371 Figure 4- Near Gravity Material (NGM) vs Specific Gravity curve In DMC, centrifugal force is critical to obtaining higher separation performance and is also key to setting the lower particle size limit for efficient separation. For coal particles that are fine, the settling of the particles must be controlled by the equilibrium of centrifugal force and viscous force. Nevertheless, when the particle size comes close to 2 mm, the viscous force begins to dominant over the centrifugal force. For particle diameters less than 1 mm, the viscous force becomes predominant, DMC performance decreases and density cut-point (offset) increases significantly [9]. Therefore, the lower particle size is chosen as 1 mm in coarse coal separation by DMC. Among Intermediate Size Beneficiation, fine dense media cyclone is no doubt the best equipment (very sharp separation, Ep: 0.05-0.08) till date, but due to complexity in operation – precise control of medium density, pressure stability, efficient magnetite recovery circuit, etc it is not widely used unless very low ash product is required and NGM is very high. Due to more auxiliary equipment required for its operation, its capex & opex is generally high. A very high quality and super fine grind magnetite power is required and loss of magnetite is also more during its operation. Due to this challenges, Fine DMC is not commercially installed anywhere in India. Recently, Tata Steel in its Jharia CPP, Jamadoba Plant has successfully done a trail run of Fine DMC (0.5 – 0.25 mm) to recover clean coal ash of 14% @ 69% yield from 26% feed ash. Globally, it was used upto 1980’s, with the development of Spirals, use of fine DMC is becoming restricted. But again after 2005, reconsideration of fine DMC has started to produce fine coal at a higher product quality with maximum recovery, which no other equipment can deliver. As there is need of hour to obtain maximum recovery of the available resources, so fine DMC is gaining popularity. A lot of research work is going on to optimise its performance and reduce the operating cost. Another advanced beneficiation technology for intermediate size is Teeter Bed Separator (TBS). It is a gravity separator developed from the conventional Hydrosizer technology and operates based on principles of hindered settling and fluidization to accomplish separation of particles. An upward water supply flow fluidizes a top-fed bed of fine particles that is forming an autogenous medium that is suitable for separation of the particles according to their size and densities. This process takes advantage of the inherent settling velocity differences between various sizes and densities of particles exhibited. Heavy particles settle more quickly in a multi-particle system formed as a result of the suspension of water and middle-mass particles in the separator. As a result, heavy particles are separated from light particles by settling rapidly through suspension. Lighter particles, on the other hand, float through suspension due to their lower settling rates. However, some small size, high density particles and larger size, low density particles report to overflow and underflow. Thus, there is misplacement of particles and separation efficiency (Ep: 0.12-0.18) is reduced due to his
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