Anodes and cathodes content impurities that influence the efficiency of electrolytic process. In this way, the impurities in the gold anodes are classified in two groups, those forming soluble chlorides, mainly copper, zinc, platinum and palladium, and the second group includes insoluble residues, chiefly silver, iridium, osmium and ruthenium. The soluble impurities remain in solution and do not plate at the cathode, while the insoluble are deposited at the bottom of the cell or in anode bags. The cathode deposit is basically rough and somewhat nodular. Mechanical entrainment of silver chloride can present a problem which may be solved by the use of anode bags to contain the sludge. Silver remains, however, the major impurity in the electrolytic gold. The electrolyte of course is saturated in silver chloride, which has an appropriate solubility at high hydrochloric acid concentrations. If the silver content of the anodes is too high, the silver chloride coats the anode and it polarizes. In order to prevent this, an alternative current may be superimposed on the plating current to give momentary reversal of the polarity, causing the silver chloride to fall away. In this way it is possible to electrolyze bullion containing as 9-12% silver, and by se of anode bags to produce a 99.99% gold cathode.

Electrolysis is performed in glazed ceramic cells arranged in one or more banks at a temperature of 60 ^oC. Heating is carried out by individual immersion heaters in each cell, while in the main production banks the electrolyte is circulated through a glass pipe network and heated in a sump. The heater consists of probes of silver/copper alloy, carrying an alternating current of 400-420 A at 22-26V. The busbars are made of 71.5% silver and 28.5% copper alloy. The five cathode bars and four anode bars form an integral unit with a single pair of electrical connections. The anode and cathode starts are assembled on a frame, which is lowered into the cell. After electrolysis the frame is carried to the wash tanks where the anode stubs and cathode deposits are passed through three water washes. The stubs are removed for recasting and the cathodes given an ammonium thiosulphate/ sodium sulphite or ammonia wash to remove adhering silver chloride. Two further water washes complete the treatment. The cathode deposits are then melted and cast into bars of which about 8-12% are rolled into 0.5 mm strips and recycled as cathode starters.

The anode sludge is largely metallic gold which is regularly collected, refined by chlorination to 99.6% gold and recast into anodes. When the platinum group metals concentration becomes appreciable, the sludge is treated with chlorine gas and hydrochloric acid to dissolve gold, which is removed by filtration, and the fine silver chloride carefully decanted, leaving a concentrate containing iridium, osmium and ruthenium in the approximate ratio 5:5:1. The soluble platinoids are recovered from the electrolyte by usual precipitation methods. Typical operating data is shown below,

                                                              Operating Data

ELECTROLYTE
Au, g/L	70-90
HCl, g/L	80-110
Temperature, oC	55-65
Circulation	Paddles
Heating	Immersion or AC probes
CURRENT
Current per cells, amp	300
Voltage per cell, volts	0.5-0.7
Anode density, amp/m2	600
ANODES
Composition Au, %	99.5
Weight, kg	12
Length x Width x Thickness, mm	280 X 230 x 12
Number per cell	4
Life, h	48
CATHODES
Composition Au, %	Rolled gold 99.99%
Length x Width x Thickness, mm	300 x 75 x 0.25
Number per cell	20 (in 5 rows of 4)
CELLS
Material	Glazed porcelain
Length x Width x Depth, mm	465 x 405 x 350