Purification mechanism of microcrystalline graphite and dissolution of non-carbon impurity during alkali autoclave-acid leaching

Low impurity content is crucial for graphite applications and microcrystalline graphite is an important candidate material. In this study, natural microcrystalline graphite, with a fixed carbon content of 76.65%, was purified by an alkaline autoclave-acid leaching method. The effects of the mole ratio of NaOH to Si and Al in graphite, the liquid–solid ratio of NaOH solution and graphite, alkali autoclave temperature and reaction time on the purity of microcrystalline graphite were studied. Results showed that the dissolution and phase transformation of non-carbon impurities were closely related to the purification process. During the alkali autoclave stage, complete dissolution of quartz was observed. The Si–O tetrahedra and Al–O octahedra structures in aluminosilicate minerals were damaged and [Al (OH)₄]⁻, [H₂SiO₄]²⁻ and [SiO₂ (OH)₃]⁻ were released. The soluble silicate and aluminate ions underwent recrystallization, producing cancrinite and sodalite that could be dissolved by acid leaching, resulting in purified microcrystalline graphite. The purity of microcrystalline graphite was further improved due to the autoclave treatment allowed NaOH solution to penetrate into the cracks of microcrystalline graphite aggregates under high pressure. In addition, the acid solution could enter the micropores left by alkali etching to dissolve the residual impurities. The fixed carbon content of microcrystalline graphite could be increased to 99.9% through the alkaline autoclave-acid leaching method.

Graphical abstract