liquation

Liquation is the controlled partial melting of an alloy or ore mixture to exploit differences in melting point. When a two-metal system is heated to a temperature between the melting points of its constituents, the lower-melting component begins to liquefy while the higher-melting component remains solid. The liquid phase can then be drained or expressed away, leaving behind a solid residue enriched in the refractory metal. This achieves separation without fully melting the entire mass.

The classic historical application was the parting of gold and silver. A gold-silver alloy heated to around 950°C to 1000°C would allow molten silver to drain through or be squeezed out of the partly-solid gold sponge. Lead served as a third component in some processes: tin added to a copper-silver mixture lowers the melting point of the silver-rich phase, making separation more efficient. The technique remains relevant in refining operations where cost-effective chemical separation is impractical at laboratory or small-batch scale.

Modern variants and limitations

Modern liquation typically uses gravity drainage or mechanical pressing. The ore or alloy is heated in a furnace with a sloped hearth or is held in a porous bed that allows liquid to collect and run off. Temperature control is critical: too cool and nothing flows; too hot and you lose the selectivity that makes the process work. Furnace linings must resist attack from both the liquid and solid phases, often requiring ceramic materials.

Liquation works best when the melting points differ by at least 100°C and the solid and liquid phases do not dissolve in one another significantly. Modern extraction favors hydrometallurgy or pyrometallurgy with clean slag separation, but liquation persists in small-scale precious metal recovery and in some specialty alloy production where high purity in the final product justifies the slower rate of throughput.

The term derives from the Latin liquare, to melt or make liquid. It should not be confused with liquefaction, which is the melting of a single pure substance, or with leaching, which is dissolution in a solvent rather than melting.