Coal washability by float-and-sink testing\nA float–and–sink (washability) test determines the suitability of cleaning coal by a process based primarily on what principle?

Introduction / Context:
Coal preparation routes include gravity separation, froth flotation, magnetic separation, and others. Washability studies guide the choice of route and operating cut points by showing how coal and associated mineral matter partition by density under ideal conditions.

Given Data / Assumptions:

• Float–and–sink tests use heavy liquids of known specific gravities.
• Coal particles with different ash contents exhibit different densities.

Concept / Approach:
In the test, a sample is divided into density fractions by immersion; the mass and ash of each density class are measured. Plots of cumulative float and sink versus density (washability curves) reveal potential yield and ash of clean coal for a chosen density cut. This directly informs gravity-based separation equipment such as jigs and dense-medium cyclones. The basis is density difference, not surface wettability (flotation), shape, magnetic, or electrostatic properties.

Step-by-Step Solution:

Verification / Alternative check:
Preparation plant design uses washability data to select cut densities (e.g., 1.4–1.8) for cyclones and to predict product yield and ash.

Why Other Options Are Wrong:

• Wettability is used in flotation, a different test.
• Shape is secondary and not the basis of float–sink partitioning.
• Electrostatic and magnetic methods are niche and not addressed by washability curves.

Common Pitfalls:
Confusing “float” in heavy liquid with “float” in froth flotation; the former refers to buoyancy, not bubble attachment.

Final Answer:
Gravity (density) separation