Effects of purification on the properties and microstructures of natural flake and artificial graphite powders

As the main components of the matrix graphite of pebble fuel elements for HTR-PM, the natural flake graphite powder (NFGP) and artificial graphite powder (AGP) must present suitable microstructural features such as specific surface area (SSA), particle size distribution (PSD), and ash contents. As a regular method, the graphite powders are purified at high temperature of ~2500 °C with the purge of halogens to obtain their high purity prior to the powder treatments such as pulverizing, shaping, and particle classification, which may introduce impurities into the graphite powders during the powder treatment process. Therefore, the purification of NFGP and AGP should be carried out after the powder treatment to prevent the graphite powders from being contaminated and reduce the cost. In this manuscript, the microstructural features of NFGP and AGP before and after purification were comparatively analyzed to figure out the effects of purification process on the microstructures of NFGP and AGP. Results showed that purification had great influences on the ash contents and the contents of impurity elements, SSA, and degree of graphitization of graphite powders. However, the purification had little influence on the PSD of graphite powders. The significant decrease of SSA of graphite powders through the purification was due to the reduction of BJH adsorption cumulative volume and porosity. Comparing with those of the NFGP, the properties and microstructure of the AGP changed more through the purification process, which was probably due to the original un-graphitized coke of AGP before purification.