This is not the place to provide a thorough and general discussion of electrolysis. We focus on the electrolysis of fluorides, and more specifically on the separation of lanthanide fluorides from thorium fluoride. The simplest method for separating various elements is to apply decreasing voltages to a batch mixture of the salts. Other, more elaborate, methods make use of the time variation of the electrolysis current when varying voltages are applied. However, all methods need a batch treatment.

The electrolysis process can be compared with the reduction process by Li which was described in section 9.3.3. Consider the metallic fluoride salt Mn+Fn. The reduction reaction reads:

MFn + nLi ^ M + nLiF [Qred] (9.62)

and the electrolysis reaction

Mn+ + ne— =) M [Qel]. (9.63)

Reaction (9.62) can be written as

Mn+ + ne— + nF— — ne— + nLi =)

M + nLi+ + ne—+ nF—— ne— [Qred]. (9.64)

Using equation (9.63), equation (9.64) can be written as

nLi ^ n(Li++ e—) [Qred — Qel]. (9.65)

It is seen that the difference between the reduction by Li and electrolysis reaction energies is expressed as a function of the Li ionization energy and of the ionization state of the metal:

Qred — Qel = nQ[Li =) Li+ + e ]. (9.66)

It follows that if some elements with different oxidation states are difficult to separate with the Li reduction technique, the addition of an additional electrolysis step may be helpful. An example is the separation of Th and La. The most stable oxidation state of Th is 4+ while that of La and other lanthanides is 3+. It follows that

Qel[Th]- Qel[La] = Qel[Th] — Qd[La]- Q[u)u++ e-]. (9.67)