its a good thing WR isn't here to answer.
'Contaminated with sulphides'
You are confused with iron ore. You got the wrong sector there bub. generally when you have less porous rock and smaller fissures to travel through you get your higher grades and higher purities. the impuriites for graphite have nothing to do with sulphides. Ash is more of a concern than sulphides
The geological setting with granite leads to lower grades and a mixing of the elements especially compared to higher qulaity vein deposits in Sri Lanka.