Coal leachate, pyrite oxidation

Note Values are in mg/kg except as indicated. All samples except the Bluewater Mill sample acquired their initial acidities and solutes because of pyrite oxidation. The Bluewater Mill tailings solution is derived from sulfuric acid leaching of uraniferous sandstone ore. Although listed as Fe(III), iron concentrations in the Bluewater Mill solution and Coal A leachate are total iron values. Except for Bluewater Mill and Coal A laboratory studies. Eh and pH values were measured in the field. All samples were filtered through 0A5foa filters prior to acidification, except for Well 201. 

This principal environmental problem posed by coal-cleaning waste is that the pyrite and marcasite in the waste are oxidized to sulfuric acid in the presenee of air, water, Ferrobacillus ferrooxidans, and Thiobacillus ferrooxidans. The sulfuric acid is usually sufficiently concentrated to dissolve numerous metallic constituents and large quantities of iron from the pyrite in the leachate. Any ECT intended to prevent sulfuric acid formation must eliminate either the air, the water, or the oxidizable sulfur compounds in the waste, or inactive Ferrobacillus ferrooxidans and Thiobacillus ferrooxidans by maintaining alkaline conditions. Post-treatment of pile drainage comprises ECTs designed to neutralize the acid in the effluent and remove the metal ions by some sort of precipitation, adsorption, flocculation, or ion-exchange phenomenon. 

The SI-FA leachate had an initial pH of 5.5, with a rapid increase to a pH of 11. The coal from which this ash was formed had not been washed prior to burning. The initial acidic pH is due to the rapid oxidation of sulfides (e.g., pyrite) present in the ash as a result of incomplete combustion of the... 

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