Which refining process is generally used in the purification of low melting metals?

We first recall that a refining process is selected by comparing the physical or chemical properties of the required metal with those of its common impurities. One such important physical property is the melting point.

The method called liquation is based on the principle that, if a metal melts at a much lower temperature than its accompanying solid impurities, then gentle heating can melt the metal while leaving the impurities unmelted. The molten metal can then be separated from the solid residue by simple flow or by skimming.

Mathematically we say that the refining will work well when

$$T_{\text{m,\,metal}} \;<\; T_{\text{m,\,impurities}}$$

where $$T_{\text{m}}$$ denotes the melting point. Because the inequality $$T_{\text{m,\,metal}}\;<\;T_{\text{m,\,impurities}}$$ is satisfied for low-melting metals like Sn, Pb and Bi, these metals are purified economically by liquation.

On the other hand,

• The chromatographic method exploits differences in adsorption, not melting points.
• Electrolysis relies on differences in electrode potentials.
• Zone refining uses repeated melting and solidification in a moving zone; it is best suited for very high-purity, high-melting semiconductors such as Si or Ge.

Hence, for metals whose melting points are low, the generally used refining process is liquation, which corresponds to Option D.

Hence, the correct answer is Option D.