Mechanism and kinetics of the reaction 1 dolomite + 2 quartz = 1 diopside + 2 CO 2 investigated by powder experiments

Abstract

Experimental investigation of the forward reaction I dolomite + 2 quartz = I diopside + 2 C02was carried out in a conventional hydrothermal apparatus, in order to determine the mechanism and the kinetics at a total pressure of 500 MPa and 680'C. The CO2 contents of the fluid phase, consisting of carbon dioxide and water, was found to vary from 80 to nearly 100 mol9o. The experiments lasted up to 365 days (8,760 hours). SEM examination of the reaction mixtures clearly shows a dissolution - crystallization mechanism operating within the whole range of X(CO), even at almost dry conditions. The data on degree of conversion ta,'sas time, results of SEM studies of experiments, and a variation of surface area of reactants and products, have shown that rateJimiting processes are always interface-controlled as long as the diopside crystals do not cover the dolomite surfaces completely. During the first 6@-800 hours, the rate of the heterogeneous reaction is controlled by the interplay of dolomite dissolution and diopside crysta! lization (including nucleation and growth). Compared with these processes, quanz dissolution appears to be rapid in any case under the experimental conditions applied. All of these dissolution and crystallization steps are themselves complex processes that consist of several elementary reactions. The experimental results show that the rate-limiting process changes during the course of reaction, even if P-T-X(CO) conditions are constant, or nearly constant. At the beginning of the reaction, diopside crystallization is rate-limiting, but later, dolomite dissolution becomes rate-determining; finally, when the dolomite-armoring diopside rim is complete (-800 hours, -3590 conversion), the transport of