Sand blaster issues

Ok I have a pressure type sand blaster. I have found that you need three things to keep it working.
1 dry sand, 2 dry air, and sifted sand.
I bought a few bags of Black Diamond 20-40 grit. When I went to screen/sift it I found that about half the sand would not go through the screen. I don’t want to throw this away, so I thought that I could grind it some how. Has anyone run into this situation and if so any thoughts on how to grind it down to a usable grit?

Are you in a country area? I have struck this before when using blasting sand that stuck in lumps. I used a machine called a Corn husker which i bought a farm clearance sale for $50. I could throw the lumps into it's hopper and it crush the it and send out loose material. I sold it to a guy who used it to make bread crumbs.