Kinetics of hydrothermal reactions of minerals in near-critical and supercritical water

This work presents new experimental results on the kinetics of mineral dissolution in near-critical and supercritical water in a temperature range (T) from 25 to 400 degrees C and a constant pressure of 23 MPa. Kinetic experiments were carried out by using a flow reactor (packed bed reactor) of an open system. The dissolution rates of albite and magnetite were measured under these experimental conditions. Na, Al and Si release rates for albite dissolution in water were measured as a function of the temperature and flow velocity in the reaction system. The maximum release rates of Na, Al and Si of albite dissolution in the hydrothermal flow systems under different flow velocities were always obtained at 300 degrees C, that is to say, the maximum albite dissolution rates in the flow systems, regardless of different flow rates, were repeatedly measured at 300 degrees C. Results indicate a wide fluctuation in albite dissolution rates occurring close to the critical point of water. The dissolution rates increased when the temperatures increased from 25 to 300 degrees C and decreased when the temperatures increase from 300 to 400 degrees C. At some flow velocities, the dissolution rates rose as the temperature surpassed 374 degrees C. Albite dissolution was incongruent in water at most temperatures. It was only at 300 degrees C that albite dissolution was congruent. The albite dissolution from 25 to 300 degrees C (at 23 MPa) will change from incongruent to congruent, whereas from subcritical 300 to 400 degrees C (at 23 MPa), the dissolution will change from congruent to incongruent. The release ratio of Al/Si (or Na/Si) is positive at T<300 degrees C, and it is negative at T>300 degrees C. The dissolution rates of magnetite in water increased with increasing T until T at the critical point of water or around it. The authors believe that this is caused by the wide fluctuations in water properties under the conditions from the near-critical to supercritical state.