Kinetics of pyrite oxidation in sodium hydroxide solutions
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- d N / d t = Sbk"pO~176
+ k'"pO~5)
where N represents moles of pyrite, S is the surface area of the solid particles, k" and k" are constants, b is a stoichiometric factor, p O 2 is the oxygen partial pressure, and [OH-] is the hydroxyl ion concentration. The corresponding fractional conversion (X) vs time behavior follows the shrinking particle model for chemical reaction control: 1 - (1 - X ) I/3 = k , t
The rate increases with the reciprocal of particle size and has an activation energy of 55.6 kJ/mol (13.6 kcal/mol). The relationship between reaction rate and oxygen partial pressure resembles a Langmuir-type equation and thus suggests that the reaction involves adsorption or desorption of oxygen at the interface. The square-root rate law may be due to the adsorption of a dissociated oxygen molecule. The observed apparent reaction order with respect to the hydroxyl ion concentration is a result of a complex combination of processes involving the oxidation and nydrolysis of iron, oxidation and hydrolysis of sulfur, and the oxygen reduction.
I.
INTRODUCTION
T H E reaction of pyrite in alkaline solutions has received very little attention in comparison with the several studies carried out in acidic media, tl-221 As a result, little and often contradictory information is presently available concerning the kinetic parameters, e.g., reaction orders, activation energies, and controlling mechanisms. Earlier studies, conducted by Gray, tq Warren, t21 Stenhouse and Armstrong, t31 and Woodcock, 141indicated a reduction in the reaction rate with the increase of pH. The authors attributed this trend to a change in reaction mechanism from chemical control, in acid solutions, to control by diffusion through an oxide film in basic solutions. These conclusions, however, were based on isolated experiments, not on systematic investigations over a broad pH range and under clearly defined conditions. Contrary to the preceding results are those of Smith and Shumate, tSJ Majima and Peters, t61 and GoldhaberJ 71 Smith and Shumate I5} investigated the effect of pH (from 1.5 to 10) on the oxidation of coal pyrite. The reaction was found to be practically independent of pH in acid solutions, but increased rapidly above pH 4. Goldhaber t71 reported similar results. Other sulfides, e.g., stibnite (5b2S3), covellite (CuS), and chalcopyrite (CuFeS2), also showed enhanced oxidation rates in buffered alkaline solutions. 161
The discrepancies between the two groups of investigations are partially explained by the different experimental conditions. For instance, Gray tq and Warren tzj increased the pH with calcium carbonate, thus providing calcium ions that can precipitate as CaSO4 o r C a C O 3 , while Smith and Shumate 15J and Goldhaber 171 used sodium hydroxide solutions. Stenhouse and Armstrong 13j worked at elevated temperatures and pressures, which might have caused a change in the reaction mechanism to diffusion control. Inadequate experimental methods may also be partly responsible for the differences among the various studies. For
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