Coal is divided into four classes based on its carbon content: anthracite (hard coal), which has the highest carbon content; bituminous coal (soft coal); sub-bituminous; and lignite (brown coal), which has the lowest carbon content. Each type of coal has a different degree of brittleness, which means it is hard, brittle, and breaks in a way. This is measured by the Hardgrove Grindability Index (HGI).
Once mined from the ground, coal can be transported to a plant for cleaning as raw coal, or crushed to a suitable size at the mine and transported to its intended use. The two main uses of coal are for power generation and steel production. For maximum thermal and economic efficiency when transported directly, coal is typically crushed to 2" x 0 to maximize truck and train transport capacity. It is then transported to power plants and steel mills with minimal volume loss.
Ideally, buyers and sellers want the coal to contain as little waste or fines (-30 mesh) as possible to optimize the value of the product.
At the crushing stage, several steps can be taken to minimize the production of fines and increase product yield.
Whenever coal is handled, either manually or by machine, it degrades. The tiny particles that form coal dust are the result of coal degradation. It stands to reason, therefore, that the gentler the coal is handled, the less it degrades. Even if you are crushing coal with the intention of degrading the material as part of the necessary process, you can expect fewer fines to be produced with slower action or rotation of the crushing equipment.
When the crushing ratio of coal can be reduced to 4:1, 3:1, or 2:1 (e.g. in a roll crusher), the feed material can flow more easily through the crushing zone in an unobstructed and continuous manner. This means that the material only comes into contact with the crushing element briefly and does not come into contact with the crushing teeth for more than one rotation.
If the reduction ratio exceeds 4:1, the incidence of tooth contact with larger pieces is higher and takes longer to pass. This results in the grinding of the coal and the production of a higher percentage of fines.
Crushing the coal to a reduced ratio may require additional crushing stages to obtain the final product. Although adding a third crushing stage at the beginning may increase capital expenditure, higher throughput and reduced crusher wear over a sustained period of time may pay dividends in the future.
As mentioned earlier, the less coal is handled and the gentler the handling, the less likely it is that fines will be produced. A popular thought related to this premise is, "Why to crush coal further that has already reached a certain size?" By using a vibrating screen to separate and bypass coals of a size that have been further crushed, you are not degrading the material further.
Screens of the right size can be specified to allow for the required separation after any crushing stage. The use of screens also reduces the capacity required for one or more crushers, thus allowing smaller, cheaper crushers to be used in one or more stages of the circuit.
All of the above methods of operation have been shown to reduce or minimize the production of fines in coal crushing circuits. The considerations for their implementation are
Weighted importance of production capacity and percentage of fines in the product
The capital budget of the plant owner
Long term expectations and economic benefits
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