ETTRINGITE FORMATION IN LIME-REMEDIATED MINE TAILINGS .1. THERMODYNAMIC MODELING

The optimal geochemical parameters required to form ettringite in a li me-remediated site have been investigated with the use of thermodynami c modelling. The speciation of suphate and 10 metals analyzed in sampl es of a mine tailing from northern Quebec was calculated with a comput er model. The model also includes calcium, which would result from the addition of lime to the system. Two speciation calculations were done . The first one is a hypothetical speciation before the onset of preci pitation and was used to devise the stability fields of mineral suscep tible of forming in high pHs. The stability fields were determined by building activity ratio diagrams with sulphate, aluminum and calcium s pecies concentrations for pHs ranging from 4 to 14. The investigation was carried out for a 25 degrees C solution with an ionic strength les s than 0.12, Thermodynamics predict that an optimal stability field fo r ettringite in the mine tailing samples containing 10(-3) mol/L of SO 4-2, 10(-2) mol/L of Al(OH)(4)(.) requires at least 10(-2) mol/L of Ca +2 and a pH larger than 11.6. The sulphate contents of the mine tailin g samples, however, limit the precipitation of ettringite and monosulp hate could be art end-precipitate. The second speciation is in equilib rium with precipitates and provides information on the speciation of t he residual metals. If ettringite could be precipitated as a stable fo rm, it could serve as a sorptive agent for residual metals, and, due t o its binding properties, consolidate tailings.