The Catalyzed Dissolution of Silicate Minerals By Hydrofluoric Acid.
Kline, William Edward
1980
Abstract
The dissolution of silicate minerals in hydrofluoric acid has been studied using a slurry reactor. The kinetics of dissolution have been described with reference to the chemical species in the bulk solution, as well as the composition and structure of the solid phase. Compilation of the experimental data has produced the first extensive tabulation of silicate dissolution rates in hydrofluoric acid. In addition, a series of kinetic models has been developed that provide insight into the fundamental processes of dissolution. The rate laws describing the dissolution in aqueous hydrofluoric acid indicate that the rates of reaction are proportional to the surface concentration of the HF molecules. The attack by the F('-) and HF(,2)('-) ions in solution, as well as by fluorine chemically adsorbed at the surface, is negligible. The dissolution of the layered silicates was investigated by slurry reactor experiments and by the examination of partially dissolved particles using a scanning electron microscope. The rates of particle dissolution per unit mass were modeled as a function of the fraction X dissolved by an equation of the form (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) For the three layer silicates the exponent parameter n takes on a value of 1/2, reflecting radial attack at the unit cell edges. In contrast, the two layer kaolinite structure dissolves at both the layer edges and the planar faces. It was concluded that attack is preferential at the surfaces where the octahedral sheets are exposed. The most important result of this work has been to show that silicate dissolution by hydrofluoric acid is a catalyzed system. The rates of dissolution, for instance, can be accelerated by as much as 900% by the presence of strong acids such as HCl. This catalysis by strong acids is attributed to the adsorption of H('+) ions at the silicate surface. The overall dissolution was modeled as superimposed catalyzed and uncatalyzed reactions, giving rate laws of the form (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) where -r(,o) is the rate of the uncatalyzed reaction, a(,H) is the activity of the hydrogen ion, and K(,A) is the equilibrium adsorption constant. The catalysis parameters k is a strong function of the nature of the silicate surface, a fact which has potential application in the area of selective dissolution from mixed mineral systems. Further investigation has resulted in the major discovery that these reactions can also be catalyzed by salts such as NH(,4)Cl, NaCl and LiCl. This catalysis is likewise attributed to the adsorption of cations at the silicate surface, and it is competitive with the catalysis by the H('+) ions. A parameter called the point of zero salt catalysis has been identified, which is the hydrogen ion activity at which the net catalysis by any concentration of salts goes to zero. The dissolution in mixtures of HF, strong acids and salts is described by a rate law of the form (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) where k(,o), k(,1) and k(,2) are the rate constants for the uncatalyzed reaction and the reactions catalyzed by acids and salts, respectively, a(,H) and a(,M) are the activities of the H('+) and salt cations, and K(,1) and K(,3) are the respective adsorption constants.Types
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