I'd like to re-iterate: the last thing you want to be doing is grinding dry PPD if you can avoid it. Use a Paterson-type stirrer (the type with a flat bottom to permit grinding) and break up the lumps under water if necessary. I found that I didn't need to do that with the PPD I received from the Formulary.
With any dry chemical it should be remembered that the particles that are going to go into your lungs are invisible. You can't see respirable dust. Respirable dust settles very very slowly. Grinding lumps when wet, as opposed to when immersed, does not prevent the creation of airborne respirable particles.