The kinetics and mechanism of the pyrite-to-pyrrhotite transformation
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I.
INTRODUCTION
THE thermal decomposition of pyrite (FeS2) has been investigated by many researchers. The apparent activation energy for the decomposition has been reported in the range of 120 to 275 kJ/mol.[1–3] The rate-determining step has been attributed by previous researchers to the dissociation [1] the coordinated lattice destruction,[1] the of an S* 2 anion, movement of the pyrite/pyrrhotite interface,[2] and gaseous diffusion.[3] The kinetics of the reduction of pyrite in H2 has also been investigated by many researchers.[1,4–10] They have reported apparent activation energies for the reduction between 70 and 200 kJ/mol. Most authors have reported that in the presence of H2, FeS2 is converted to FeS with the production of H2S. Few authors have attempted to include the fact that FeS2 is not converted directly to FeS and have neglected to account for the intermediate pyrrhotites which form during the reduction. One author[9] has proposed the following mechanism: 2FeS2 1 H2 ——> Fe2S3 1 H2S
[1]
Fe2S3 1 H2 2FeS 1 H2S
[2]
The formation of Fe2S3 is unlikely based on present knowledge of the thermodynamics stability of iron sulfides.[11–14] Another, somewhat more plausible, mechanism has been suggested by Niwa et al.:[10] FeS2 1 H2 ——> FeS 1 H2S
[3]
FeS 1 xS (in FeS2) ——> FeS11x
[4]
FeS11x 1 xH2 ——> FeS 1 xH2S
[5]
These authors defined FeS11x to be FeS1.15 based on their experimental observation that this was the only pyrrhotite
J.M. LAMBERT, Jr., Engineer, is with the Parr Instrument Company, Moline, IL 61265. G. SIMKOVICH and P.L. WALKER, Jr., Professors Emeriti, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted February 10, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
which appeared. They further concluded that the second reaction was rapid compared to the other two, since no FeS appeared when FeS2 was present. An apparent activation energy of 120 kJ/mol was reported. Other authors,[7,9] who also reported the apparent activation energy for the reduction to be 100 kJ/mol, concluded that the rate was controlled by the diffusion of sulfur in the pyrite lattice and by the availability of H2 to remove sulfur from the pyrite surface. They proposed the following twostep mechanism: FeS2 ——> FeS 1 S
[6]
S 1 H2 ——> H2S
[7]
It is the purpose of this investigation to determine the rate and mechanism by which pyrite is reduced to pyrrhotite both thermally and in H2, and to attempt to elucidate the cause of the widely varying activation energies reported in the literature for this transformation. II.
THEORETICAL CONSIDERATIONS
Prior to beginning the experimental work, the thermodynamic region of stability was examined to establish the limits within which the kinetics were to be investigated. According to accepted phase equilibria of iron-sulfides,[11] pyrite (FeS2) decomposes to pyrrhotite (FeSx, 1.00 , x , 1.23) in the temperature range of 523 to 1016 K. As can be seen in Figure 1, the composition of the pyrr
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