General widespread interest in fine coal cleaning circuits did not occur in the United States until the mid 1960’s. Up until this period the relatively low price for which bituminous coal was sold did not justify for many operators the capital and maintenance costs associated with the beneficiation and drying of fine coal. However, with current growing interest in this area, the introduction of new types of process equipment along with modifications to existing components is occuring to maximize the recovery of coal fines.
The sandwich screen consists of two independent fine, square, woven wire screens held together in a common binding in such a manner as to allow cross wires of the bottom surface to create an interference in the free openings of the top screen. By adhering to correct opening vs. wire diameter ratios the blinding of nearsize-oversize particles experienced with conventional fixed opening woven wire and profile screens may be prevented. Product separation is thus achieved when fine particles pass through a nominal opening created by cross wires from both fine screen surfaces.
Feed to the machine is via a three way flow divider mounted on a supporting tower above the live frame body. The device is specially constructed to provide uniform feed streams to distribution boxes located at the head of each screen section. Also, a single high speed (3600 RPM) vibrator is mounted in the center of the live frame which aids the sizing process by rapidly breaking fluid surface tension at the screen surface and conveying oversize solids away from the oncoming load.
Multifeed screens are also currently being used in 28 mesh x 0 fine coal dense medium circuit applications. Screen oversize is reslurried and passed through double drum magnetic separators for further recovery of magnetite values. Similarly, on the clean coal side heavy medium cyclone overflow reports to six units for magnetite drain screening at 150 mesh. Screen underflow returns to the dense medium sump while oversize is reslurried and fed to double drum separators. Finally, 28 x 100 mesh clean coal from the magnetic separators is prethickened on four additional. Multifeeds before discharging into screen bowl centrifugal dryers.
Efficient desliming of 28 mesh x 0 raw coal is of obvious importance in this circuit since recirculation of excessive non magnetics back to the dense medium sump with drain screen effluents will adversely effect system performance. Also of importance is highly efficient removal of fine magnetite through the clean coal and refuse drain screens. By maximizing recovery here magnetite losses are reduced and the job for the magnetic separators is made easier.
