Experimental study of dissolution rates of actinolite in CaCl2–HCl–H2O up to 400 °C

Abstract

Actinolite dissolution rates in aqueous HCl and aqueous CaCl2–HCl have been measured at temperatures ranging from 25 to 400°C at 23MPa, under conditions far from equilibrium, using a flow-through reactor. The relative release rates for the various elements in actinolite due to breaking of metal–oxygen bonds are all different. All of the release rates vary with temperature and pH. The release rate of Si increases as temperature rises from 20 to 300°C, but then decreases at higher temperatures. Other metallic ions, such as Ca, Mg, Fe, and Al, are all released more rapidly than Si at temperatures between 25 and 300°C, but more slowly at temperatures ⩾300°C. The ratios of mCa/mSi and mMg/mSi in the effluent at 200°C are close to the stoichiometric proportions of these elements in actinolite, but differ from stoichiometric values at temperatures both above and below 200°C. Actinolite dissolution in aqueous HCl at 100°C yields element ratios close to stoichiometric values in the mineral, but again the ratios vary at higher and lower temperatures.Our experiments demonstrate that, under conditions far from equilibrium, dissolution rates depend on both pH and the activity of aqueous MiZi+, where Zi is the valence of the metal Mi in the mineral and MiZi+ refers to Ca2+, Mg2+, or Al3+. The ion-exchange model suggests that metal–hydrogen ion exchange reaction on the surface is a key step for mineral dissolution, and that a Si-rich, Mi-deficient precursor complex is formed on the mineral surface at temperatures <300°C. Using our experimental data, we express the release rate of Si (r) as follows:-r+=Aexp(-EA/RT)((aH+)Zi/aMiZi+)αwhere α is the order with respect to the ratio of activity of hydrogen ions and dissolved metal ions Mi. The Mg–H exchange coefficient, α, is 0.55 at 100°C. The apparent activation energy EA=14.4kJ/mol, and A=0.23×10−3mcm−2s−1.Our results show that water properties vary strongly within the critical region, which affects the dissolution mechanism. Thus, the release rates for various elements at temperatures above 300°C are different from those at temperatures below 300°C. High resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) studies indicate that a Fe(Al)-rich, Si-deficient layer is formed on the surface at temperatures above 300°C. X-ray photoelectron spectroscopy (XPS) analyses show that the surface layer that formed by reaction with aqueous HCl at 400°C is a hydrated silicate, composed of Si–OH and Mi–OH, as well as Si–O and Mi–O bonds.