Lithium molten-salt electrowinning

Table 4.7. Lithium metal molten-salt electrowinning...

Other Metals. AH the sodium metal produced comes from electrolysis of sodium chloride melts in Downs ceUs. The ceU consists of a cylindrical steel cathode separated from the graphite anode by a perforated steel diaphragm. Lithium is also produced by electrolysis of the chloride in a process similar to that used for sodium. The other alkaH and alkaHne-earth metals can be electrowon from molten chlorides, but thermochemical reduction is preferred commercially. The rare earths can also be electrowon but only the mixture known as mischmetal is prepared in tonnage quantity by electrochemical means. In addition, beryIHum and boron are produced by electrolysis on a commercial scale in the order of a few hundred t/yr. Processes have been developed for electrowinning titanium, tantalum, and niobium from molten salts. These metals, however, are obtained as a powdery deposit which is not easily separated from the electrolyte so that further purification is required. 

A number of electrolytic processes are used for the industrial production of metals. Some metals such as zinc, copper, manganese, gallium, chromium, etc. are electrowon from aqueous baths. Another common electrolytic process used is molten salt electrolysis. The most important application of molten salt electrolysis till now has been in the electrowinning of metals. Today aluminum, magnesium, lithium, sodium, calcium, boron, cerium, tantalum, and mischmetal are produced in tonnage quantities by molten salt electrolysis. As a representative example, the electrowinning process for aluminum is taken up. 

The production of pure alkali metals is difficult due to their extreme reactivity with commonly used substances, such as water. The alkali metals are so reactive that they cannot be displaced by other elements, and molten salt electrolysis is therefore an option for producing many of these metals. Sodium and lithium are the most important alkali metals produced by electrolysis, while magnesium and calcium are the most important alkaline earth metals produced by electrolysis. For calcium and magnesium, thermal processes are currently more important than electrowinning. Magnesium, being the only of these metals that can be used for structural purposes, is a major industrial product. Therefore, a separate chapter is devoted to the electrolysis process for magnesium. 

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