The production of high quality silicon requires the use of ultraclean coal containing less than 1.5% ash. The magnetite used to clean the coal in a dense medium process is a contaminant that seriously impacts the quality of the final silicon product. As such, research has been conducted to evaluate the potential to substitute the magnetite with fine silica–based alternative material generated during the silicon production process. Dense medium cyclone tests were performed based on a statistically designed program to determine the optimum conditions that maximize organic efficiency and minimize probable error and low–density bypass. The results revealed that a clean coal product with less than 1.5% ash can be produced using a medium formed from the silicon production waste with an organic efficiency value of around 99% and a probable error value below 0.02. There was no measurable bypass of high density particles into the product stream or low–density particles into the reject stream.

Dense medium cyclones are used extensively in the mineral processing industry to beneficiate various minerals including coal, diamonds and iron ore, amongst others. According to Reeves (2002), .the cyclone has been installed in over one-quarter of the coal preparation plants worldwide.. Dense medium cyclones have the ability to achieve high capacities, and simultaneously obtain sharp separations and high separation efficiencies. However, this piece of equipment does have a shortcoming in that its capacity is constrained by the solids carrying capacity of the spigot. This is termed the spigot capacity. There is uncertainty on whether the spigot capacities specified by DSM (Dutch State Mines), the original developers of the dense medium cyclone, can be increased or not, and how these capacities were determined. The purpose of this study is to establish a methodology to determine the spigot capacities of dense medium cyclones, and determine the parameters that influence these capacities. In order to illustrate the significance of increasing the capacity of dense medium cyclones, the following coal example is used: In 2005, South Africa produced about 245Mt of coal valued at R35.86 billion. A significant proportion of this coal is beneficiated through dense medium cyclones. Therefore, an increase in the cyclone capacity, even if relatively small, represents a large number in terms of tonnages of coal produced or monetary gains. It has been established clearly in this investigation that the maximum spigot capacity is reached at the onset of roping. A critical sinks ore concentration at which spigot overloading occurs has been observed. The simplest and best indicator of possible spigot overloading has been established to be the sinks ore concentration, measurement of this parameter could, however, prove challenging on most industrial cyclones. Further, spigot overloading of a dense medium cyclone can be detected visually by observing the discharge type at the sinks and monitoring particle misplacement to the floats stream. A regression model that quantifies the spigot capacity, in terms of ore and slurry, has been developed. Various parameters were considered in the model, these include: cyclone geometry, feed head, medium density, and medium grade. Parameters that influence the spigot capacity of dense medium cyclones have been established, and their effect on the spigot capacity has been quantified. The spigot capacity values obtained in this investigation were compared with those specified by DSM, and it was concluded that there is large potential to increase the .spigot capacities. specified by DSM. / Dissertation (MEng (Metallurgical))--University of Pretoria, 2008. / Materials Science and Metallurgical Engineering / unrestricted

Coal preparation plants are required in some cases to produce a high-grade product using a low specific gravity cut-point. For these situations, a second higher gravity separation would be desirable to generate a mid-grade product that can be utilized for electricity generation thereby maximizing coal recovery. A study was conducted to evaluate the potential of achieving efficient separations at two different density cut-points in a single stage using a three-product dense medium cyclone. Variations in density cut-point and process efficiency values were quantified as a function of the feed medium density, feed medium-to-coal ratio, and feed pressure using a three-level experimental design program. Results indicate the ability to effectively treat coal over a particle size range from 6mm to 0.15mm while achieving both low- and high-density cut-points up to 1.95 relative density. Ash content decreased from 27.98% in the feed to an average of 7.77% in the clean coal product and 25.76% in the middlings product while sulfur content was reduced from 3.87 to 2.83% in the clean coal product. The overall combustible recovery was maintained above 90% while producing clean coal products with ash and total sulfur content as low as 5.85 and 2.68%, respectively. Organic efficiency values were consistently about 95% and probable error values were in the range of 0.03 to 0.05, which indicates the ability to provide a separation performance equivalent to or better than traditional coal cleaning technologies.

Korean coals are typically high rank anthracite characterized by high ash content and difficult cleaning characteristics. The main objective of the study was to evaluate the feasibility of treating various size fractions within the coal using an assortment of physical coal cleaning technologies. Dry cleaning is preferred due to the friability of the coal. As such, three pneumatic processes were tested including Ore Sorting for the plus 10 mm material, Air Table Separation for 10 x 1 mm fraction and Tribo-electric Separator for - 1 mm fraction. The Dense Medium Cyclone is known to be one of the most efficient separation processes and thus was evaluated for the cleaning of 10 x 1 mm coal. To realize the optimum performances from the Air Table and Rotary Tribo-electric Separator, their operational variables were systematically studied using a parametric experimental design. In addition, the dense medium cyclone and X-ray Transmission Sorting trials were performed under various medium densities and separation settings, respectively. A comparison of the cleaning performance revealed that the Dense Medium Cyclone and X-ray Transmission Sorting proved to provide the most effective results with maximum ash rejection and combustible recovery. The tribo-electric separation process was ineffective while the air table provided modest ash reduction potential.