The Kinetics of the Thermal Decomposition of Pyrite

Abstract

The kinetics of the thermal decomposition of purified pyrite to pyrrhotitc and sulfur in a dynamic argon atmosphere have been studied over the temperature range 600 to 653 "C. T h e reaction was follo~ved by the rate of weight loss as indicated by a quartz spring balance and also by the rate of progression of the pyrite/pyrrhotite interface into a cylindrical, compressed, polycrystalline pellet. The temperature coefficient of the reaction was foiind to be 69.5 f 5.9, 64.7 f 3.3, and 66.9 f 5.1 kcal mole-I, when the results were processed in three different \\lays.The pyrite/pyrrhotite interface was found to progress a t a linear rate into the pellet a t a given temperature; eq~~atiorrs were derived to express the variation of reaction rate with temperature.--lttempts to follow the early nucleation stage ol the decomposition, using massive mineral crystals, proved unsuccessful.The thermal deconlpositioil of the mineral pyrite (FeS2) to yield pyrrhotite (FeS1+,) and sulfur has been made the subject of many investigations (1-13) ; this work has recently been reviewed by Kullerud and Yoder (1). Most studies have been made by measuring sulfur vapor pressure values ( p s ) under equilibrium conditions over a range of temperatures. The enthalpy, A H , of the reaction, obtained froin these measurements, has given values ranging fro111 39 to 104 lccal ~nole-I for various investigators, most values lying in the 05 to 85 ltcal mole-' range. 31Iost \vorl;ers employed static conditions a t less than 1 a t m pressure; I<ullerud and Yoder ( I ) , ho\vever, extended their measurements to higher pressures. The only investigation to be made under the dynamic conditions of a flow of inert gas appears to be that of ITdiiltseva and Chufarov ( 1 3 , \\-I10 used both single crystals and polycrystalline cubes of pj~rite.The difficulties associated 11rit11 the study of this reaction arise fro111 tn-o sources: (a) variability of composition of the resultant pyrrhotite, \vhich is a nonstoichiometric body ; Bog and Rosenqvist (9) expressed the reaction by the equation for temperatures of ,500 to 530 "C, \\here x varies \\-it11 temperature; (b) IVunderlich (13) has pointed out that the degree of association of the sulfur also varies \\it11 temperature, higher pressures and lower temperatures of reaction favoring the formation of S s and S6 molecules over the Sz indicated in eq. [I].In the prese~it study, the conditions employed were those of constant gas f l o~ similar to those that might be expected to obtain in an industrial manufacturing process for sulfur fro111 pyrite. The aiin of this \\-orIc was to determine the kinetic parameters for the reaction of small, polycrystalline cylindrical pellets of pyrite that would approximate in size the pieces of ore that \vould be used in the industrial process. In addition, this type of study was amenable to the same type of anal>.tical treatment as that employed by i\IcI<ewan (14) in the reduction of iron oxide pellets by hydrogen.' I V

The kinetics and mechanism of the pyrite-to-pyrrhotite transformation

“…The apparent activation energy for the decomposition has been reported in the range of 120 to 275 kJ/mol. [1][2][3] The rate-determining step has been attributed by previous researchers to the dissociation of an anion, [1] the coordinated lattice destruction, [1] the S* 2 movement of the pyrite/pyrrhotite interface, [2] and gaseous diffusion. [3] The kinetics of the reduction of pyrite in H 2 has also been investigated by many researchers.…”

Discharge Behavior and Thermal Stability of Synthetic FeS2 Cathode Material

“…where C = pR/k [7] where A, B, and C are constants, t is time, p is the density of pyrite, R is the initial radius of the particle, k is the firstorder rate constant for the surface reaction, and X is the fractional conversion. In this analysis, the end point of the pyrite decomposition (X = 1) is taken at the composition of FeSl.14 which corresponds to the composition in the Fe-S binary phase diagram (6) in equilibrium with FeS2 in the temperature range of this study.…”

“…[7] and the nonlinearity in the fit, the rate constant is not determined from plots of 1 -(1 -X ) 11~ v s . time.…”

“…[7]) is calculated according to the following rate equations for first-order kinetics Table I. There is a slight increase in decomposition rate going from -70+100 mesh to -140+200 mesh particles.…”

The Thermal Decomposition Behavior of Pyrite-Pyrrhotite Mixtures in Nitrogen Atmosphere

“…Considerable research has been carried out on the thermal decomposition behavior of pyrite in an inert atmosphere using a wide range of methods. Many of these results show that the thermal decomposition of pyrite is controlled by a chemical reaction and is a surface first-order reaction [12,13]. Generally, it is considered that the thermal decomposition process is as follows: pyrite ⟶ pyrrhotite ⟶ troilite (FeS) ⟶ Fe, and this process is controlled by the temperature and total S gas pressure in the system [14].…”