Coal Preparation

What is coal preparation?

Definition

Method used to improve the quality of a coal resource by removing inorganic impurities, regulating the size, ash content, moisture and other properties with the objective to improve the value of the coal resource.

When do you need coal preparation?

• Thin seams or seams difficult to mine because of the risk of contamination.
• “Dirty seams” with included stone bands.
• Coal contaminated by roof or floor material
• Seams with impurities – pyrites, calcite etc.
• When it is possible to make a specialised product e.g. coking coal.
• When product specifications demand for tight quality control.
• Maximise coal reserve potential - financially.

How does Coal Preparation work

Simply, coal preparation uses the principle that coal is less dense than rock. The separation processes used in coal preparation are designed to separate on relative density.

Coal Preparation System

Crusher/Breaker Station

• Coal Preparation plants must treat coal at a specified top size dependent on design parameters for the plant.
• Depending on CPP configuration, bottom size may be important – but not usually.
• Types of size reduction units – roll crushers, sizers, rotary breakers

Coal Preparation Plant

Consists of :

• Sizing screens to divert coal to the correct process streams at the correct sizing.
• Process separators designed for specific size ranges and applications.
• Magnetite recovery systems.
• Dewatering screens & centrifuges
• Pumps, pipes, chutes.
• Combination of 2, 3 or more separate processes to produce one or more product coals e.g. DM Bath, DM Cyclone and Spirals making both coking and thermal products simultaneously from the one CPP.

Treatment of Large Coal

• Easiest to separate but the separation methods are not always the most efficient processes.
• Tend to see coal and rock adhering as a single lump.
• Methods include Jig Separators and Dense Medium Baths
• Jigs require the use of a pulsating flow of air to create a buoyant bed. The product coal is “lifted” upwards through the reject material. The rejects settle to the bottom where they are removed by mechanical means.
• Dense Medium Baths use a “dense” media to control separation. Media is usually magnetite but historically sand has been used. The product coal tends to float if the RD of the particles is less than that of the dense media. Rejects are more dense and settle to the bottom where they are removed by mechanical means

Jigs and Baths

• Can treat feed up to 100mm for jigs and 150mm for DM Baths
• No constraints on bottom size for jigs as long as there is a treatment process for slimes generation.
• DM Baths have limitation to bottom size to cope with magnetite recovery systems. Also there are more efficient methods of treating small coal fractions using dense medium techniques.

Small Coal

• The Dense Medium Cyclone (DMC) is the most efficient separation process.
• DMC’s consist of a cylindrical section connected to a truncated cone.
• Feed enters tangentially into the cylindrical section under pressure. Centrifugal forces separate the reject from the coal.
• The coal reports to the overflow and the rejects the underflow.

Dense Medium Cyclones

• Efficient separators from 0.5 mm up to 50mm.
• Requires a magnetite recovery system – if excessive fine coal (minus 0.5mm) is treated in DMC’s, this material reports to the magnetite recovery circuit (and not the product conveyor!). Results in lost product yield and poor magnetite consumption.
• Many operators crush ROM to 50mm top size prior to CPP and use DMC’s as main separation process.

Fine Coal

• Most difficult size fraction (-1.0 +0.063mm) to beneficiate efficiently. Separation processes do not include magnetite for density control
• Methods include spirals (best option) and water washing cyclones
• Spiral – acts like a radial water slide. Separation occurs as coal follows the stream of water down the spiral. Reject material, being more dense, falls out of the water stream and is removed by splitters.
• Water washing cyclones – similar to DMC’s but do not use magnetite. Cyclone geometry is also different to that of DMC.

Tailings

• The action of water on raw coal generates tailings – often called slimes. Contains high quantities of clay and also includes fine coal particles.
• If the tailings have a high proportion of fine coal, the method to recover the coal is by using flotation technology.
• Tailings are treated in settling thickeners using flocculant to settle the solids. Water is returned to the CPP for reuse.
• Tailings are not usually used as a product coal – high ash, high total moisture, very fine size distribution and high sulfur.

Dewatering

• It is necessary to dewater product and reject streams.
• Handleability of product/reject streams
• Recover magnetite
• Reuse of process water
• Product quality control – total moisture
• Large coal - dewatering screens
• Small coal – dewatering screens and centrifuges (basket type)
• Fine coal – high frequency dewatering screens and/or centrifuges (scroll type)

Magnetite Recovery

• Magnetite is required for DM Bath and DM Cyclone separation processes.
• Magnetite is supplied as a finely ground product (nominally minus 53micron) in bags or bulk.
• Magnetite has a relative density of 4.8g/cm3
• It makes a stable media when mixed with water. The media can be maintained at a consistent relative density if kept agitated.
• Magnetite is used because it is magnetic and can be recovered from the process water streams using magnetic separators.
• Consumption rates of magnetite using modern technology is expected to be less than 0.5kg/tonne feed coal.

Controllables

Environmental Issues

• Dust – a problem in the raw coal materials handling area. Major issue at dump hoppers, stockpile areas, transfer stations, breaker stations and “dry” screening stations.
• Water – run-off water from stockpiles is saline. If coal has high concentration of pyritic sulfur, expect that the process water is acidic.
• Noise – CPP’s are noisy. There are many screens, sirens, dozers, trucks, trains and loaders. This must be taken into consideration if the CPP is near a built up area.

A Few Know’s

• Know your coal reserves.
• CPP configuration – what is the best way to process your coal
• CPP capacity – how much coal will be processed per year
• ROM quality variation – how do you manage feed input to achieve quality output?
• Know your market.
• CPP configuration – when you can make a specific product at the “right” price!
• Quality control of product – how tight is your contract specification? How do you manage your quality?
• Know your costs.
• If you can sell your raw coal as a product at a better price than your washed product (adjusted for yield from CPP and CPP operating costs) then don’t wash the coal.
• Manage your maintenance costs. Good maintenance practices are essential.
• What is the best way of stockpile management? Rehandling is expensive.
• Know your suppliers
• Spare parts, replacement parts, consumables (magnetite, flocculant), water, power, etc.