The refining method used when the metal and the impurities have low and high melting temperatures, respectively, is

We start by recalling the basic principle involved in the choice of a refining method. When a crude metal contains impurities whose melting point is much higher than that of the metal itself, we prefer a method in which the metal is gently melted so that it flows away from the still-solid impurities. Mathematically, we can express the condition as $$T_{\text{m,\,metal}} < T_{\text{m,\,impurity}}$$ where $$T_{\text{m}}$$ denotes the melting temperature.

The technique that exactly uses this inequality is called liquation. In liquation, the impure solid is placed on the sloping hearth of a reverberatory furnace and is heated to a temperature $$T$$ such that $$T_{\text{m,\,metal}} < T < T_{\text{m,\,impurity}}$$. Because the chosen temperature $$T$$ lies between the two melting points, only the metal melts, while the high-melting impurities stay solid. The molten metal then flows down the slope and is collected separately, leaving the impurities behind.

Other listed techniques are ruled out once we match their operating principles with the given temperature condition:

• Vapour-phase refining demands that the metal form a volatile compound or itself be volatile, which is not stated here.

• Zone refining relies on repeated melting-solidification of a narrow zone and is typically used when both metal and impurity have close melting points but different distribution coefficients.

• Distillation is for metals that are already volatile at moderate temperatures relative to impurities, again not the criterion described.

Therefore, the process that satisfies the requirement of “metal has low melting temperature, impurities have high melting temperature” is liquation.

Hence, the correct answer is Option A.