The only process for directly refining gold electrolytically that is known to have been reduced to practice is that patented by Dr. Kmil Wohlwill. Hamburg, Germany, United States patents 625,863 and 625,864.
The feature protected by patent is the electrolyte. This is composed of gold chloride solution rather strongly impregnated with free hydrochloric acid, and it is in this latter point that the significance of the invention lies. If a gold anode be placed in a neutral or only slightly-acid solution of gold chloride, opposed by a suitable cathode, gold will be deposited by an electric current, but little or no gold will be dissolved from the anode. Free chlorine is given off; a result not to have been anticipated since chlorine is an active solvent for gold. Upon the addition of free hydrochloric acid to the bath, a point is reached where the chlorine at the anode is suppressed, and gold passes into solution, equivalent for equivalent, to that deposited. The strength of the electrolyte is easily maintained by such additions of gold chloride from time to time as will be equivalent to the copper, platinum, etc.,
Second: The cells, which are of white porcelain (Berlin ware), arc 15 inches long x 11 inches wide and 8 inches deep. These are filled with a prepared solution of gold tri-chloride containing 30 grammes of gold per litre (3 8-10 Troy ounces per gallon), the depth of the solution being such as not to entirely submerge the anodes. In each of these cells arc suspended 12 anodes and 13 cathodes, in multiple. The anodes are each 6 inches in length, 3 inches wide and }A inch thick. The corresponding cathodes are of fine gold and rolled down to 1-100 of an inch in thickness. The distance between anode and cathode is ij-j inches. In our present working seven cells are placed end to end on a bed of sand, underlying which is a series of steam pipes by which the temperature of the bath may be raised and maintained at any desired degree. At present our cells are kept at from 500 to 55° C. The increased temperature serves to reduce the voltage required, and at the same time to diminish the amount of free acid necessary to suppress evolution of chlorine at the anodes.
Circulation of the electrolyte by mechanical means is necessary to secure uniform disintegration of the anode and deposition of the gold.
The seven cells in use are connected up, in scries, with the dynamo, which is at present regulated to furnish 100 amperes. The tension between the terminals of the seven cells varies from 4}4 to 5 volts, dependent upon temperature, strength of solution, etc.
The chemical equivalent of gold is very high, and the electromotive force required for the deposition of gold is very low, so that the power necessary for running the plant is insignificant. With only seven cells of our plant in operation, we are refining about 5,000 ounces per week, with the expenditure of about 1 horse-power. One attendant suffices to manage the work, with the occasional assistance of a second workman.
The cost for hydrochloric acid in the hath is 20 cents per 1,000 ounces of deposited gold.
We can increase our refining capacity to 50,000 ounces per week with our present plant.
Beside the advantage the process offers for the refining of gold with an insignificant consumption of acid, with little labor, and small expenditure of power, an important one should be mentioned, viz., the recovery of any platinum present in the bullion. This dissolves, but is not deposited with the. gold. When the electrolyte becomes sufficiently charged with platinum, the gold is first precipitated by sulphur dioxide, and removed. Then the platinum remaining in solution is separated as ammonium platinum chloride. Lastly, the copper is recovered by passing the wash waters over iron scrap.
With these advantages to its credit, the process has limitations which are serious ones. Any silver in the anodes will, of course, be converted into insoluble chloride, and if the percentage be small will fall to the bottom of the containing tank as "slimes." But if the amount of silver exceed 5 per cent, the chloride tends to adhere to the anode, as a crust, which must be mechanically removed.
Furthermore, any considerable quantity of copper present in the bullion to be treated is undesirable, since the electrolyte, in that case, must be renewed inconveniently often.
http://books.google.com/books?id=LCnmAAAAMAAJ&pg=PA158&dq=Wohlwill+Process&hl=en&ei=svXRTdGGIIeztwfSsMGiCg&sa=X&oi=book_result&ct=result&resnum=2&sqi=2&ved=0CC0Q6AEwAQ#v=onepage&q=Wohlwill%20Process&f=false
8) )
