The Study on the Purification of Waste Graphite Thermal Field Material of FZ-Si Crystal

The rapid development of the photovoltaic industry has led to increased demand for monocrystalline silicon. Numerous waste graphite devices are generated in the process of crystal pulling, and the waste graphite thermal field contains the largest proportion of graphite. Reports on graphite purification from the waste graphite thermal for photovoltaic crystal pulling are relatively rare. In this study, we purified waste graphite using a combined process of ferric induction roasting and acid-leaching. Through thermodynamic analysis, it is found that the appropriate material ratio is conducive to generating the appropriate target product, namely Fe₃Si. Using ball milling mixture as raw material, the calcined product was obtained at 1300℃, 30% iron ratio and 90 min roasting time. XRD, SEM–EDS and XPS analysis methods showed that the calcined product phase was graphitic carbon and Fe₃Si, which was consistent with thermodynamic calculation, and then HCl and HF leaching were carried out. High-purity graphite carbon (HPG) with fixed carbon content greater than 99.9% was obtained. The removal rates of Si, Fe, Al, Mg, Zn, Ni, Ti Ca, and Na were 99.58, 99.56, 99.89, 98.32, 99.26, 99.47, 99.62, 98.48, and 98.98%, respectively. Meanwhile, the HPG products with a purity of 99.98% were obtained.