33.Tell about comparison of silver refining in the Moebius and Balbash – Thum cells.
Comparison of silver refining in the Moebius and Balbach – Thum cells
Feature |
Moebius cell |
Balbach-Thum |
Anode scrap |
Leaves stumps which must be remelted |
Consumes all anode material |
Cathode scraping |
May be almost continuous and automatic |
Needs manual scraping at intervals |
Anode slimes |
Anodes must be bagged individually and carefully positioned; relatively intolerant to anode slimes |
A removable one-piece filter cloth may be used with a permanent, coarse mesh; tolerant to anode slimes |
Electrolytic power requirement |
Closely spaced electrodes lead to a low cell voltage |
Larger interelectrode spacing gives a high cell voltage |
Floor space |
Small |
Large (up to 5 times that of a Moebius cell) |
Electrolyte inventory |
Small |
Large, but impurity levels (e.g. Cu) may be lower, allowing more anodes of lower purity to be used |
Current density |
High |
Low (but this may give a lower codeposition of noble metals) |
Maintenance |
Moving scrapers may give maintenance problems |
Ni moving parts gives fewer maintenance problems |
34.Express the rotating-cylinder cell for electrorefining of silver, and features, cell structure.
The Moebius 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.
A number of rotating cylinder cathode cells have been used as an alternative to the above outdated designs. Figure 4.14 shows one version of such a cell. While a membrane-divided rotating cylinder electrode cell facilitates automatic scraping, provides the convenience of a single cathode and gives a uniform silver product, there are several possible disadvantages which must be considered in cell design:
1.Increases in maintenance levels due to the rotational drive and electrical brushgear. 2.Increased catholyte stirring, if excessive may entrain any insoluble impurities into the deposit or increase the rate of codeposition of any soluble, noble metals present.
3.Increased Joule heating and maintenance problems of a membrane, e.g. silver may lodge against the membrane, causing early membrane failure due to locally high current densities.
Fig. 4.14 A rotating-cylinder electrode cell cell electrorefining of silver. (a) Sectional view. The metal grows as flake, is dislodged continuously by a wiper blade and may be withdrawn at the cell bottom. (b) Plan view.
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