Coal can be washed free of rock by exploiting the density gap: coal is about 1.3 times as dense as water and rock about 2.5 times, so both sink in plain water. Separate them either in a pulsed-water jig, which stratifies the load, or in a heavier clay slurry of about 1.6 kg per litre in which coal floats and rock sinks.
Part of Coal Surface Mining
Density-based separation of coal from rock and clay using water and a weighted slurry, with the reasons plain-water flotation fails.
Plain Water Does Not Float Coal
The parent article describes washing as agitating crushed coal in a trough, with coal floating or staying near the surface and rock sinking. That cannot work in plain water. The parent puts coal at a relative density of about 1.3 and rock at about 2.5, and water is 1.0. A body floats only if it is lighter than the fluid, and 1.3 is heavier than 1.0, so coal sinks in water just as rock does. What works is a way of making the heavy rock settle faster or lower than the coal, or a fluid that is denser than coal and lighter than rock.
Three Working Methods
1. Hand jig. A box with a perforated or screen bottom, dipped and lifted repeatedly in a tub of water. Each pulse lifts the load briefly and it settles back in order of density: heavy rock to the bottom and lighter coal above it. Scrape off the top layer as coal and the bottom layer as waste. Mixed shaly pieces (middlings) end up between and are re-run or discarded.
2. Sluice or launder. A sloped trough with flowing water. Light, coarse coal is carried further down the trough than the heavier rock, which stops early. Set the slope and the flow by trial.
3. Dense slurry (sink-float). Stir fine clay or heavy sand into water until a litre of it weighs about 1.6 kg, which makes the density 1.6. Coal (1.3) floats on it, rock (2.5) sinks. Check the density by weighing a measured litre: 1.6 kg per litre is 1.6. The slurry needs constant stirring to stay uniform, and coal contaminated with clay must be rinsed.
Worked Example: Choosing The Slurry Density
Inputs: coal 1.3; rock 2.5; any slurry between these will float coal and sink rock. Pieces of shaly coal are assumed at 1.8 to 2.0, between the two.
- A slurry at 1.6 floats pure coal (1.3 < 1.6), sinks stone (2.5 > 1.6), and sinks the shaly pieces (1.8 > 1.6).
- Raising it to 1.9 would also float some of the shaly pieces, giving more coal and more ash.
- The choice is a trade: a lower density gives cleaner coal and a smaller yield.
Common Mistakes
- Expecting coal to float in a trough. It does not; check the density.
- Skipping the settling pond. Fine coal and clay leave in the wash water and clog the drain.
- Drinking-water contamination. Runoff from coal washing and spoil can be acid, because pyrite in the coal oxidises to sulphuric acid. Keep wash water and spoil drainage away from wells and streams.
- Washing coal you will store wet. Dry it first (see the storage article).
Safety
Wet work and fine coal
Wet coal fines in a settling pond hold heat and can ignite when they dry. Keep them out of dry heaps. The slurry is slippery and heavy, so keep trays and drains clear and do not let children or livestock near a wash pond.
What’s Next
- Learn next: Storage & Weather Protection — drying and keeping washed coal
- Combine with: Sorting & Grading — what to try before washing