Coal Preparation » Gravity Separation
The plunging flow at the base of the downcomer in Jameson cell generates high shear stress, leading to the detachment of some particles from particle‐bubble aggregates, particularly for coarse and/or less hydrophobic particles. Laboratory tests of a downcomer modified with a cone type pipe diffuser and a deflection plate have shown the potential of improving the flotation performance of Jameson cells. The aim of this project was to carry out large pilot plant investigations of the modified downcomer at a mine site for a comprehensive assessment of improved combustibles recovery.
The use of the modified downcomer converted the downcomer exiting flow from the vertical plunging flow with a line velocity of 0.35‐0.5m/sec into a sideward or radial flow with a velocity of less than 0.1 m/sec. This significantly reduced velocity of the exiting flow reduced the shear stress by more than 75% and mitigated the channelling flow or short‐circuiting between the downcomer exit and the tailings exit. Test results have proved that the reduced shear stress decreased the chance of the disruption of particle‐bubble aggregates, particularly for coarser and/or less hydrophobic particles.
Results from plant‐based trials of the pilot scale Jameson cell test rig have demonstrated that the use of the modified downcomer can increase the combustibles recovery by 1‐7% for coals with low floatability and 3.9 - 8.0% for coals having intermediate floatability. The use of the modified downcomer improved the combustibles recoveries not only in the +250 μm size fraction, but also in the ‐250 μm size fraction, and the increased combustibles recoveries with the use of the modified downcomer increased with increasing particle size. Ash values of concentrates obtained with the use of the modified downcomer were comparable to that from the original downcomer and aligned with the normal findings that the ash value of concentrate generally increases with the combustibles recovery.
No adverse effects were observed with the modified downcomer on the pilot scale Jameson cell. Modifications of the downcomer with a cone section and a deflection plate are low‐cost and easily retrofittable into exiting cells. The use of the modified downcomer on Jameson cells for the flotation of fine coal could have an attractive economic benefit.