Thermal and gas purification of natural graphite for nuclear applications

Natural graphite such as flake graphite and microcrystalline graphite have important applications in nuclear engineering, especially in high-temperature gas-cooled reactors. Owing to requirements for low ash content and total equivalent boron content, thermal and gas purification is necessary for producing natural graphite powder with nuclear-grade purity. In this study, representative Chinese flake graphite and microcrystalline graphite were purified thermally or with halogens at graphitization temperatures. By thermal purification at 3000 °C, natural flake graphite and microcrystalline graphite containing at least 99.9% carbon was achieved. The metallic impurities remaining after thermal purification were mainly elements that formed carbides with extraordinarily high melting/boiling points, such as B, Ti, Ta, V, W, and Mo. Gas purification with halogens further increased the purity of natural flake graphite and microcrystalline graphite to nuclear grade, that is, the ash content and total equivalent boron content were less than 50 ppm and 0.01 ppm, respectively. Meanwhile, an investigation of the purification mechanism helped to understand the chemistry and thermodynamics of thermal purification of natural graphite.