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Coal separation technologies
Several different technologies take advantage of the density difference between coal and unwanted rock and mineral fragments mixed with or attached to the coal. Coal has relatively low density or specific gravity (1.2 to 1.5) compared to other rocks such as shale (2.4 to 2.8) or minerals such as pyrite (4.9 to 5.2). Different technologies such as jigs, heavy (dense) media separators, spiral concentrators, and teetered beds, use these differences in density to separate coal from rock. Different technologies are used for different particle sizes.
- Dense-medium separators
Coarse particles are often run through a “heavy” or “dense” medium vessel, drum, or cyclone. In this technology, a suspension of ultra-fine magnetite (specific gravity = 5.2) is made in water. The density of the mixture (the medium) can be adjusted by adding or removing specific amounts of the magnetite powder. The density is adjusted based on sample testing (float-sink analyses) of the mined coal so that less dense coal should float and denser non-coal particles should sink This technology is particularly useful for separating pieces of mostly coal from mostly rock (e.g., pieces of floor, parting, and roof rock, cleat and fracture-filling vein minerals). Cleaned coal particles are dewatered with screens or dryers. Magnetite particles are removed with magnets and recycled. The material that sinks is removed as coarse reject (Llewellyn, 1973; Nunenkamp, 1976; Singh and Peterson, 1979; Gluskoter and others, 2009; Luttrell and Honaker, 2012).
- Spiral concentrators
Fine particles are commonly run through spiral concentrators at coal preparation plants. Spirals are tall (10-15 ft), helically coiled chutes. Fine particles and water are pumped into the top of the spiral chutes and then gravity and centrifugal force separate less-dense coal particles and most of the water to the outer edges of the chutes, and denser particles to the inner edges. Cleaned coal is sent from the spirals to screens or sieve bends for dewatering. Denser rock and mineral particles are removed in a water slurry (McCandless and Shaver, 1978; Honaker and others, 2007; Gluskoter and others, 2009; Luttrell and Honaker, 2012).
Aside from density (specific gravity) differences, coals can also be hydrophobic. This is a property which means their surface repels water. The froth-flotation process uses this surface property to separate fine and very fine coal particles from rock and mineral particles.
- Froth-flotation processes
The froth-flotation process uses coal’s hydrophobic surface property to separate fine and very fine coal particles from rock and mineral particles. At this size, non-coal components are sometimes intimately associated with the coal particle itself. Air and reagents (sometimes called frothers) are mixed with water to create bubbles in a series of tanks (called cells). Mixed particle types enter the tanks and the coal particles stick to the bubbles (because of the coal’s hydrophobic properties) and rise to the top of the tanks as a bubbly froth. At the end of the process, coal particles can be skimmed or decanted with the froth. The froth is passed through a filter (often a large disc filter) to separate out the coal particles. Non-hydrophobic rock and mineral particles sink and are removed as reject in a water slurry. The slurry is generally sent to a thickening tank (Llewellyn, 1973; Nunenkamp, 1976; McCandless and Shaver, 1978; Singh and Peterson, 1979; Honaker and others 2007; Gluskoter and others, 2009; Luttrell and Honaker, 2012).
Dewatering and drying. Many density separation techniques and froth flotation use water to carry the rock and coal particles, so a variety of dewatering mechanisms (drain and rinse screens, sieve bends, dryers, a variety of centrifuges, and a variety of filters) are used at the end of many preparation circuits. Chemical flocculants may be added to enhance particle separation. Preparation plants may also have a large circular thickening tank at the end of the fine or ultra-fine particle dewatering circuit for clarifying water to recycle through the plant (Nunenkamp, 1976; Gibbs and Hill, 1978; McCandless and Shaver, 1978; NRC, 2002; Luttrell and Honaker, 2012).
- Thickening tank
Thickening tanks (also called static thickeners or thickeners) are large (up to 200 ft wide) tanks, often located outside, but adjacent to the preparation facility. Feed slurry, which contains fine to very fine particles, enters the tank through a pipe. When the particles enter the tank, some fall to the bottom while others remain suspended in the water because of their grain size. The large size of these tanks means that suspended particles eventually settle towards the bottom of the tank. Coagulants and flocculants may be added to promote aggregation of very fine particles with each other to maximize settling of the particles out of the water column. The water in the tank becomes relatively stratified with sediment at the bottom and decreasing particles in the water upwards through the tank. The upper part of the water column is clear. Slowly rotating rakes at the bottom of the tank collect the sediment (the underflow) towards a central discharge, where it can be pumped out through pipes to a disposal slurry pond. Clear water (called clarified water) at the top of the tank can be recycled into the plant via an overflow or outflow pipe (Gibbs and Hill, 1978; Gluskoter and others, 2009; NRC, 2002; Honaker and others, 2007; Luttrell and Honaker, 2012).
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Last Modified on 2024-04-22
