Dissolution kinetics of dolomite in water at elevated temperatures

Kinetic experiments of dolomite dissolution in water over a temperature range from 25 to 250 degrees C were performed using a flow through packed bed reactor. Authors chose three different size fractions of dolomite samples: 18-35 mesh, 35-60 mesh, and 60-80 mesh. The dissolution rates of the three particle size samples of dolomite were measured. The dissolution rate values are changed with the variation of grain size of the sample. For the sample through 20-40 mesh, both the release rate of Ca and the release rate of Mg increase with increasing temperature until 200 degrees C, then decrease with continued increasing temperature. Its maximum dissolution rate occurs at 200 degrees C. The maximum dissolution rates for the sample through 40-60 mesh and 60-80 mesh happen at 100 degrees C. Experimental results indicate that the dissolution of dolomite is incongruent in most cases. Dissolution of fresh dolomite was non-stoichiometric, the Ca/Mg ratio released to solution was greater than in the bulk solid, and the ratio increases with rising temperatures from 25 to 250 degrees C. Observations on dolomite dissolution in water are presented as three parallel reactions, and each reaction occurs in consecutive steps as CaMg(CO3)(2)(s) = MgCO3(s) + Ca2+ + CO32- MgCO3(s) = Mg2+ + CO32- where the second part is a slow reaction, and also the reaction could occur as follows: Rate = Sigma r(ij) = Sigma k(ij)(a(i))(n) where Sigma r(ij) refers to one of each reaction among the above reactions; k(ij) is the rate constant for ith species in the jth reaction, a(i) stands for activity of ith aqueous species, n is the stoichimetric coefficience of ith species in the jth reaction, and define n = nij. The experiments prove that dissolved Ca is a strong inhibitor for dolomite dissolution ( release of Ca) in most cases. Dissolved Mg was found to be an inhibitor for dolomite dissolution at low temperatures. But dissolution rates of dolomite increase with increasing the concentration of dissolved Mg in the temperature range of 200-250 degrees C for 20-40 mesh sample, and in the temperature range of 100-250 degrees C for 40-80 mesh sample, whereas the Mg2+ ion adsorption on dolomite surface becomes progressively the step controlling reaction. The following rate equation is suitable to dolomite dissolutions at high temperatures from 200 to 250 degrees C. -r(Ca2+) = k(m(Ca2+))(n) + k(ad)(K(Mg2+)m(Mg2+))/(1 + K(Mg2+)m(Mg2+)) where -r(Ca2+) refers to dissolution rate (release of Ca), m(Ca2+) and m(Mg2+) are molar concentrations of dissolved Ca and Mg, k(ad) stands for adsorption reaction rate constant, K-Mg refers to adsorption equilibrium constant. At 200 degrees C for 40-60 mesh sample, the release rate of Ca can be described as: -r(mol m(-2) s(-1)) = 0.55 x 10(-4) (m(Ca))(-0.36) + 0.135 x 10(-4) (m(Mg))/(1 + 0.14m(Mg)).