There are many methods of purification of graphite powders . Most of them involve acid or alkali treatments while several involve either expansion processes or physical treatments.
Article Purification process of natural graphite as anode for Li-ion...
Article PREPARATION OF EXFOLIATED GRAPHITE USING NATURAL GRAPHITE OF SRI LANKA
Conference Paper HF Acid Digestion for Purification of Vein Graphite
Article USE OF CRYSTAL MORPHOLOGIES TO UNRAVEL THE ORIGIN OF VEIN GR...
I think this depends upon the aim of the graphite.If you want to use it to produce nanotubes or graphene sheets, it needs a purification which uses strong acids. Protocols for this are numerous, but I think I could help you once I know what the purpose of your research is.