I've made black powder with charcoal. To turn it into dust, I put it inside a suede leather bag, cinched the bag shut tight, and pounded it with a brick against a concrete curb. The suede kept the charcoal contained, and was tough enough not to be punctured in the process. When I was done, I had about 4 cups of charcoal powder that was as fine as sand or better. It took me maybe 10 minutes all told. I wasn't much concerned with production rate, so probably could've gone quicker.