Ultrafine coal classification using 150 mm gMax cyclone circuits

A two-stage classification circuit using 150 mm diameter gMax cyclones was installed and evaluated in a coal preparation plant in an effort to achieve a clean coal product without the use of froth flotation. Particle size separations of around 37 μm were achieved while limiting ultrafine bypass to less than 10% in the circuit underflow stream. As a result, approximately 81% of the ash-bearing material in the circuit feed was rejected to the circuit overflow stream. The feed ash content was reduced from around 50% to values in the range of 22–30% in the circuit underflow stream with a mass recovery of about 30%. Further reductions in the coarse product ash content were limited due to the particle density effect and the remaining presence of a significant quantity of high-ash slime material in the coarse product. The typical D50 for the coal particles was 40 μm while the estimated value for mineral matter was 17 μm. Based on the findings of the study, the use of classification to recover a low-ash, coarse fraction in the feed of a fine coal circuit is limited by the density effect regardless of the ability to eliminate ultrafine bypass.