Dissolution and precipitation kinetics of gibbsite at 80°C and pH 3: The dependence on solution saturation state

Abstract

Dissolution and precipitation rates of gibbsite were measured in dilute aqueous solutions at pH 3 and 80°C as a function of solution saturation state using stirred-flow reactors. Saturation state ( Q K eq = exp( ΔG RT ) ; where Q is the activity quotient, k eq , is the equilibrium activity quotient (or solubility), ΔG r is the deviation of the Gibbs free energy of the reaction (kcal mol −1) from the equilibrium value, R is the gas constant, and T is temperature in Kelvin) was determined with respect to the overall reaction Al(OH) 3(cr) + 3H + ⇔ A1 3+ + 3H 2O. The equilibrium solubility at 80°C for this reaction was tightly constrained using the solution saturation states from the experiments with the five slowest dissolution and precipitation rates. The calculated equilibrium solubility ( K eq = a Al 3+ A H+ 3 ) is 10 5.00±0.08, in excellent agreement with published values despite differences in thermodynamic models and the definition and measurement of pH. The variation in dissolution rate (mol m −2 sec −1) with ΔG r over the range −1.14 ≤ ΔG r ≤ 0 kcal mol −1 is given by Rate diss = −(4.72 ± 0.28) × 10 −10[1 − exp {(−8.12 ± 1.02) g (3.01±0.05)}] where g  ¦ΔG r¦ RT . The precipitation rate (mol m −2 sec −1) varies with ΔG r over the range 0 ≤ ΔG r ≤ +0.467 kcal mol −1 according to Rate ppt = −(2.07 ± 0.63) × 10 −10[1 − exp { g (1.20±0.31)}] or to Rate ppt = (1.94 ± 1.55) × 10 −10 g (1.10±0.11) The variation of the dissolution rate with ΔG r can be separated into three regions. Near equilibrium (0 ≥ ΔG r > −0.200 kcal mol −1), the rates increase gradually with increasing undersaturation according to an approximately linear function of ΔG r . Over the range −0.200 > ΔG r < −0.500 kcal mol −1, the rates increase sharply as ΔG r becomes more negative. Far from equilibrium, at ΔG r < −0.500 kcal mo −1, the dissolution rates are constant at their maximum value. On the other hand, precipitation rates are nearly linear with ΔG r for 0 ≤ ΔG r ≤ +0.467 kcal mol −1 (two times saturation). The complex functional dependence of dissolution rate on ΔG r indicates that the measured precipitation rates cannot be obtained from the far-from-equilibrium dissolution rate using transition state theory and the principle of detailed balancing. However, near equilibrium (−0.200 < ΔG r < +0.200 kcal mol −1), the approximate linear dependence of both dissolution and precipitation rates on ΔG r supports the application of transition state theory to the overall reaction and indicates that, only over this range of ΔG r , the same set of elementary reactions may control the overall rate. The sharp increase in dissolution rate from 0 ̄ .200 > ΔG r > −0.500 kcal mol −1 is suggestive of a surface phase change corresponding to a change in dissolution mechanism. A plausible dissolution mechanism over this range of ΔG r involves the opening of dislocation cores to form etch pits and is supported by theoretical calculations and SEM observations.