Kinetics of pyrite oxidation in sodium carbonate solutions
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- d N / d t = SbkpO ~ [ O H - ] ~ where N represents moles of pyrite, S is the surface area of the solid particles, b is a stoichiometric factor, k is an apparent rate constant, pO2 is the oxygen partial pressure, and [OH-] is the hydroxyl ion concentration. The experimental data were fitted by a stochastic model for chemically controlled reactions, represented by the following fractional conversion (X) vs time (t) equation: (1
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X) -2/3
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1 = ksrt
The assumption behind this model, i.e., surface heterogeneity leading to preferential dissolution, is supported by the micrographs of reacted pyrite particles, showing pits created by localized dissolution beneath an oxide layer. In addition to the surface texture, the magnitude of the activation energy (60.9 kJ/mol or 14.6 --- 2.7 kcal/mol), the independence of rate on the stirring speed, the inverse relationship between the rate constant and the initial particle diameter, and the fractional reaction orders are also in agreement with a mechanism controlled by chemical reaction.
I.
INTRODUCTION
F O R many decades, the aqueous oxidation of pyrite, the most common and widespread sulfide mineral, has been studied in order to understand and control the reaction in the various stages of mineral deposition, ore processing, metal extraction, coal processing, and acid mine drainage. However, the majority of these studies were conducted in acidic systems; limited information is available on the reaction kinetics in alkaline solutions, and rate parameters have not been well established3 ~-35] Pyrite decomposition in alkaline solutions may be represented with an overall reaction such as FeS2 + 15/402 + 5 / 2 H 2 0 = FeOOH + 4H + + 2SO 2-
[1]
The addition of a base, e.g., NaOH, to the system neutralizes the acid formed by this reaction and maintains a favorable pH condition for the precipitation of iron. The rate of pyrite oxidation has been found to be affected by the nature of the reagent used to control the pH. Among these reagents, sodium carbonate is claimed to be particularly effective in enhancing the decomposition of pyrite in aerated solutions, t3,4AL~41 Wheelock I"1 investigated a process for coal desulfurization involving pressure oxidation in sodium carbonate solutions. In another investigation, In1 higher oxidation levels of pyrite and V.S.T. CIMINELLI, Associate Professor, is with the Department of Metallurgical Engineering, Universidade Federal de Minas Gerais, Belo Korizonte MG-30160-030, Brazil. K. OSSEO-ASARE, Professor, is with the Department of Materials Science and Engineering, Penn State University, University Park, PA 16802. Manuscript submitted May 19, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
arsenopyrite as well as higher gold recoveries, in comparison with those observed in NaOH solutions, have been reported for the pressure oxidation of a refractory gold concentrate. Recently, oxidation in alkaline solutions has been adopted for the treatment of refractory gold ore on an industrial scale, tl~ The main objective of the present work was to
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