Kinetics of the Dissolution of Silica in Aqueous Sodium Hydroxide Solutions at High Pressure and Temperature

The kinetic of the reaction of sand with aqueous NaOH corresponding to the ratio SiO2/Na2O = 2 was studied in a pressure vessel at 220°C and 2.7 MPa. Since the kinetic curves could not be obtained directly from the experimental data, a new method is proposed to plot the entire kinetic graph from experimental data. An analytical expression of the type α = A [1 -exp(-Bt)] describes the system perfectly. The constants A and 5 were calculated for a silica sample having a narrow granulometric distribution (range i.e. 300-315 μn). The value of A is found to be almost constant, between 0.95 to 0.99 and B ranges from 0.03 to 0.14 when [OH-] increases from 0.5 to 12.5 mol/L. The kinetic order with respect to OH- is equal to 0.470 ± 0.013 and the kinetic constant at 220°C is 3.933 × 10-6 g/m2 .s.