Immobilization of fluorine in aqueous solution by calcium aluminum ferrite and the mixture of calcium aluminate and gypsum

Abstract

The immobilization of fluorine by calcium aluminum ferrites has been studied using 20 mass ppm F-containing aqueous solution. It is found that the rate of the F immobilization by Ca2(Al,Fe)2O5 is initially slower than that by 3CaO·Al2O3 at the solid/liquid ratio of 1/200, but a similiar immobilization degree is obtained at later stage. The hydration product contributed to the F immobilization is the Ca3(Al,Fe)2(OH,F)12 phase, which is confirmed by the combination of XRD and electron microprobe analysis. For a ferrite phase of Ca2(Al0.68Fe0.32)2O5, the hydration product has a chemical composition of Ca3Al1.54Fe0.46[(OH)1-xFx ]12 (x=0-0.13). The F immobilization by 2CaO·Fe2O3 does not occur to a significant extent, but the F immobilization by 12CaO·7Al2O3 is accelerated with the addition of 2CaO·Fe2O3 due to the formation the Ca3(Al,Fe)2(OH,F)12 phase. It is also observed that F can be immobilized by the formation of ettringite (3CaO·Al2 O3·3CaSO4·32H2O, 3CaO·Fe2O3·3CaSO4·32H2O) through the substitution of F ion for the SO42 site.