Silver anodes produced by the leaching and precipitation processes and a smaller proportion of anodes obtained by the by-products recovery process are sent to the electrolytic recovery process. The silver is refined electrolyzing in a solution of slightly acidic silver nitrate, with some copper and potassium nitrate present. The relatively pure leach anodes are refined in Moebius cells, with anode bags and cathode scrapers. The by-products recovery anodes are high in gold and produce a considerable sludge fall out. In this way, they are refined in Thum cells. The composition of these anodes varies, for example a typical content is 75-90% Ag, 8-17% Au, 0.2-0.5% Cu and 0.02-0.3% Pb. The refined crystals deposited at the cathodes content 99.99% Ag and are washed free of electrolyte, dried under vacuum and melted into 34 kg bars. In both cells the basic electro-chemistry is similar. Electrolyte compositions are as follows,
|
Component
g/L |
Moebius Cell |
Thum Cell | ||
|
Fresh |
Discard |
Fresh |
Discard | |
|
Silver |
25-30 |
30-38 |
25-30 |
99-105 |
|
Copper |
2-5 |
60-65 |
6-12 |
70-85 |
|
Lead |
0.1-0.5 |
1.8-2.2 |
0.2-0.8 |
9-12 |
|
Potassium Nitrate |
13-15 |
15-18 |
--- |
--- |
|
Nitric Acid |
8-10 |
10-15 |
16-20 |
16-20 |
Copper is usually added to fresh electrolyte in order to improve conductivity and is allowed to build up to high levels before the electrolyte has to be stripped. The presence of copper and free nitric acid increases the conductivity of the electrolyte. Copper will not plate in preference to silver. It is important to indicate that with high copper concentrations, there is a potential mechanical entrainment. This is with cathode deposits such as are produced by silver electrolysis since crystal growth is not uniform and large dendritic crystals are formed. Electrolytic reduction of silver ions at the cathode can be noted, in which case copper would plate. The problem is solved by employing mechanical scrapers to break off the accumulation of crystals. Also, scrapers prevent growth right across the cell, which could cause shorting and at the same time gently agitate the electrolyte.
All aqueous electrolytes have a lower electrical resistance at higher temperatures, when the conductances of different ions tend to approach a common value under the same potential gradient. In this way, electrolysis is conducted at an elevated temperature, heat being provided from the power dissipated by the internal resistance of the cell. If the electrolyte were left to itself, a slow reduction of silver would result, due to silver and impurities are dissolving at the anode, while silver only is deposited on the cathode. The excess acidity present allows for the dissolution of some silver directly into the electrolyte and this acidity is maintained by performing daily analysis and adjust with fresh nitric acid. The acidity is higher in Thum cells since the anodes are less pure. In order to improve crystal properties, tartaric acid is added to Moebius and Tum cells. Since tartaric acid is a polycarboxylic acid, it is possible that some complex formation takes place adjacent to the cathode surface modifying the crystal formation.
Crystals from both cells are washed in a centrifuge, first with water acidified with 1% nitric acid, and then with further water washes. Some gold refineries employ a rotary vacuum device to remove the last traces of moisture, after which the crystals are charged into a induction furnace for melting. Typical furnaces have a capacity of 350 kilos and approximately 70-80 minutes are needed to melt the charge. The silver is then cast into standard 34 kg bars that assay 99.99% Ag, 2-12 ppm Cu, 9-17 ppm Fe, 0.5-2.5 ppm Pb and 0.5-1.5 ppm Zn.