Coal preparation – cleaning very fine coal For cleaning fine coal dust smaller than 0.5 mm in size, which unit operation is most suitable?

Introduction / Context:
Coal beneficiation separates valuable coal from mineral matter. The choice of separator depends strongly on particle size. Fine particles become difficult to treat by gravity alone due to low settling velocities.

Given Data / Assumptions:

• Particle size < 0.5 mm (500 microns).
• Clean coal must be separated from ash-forming minerals.
• Water-based processes considered.

Concept / Approach:
Froth flotation exploits differences in surface hydrophobicity. Fine coal is naturally hydrophobic (or can be made so with collectors) and attaches to air bubbles, rising to the froth, while hydrophilic mineral matter stays in the slurry. This makes flotation ideal for ultrafines where gravity methods fail.

Step-by-Step Solution:

Verification / Alternative check:
Plant practices commonly route −0.5 mm fraction to flotation cells or columns; coarser fractions go to jigs, spirals, or dense medium cyclones.

Why Other Options Are Wrong:

• Trough washers and Baum jigs rely on hindered settling—inefficient for ultrafines.
• Spiral separators handle moderately fine sizes (typically 0.15–3 mm), not the finest fractions as effectively as flotation.

Common Pitfalls:
Insufficient reagent conditioning or poor air dispersion reduces recovery; thick froths may entrap ash unless froth washing or column flotation is used.

Final Answer:
Froth floatation