Moebius and Thum electrolytic cells are used to recover silver. In some refineries there are 24 Moebius or vertical electrolytic cells arranged in two banks of 12. The cells are built of PVC, coated externally with fiberglass, with a capacity of 200-300 L each. The anode/cathode spacing is 45-55 mm and each cell is equipped with five cathodes and four anode baskets per cell. Each basket contains two anodes, whose weight is from 5 to 10 kg. The anode bags are fabricated from terylene mounted on a PVC frame, and the cathode is composed of 316 stainless steel. During operation, the voltage drop across each cell, which is operated at 30-37 oC, is 1.5-2.5 V at a current of 400 amperes. The silver is removed daily by scraping the fallen crystals from the bottom of the cell up the one inclined side of the cell and into a stainless steel trolley. Sludge is removed from the anode bags monthly. Anode scrap amounting to a maximum of 12-17% of the original anode mass is produced and is electrolyzed in special cells equipped with anode baskets made of perforated titanium plate.
Normally, anodes take 45-48 hours to dissolve. The anode sludge which is chiefly gold is collected by removing bags from the cells, fitting them into a special inclined stainless steel frame and washing out the contents under pressure with spent electrolyte. The sludge is then filtered, dried, melted and cast into bars. These are return to the gold refinery for chlorine refining.
Thum cells are known as horizontal cells and refine the by-products recovery anodes. These cells are constructed of PVC coated externally with fiberglass, with a capacity of 100-150 L and are operated at 38-42 oC. The anode/cathode spacing is 90-130 mm and the anode/cathode ratio is 1/1.8. On the base of the cell is a stainless steel sheet that acts as the cathode and above it up to 30-50 kg of anodes are suspended in a terylene cloth supported in a PVC framed basket. A carbon rod works as anode connection. The voltage drop across each cell is 3.5-5.5 V at a current of 250-300 amperes. Anode life is 24-40 hours and there is no scrap production. The relatively large quantity of anode sludge produced is removed every 4 to 6 days and treated in the same way as the sludge from the Moebius cell.