International audienceThe process under study aims at removing dissolved fluorides from effluent waters through the addition of calcium and the precipitation of fluorite in a fluidized bed. In the concept developed at COMURHEX, the effluent is a highly concentrated KOH solution and the calcium is provided as a suspension of portlandite with calcite as an impurity. COMURHEX's target is to achieve an efficient defluorination of KOH solutions prior to recycling and to control the growth of the fluorite grains. Since building a fluorite crystallisation model requires a thermodynamic basis, we started exploring portlandite, calcite and fluorite solubility in concentrated KOH solutions. We are primarily concerned with equilibria in the system Ca(OH)2-CaCO3-CaF2-KOHH2O at temperatures 15 - 35°C in KOH solutions up to 6M (340g/kg). Suspensions and mixtures are prepared in a 3 L stirred reactor under N2 atmosphere and the temperature is varied. Liquid samples are taken from the reactor, filtered, diluted, and analysed by ICP-AES for cations and ionic chromatography for anions, while the solids are washed in ethanol and observed by SEM. In order to choose materials and improve techniques, this work was started with the Ca(OH)2-CaCO3-KOH-H2O subsystem, where solubility data and speciation models are already available from the literature. In this subsystem we experienced the analytical and kinetic difficulties already quoted by other authors. Albeit ICP-AES is very sensitive, Ca analysis is difficult in KOH solutions, because portlandite solubility is low and a matrix effect of potassium weakens the Ca signal, so that its calibration must be performed using KOH solutions rather than conventional (dilute) standards. This problem was overcome by using the standard addition method where the calibrating solutions are prepared at the same KOH concentration as the unknown samples and spiked with increasing amounts of calcium. This method enables the absolute calcium variations to be calibrated and the calcium present as impurities in the KOH parent solution (and thus in the standards) to be evaluated by regression
