KINETIC APPROACH TO THE MINERAL REACTION PROCESSES DURING HYDROTHERMAL TREATMENT OF A SAPONITIC CLAY

In the course of hydrothermal experiments with a saponitic clay, evide nce for the dissolution of the accessory sepiolite and the formation o f smectite has been detected above 120 degrees C. Hydrothermal reactio ns with a clay to water ratio of 1:3 were performed at temperatures of 60, 90, 120, 175 and 200 degrees C with time intervals of one month t o one year. The BET surface area and cation exchange capacity (CEC) ar e correlated with the sepiolite and the smectite content determined fr om XRD data. These relations have been used to recalculate the time de pendence of the mineral contents in the time/temperature conditions of the experiments. The E-a Values obtained for sepiolite dissolution (7 -18 kcal/mol) or smectite formation (4.8-5 kcal/mol) indicate that sep iolite dissolution controls the rate of the process. Both results fit an apparent first-order reaction and the system seems to evolve to a s table mineral composition in a short time period, ranging from one to 10 years as temperature decreases.