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The kinetics of the leaching of natural magnesite with hydrochloric or nitric acid has been studied. Relatively high values of the apparent activation energy (from 46.5 to 70.0 kJ mol−1), but especially the low values of the apparent reaction order for H+ cations, $n$ n , have shown that the whole process is controlled by the surface chemical reaction. The value of $n$ n decreases continuously with increasing activity of H+ cations in the range of pH 5 to −1.4. At ${\rm{pH}}\,\lt 2.6$ pH < 2.6 there is a transition from the kinetic regime characterized by $n \cong $ n ≅ 0.5 to another one with $n$ n close to zero. [ABSTRACT FROM AUTHOR]

Leaching kinetics of natural magnesite with nitric acid was investigated, and the results of a detailed critical analysis are presented. The reaction system MgCO3-HNO3-H2O has been studied with special regard to the production of pure magnesium salts. The generalised non-porous shrinking particle model considering the n-th order liquid-solid reaction was fitted to the measured kinetic data and the values of apparent activation energy, EA, and reaction order, n, were calculated. Relatively high values of EA (43.7 and 58.5 kJ mol-1) obtained for both "low" (0.01-0.1 M) and "high" (1-6 M) concentrations of HNO3, respectively, lay within the range of values presented by other authors and indicate that the overall process is controlled by the chemical reaction in the whole range of reaction conditions considered (temperature from 70 to 100°C and concentration of HNO3 from 0.01 to 6M). This hypothesis has been confirmed by the calculated values of n, which considerably differ from 1. Until now, it has been assumed in the literature that n=1 and no attention has been paid to the actual mechanism of the intrinsic chemical reaction. In this work, the value of n=0.22 was calculated for the solutions with "low" HNO3 concentration, and n=0.05 was obtained for the concentrated HNO3 solutions. These values of n are considerably lower than the value of n=0.5, predicted by the theory. It is hypothesised that the decrease in n with increased acid concentration might be related to the formation of CO2 and its transfer from the liquid to the gaseous phase. [ABSTRACT FROM AUTHOR]