Physics
Scientific paper
Feb 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54..395n&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 2, pp.395-402
Physics
12
Scientific paper
The kinetic behaviour of pyrite oxidation in the laboratory was studied over a period of about 10,000 hours in reactors through which a carbonate-buffered solution and air (20% O 2 ) flowed continuously. Three grain size fractions were monitored. The concentration of sulphate and the mass of the effluent solution were measured periodically to calculate oxidation rates. The results indicate that the rates of reaction decreased significantly with time. The rates initially exhibited an inverse dependence on grain size (within 400 h) then became more linear with the square of the inverse grain size at later times (after 8000 h) suggesting a surface-layer control of the reaction with time. Surface analysis by X-ray Photoelectron Spectroscopy revealed the presence of ferric oxide on the pyrite surfaces, and ion boring with auger electron spectroscopy indicated a layer thickness on the order of 0.6 microns on the 215 micron grains. The data are represented by a shrinking core model which includes the effects of the surface rate constant plus the diffusive resistance to oxygen transfer through the accumulating reaction layer as oxidation proceeds. The three grain sizes (representing different specific surface areas) exhibited consistent estimates of the surface rate constant ( K s = 3.07 × 10 -6 m h -1 ± 46%) and the diffusion coefficient for oxygen through the oxide layer ( D s = 1.08 × 10 -12 m 2 h -1 ± 30%). The estimated thickness of the oxide layer at the end of the experiment agreed well with the measured value. Oxide accumulation on the pyrite surfaces under neutral pH conditions results in a significant reduction in oxidation rates over time. This behaviour has important implications for the reduction of the rate of release of oxidation products, including hydrogen ions, to environments where sulphide mineral wastes are exposed to the atmosphere.
Gillham Robert W.
Nicholson Ronald V.
Reardon Eric J.
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