the plant being only 40 feet x 45 feet.

At the Guggenheim Smelting Company, New Jersey, a continuously moving endless silver belt was formerly used as cathode, the deposited silver being brushed off by fixed brushes, so that it falls into a separate tank. The capacity is 100,000 ozs. per day, and the cost, exclusive of superintendence, office expenses, and royalty, is stated to be about 1/8 cent per oz.

Electrolytic refining presents many advantages over acid parting, among them being the absence of noxious fumes, except during acid treatment of the gold residues, the small amount of labour and chemicals, the cleanliness and rapidity of working, and the absence of bye-products, so that the loss of metal is reduced to a minimum. In consequence of these advantages, dore silver is being more and more treated by this method.

In the Wohlwill’s Electrolytic Process the anode consists of impure gold, 4 mm. thick, and the electrolyte is a solution of gold chloride, containing 25 to 30 grammes of gold per litre, and from 20 to 50 c.c. of concentrated hydrochloric acid per litre, according to the strength of the current. The liquid is heated from 60° to 70° C. If the temperature is allowed to fall, or the acid is omitted, chlorine escapes. The cathode consists of pure sheet gold, and the current density is from 400 to 500 amperes per square metre. The gold, platinum, and palladium are dissolved, and the residue consists of iridium, silver chloride, and some finely- divided gold derived from the decomposition of AuCl. Nearly pure gold is deposited in an even coherent form on the cathode, and the solution becomes weaker in gold owing to the dissolution of impurities at the anodes, so that gold chloride must be added from time to time. The operation is carried on continuously until the amount of platinum in solution is twice that of the gold (or the palladium in solution exceeds 0.5 per cent.), when it is precipitated with ammonium chloride. The anode is stated to be reduced to one-tenth of its thickness in 24 hours, and is then melted down and recast. The electrodes are 3 cm. apart. Silver, lead, and bismuth are the cause of difficulties necessitating the brushing of the anodes and the addition of sulphuric acid.

This method offers a cheap way of preparing proof gold. The electrolytic process is especially useful for:

  1. Gold mud such as that obtained from the Moebius process;
  2. Argentiferous gold, if the silver and lead do not together exceed 150 parts per 1,000 and lead alone is not more than 50 parts.

The consumption of acid is about 2 per cent, of that which would be required to dissolve gold in the ordinary chemical way. There are practically no fumes, and the cost of electricity is only about three pfennige, or one penny per kilogramme of gold in Hamburg.

The platinum is accumulated until it amounts to 50 or 60 grammes per litre of solution and is then precipitated. In the year previous to May, 1900, the Hamburg refinery treated 2,000 kilogrammes of gold from all parts of the world and accumulated 1½ kilogrammes of platinum, which is worth four times the cost of refining the gold. Iridium is also completely separated from the gold. The costs at Hamburg, exclusive of superintendence and general expenses, are 1s. to 1s 6d. per kilogramme (32 ozs.) of fine gold, when working fine mud from the Moebius process, and 2s. to 2s 6d. per kilogramme of argentiferous gold. The cost of a plant for refining gold to the value of £10,000 per day would be about £1,000 in Hamburg, including £60 for a dynamo giving 150 amperes at 15 volts, and £250 for a gas engine giving 5 H.P. The space required would be about 150 square metres. A licence to use the process must be obtained from the Norddeutsche Affinerie.

The process was introduced at the Philadelphia Mint in 1902, owing to the foiling off in the amount of dore silver received. Seven cells of Berlin porcelain, each 15 inches long, 11 inches wide, and 8 inches deep, are used, filled with a gold solution containing 30 grammes of gold to the litre. In each cell are 12 anodes in multiple, each 6 inches long, 3 inches wide, and ½ inch thick, and 13 cathodes of fine gold rolled out to 0.01 inch thick. The cathodes appear to be larger than the anodes which are not completely submerged. The cells are heated from 50° to 55° on a bed of sand by means of steam pipes. The seven cells are in series with the dynamo and 100 amperes are used, the difference of potential between the terminals being from 4½ to 5 volts. One attendant is said to be enough to manage the work with the occasional help of a second. About 5,000 ozs. per week are refined with the expenditure of one horse-power.

It is evident from this account that the current density is somewhat less than 33 amperes per square foot of anode, or about 350 amperes per square metre. The gold anodes must, from their dimensions, weigh about 6,000 or 7,000 ozs. when new, so that the time of treatment is not less than a week. The cost for acid (20 cents per 1,000 ozs. of deposited gold) and power is trifling, but the labour costs are not inconsiderable, and the length of time of treatment appears to detract seriously from the value of the process.