Coal preparation refers to the treatment of raw coal to enhance its suitability for particular end-uses. The extent and method of treatment depends on the properties of the coal and its intended purpose. It may require only simple crushing or it may need to go through a complex treatment process to reduce the inorganic mineral impurities.
Around 80 percent of all coal mined in Australia, and most of the black coal destined for export, is washed to provide a twofold advantage: it upgrades the quality of the coal; and it improves the economics of transportation by removing most of the non-combustible material.
The Australian coal industry has developed considerable expertise in coal preparation. This has enabled it to achieve a high degree of quality control in order to meet customer specifications.
Coal preparation is now a sophisticated process. Routine underground sampling of the coal to be mined provides advance information on coal washability and product quality parameters. This information is used by technical staff in the coal preparation plants to predict washery performance and product quality in the medium to long term – a vital factor in securing markets in advance of production and, therefore, enabling forward planning by coal companies.
Reject disposal methods involve isolating both coarse reject material and fine reject material or tailings produced by the coal processing plant. Tailings usually consist of a mixture of fine particles of coal and clay minerals, suspended in water. Tailings dams are constructed to permit the solid matter to settle out of suspension and allow the clean water to be collected and re-used in washing or other mine processes.
Coarse reject material is normally carried from the preparation plant to the disposal site in trucks and, in the case of open-cut mines, may be used to fill the workings from which the coal was extracted. Alternatively, it can be moulded into the topography of the area to be covered later with topsoil and revegetated as part of the mine rehabilitation process.
In summary, effective preparation of coal prior to combustion:
1. improves the homogeneity of coal supplied
2. reduces transport costs
3. improves utilistation efficiency
4. produces less ash for disposal at the powerplant, and
5. reduces emissions of oxides of sulphur
Coal preperation plant
A Coal preperation plant, nestled among the mountains. Coal Preperation plants generally use gravity process equipment to separate the refuse from the product (coal). Coal has a specific gravity between 1.35 and 1.5, while the refuse rock has a Specific Gravity of 2.1 to 2.3. Heavy Media is the most popular method of cleaning coarse sizes, jig plants are probably the second most common method used for coarse coal. Heavy media cyclones are being used more often for fines size fractions. Flotation is generally used to clean the -28 mesh size fraction, although spirals and heavy media cyclones have shown success in cleaning down to 100 mesh coal feed. Spirals are generally used for middling sizes ( 10 mesh to 60 mesh)
flow diagram of a 400 ton per hour coal plant that produces both metallurgical coal and steam coal for power generation. Generally speaking, metallurgical coal is cleaner , has higher carbon content and is easier to process than the lower rank steam coals.
At this prep plant, all feed coal (ROM) is crushed to -2″ before entering the plant circuits. The 2″ x 10 mesh coal is screened and goes to the primary dense medium cyclone circuit. The 2″ x 10 mesh coking coal is recovered from the refuse here. The middlings (-10 mesh x 60 mesh) are processed in the Secondary Dense Medium Cyclone Circuit to recover the coal here, this produces a higher ash coal and is used as steam coal. The coking coal product conveyor has an intermediate screen that enables material to be diverted in varying proportions to steam coal if necessary to enhance the coking properties.
The -10 mesh coal is deslimed at 100 mesh and then sized at 10 mesh x 100 mesh is processed in the spiral circuit to produce coking coal. The -100 mesh slimes and -60 mesh coal is then processed in the flotation circuit to produce coking coal.
The cleaned coal is dried in centrifuges and a belt filter press. Coarse rejects are crushed and then recombined with the fine tailings for disposal to the refuse fill. All ROM coal production is beneficiated in the coal preparation plant. Approximately 2,000,000 tonnes of cleaned coal is produced annually, with the plant processing about 3,000,000 tons of raw coal annually.To view a larger image of the Flowsheet, click on the text above the Flowsheet.
This graphic shows the general process of a coal plant. First, the mined coal is loaded into a stockpile, with a reclaim tunnel beneath it. Then, the coal is transported to a raw coal silo, usually 10,000 ton capacity, for feed to the plant at a constant rate. In this instance the first stage is a crushing/screening plant, with heavy media processing (for coarse coal sizes – 2″ x 10 mesh), spirals for the middling sizes (10 mesh x 60 mesh), flotation for the -60 mesh fine coal feed. The cleaned coal is then transported to the next plant, which is a series of thermal dryers for reducing the moisture content and thereby raising the BTU value of the coal. The final dried coal product is then transported to the clean coal silo’s for loading onto a train for transport to the end user. In this case, a steel mill and a steam power generation plant.