Starting to process clay

We have a blunger gifted to us from the late, great Norwich Potter, Robin Dauncey. A blunger is a large machine a little like a food processor. I have started to process the clay with this but I am finding the clay contains large rock deposits which were missed. This is playing havoc with the blades of the blunger. Not only that the clay is sticky and binding onto the blades rather than mix up the material for sieving .