Coal Preparation » Fine Coal
The objective of this study was to establish a major step change in the technology used by the coal preparation industry to beneficiate fine particles, especially particles smaller than 0.200 mm. We use the term “Inverse Flotation” to describe the new method of fine coal beneficiation. Particles in a gaseous dispersion interact from the gas side of the gas-liquid interface, either at a planar interface or at the surface of falling drops.
In conventional flotation the particles approach the gas-liquid interface from the liquid side. This limits the efficiency of recovering fine particles, because fine particles tend to follow the fluid stream lines and hence have a low collision efficiency with the bubbles. Due to the low viscosity of air compared to water, inverse flotation processes should have a much higher collision efficiency.
The aim of this project was to perform laboratory scale experiments to test the feasibility of two alternative inverse flotation methods. The first method involved contacting the particles with a moving liquid interface in a trough. Hydrophilic particles should sink, and the hydrophobic coal particles should float and be entrained in the overflow. This work was carried out at the University of South Australia. The trough showed the potential to separate particles based on their hydrophobicity. For instance, the trough could split a -106 um coal sample with a head ash of 55 % into an overflow product with an ash of 23 % and underflow with an ash of 61 %, at a yield of 57 %. This work was compared with that achievable using a laboratory Denver flotation cell. After 20 minutes, in the absence of a frothing agent, this cell was able to separate the same feed sample and achieve a product with 11 % ash at a yield of 39 %. However, with MIBC frothing agent, a product with 14 % ash at a yield of 62 % was achieved. This result was much better than that obtained using the trough.
The second method involved the interaction of a dry dispersion of feed with falling drops. This work was carried out at the University of Newcastle. The feed plume of dispersed particles was projected horizontally into the collision chamber. Water drops were released to fall vertically through the chamber. The dispersed particles then collided with the water drops, with selective capture of the hydrophilic particles. The water collected at the base of the chamber, and then drained into the underflow collection tank.
These experiments produced a linear correlation between the recovery of the particles in the underflow and the water flux. The difference in underflow recovery for two types of particles provides a measure of the selectivity of the process. For glass ballotini (spheres) in 38-45 um size range, the cleaned ballotini had a recovery about 30 % higher than the uncleaned ballotini. Clean (hydrophilic) crushed silica particles had a 45 % higher probability of being captured than coal particles of a similar size. Hence, this work proved that there is selectivity between hydrophilic and hydrophobic particles.
Overall, there was some clear selectivity measured, dependent on the particle size and other surface properties of the material. However, given the limited magnitude of the observed selectivity, it is unlikely that either of these concepts will develop into a new technology competitive with conventional froth flotation. The trough also suffered from a very limited throughput capacity. The multiple falling drop apparatus had serious operational problems when trying to disperse fine particles into a plume, due to their tendency to aggregate. These problems would only become worse with real feeds.
As a result, it is recommended that no further investment be made into developing either of these two processes.