Dense Medium Separation

Dense Medium SeparationDense medium separations include those coal preparation processes which clean raw coal by immersing it in a fluid having a density inter-mediate between clean coal and reject. As there is a general correlation between ash content and specific gravity, it is possible to achieve the required degree of removal of as h-forming impurities from a raw coal by regulating the specific gravity of the separatin g-fluid.Dense medium processes offer the following advantages over other coal cleaning processes:1)Ability to make sharp separation at any specific gravity within the range normallyrequired even in the presence of percentages of the feed in the range of ±0.1 sp grunits.2)Ability to maintain a separating gravity that can be controlled with±0.005 sp gr units.3)Ability to handle a wide range of sizes, up to 35.6 cm.4)Relatively low capital and operating costs when considered in terms of high capacityand small space requirements.5)Ability to change specific gravity of separation to meet varying market requirements.6)Ability to handle fluctuations in feed, both in terms of quantity and quality.1 Dense Medium Separation of Coarse CoalCommercial application of the dense medium process is a practical extension of the familiar of laboratory floa t-sink test, which is used as a standard (100% efficiency) gravimetric separation. Commercial plants do not exactly duplicate laboratory float-sink separations for the following reasons: suspension, rather than true liquids, usually are used as separating media; the introduction of feed and removal of the float and sink introduce disturbances in the separating medium; in suspension; and the practical need for high throughput does not allow sufficient retention time for separating near-gravity material perfectly.Theoretically, any size particle can be treated by dense medium processes; practically, the sizes treated range from about 0.5 mm to about 150 mm, although sizes ranging up to 35.6 cm occasionally are washed. Sizes coarser than about 6.3 mm normally are treated in static dense medium separators, while the size range 0.5 mm to about 6.3 mm normally is washed in separators employing centrifugal force, for example, the dense medium cyclone washer.The ideal separating medium would be a true liquid having the following properties: low in cost, miscible with water, capable of adjustment over a wide range of specific gravities, stable, nontoxic, noncorrosive, and low in viscosity. Although no ideal medium exists, a variety of dense media have been developed and currently are being used commercially to separate raw coal into refuse and salable clean coal.From a practical viewpoint, any medium should have the following characteristics; cheap at the point of use, physically stable so it does not decompose or degrade in the process, chemically inert so it does not react with the coal, easily removable from the product, easily recoverable for reuse, low in viscosity at the desired specific gravity of separation, and a stability capable of being maintained over the range of specific gravity likely to be required.1.1SuspensionsA suspension may be defined as any liquid in which insoluble solids are dispersed and kept in a state of fluid energy. The stability of suspensions used in coal preparation range from nearly stable suspensions using ultrafine magnetite to highly unstable suspensions of relatively coarsesand in the Chance process.The specific gravities of separation for coals range from about 1.30~1.90. To achieve this range of specific gravity while keeping the volumetric concentration at a reasonable level, it is necessary either to select high specific gravity solids or to introduce upward currents in the separating vessel. As the usually accepted volumetric concentration of the suspended solids is between 25 and 45%, their size and specific gravity must be selected to provide the same time having the required medium stability. The coarser the solids, the higher the settling rate, the lower the viscosity, and the easier it is to recover the medium. The finer the solids, the lower the settling rate (hence, the greater stability), the higher the viscosity, and the more difficult it is to recover the medium. Additionally, the higher the specific gravity of the suspended solids, the lower the volumetric concentrations for a given specific gravity. Hence, it is possible to select the specific gravity, size consist, and volumetric concentration of the suspended solids to achieve medium characteristics that provide overall optimum performance and economy.Control of density, viscosity, and settling rate of a suspension is necessary for efficient separation of coal and impurities.The following equation determines the density of a suspension:D= D=100／<(100-C)+C／d>Settling rate of a semistable medium with added slimes(suspension densit y—1.50)Proportion of slimes, Setting rate,weight % min per in. (sec/mm)0 1.78(4.20)10 3.98(9.40)15 6.31(14.90)20 9.55(22.55)25 14.13(33.38)30 21.38(50.50)35 31.62(74.69)40 47.86(113.06)Where D is density of the suspension (g/cm3), d is density of the solids (g/cm3), and C is weight concentration of the solids in suspension (%). From this equation, it will be seen that the density of a suspension can be varied at will by adjusting the proportion of medium solids persent in the suspension. There is, however, a natural limit to the proportion of solids imposed by the volume concentration, since it is important that the suspension should have a relatively low viscosity to permit the free travel of coal and impurities in it. This limiting volumetric concentration is similar for all suspensions with particles of similar size range, but can be modified to a limited extent by change in the size grading of the medium solids.The equation also shows that the density of the suspension is a function of the specific gravity of the medium solids, and it naturally follows from this that media composed of higher specific gravity solids will have a higher density at the limiting volume concentration than suspensions composed of lower specific gravity solids.High density solids, such as magnetite (5.0 sp gr) and barites (4.2 sp gr) give suspensionswhich generally are unstable but which permit suspension densities up to 2.0. Fortunately, however, the stability of such medium can be considerably modified by control of the size grading and by the amount of coal and shale slimes which are present in the suspension, it is found that by fine grinding in the presence of 30%~40% of slimes, semistable suspension can be achieved at densities as low as 1.30~1.35.The influence of added clay to an unstable medium can be seen from Table 4-1, which shows that the settling rate is approximately halved with each 10% increase in slimes concentration and that with the addition of 30% of slimes, the settling rate is so far reduced that the medium can be considered stable. At higher concentrations of slimes, however, the viscosity increases rapidly. Generally ,it is impossible to operate with more than 50%~60% of slimes, as shown by Geer, at al., the influence of clay on viscosity depends on its mineralogical composition; if the clay is bentonitic, only a few percent can be tolerated.Viscosity is the “resistance to flow”of a true Newtonian liquid; however, the definition generally is accepted to include suspensions. The viscosity of a medium in the coal cleaning process has little effect on the light coal particles or the heavy reject particles, but becomes critical in the separation of material of a density equal to near that of the medium. As the falling velocity of a particle in a fluid is proportional to the density difference between the particle and the fluid, a low-viscosity medium must be maintained to separate near-density material at a high rate of feed.Because the density composition of coals varies widely and coals are separated over a range of specific gravities, the effect of viscosity on the sharpnesss of separation will vary with the coal and the separating density.A variety of material has been used as the solids suspended in water for dense media. Included are sand, magnetite, ferrosilicates, pyrites, shales, barites, loess, and mill scale; of these, only sand and magnetite are used extensively in the United States for coal cleaning.2 Separatory V esselsV essels for washing coals may be categorized as cones, drums, and troughs, and include solid cylindrical drums, perforated and solid cylindrical drums rotating in a tank of dense medium, conical tanks, and parallelepipeds.In some vessels, all the medium enters near the top of the tank; in others, a portion enters the bottom of the tank to provide an upward current; in still others, a portion of the medium enters at various levels to form horizontal currents or maintain homogeneity of the medium throughout the tank.The feed generally is introduced at the top of the medium bath. In some vessels, however, the feed is forcibly submerged before being released into the medium bath. In most washers, the clean coal is removed with a portion of the medium overflowing a weir; in a few washers, the floating layer of coal is skimmed off the top of the bath. In cone-type washers, the reject is removed either through an internal or external airlift or though a lockhopper arrangement; in drum-type washers, the reject usually is removing drum; in trough-type washers, the sink usually is carried out of the vessel by a chain conveyor which, in some case, is the same conveyor that is used to skim off the float coal.The function of the separatory vessel is to separate a feed into two or more products. In most instances, the feed is a presized, prewetted, raw coal. However, the feed may be the sink product from a primary separator for secondary separation into a middling and a refuse product. Inasmuch as the separator is the heart of the preparation plant, its design has been given considerableattention by coal preparation plant equipment manu facturers and by coal operators.Requirements for designing a vessel include: it must have the ability to process a high tonnage of coal per unit area of floor space; it should required a minimum volume of circulating medium; the specific gravity of the entire volume of medium in the vessel generally must be kept uniform; hydraulic currents inside the vessel must be minimized to approach a true float-sink separation; the vessel must be designed for the efficient entry of feed to the washer, recovery of the clean coal product, and the elimination of the refuse; and the vessel should the clean coal product, and the elimination of the refuse; and the vessel should be capable of accepting a feed varying size consist and percentage of impurities without adverse effects on the cleaning efficiency.Ideally, the separatory vessel should be capable accepting a feed containing particles of material varying widely in size, shape, and specific gravity, and separating these particles on the basis of specific gravity. All material having a specific gravity lower than desired would be recovered as a float product; all material having a specific gravity higher than desired would be rejected as a sink product.。