KINETICS AND MECHANISM OF CHALCOPYRITE FORMATION FROM FE(II) DISULFIDE IN AQUEOUS-SOLUTION (LESS-THAN-200-DEGREES-C)

Chalcopyrite forms from the reaction between aqueous Cu(II) solutions and pyrite by the overall reaction 2Cu(+) + FeS2 = CuFeS2 + Cu2+. 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 X 10(-8) cm(-2).s(-1) at 100 degrees C and 180.9 X 10(-8) cm(-2).s(-1) at 157 degrees C. At 125 +/- 2 degrees C the apparent first order rate constant is 5.86 +/- 0.31 x 10(-8) cm(-2).s(-1) for all observed pyrite reactant size fractions. Interpolation of the rate data to 25 degrees C suggests an apparent first order rate constant for the reaction of 6.47 X 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: CU2+.FeS2(surface) --> CU+.FeS2(surface) (fast) CU+.FeS2(surface) --> CUFeS2(surface) (slow). Chalcopyrite formation through the reaction between aqueous copper and pyrite is very slow at temperatures below ca. 100 degrees C where the reaction with Fe(II) monosulphides is faster. However, it becomes the faster reaction at temperatures above 100 degrees C and is predicted to be extremely rapid at temperatures in excess of 200 degrees C. The results suggest an alternative, kinetic origin for chalcopyrite parageneses in many natural systems.