THE EFFECT OF GYPSUM COATINGS ON THE DISSOLUTION RATE OF CALCITE IN AMD SOLUTIONS

A common economical way to treat acid mine drainage (AMD) is to react it with limestone to neutralize its acidity and increase its net alkalinity.

                        CaCO₃ (cal) + 2H⁺=Ca²⁺ (aq) + H₂CO₃ (aq)                         (1)

During treatment, secondary mineral coatings form on the limestone (armoring), which may affect its dissolution rate and the effectiveness of this treatment. Anoxic limestone drains (ALD) are used to reduce the oxidation of ferrous iron, which when oxidized to ferric iron, will precipitate as ferric iron oxyhydroxide coatings due to the increase in pH attributed to dissolving limestone. Recent studies (Hammarstrom et al. 2003) show that gypsum along with iron oxyhydroxides precipitate on the surface of limestone in treatment facilities.

                        Ca²⁺ (aq) + SO₄^2- (aq) + 2H₂O=CaSO₄·2H₂O (gyps)             (2)

This raises the question of how much does the formation of gypsum coatings affect calcite dissolution rates.

An array of batch experiments measured the dissolution rates of Iceland spar calcite (A_sp=0.015m²/g, 40-60 mesh fraction) in sulfate rich solutions where gypsum could precipitate onto calcite surfaces. These experiments were conducted at room temperature using 0, 0.1, 0.3, and 1.0 M solutions of sodium sulfate with pH values 1.5, 2.0, 2.5, 3.0 and 3.5. Scanning electron microscopy (SEM) shows the distribution and type of coatings. Rates are reported in terms of moles of H⁺ consumed and moles of Ca released versus time. The rate, r_i=dm_i/dt (mol/m²sec) was calculated from the amount of Ca released into solution or the amount of H⁺ ions consumed. The concentration of H⁺ or Ca²⁺ (m) at any time (t) follows an equation of the form m=at^1/2 + b. The r_i=dm_i/dt=1/2at^-1/2, decreases over time. The decline in the rates with time can be attributed to: (1) H⁺ ion consumption over the course of the experiment, (2) change in surface area as a function of extent of reaction, (3) diffusion of H⁺ ions though a growing layer of gypsum, or a combination of these factors.

At pH=1.5, below the pK_a for bisulfate and where gypsum coatings form on the surface of calcite, the ratio of the rate of calcite dissolution with 1.0 M sulfate to the rate with 0 M sulfate is 0.9 and at pH=3.5 the ratio is 12.6.