I believe you are talking about Iron(III) chloride which is typically used for flocculation or for organic synthesis.
There are many different types of grinders. It really depends on the volume and size at which you want to grind the crystals. To grind a small volume (a few grams), just use a mortar and pestle, for quantities over 20 tons per day, you may want to use a ball mill. There are many variations of grinders in between these volumes.
Caution: Depending on your iron chloride synthesis method, ensure that all volatiles are removed prior to grinding, as fires are started this way.
Take care, and I wish you the best.