Coal Preparation » Gravity Separation
Conventional banana-screen design continues to rely on the 1960s DSM methodology, carried into modern practice largely through simple multipliers applied in the ACARP DMC Handbook. These legacy models overlook several key operating variables.
Recent plant experience has shown the above factors can substantially affect drainage rates, screening efficiency, magnetite consumption, and medium-density stability. This project addresses these limitations by extending earlier QCAT test-screen investigations, expanding the operating envelope, decoupling stroke and frequency effects, and evaluating a broader range of apertures.
Static testing showed that panel performance is uniform at low feed rates, with divergence appearing above 150 m³/h/m² as lower-open-area panels (i.e. 0.5 mm ww) begin to flood. Flooded drainage rates were significantly higher than predicted by DSM, and a squared correlation was established between maximum drainage rate and open area.
Screen-angle testing revealed a consistent U-shaped efficiency trend with optimal performance at 0° and 25°, aligning with previous findings. Frequency effects were minimal at low angles and large apertures but became significant when apertures were small or angles high, where vibration promoted material carry-over.
Conventional wisdom held that higher bed depths should reduce drainage rates, however this study showed that higher bed depths were producing lower moistures. Velocity measurements further demonstrated that higher particle velocities correlate with poorer screening performance, challenging long-held assumptions about the benefits of increased throughput.
Optimal operating conditions were identified as a 100 - 200 mm bed depth at 10 - 12 mm stroke and
11 Hz frequency, producing a particle velocity of approximately 0.15 m/s. Final solids trials confirmed the importance of weirs for slowing material flow and reducing medium carry-over.
Overall, the project demonstrates that residence time, not bed depth, is the primary determinant of screening efficiency and provides improved guidance for screen selection, circuit control, and magnetite management.