Study on mineral surface reacted with water at temperatures above 300°C and 23 MPa

Abstract

In this work, we carried out experiments on silicate mineral dissolution using a flow-through reactor from 20 to 400°C at 23 MPa. The dissolution of silicate minerals such as actinolite, diopside, and albite in water may require the breaking of more than one metal–oxygen bond type. Different metal elements in silicate minerals have different release rates and the dissolution product is often non-stoichiometric. Na, Mg, Fe, and Ca dissolve faster than Si at T < 300°C. At T ≥ 300°C, the release rate of Si is higher than that of the other metals. The molar concentration ratios of the dissolving metal Mi versus Si such as Mg/Si, Ca/Si and Al/Si, which are the release ratios in the effluent solutions, are often different from the molar ratios of these elements in the minerals. The results show that the incongruent dissolution of minerals is related to surface chemical modifications. Non-stoichiometric dissolution is caused by the formation of a non-stoichiometric leaching layer at the surface or by the presence of a secondary mineral at the surface. Our experiments indicate that the dissolution of most silicate minerals is close to stoichiometric at 200 and 300°C, e.g., for actinolite and albite at 300°C. The surfaces after reaction with aqueous solutions were investigated using SEM and TEM. At T < 300°C, the mineral surfaces (e.g., for actinolite) after the reaction with water are slightly Si-rich and slightly Fe (and/or Mg, Ca) deficient. In contrast, at T ≥ 300°C, the surfaces after reaction with water are slightly Fe-rich and somewhat Si deficient.