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P. H. Tewari and A. B. Campbell
Dissolution of Iron Sulfide (Troilite) in Aqueous Sulfuric Acid Param H. Tewari* and Allan 8. Campbell Research Chemistry Branch, Atomic Energy of Canada Limited, Whiteshell Nuclear Research Establishment,Pina wa, Manitoba ROE ILO, Canada (Received February 3, 1976) Publication costs assisted by Atomic Energy of Canada Limited
The dissolution of iron sulfide (troilite) was studied in dilute sulfuric acid, sodium hydroxide, and H2S saturated solutions of different pH. The dissolution kinetics of the reaction FeS 2H+ s Fez+ H2S (8) was studied by following the depletion of H+ from the bulk solution in which disks holding FeS pellets were rotated. These studies showed that the rate of reaction is not controlled by transport of the reactants or the products. The activation energy for the dissolution of FeS is 60 f 7 kJ molW1.The forward reaction is directly proportional to the acid concentration and the reverse reaction is one-half order with respect to both the concentration of Fez+ and H2S pressure. Thermodynamic quantities for the dissolution reaction are reported.
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Introduction Our interest in the dissolution kinetics of iron sulfide stems from its importance in the transport and deposition of iron sulfides in heavy water plants. However, the dissolution of metal sulfides is important in hydrometallurgical operations and semiconductor technology as well. The dissolution kinetics of semiconductor sulfides such as ZnS, CdS, ZnSe, etc. were studied by Locker and de Bruyn' by following the changes in H2S pressure generated in a reaction such as ZnS 2H+ @ Zn2+ H2S (g). Korkowski2 reported that the rate of dissolution of ZnS in H2SO4 solutions is independent of the method of preparation of the sulfide whereas the extent of dissolution does depend upon the method of preparation of ZnS. Poh13 studied the kinetics of dissolution of FeS by following the disappearance of solid FeS from the dissolving mixture, but the method was qualitative and no rate constants were reported. We report here the rate of dissolution of FeS under various pH conditions (pH 2-5) and temperatures (10-55 "C), and the thermodynamic and kinetic constants of the reaction.
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Experimental Section
Materials. Commercial (Cerac, Cerac-Pure Inc., Butler, Wisc.) FeS was used, which on x-ray analysis was found to be hexagonal troilite with a trace of cubic FeS and FeO. The surface area of the powder determined by BET, low temperature (77 K) nitrogen absorption was 0.2 m2/g (assuming 15.8 X m2 as the cross-sectional area of the N2 molecule). The absolute accuracy of the method was judged to be f 1 0 % or better based on the measurements of powders of known surface areas supplied by Particle Information Services, Los Altos, Calif. FeS pellets for the rotating disks were made by compressing fine FeS powder in a pelletizing press at -52 MPa. The pellets were firm and did not flake in the dissolution experiments. X-ray diffraction spectra of the pellets were obtained before and after the experiments to determine structural changes (if any) of the FeS during dissolution. All acidic solutions were made with analytical grade HzSO4. NaOH solutions were COz free. Triple distilled water was used for all solutions. Procedure. The method was similar to that employed by The Journal of Physical Chemistry, Voi. 80, No. 17, 1976
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Vermilyea4 for MgO. The dissolution rate for the net forward reaction FeS
+ 2H+
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Fe2+ H2S (8)
(1)
was followed by the rate of change of pH of the bulk solution. The pH was measured with a Beckman research pH meter fitted with a combination pH and reference electrode. The equilibration time for the p H electrode was obtained for solutions of known acidity or basicity and, in most cases, was
