Fine Coal Classification

This project formed part of a major program under the CMTE, with the overall objective of enhancing coal recovery by developing new or improved technology for fine coal classification.

The project had the following components.

1. Baseline testing at three coal preparation plants to define the actual performance of size classification unit operations under typical operational conditions. These were selected on the basis of differing types of plant feed and the fine coal circuits employed.
2. Best practice data was obtained from equipment suppliers, available research information and other sources.
3. A number of recently developed commercial size classification devices were made available to the project. These units were evaluated in the pilot facility at QCAT under controlled conditions and a detailed understanding of their capability was obtained. This information is available for a comprehensive comparison of potential options for classification.
4. The most promising option in terms of selection of unit operations and circuit configuration has been investigated at a realistic scale in an operating plant.

Hydrocyclones provide a cost effective method of classification, but two problems with this device were identified:

• ultra-fine particles report with the oversize flowstream in proportion to the water split
• coarse coal particles report to the undersize flowstream due to the adverse effect of the wide range of particle densities found in fine coal feeds

Approaches to overcome the first problem such as using two cyclones in series tend to exacerbate the density effect problem and result in unacceptable loss of coarser coal particles. Pilot plant investigations and computer simulations have identified the 'Three Stage Counter Current Cyclone Circuit' (CCC) as the best option for overcoming these two problems.

Three alternative separation units were tested at the QCAT pilot plant.

1. Whirlsizer - Not suitable for classification of fine coal
2. Varisieve - Technically superior to the hydrocyclone but requires high screen area and regular maintenance
3. Water Injected Cyclone - Shown to be able to significantly reduce the amount of ultrafines, but leads to a large increase in the separation size of low density material such as coal. It may also worsen the density effect.

A 'Three Stage Counter Current Cyclone Circuit' was constructed with commercial sized cyclones (350 and 250mm diameter), and a detailed investigation was carried out at one of Australia's largest export coal mines. This novel circuit has achieved considerable improvement in classification efficiency when compared with a conventional single cyclone. The amount of ultrafine particles in the oversize flowstream was reduced by over fifty percent and this also led to a reduction in the amount of very fine sized pyrite particles in this flowstream. The size of separation was similar (0.09 compared with 0.10mm), but the sharpness of the partition curve was increased with the CCC circuit and the loss of clean coal in the minus 0.250 plus 0.063 size fraction reduced by about 40%.

All three stages can be implemented with only a single pumping station and gravity feeding of subsequent units if there is sufficient height in the plant, hence maintaining simplicity of the plant circuit.