An investigation of the hydrodynamics of the teetered bed separator for fine coal recovery.

Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2005.The South African coal industry produces a large quantity of coal per annum. The rejects from various unit operations, such as spirals, consist of fine coal that joins the plants tailings dam waste. As existing high quality resources become depleted, the need to improve recovery of this fine coal grows. This project investigates the use of a teetered bed separator (TBS); a hindered settling gravity concentration device for fine coal recovery. This device has proven successful in the United Kingdom and in Australian collieries for fine coal separation in the size range between 2mm and 0.3mm. It has also been used for decades as a classifying device for silica sand and tin. The TBS operates in the size range of water-only cyclones and spiral concentrators, and could potentially be used to separate a broader size range of coal fines so as to offer a lower footprint device for the fines recovery section of a plant. Spiral concentrators cannot always be operated efficiently at a separating specific gravity of lower than 1.6; a TBS may also extend the density range for separation and thus improve recovery. The objective of this project was to gain a full understanding of the TBS from fundamental particle interaction and develop a lab scale unit, which is capable of separation to about 0.1mm at optimum conditions. This involved the development of design parameters based on the various distributor plates and flow pattern modelling. The hydrodynamics of the separator were investigated using the Eulerian-Eulerian modelling approach of commercial CFD package, Fluent 6.1. Seven distributor plates of varying aperture size and geometric arrangement were considered. Coal and shale particles, sized between 2mm and 0.038mm with a specific gravity (SG) range of 1.2 to 2.0, were separated using the laboratory scale unit. The results of both the simulations and the laboratory tests were then compared. The simulations revealed that Plate 3 was the best option for implementation. It had an even upward velocity profile compared to the other plates, with minimum wall effects and disturbances. The upward water flow rate (teeter water) was varied experimentally and the composition of the teeter bed, underflow and overflow were analysed using 1.5, 2 and Smm cubic density tracers with an SG range of 1.2-2.0. Analysis of the partition curves of the distributor plates revealed that Plate 3 had the lowest Ecart Probable (Ep) and cut- point densities. The comparison of simulated results and experimental results show that the simulator could predict the distributor plate design with the lowest Ep in practical tests. The simulator could be beneficial when optimising an industrial scale unit, by allowing prediction of improved segregation patterns and thus separation efficiency

Optimal design of a secondary milling circuit for treating chromite-rich UG-2 platinum ores.

Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2011.Extraction of platinum group elements (PGE) is a major source of revenue in South Africa and the reserves represent about 75 per cent of world reserves. Most of the remaining Platinum Group Mineral (PGM) reserves are located in the UG-2 chromitite layer of the Bushveld Igneous Complex. Platinum concentrators experience significant losses of PGE in their secondary milling circuits due to insufficient liberation of platinum-bearing particles. The chromium oxide (Cr2O3) content in UG-2 concentrates is typically 3%, which results in operational problems in the downstream smelting process. Ways of improving the design of the secondary milling circuit were investigated, with the purpose of improving PGE recovery and reducing Cr2O3 entrainment in the subsequent flotation stage. Batch-scale laboratory and pilot plant tests were carried out to investigate the optimal design of a secondary milling circuit configuration. The optimal design consisted of a conventional hydrocyclone to de-slime the feed, followed by gravity separation with a spiral concentrator circuit to separate the ore into lights (silicates-rich) and heavies (chromite-rich) fractions. Separate milling of the light and heavy fractions made it possible to grind the silicate-rich fraction finer and to avoid over-grinding of the chromite. The total milling energy was redistributed between the silicates and chromite ball mills with 88% of the energy input to the silicates mill and 12% to the chromite mill thus reducing chromite over-grinding. The effects on the recovery of PGE, and the entrainment of Cr2O3 were measured in combined batch rougher flotation tests. The results indicated a 2% improvement in the secondary rougher flotation PGE recovery for the densifier underflow sample as compared to the standard MF-2 circuit, and most significantly the Cr2O3 entrainment was reduced by over 30% overall. Attritioning of the chromite-rich heavies fraction and ball milling of the silicates-rich lights fraction resulted in a 52% reduction of Cr2O3 in the rougher flotation concentrate and a 0.4% increase in PGE recovery (0.4%) as compared to the standard circuit. The improved reduction in chromite entrainment may be attributed to the lower fines generation with attritioning (52.8%- 106μm) as compared to ball milling with a 12% energy input (83.6% -106μm). Over 50% of the chromite minerals remained in the +106μm of the attritioned heavies product as compared with 21% for the ball milled spiral heavies stream. This accounted for a significant proportion of the overall chromite reduction in the flotation concentrate and supported the motivation for the inclusion of a separate grinding circuit for the chromite and silicate particles. Pilot plant testwork on a VHG (very high grade) spiral concentrator circuit followed by laboratory milling and rougher flotation tests confirmed the above conclusions. A 3.7% improvement in PGE recovery was noted with a 32% Cr2O3 reduction in the secondary rougher flotation concentrate as compared to the standard circuit. The statistical reliability of the laboratory and pilot plant data were quantified at various stages of the testwork due to the heterogeneous nature of the feed material and representative sampling. The repeat analyses on selected flotation tests for the high grade ore revealed that the variances were below 0.5%, 4%, and 7% for the head grades, PGE and Cr2O3 recoveries respectively. The flotation results for the standard and significantly improved milling circuits had variances in the 4E recoveries for the low grade ore and pilot plant ore of below 5.5% and 1% respectively. Low variances (<1%) in the Cr2O3 recoveries were noted for the low grade and pilot plant ores. A preliminary cost estimate was undertaken based on the pilot plant data to determine what value the proposed circuit could add for an additional 3.7% PGE recovery. An additional revenue of approximately R50 000 per day could result based upon the platinum mineral recovery only. The other precious metals, i.e. palladium and rhodium were neglected and would further increase the overall revenue. The minimum payback period for the estimated capital investment would be approximately 4 years. This confirmed the benefit of this improved secondary milling circuit design as a viable option. A closed-circuit operation of the silicates mill should offer more significant benefits compared to the open circuit option; however, this was not considered in the current testwork. This project has confirmed the benefit of separate ball milling and the use of a spiral concentrator as an effective gravity separation device in the secondary milling circuit for a chromite-rich (>50%) UG-2 platinum ore