Washability Analysis
Also known as: Coal Washability, Density Separation Analysis, Float-Sink Analysis
Washability analysis is a laboratory method that determines the feasibility and efficiency of density-based separation for coal or mineral beneficiation. By fractionating ore or coal into density bins using sink-float tests and assaying each fraction, engineers can optimize design of separation plants (dense-medium cyclones, jigs, spirals) and predict clean product quality. Washability curves are essential tools for pre-feasibility and detailed design studies.
Key highlights
- Direct empirical measurement of separation feasibility; removes guesswork from circuit design
- Results directly transfer to design of industrial separators (dense-medium cyclones, spirals)
- Identifies products at each density that can be marketed or further processed
- Sensitive to ore property changes (weathering, hydration); repeated tests track ore variability
- Low cost compared to pilot-scale testing; rapid turnaround for multiple samples
Intuition
This section is available to Pro members. Upgrade to Pro
How it works
This section is available to Pro members. Upgrade to Pro
When to use it
Use washability analysis when developing coal or mineral beneficiation processes, especially for ores where gravity separation is feasible (density contrast between ore and gangue >0.3). Assume ore mineral densities are well-known. Prefer alternatives like flotation kinetics if density separation is not technically feasible or cost-effective.
Strengths & limitations
- Direct empirical measurement of separation feasibility; removes guesswork from circuit design
- Results directly transfer to design of industrial separators (dense-medium cyclones, spirals)
- Identifies products at each density that can be marketed or further processed
- Sensitive to ore property changes (weathering, hydration); repeated tests track ore variability
- Low cost compared to pilot-scale testing; rapid turnaround for multiple samples
- Laboratory separation at single size does not capture size-density interactions; real plants operate on size distributions
- Assumes perfect separation at target density; practical separators have misplaced material (middlings)
- Tests require access to heavy liquid solutions which are expensive and sometimes toxic
- Results are empirical; do not provide physical understanding of separation mechanisms
- Sample size is limited (typically <500 grams); may not capture rare coarse or fine particle behavior
Common pitfalls
This section is available to Pro members. Upgrade to Pro
Applications
This section is available to Pro members. Upgrade to Pro
Frequently asked
What density range should I test in washability analysis?
For coal, typically 1.3 to 1.6. For mineral ores, use densities that span your ore (target mineral) and gangue. Iron ore might be 2.6-3.2; copper sulfides 2.8-3.3. Use published mineral densities to guide range selection.
How do I handle middlings (material in the 1.4-1.5 density range)?
Middlings are partially floatable/sinkable material. In practical plants, middlings are recycled back to the separator or processed separately. In washability analysis, assign them based on your circuit design (most commonly, treat middlings as float product, i.e., reject).
Can washability predict the performance of actual dense-medium cyclones?
Washability provides the theoretical limits. Actual cyclone performance will be 80-90% as good due to density overlaps, size segregation, and other plant losses. Use washability data as a guide but conduct pilot tests before full-scale investment.
How often should I update washability curves during mining?
At least annually, especially early in mine life. If ore type (zone, oxidation state) changes significantly, re-test. Washability parameters (cutoff density, expected recovery) may drift by 5-10% as mining progresses.
What is the typical ash reduction achievable by coal washability optimization?
Raw coal ash content often ranges 15-40%; washability can reduce it to 8-15% depending on density cutoff. Better reduction (to <10% ash) requires coarser crushing or selective flotation of fines.
Sources
- 1.McCullough, R. B. (1963). The theoretical basis and practical application of coal washability studies. Transactions of the Society of Mining Engineers, 226, 13-26.
- 2.Clarkson, T. E., & Trevits, M. A. (2000). Coal preparation and resource utilization in United States. Bureau of Mines Information Circular IC 9413.
You have read it. What now?
Cite this page
ScholarGate. (2026, June 3). Washability. ScholarGate. https://scholargate.app/mining-engineering/washability