Here is my "mulling" process (FWIW): I bought 200 lbs of silica sand from a local supplier, added the appropriate amount of bentonite and mixed the dry ingredients using a concrete mixer. I poured that concoction, about 20-30 lbs at a time, into a 5 gallon bucket and started adding a bit of water. I then used a paint paddle blade attached to a 1/2" Milwaukee drill to churn up each small batch. The small batches were then dumped into a 55 gallon plastic barrel and mixed once more. After each casting run, I throw the hard baked sand into the cement mixer with a short piece of heavy shafting and let that beat the clods back into fines. The dust is poured back into the big barrel with a bit more water as necessary and recombined using the drill and paddle mixer. When I'm not filling a mold, I keep an industrial weight trash bag over the top with a bungee cord to keep the moisture inside.

Again, not a traditional muller method but seems to work well enough and the sand stays pretty consistent. I have cast about 30-40 molds on this last batch of greensand and had no issues.