Kinetics and mechanism of chalcopyrite formation from Fe(II) disulphide in aqueous solution (

Abstract

Chalcopyrite forms from the reaction between aqueous Cu(II) solutions and pyrite by the overall reaction 2 Cu + + FeS 2 = CuFeS 2 + Cu 2+ The result resolves a long standing controversy about the origin of the electron balance in the reaction. The reaction is a surface chemical reaction-controlled process. The rate is proportional to a fractional power of the geometric pyrite surface area and directly proportional to the dissolved copper concentration. The rate expression may be represented as −d([Cu] aq) dt = kA 0.55([Cu] aq) , where −d([Cu] aq) dt is the rate in moles [Cu] · dm −3 · s −1, [Cu] is the total concentration of dissolved copper, and k is the apparent first order rate constant in cm −2 · s −1 which varies between 1.27 × 10 −8 cm −2 · s −1 at 100°C and 180.9 × 10 −8 cm −2 · s −1 at 157°C. At 125 ± 2° C the apparent first order rate constant is 5.86 ± 0.31 × 10 −8 cm −2 · s −1 for all observed pyrite reactant size fractions. Interpolation of the rate data to 25°C suggests an apparent first order rate constant for the reaction of 6.47 × 10 −12 cm −2 · s −1. The apparent Arrhenius activation energy for the reaction is 94.29 ± 0.07 kJ mol −1 · K −1. The experimentation suggests a strong rate dependence on the concentration of Cu(I) in solution suggesting the following mechanism: Cu 2+ · FeS 2( surface) → Cu + · FeS 2( surface) (fast) Cu + · FeS 2( surface) → CuFeS 2( surface) (slow). Chalcopyrite formation through the reaction between aqueous copper and pyrite is very slow at temperatures below ca. 100°C where the reaction with Fe(II) monosulphides is faster. However, it becomes the faster reaction at temperatures above 100°C and is predicted to be extremely rapid at temperatures in excess of 200°C. The results suggest an alternative, kinetic origin for chalcopyrite parageneses in many natural systems.