OXIDATIVE DISSOLUTION AT BASIC pH OF A PYRITE-RICH SLUDGE DOPED WITH FLY ASH

A study of sulfur-rich sludge (71.6% pyrite) dissolution at alkaline conditions (pH ca.9) was investigated using non-stirred and stirred flow-through experiments at room temperature (26 ± 1.2oC) and O2 partial pressure of 0.20 atm. The dissolution rate of pyrite-rich sludge was calculated based on the S release (aqueous SO4) at the steady-state. Results show that: (1) in the nonstirred reactor the sludge dissolution is practically nil as a result of a Fe-mineral coating on the pyritic grains; (2) In the stirred reactor, such a coating is not observed for 600 h, and the sludge dissolution rate is 6.05x10 mol m s. The dissolution kinetics at alkaline pH of the pyrite-rich sludge was also studied adding a fly-ash material (residue of coal combustion) to the mining waste using saturated column experiments. It is observed that: (1) A leaching experiment with a fly-ash column and neutral pH eluent enhanced the neutralization capacity of this material, yielding a leachate pH of approx.10; (2) In a saturated column, consisting of an initial thick layer of fly-ash material and a layer of pyritic sludge, attenuation of the pyrite-rich sludge oxidation is attained at pH approx.10 due to Fe-mineral coating on the pyrite grains (e.g., pyrite encapsulation); (3) In a saturated column experiment solely filled with the pyritic sludge, oxidation occurred favourably at pH approx.3.6. Additional