Whitening and purification of calcined talc powders from black talc by the milling and reactive calcination with magnesium precursors

As the main raw material for ceramics and coatings, black talc contains high contents of organic carbon and quartz, which deteriorates the whiteness and purity of calcined talc. In this work, calcined talc powders with high whiteness and purity have been prepared by the milling and reactive calcination of black talc with magnesium precursors. Black talc and calcined powders were analyzed by the crystal and chemical compositions, particle size distribution, thermal decomposition, and microstructural observation. The Gibbs free energy of solid reactions was calculated to evaluate the product phases in the reactive calcination. The milling of black talc released organic carbon and colored minerals, which were mostly removed in the magnetic separation and calcination, thus greatly increasing the whiteness of calcined powders. The protoenstatite phase in the calcined powders is continuously enhanced with increasing calcination temperature, whereas the total amount of quartz and cristobalite are the opposite. In the calcination process, magnesium precursors reacted with the quartz phase to produce the protoenstatite and forsterite phases, resulting in the reduction of the quartz phase and purification of the calcined powders. With the addition of 23% nano-MgO, the powder mixture calcined at 1250 °C has a protoenstatite and forsterite content of 84.5% and 14.4%, respectively, and the quartz and cristobalite amount is as low as 1.1%, with a powder whiteness of 93%. Compared to the present physical purifications, the combination of milling and reactive calcination greatly increases the grade of calcined talc powders and extends the commercial applications of black talc.