The acid mine drainage problem in Pennsylvania is of continuing and growing interest to the coal mining industry, the state government and the citizenry at large. Approximately one-third of the total volume of mine drainage presently polluting the streams of the Commonwealth originates from active coal operations and two-thirds come from “abandoned mine discharges.”
The acidic contamination of surface drainage and mine drainage water from coal mines is normally caused by the oxidation of iron pyrites. Acidic drainage waters contain dissolved iron salts and varying proportions of other dissolved metals. These waters are slightly cloudy, although suspended iron oxides, coal dust, etc., may also be present.
Technically, any acidic effluent from any coal mine could be purified by chemical processes to any desired standard of purity. Chemical treatment has been applied successfully to similar effluents in other industries in order to make them suitable for discharge to municipal sewers or to natural water courses and investigations have confirmed that these processes could be applied to the acidic effluents under review.
The demonstration plant was designed so that it could be readily moved over existing highways and roads in the Commonwealth without special permits. The pilot plant was constructed in a van type trailer that is 40 feet in length, 8 feet in width, and rises to a height of 12-½- feet above the ground.
Acid mine drainage is brought into the pilot plant by the means of a self-priming centrifugal pump and is pumped through a flow meter into a flash mixer where hydrated lime is added from a screw feeder to neutralize the mine water. By means of gravity the neutralized mine water and suspended solids, consisting primarily of ferrous hydroxide and gypsum, flow into an aerator. Air is brought into contact with the ferrous hydroxide coverting it to ferric hydroxide by the means of a turbine mixer and an air blower. After aeration the slurry, consisting of neutralized mine water and suspended solids, flows by gravity into a thickener. Chemical flocculating agents can be added to this slurry to increase the settling rate of the suspended solids and improve the effluent clarity if desired. The suspended solids in the neutralized mine water slurry settle to the bottom of the thickener while the clear effluent overflows the thickener and is considered treated mine water. The flocculated iron bearing sludge is then pumped to a solid bowl centrifuge.