Stirring effect on kaolinite dissolution rate

Experiments were carried out measuring kaolinite dissolution rates using st irred and nonstirred flow-through reactors at pHs 2 to 4 and temperatures o f 25 degreesC, 50 degreesC, and 70 degreesC. The results show an increase o f kaolinite dissolution rate with increasing stirring speed. The stirring e ffect is reversible, i.e., as the stirring slows down the dissolution rate decreases. The effect of stirring speed on kaolinite dissolution rate is hi gher at 25 degreesC than at 50 degreesC and 70 degreesC and at pH 4 than at pHs 2 and 3. It is suggested that fine kaolinite particles are formed as a result of sti rring-induced spalling or abrasion of kaolinite. These very fine particles have an increased ratio of reactive surface area to specific surface area, which results in enhancement of kaolinite dissolution rate. A balance betwe en production and dissolution of the fine particles explains both the rever sibility and the temperature and pH dependence of the stirring effect. Since the stirring effect on kaolinite dissolution rate varies with tempera ture and pH, measurement of kinetic parameters such as activation energy ma y be influenced by stirring. Therefore, standard use of nonagitated reactio n vessels for kinetic experiments of mineral dissolution and precipitation is recommended, at least for slow reactions that are surface controlled. Co pyright (C) 2001 Elsevier Science Ltd.