What is the name of the unit operation that is essentially the reverse of fractional crystallisation (i.e., selective dissolution of a component from a solid)?

Introduction / Context:
Fractional crystallisation separates components by crystallising one (or more) components from solution. The reverse idea is to selectively dissolve targeted constituents from a solid matrix using a suitable solvent—an operation ubiquitous in hydrometallurgy and food processing.

Given Data / Assumptions:

• Focus on a unit operation that removes solubles from solids using a liquid solvent.
• Differentiation from gas–liquid mass transfer operations is required.

Concept / Approach:
Leaching involves contacting a solid mixture with a solvent to dissolve desired solute(s), leaving behind an insoluble residue. It is the conceptual opposite of crystallising a solute from solution to separate it in solid form. The other listed options are gas–liquid or distillation operations, not solid–liquid extraction of solubles.

Step-by-Step Solution:

Verification / Alternative check:
Textbook definitions: Leaching (a.k.a. lixiviation) is used to recover sugar from beet, copper from ores, etc.—all involve dissolving solute from a solid matrix.

Why Other Options Are Wrong:

• Stripping/absorption: Gas–liquid mass transfer operations.
• Differential distillation: Vapour–liquid separation based on volatility, not dissolution from a solid.

Common Pitfalls:
Confusing liquid–liquid extraction with leaching; here, the feed is a solid containing soluble species.

Final Answer:
leaching