The effect of limestone treatments on the rate of acid generation from pyritic mine gangue |
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Authors: | Ronald A Burt Frank T Caruccio |
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Institution: | (1) Department of Geology, University of South Carolina, 29208 Columbia, SC, U.S.A. |
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Abstract: | Surface water enters the Haile Gold Mine, Lancaster County, South Carolina by means of a small stream and is ponded behind
a dam and in an abandoned pit. This water is affected by acidic drainage. In spite of the large exposures of potentially acid
producing pyritic rock, the flux of acid to the water is relatively low. Nevertheless, the resulting pH values of the mine
water are low (around 3.5) due to negligible buffering capacity. In view of the observed low release of acidity, the potential
for acid drainage abatement by limestone ameliorants appears feasible.
This study investigated the effects of limestone treatment on acid generation rates of the Haile mine pyritic rocks through
a series of leaching experiments. Below a critical alkalinity threshold value, solutions of dissolved limestone were found
consistently to accelerate the rate of pyrite oxidation by varying degrees. The oxidation rates were further accelerated by
admixing solid limestone with the pyritic rock. However, after a period of about a month, the pyrite oxidation rate of the
admixed samples declined to a level lower than that of untreated pyrite. Leachates produced by the pyrite and limestone mixtures
contained little if any iron. Further, in the mixtures, an alteration of the pyrite surface was apparent.
The observed behaviour of the treated pyrite appears to be related to the immersion of the pyrite grains within a high alkalinity/high
pH environment. The high pH increases the rate of oxidation of ferrous iron which results in a higher concentration of ferric
iron at the pyrite surface. This, in turn, increases the rate of pyrite oxidation. Above a threshold alkalinity value, the
precipitation of hydrous iron oxides at the pyrite surface eventually outpaces acid generation and coats the pyrite surface,
retarding the rate of pyrite oxidation. |
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