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| Polak K. |
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| Keywords: |
| No 2 (2006), p. 111-118 |
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| Fedoročková A., Raschman P. |
| CHEMICAL DISSOLUTION OF PERICLASE IN DILUTE HYDROCHLORIC ACID |
Abstract
Kinetics of the reaction between particulate polycrystalline MgO and dilute hydrochloric acid were studied with special regard to the rate and mechanisms of chemical dissolution of MgO. The effect of process parameters viz. concentration of H+ ions, temperature and particle size was investigated. It was observed that the rate of transfer of magnesium to the solution: a) increased with increase in both HCl concentration (from 10-2 M to 10-4M) and temperature (from 25°C to 60°C); b) decreased with increase in particle size (from 63 to 355 ?m).
To obtain the temperature and concentration dependence of the rate of periclase dissolution, conversion vs. time data measured at different temperature and different initial pH of the solution were analysed. It was concluded that rate of MgO dissolution decreased with increasing pH over the whole temperature range considered in the present work (25°C – 60°C).
Since the slope of log (rate) vs. pH curve can provide a useful information on the mechanism dissolution (nature of the rate determining step), the slope values were calculated for individual experiments. Even though obtained values (0,16 < n < 0,91) are, within an experimental error, consistent with classic kinetic theories (Vermilyea, Diggle, Gorichev), the rate-controlling step could not have been assigned unambiguously. Nonlinear shape of the curve log r = f(pH) indicates that rate-determining steps of different nature may occur at different pH, or that individual effects of various rate-determining steps can be changed as pH is changed. We conclude that at pH ? 2, the rate is probably controlled by the protonation of the O2- ions to OH- (O2+H+?OH-). At higher pH values, the rate can be controlled either by protonation of surface hydroxyl groups (OH- + H+? H2O) or penetration of protons under the surface of the solid phase. At higher temperature and pH ? 4, protonation of O2- ions to form a water molecule (O2-+ 2H+? H2O) or transport of Mg2+ ions from the solid surface become probably the rate determining steps.
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| Keywords: Magnesium oxide|Hydrochloric acid|Chemical dissolution|Rate|Mechanism| |
| No 2 (2006), p. 202-208 |
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