Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite

摘要

The wollastonite far from equilibrium dissolution kinetics and the formation and evolution of surface leached layers have been measured in mixed-flow reactors at 25 ℃, 0.9 6 pH 6 12 and reaction times to more than 3000 h. Wollastonite dissolution rate decreases with increasing pH consistent with pH < 4.8: ~r+;Ca,pH≤5/(mol/cm2/s) = 10~(-10.00)a_(H~+)~(0.46) and ~r+,Si,pH≤5/(mol/cm~2/s) =10~(-10.88)a_(H~+)~(0.29) pH ≥ 4.8: /~r+,Ca-Si,pH≥5/(mol/cm~2/s) = 10~(-11.41)a_(H~+)~(0.18) Dissolution stoichiometry, and the texture, structure and chemistry of wollastonite surfaces strongly depend on solution pH and duration of reaction. At 4.8 ≤ pH ≤ 12, after an initial period of non-stoichiometric dissolution, stoichiometric release of Ca and Si was observed. No significant change in the specific surface area of reacted powder was observed using BET and Small Angle X-ray Scattering (SAXS) measurements. In accord with these observations no chemical or structural changes were observed on the surfaces of reacted powders by X-ray Photoelectron Spectroscopy (XPS), Diffuse Reflectance Infrared Fourier-Transformed (DRIFT) Spectroscopy, SAXS or Transmission Electron Microscopy (TEM). In contrast, at pH < 4, preferential calcium release to solution is observed, which produces a thick, highly porous Ca-depleted altered layer affected by an extensive crazing. Both silica and calcium releases decline with time but, whereas Ca release continuously decreases, Si release reaches steady-state after 1000 h and continues for more than 2500 h of reaction. After 3000 h of leaching at pH 1.0-2.2, a completely Ca-free solid is produced. Acid leaching induces drastic molecular and structural rearrangements of wollastonite surfaces that progressively affect the entire grain. After 100 h of reaction, most characteristic Si-O and Ca-O bands of crystalline wollastonite disappear and the DRIFT spectra of the solid strongly resembles that of amorphous silica. XPS analysis showed very strong decrease of the Ca/ Si surface concentration ratio after 10-100 h of reaction at pH 2 and the removal of almost all Ca from the surface after 120 h. XPS also evidences a decrease of the O_(1s) peak width reflecting the disappearance of non-bridging oxygens. Condensation reactions and reconstruction of the leached layer is further demonstrated by ~(29)Si MAS NMR spectroscopy which shows, as dissolution proceeds, the progressive formation of less reactive Q~3 and Q~4 structural units at the expense of initial highly reactive Q~2. As a result, wollastonite dissolution as a function of time can be correlated to the relative proportion of the different Q~i units.