Thermal and structural analysis of the reaction pathways of α-spodumene with NH4HF2

Abstract

In this work, the thermal and microstructural analysis of the reaction pathways of α-spodumene with NH4HF2 at low temperatures is presented. The thermal behavior of NH4HF2, mixtures α-LiAlSi2O6/NH4HF2 and by-products (NH4)3SiF6·F/(NH4)2SiF6, are analyzed by simultaneous thermogravimetry and differential thermal analysis (TGDTA). The study of the crystallite size and lattice strain in the solids is carried out using X-ray diffraction (XRD). The obtained results indicate that the heating rate has a remarkable influence on the processes. At low heating rates, 1 K/min, the reaction between the mineral and NH4HF2 begins at about 351 K and is completed around 426 K, approximately. The obtained products are LiF, (NH4)3SiF6·F, (NH4)3AlF6, NH3 and H2O. In the 368–429 K temperature range, the unreacted NH4HF2 is removed and (NH4)3SiF6·F starts to decompose to generate (NH4)2SiF6. Finally, (NH4)2SiF6 and (NH4)3AlF6 sublimate and decompose at 433 and 493 K, respectively. The main final product is a LiF/AlF3 mixture at 533 K.