I suggest the use of alumina balls to prevent eventual contaminations. And never steel Concerning the time that you should I apply ball milling, it depends the the start grain size, hardness, etc.…. I suggest to use zirconia or WC balls.
Grinding media, the objects used to refine material and reduce particle size, are including Attritors internally agitated ball mills and DMQX horizontal media mills, . Zirconium Oxide Balls 95% ZrO2 are the strongest, best wearing ceramic . 3.5mm, 3.6–4.1mm, 3.8–4.1mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm,