PYRITE OXIDATION IN HUMID AIR

Although mining is essential to life in our modern society, it generates huge amounts of waste that can lead to acid mine drainage (AMD). Proper management of mine wastes in the 21^st century requires an understanding of the geochemical reactions that create acidic drainage. In many areas, mine wastes occur as large piles that are open to the atmosphere so that air and water vapor can circulate through them. Our research addresses the reactions and transformations of the minerals that occur in humid air in the pore spaces in waste piles.

The rate of pyrite oxidation in moist air was determined by measuring over time the change in pressure between a sealed chamber containing pyrite plus oxygen and a control chamber. The experiments carried out at 25˚C, 96.8% fixed relative humidity, and oxygen partial pressures of 0.21, 0.61, and 1.00 showed that the rate of oxygen consumption is a function of both oxygen partial pressure and time. The rates of oxygen consumption fit the expression

dn_O2/dt=10^-6.48P^0.5_O2t^-0.5 .

The rate slows with time because a thin layer of ferrous sulfate + sulfuric acid solution grows on pyrite and retards oxygen transport to the pyrite surface.

At very short reaction times, the rate of pyrite oxidation in air is slightly faster than the aqueous oxidation rate at the same oxygen partial pressure and temperature. At greater extents of reaction, the rate slows significantly and approaches the rates reported by humidity cell studies. This slower rate of oxidation in air appears to be more appropriate than the aqueous oxidation rate for modeling pyrite oxidation in unsaturated waste piles.

At relative humidities less than 95%, a solid ferrous sulfate phase (melanterite or szolmonokite) becomes saturated and will precipitate from the ferrous sulfate + sulfuric acid solution in cracks and on the reacting surface. These solids have the potential to wedge apart the sample leading to physical disaggregation of the pyrite as is often seen in museum samples. This increase in exposed surface area could accelerate the oxidation of remaining pyrite, further increasing the amount of acid produced.