Refining

Whatever route was used to isolate a metal, the result is essentially never pure on the first pass — some impurity always tags along. Refining is the final step, and which technique is chosen depends on exactly how the metal's properties differ from those of its impurities. Six methods cover most practical cases:

(a) Distillation

Works for low-boiling metals such as zinc and mercury. The impure metal is simply evaporated, and the pure metal is recovered as the distillate — impurities with higher boiling points stay behind.

(b) Liquation

Suited to low-melting metals such as tin. The impure metal is allowed to flow down a sloping surface when gently heated: the metal melts and runs off, while higher-melting impurities remain solid and get left behind.

(c) Electrolytic refining

Here the impure metal itself becomes the anode, and a thin strip of the pure metal is the cathode, both dipped into an electrolytic bath containing a soluble salt of that same metal. On passing current:

Pure metal dissolves off the anode and re-deposits on the cathode. Impurities sort themselves by reactivity: more basic (more reactive) impurity metals stay dissolved in the electrolyte, while less basic (less reactive) ones simply fall away as a sludge at the bottom of the anode compartment — the anode mud. The whole process rests on the same electrode-potential and Gibbs-energy reasoning from Section 6.5.

Copper is refined exactly this way: impure (blister) copper anodes, pure copper cathodes, and an acidified copper sulphate electrolyte:

The anode mud from this process is valuable in its own right — it contains antimony, selenium, tellurium, silver, gold, and platinum, and recovering these can go a long way toward paying for the refining. Zinc, too, can be refined electrolytically.

(d) Zone refining

This method exploits the fact that impurities are more soluble in the molten metal than in the solid. A mobile heater is wrapped around one end of a rod of impure metal (Fig. 6.7) and slowly moved along its length:

• As the heater advances, a narrow molten zone travels with it.
• Just behind the moving heater, pure metal crystallises out as the melt cools.
• The impurities, staying preferentially dissolved in the liquid, get swept forward into the next section of melt rather than being trapped in the solidifying crystal.
• Repeating the pass several times, always in the same direction, drives the impurities steadily toward one end of the rod, which is then simply cut off.

Zone refining is the workhorse method for producing ultra-high-purity semiconductor-grade metals — germanium, silicon, boron, gallium, and indium among them.

(e) Vapour phase refining

Here the crude metal is first converted into a volatile compound, which is collected separately and then decomposed to release the pure metal. Two conditions have to be met: the compound must be genuinely volatile, and it must be easily decomposable so the metal can be recovered without difficulty.

Mond process (nickel). Nickel is heated in a stream of CO to form volatile nickel tetracarbonyl, which is then decomposed at a higher temperature:

$Ni(CO)_4 \xrightarrow{450\text{-}470\,K} Ni + 4CO \qquad(6.53)$ …