The Initial Extraction Kinetics of Cr(VI) Ion from Chromite Ore under Alkaline Hydrothermal Conditions

Abstract

The initial extraction kinetics of Cr (VI) ion from chromite ore under the hydrothermal conditions of the temperature range: 250 to 420°C, concentration of NaOH solution: 5 mol⋅dm^-3and atmosphere: 20 MPA of O₂ gas, has been investigated. The grain size and shapes of chrome ore were almost kept constant during extraction period, and the initial kinetic curves were examined in consideration of the core-model. The rate of extraction is in good agreement with the following zero-order equation:
where x is the extraction ratio of Cr(VI) ion from chrome ore, k the constant, t the reaction time. Arrhenius plots showed two slopes in the range from 360 to 420°C. The apparent activation energies were found to be about 37.2 kcal⋅mol^-1 in the range of 360∼380°C and 52.8 kcalmo1^-1 in the range of 400∼420°C, respectively. These results showed the existen ce of two different extraction processes. The apparent activation energy of extraction rate of Cr(VI)I ion from the synthesized-MgCr₂O₄ under the same conditions as the case of the chrom e ore was found to be about 14.3 kcal⋅mol^-1. It was suggested that the two extraction processes from the chrome ore were respectively expressed by the following equations:
and that iron and aluminium affected the extraction of chromium as an inhibitors.