comparative study of electrochemical and thermal oxidation of pyrite

ORIGINAL PAPER C.M.V.B. Almeida á B.F. Giannetti Comparative study of electrochemical and thermal oxidation of pyrite Received: 28 March 2000 / Accepted: 2 February 2001 / Published online: 16 October 2001 Ó Springer-Verlag 2001 Abstract The oxidation of pyrite was studied as a function of grain size, employing voltammetric and thermal analysis. The data obtained with the electrochemical experiments are consistent with the results shown in the thermogravimetry-dierential thermogravimetry TG-DTG) records. The use of both tech- niques revealed that oxidation of the mineral is aected by the grain size employed. The maximum yield of SO 2 4 was obtained from electrodes constructed with <210 lm-sized particles. The same behavior was observed during thermal decomposition. When the temperature surpasses 490 °C, the small particles <210 lm) oxidize in a single step which is not aected by changes in the rate of air ¯ow, indicative of their higher reactivity among the ground mineral electrodes. The comparison between the results obtained by the two methods suggests that, below 490 °C or 0.6 V, pyrite oxidation, either in air or in aqueous media, is similar and depends on the semiconducting properties of the mineral. Keywords Voltammetry á Thermal analysis á Pyrite particles á Electrochemical dissolution á Surface reactivity Introduction Minerals respond to the environment through their surface. The physical and chemical properties of the surfaces play an important role in metal deposit for- mation and in hydrometallurgical processes. The redox reactivity of iron sul®des, in addition to their abun- dance, accounts for the variety of geochemical processes in which they are involved. Hence, it is highly desirable to obtain some information on the reactivity of the mineral surfaces before the leaching and ¯otation processes take place. Fracturing and grinding are common processes applied to rocks and ores to separate phases of economic interest and to characterize analytically the materials. The eciency of the ¯otation and leaching processes and the mineral solubility will depend on the physical and chemical conditions of the surfaces, which restricts the reactivity between the solid and liquid phases. One of the customary methods of determining the surface reactivity of minerals is the evaluation of their dissolution rate by quantifying the content of a dissolved substance in a proper medium [1]. However, this is a quite slow and labor-consuming method. In the present work, measurements of the anodic dissolution current of carbon paste electrodes were com- pared with the results of thermal analysis. Carbon paste electrodes have already been applied to the study of electrochemical dissolution processes of dierent powder and granulate materials [2, 3, 4]. The use of cyclic voltammetry allows estimation of the relative quantity of oxidation products produced during the mineral dissolution. On the other hand, thermal analysis may be used as a tool to assist in the study of the electrochemical behavior of the mineral. This technique permits the evaluation of the surface condition when in contact with air and the oxidation process in the absence of a liquid phase. The aim of this work is to compare the results obtained by electrochemical and thermal methods regard- ing the grain size of the samples. Experimental The ore pyrite samples investigated were from Morro Velho Mine, in Nova Lima, Minas Gerais, Brazil. A quantity of the mineral was hand ground in an agate mortar and pestle. This material was then sieved to isolate the fractions containing particles <210 lm, 210± 250 lm and >250 lm in size. The three electrodes were designated Py21, Py2125 and Py25, respectively. J Solid State Electrochem 2002) 6: 111±118 DOI 10.1007/s100080100200 C.M.V.B. Almeida á B.F. Giannetti &) LaFTA ± LaboratoÂrio de FõÂsico-QuõÂmica TeoÂrica e Aplicada, Instituto de CieÃ ncias Exatas e Tecnologia da Universidade Paulista, R. Dr. Bacelar 1212, Cep 04026-002, SaÄo Paulo, Brazil E-mail: [email protected]

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