Coal washability (float–sink) curves:\nA washability curve derived from float-and-sink tests primarily assesses cleanability based on which property?

Introduction / Context:
Coal preparation depends heavily on density (specific gravity) differences between coal and mineral matter. Float-and-sink testing uses heavy liquids to partition a coal sample into density classes and then constructs washability curves that guide circuit selection (jigs, dense-medium cyclones, spirals) and predict product yield vs ash at given cut densities.

Given Data / Assumptions:

• Standard float–sink laboratory procedure.
• Coal and its associated mineral matter have distinct densities.

Concept / Approach:
By immersing particles in liquids of known specific gravity, one observes whether they float or sink and thus infers their density class. Washability curves plot cumulative float/sink mass and quality metrics (e.g., ash) versus density, enabling assessment of how effectively a gravity-based plant can clean the coal. Hence, the primary basis is density, not wettability (flotation), size (screening), volatile content (proximate analysis), or magnetic properties.

Step-by-Step Solution:

Verification / Alternative check:
Plant cut densities predicted from washability curves (e.g., 1.5–1.8) correspond directly to dense-medium separations.

Why Other Options Are Wrong:

• Wettability: flotation tests use reagents and air, not heavy liquids.
• Size: assessed via screening, not float–sink.
• Volatile/magnetic: unrelated to float–sink partitioning.

Common Pitfalls:
Confusing float–sink (density) with froth flotation (wettability); they are distinct beneficiation routes.

Final Answer:
Density separation