THE EFFECT OF GYPSUM COATINGS ON CALCITE DISSOLUTION RATES AND THE TREATMENT OF AMD

Oxidation of sulfur from pyrite produces AMD solutions with low pH and high sulfate concentrations. A common passive treatment for acid mine drainage (AMD) is to react it with limestone to neutralize its acidity and increase its net alkalinity. However, limestone drains have been reported to fail after a period of time, which has been attributed to coatings. Although these coatings are often iron and aluminum hydroxides and hydroxysulfates, gypsum coatings can form by the reaction:

CaCO_{3 (cal)} + H₂SO₄^*  + 2H₂O = CaSO₄·2H₂O _(gyp) + H₂CO₃^*            

An array of batch reactor experiments measured the dissolution rates of Iceland spar calcite (A_sp = 0.015m²/g, 40-60 mesh fraction) at room temperature in 0, 0.1, 0.3, or 1.0 M solutions of sodium sulfate with pH values of 1.5, 2.0, 2.5, 3.0 or 3.5. Scanning electron microscopy (SEM) showed that gypsum coatings formed on calcite at low pH where sulfate concentrations were high. Gypsum coating formation is controlled by the saturation of gypsum at the calcite surface.

The effect of the growing gypsum layer on the rate of acid neutralization can be derived from Fick's first law of diffusion, which when solved using appropriate boundary conditions predicts that the rate of neutralization of acid declines as the square root of time (r = kt^-0.5).These experiments confirm this behavior and predict that the diffusion coefficient of H⁺ through a gypsum layer is between 1.71x10^-15 and 4.54x10^-14 m²/sec.