Thermal transformation of pyrophyllite and alkali dissolution behavior of silicon

Thermochemical behavior of pyrophyllite was characterized by using XRD, FTIR, TG-DTA and MAS NMR techniques. Pyrophyllite experiences a series of solid-state phase transformations during thermal treatment. Dehydroxylation of pyrophyllite is completed at 880 °C, and the original layered framework is inherited in dehydroxylated pyrophyllite. Up to 1100 °C, the layered framework is collapsed thoroughly, with formation of amorphous SiO2 and mullite. Above 1200 °C, cristobalite crystallizes from amorphous SiO2, and mullitization is promoted dramatically. The alkali leaching results show that amorphous SiO2 in the thermally activated pyrophyllite is soluble in caustic soda liquor, while mullite is not, and the formation of sodium aluminosilicate hydrate of 0.95Na2O·Al2O3·3.25SiO2·4.79H2O during alkali leaching diminishes the dissolution ratio of silica. The alkali dissolution behavior of silicon demonstrates an innovative process for removing silica from pyrophyllite by using thermochemical activation (TCA) followed by alkali leaching, which can be applied to desilication of bauxite ore and preparation of alumina-base porous material. A desilication of 84% was obtained from the tested ore containing 90% pyrophyllite at the optimal thermochemical activation conditions of temperature 1100–1150 °C and time 90–20 min. The good alkali dissolution characteristic of silica also is a cogent evidence on the formation of amorphous silica during the thermal treatment of pyrophyllite.