Milling - Continued

Photo - Dryer

Filter

Underflow from the thickener was pumped to an Eimco " Agidisc", 8 ft., 10 in. in diameter with eight discs, located on the flotation pump floor. The filter was designed to produce a cake of not more than 11% moisture content.

Dryer

After removal from the filter discs, the cake falls through a discharge chute to a short horizontal conveyor which fed into the dryer.

Concentrate dryer.This unit was a rotary dryer and with the heating unit and all auxiliaries were purchased as a package from Koppers Inc., Hardinge Company Division. Dryer unit specifications call for the unit to handle a maximum of 600 t.p.d. of filter cake with 13% moisture and deliver a dried product not exceeding 5% moisture.

The dryer itself was a Ruggles-Coles XH-12 unit, 80 in. in diameter and 40 ft. long. Speed of dryer rotation is 5.3 r.p.m. with the axis inclined at about 2.5 degrees. Four shell knockers were provided. Feed was delivered from the filter via a rubber chute to a screw feeder which, as it injected the filtered material into the dryer, also functioned at the same time as an air lock.

The heater was a Peabody horizontal direct-fired unit, 5 ft. dia. and 14 ft. long. Fuel used was No.6 fuel oil fired by a mechanical-atomizing forced-draft-register burner complete with electric ignitor.

Principle of the drying operation was as follows: The heater had two independent air intakes from separate fans; one supplied combustion air through the burner register to sup- port oil combustion in the inner chamber; the other supplied quench air through the annulus to mix with the hot combustion gases in the furnace-exit throat. Quench-air volume is modulated by automatic-damper control to maintain the desired exit-gas temperature which in this application is the inlet-gas temperature for the dryer.

The heater was rated at 10 B. T .U .per hr. net output with exit gas at 1800°F and burns 595 Ib. per hr. of No.6 fuel oil.

The gas stream which exited from the dryer at about 190 degrees was passed successively through a 9-ft.-diam. low- velocity dry cyclone and then through a wet-cyclone scrubber before being exhausted to the atmosphere.

MILLING-CONTINUED