The method just described aims at leaving gold in an insoluble state at the anode. Other impurities are usually left there, and the gold sponge, or slimes, needs further refining by methods, to be indicated later. By using a suitable electrolyte the gold can be dissolved from the anode bars and deposited in sheet form on the cathode, while many metals can be left in solution. The active agent in causing the solution of the gold is chlorine; this transforms the silver in the anode to chloride, and the presence of silver chloride tends to prevent the solution of the gold by forming a protective coating on the bars. In order to hasten the operation high current densities are employed, but this tends to cause the escape of chlorine when chloride of gold is used as an electrolyte. This is prevented or minimised by using an excess of hydrochloric acid, sodium, or ammonium chloride, and heating the solution. The best temperature is found to be between 60deg. and 70deg. C, with gold contents of the solution 25 to 30 grammes per litre, and with HCl. 1.19 sp. gr. between 20 and 50cc per litre. The current density is kept above 500 amperes per square metre. Under these conditions only gold is deposited at the cathode.
Separating Platinum from Gold
Platinum remains in solution until it reaches double the amount of the gold present. The salts in the solution are necessary for a coherent deposit of gold. Some gold is deposited with the anode mud, and this is due to the formation of aurous chloride, which at once splits up in the presence of water to auric chloride and metallic gold.
3AuCl=AuCl3 + 2Au.
Silver chloride also falls together with lead and bismuth. These impurities remove a certain amount of chlorine, which is obtained from the electrolyte, the amount of gold in solution therefore diminishes, and it is necessary from time to time to add gold chloride.
The electrolyte vessels used are made of some acid resisting earthenware. The anodes are only 4mm in thickness, and are eaten away in 24 hours, the cathodes are made of thin rolled gold sheets. The distance between the electrodes, with an impure electrolyte, is 3/8- inch. This process, as stated by Dr. Rose is specially applicable to the refining of platiniferous gold.