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dewatering continued The Blue Creek prep plant washes the entire raw coal stream. The typical raw feed coal characteristics for Stockton are 58% ash and 0.4% sulfur. The prep plant produces approximately 585 tph of coarse and fine refuse. The coarse refuse consists of refuse screen over-sized material, and is transferred by refuse belts to a coarse refuse loadout. This coarse refuse stream consists of all plus 1-mm refuse material, and accounts for about 500 tph (55.5% of the plant feed). The remaining material, about 85 tph (or 9.4% of the plant feed), is fine refuse material, minus 1 mm, which reports to the thickener. Currently the thickener underflow produces 800 gpm with 32% solids (or 1.2 sg) of minus 1-mm fine coal refuse, all of which feeds the plate and frame circuit. The 800-gpm thickener underflow delivers 262 tph of slurry mass and 85 tph of solid refuse. Roughly 90% of the plate and frame feed is minus 60 mm. With this particle size distribution and quantity of slurry, Blue Creek plant had to select the appropriate plate and frame system for the application. Plate and Frame Presses Plate and frame presses (filter presses) work in a "batch" loading style. They are filled with slurry at the start of a filtering cycle. During the filtration cycle, the filter press produces a batch of solid (dewatered) material or "filter cake" and clarified water. The clarified water or "filtrate" is collected during the filtration cycle, and is either reused in the plant or discharged. After discharging the filter cake, the filter press is re-loaded with slurry for another filtration cycle. The filter press uses pressure placed on the solid-liquid material to displace the filtrate from the solid material. The increased pressure from a plate and frame system can produce moisture levels less than 25%. The plate and frame system consist of a series of plates with recessed chambers. These plates are forced together with hydraulic pressure typically in the range of 2,500 psi (19 MPa). This compression creates sealed chambers that trap the slurry until pressure builds from the feed pump. As pressure increases, water in the slurry passes through the filter media leaving the solids trapped in the concave cavity. The chamber continues to pack with solids forming the filter cake. Once the chamber fills completely and the water has been forced from the solids,the press opens releasing the solid cake. Clear water is collected in the filtrate pans and returned to the process for re-use. Membrane vs. Chambers There are two styles of filter presses: recessed chamber and membrane. In a recessed-chamber filter press, all of the plates are rigid and concave. The slurry is trapped in the chamber created by the plates being pressed together. This design relies entirely on the hydraulic pressure (usually in excess of 200 psi) of the feed pump to achieve dewatering. As water bleeds from the cake, more slurry is packed into the void until dewatering is complete. This design requires less initial capital but may lead to longer cycle times (fewer cycles per hour), greater wear on the feed pump, and higher moisture content of the cake. To reach the desired pressure, positive displacement pumps or dual-stage slurry pumps are frequently used. With membrane filter presses, every other plate is recessed (chamber plates), the rest of the plates are hollow with flexible elastomer faces (membrane plates). Once the chamber is full and pressurized to about 120 psi, the feed pump stops, feed valve closes and air (or water) inflates the membrane plates to a pressure greater than the pressure in the chamber (usually about 180 psi). The even pressure across the face of the cake forces water from the cake more rapidly, thus reducing dead-head conditions on the feed pump and reducing Figure 2: The flowsheet for the Blue Creek filter press circuit (Taggart, 2011). 36 www.coalage.com January 2013