Balbach-Thum cell

Two different techniques for silver elec-trorefining are employed, the Mobius and Balbach-Thum cells. The main difference between them is that the electrodes are arranged vertically in the Mobius cell and horizontally in the Balbach-Thum cell (Fig. 22). In the Mobius cell cast, crude silver anodes are suspended in anode bags that collect anode slime. The spent anodes are replaced continuously by new anodes. The method uses stationary or rotating stainless steel cathodes. Deposited silver forms dendritic crystals on the cathode and these are removed manually or continuously by automatic scrapers. The Mobius electrolysis process is most suitable for crude silver containing >90% Ag. With high copper content, the electrolyte must be regenerated very often, and with... 

Fig. 22 Mobius and Balbach-Thum cells for silver refining.

Silver refining to dore specification is modelled on BRM practice, and is followed by a conventional parting plant operation employing Balbach Thum cells. 

Dore is electrorefined in conventional Balbach Thum cells to produce 99.99% silver cathode which is then cast or granulated according to market requirements. Anode slimes are periodically recovered from the cells and are leached to produce a 98% gold product. 

The cells used are commonly the Balbach-Thum cell design, which uses a single anode within an upper diaphragm compartment and a single horizontal cathode sheet on the base of the cell. The cell is constructed from PVC. Silver deposits in dendritic form and needs to be periodically scraped from the cathode to prevent short circuiting. It is then removed from the cell. Anode current densities used are in the range of 450 to 650 amps/m, and for a cell with anode area of around 0.25 m the theoretical production is around 0.5 kg/h of silver per cell (Mantel, 1960). 

The Balbach-Thum cell (Fig, 4.13) is designed around a rectangular trough containing a thick, solid carbon plate cathode which covers the cell bottom. An anode basket is suspended in the top of the cell the basket is fabricated so as to support a number of silver anodes. The anodes are usually cast plates, but smaller, more irregular, pieces of silver scrap can be used. Woven cloth or polymer mesh inside the tray serves to contain insoluble anode materiaL Recovery of silver is usually by manual scraping, following cathode removal. 

The hfoebius type of design is perhaps best suited to modern automated processing, while the Balbach-Thum cell is preferred for smaller use in low-technology markets. There are many modifications and indeed hybrids of the two cell designs. 

Table 4.9 Comparison of silver refining in the Moebius and Balbach-Thum cells...

Feature Moebius cell Balbach-Thum cell... 

Balbach A variation of the Moebius process for electrolytically removing gold from silver, in which the anodes are placed horizontally in wooden trays lined with canvas to retain the slimes. The silver is deposited as crystals on graphite cathodes at the base of the cell. Invented by E. Balbach. See also parting, Parkes, Thum, Wohlwill. 

The above requirements lead to the need for relatively simple geometries usually based on tank cells where the anodes are surrounded by porous bags, baskets or membranes to contain anode debris and slime. There are two traditional cell designs which, with modern improvements in engineering design and constructional materials, persist today. The Moebius cell uses a vertical electrode arrangement, while the Balbach cell (later improved by Thum) uses horizontal electrodes. 

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