1. A-segregates are predicted to form when the Rayleigh number exceeds a certain critical value.

2. The pump aggregate includes an isolation zone which segregates all regions which include drive medium from regions where the pump medium is present.

3. A batch tower (20) is positioned adjacent the elevator (16), and the batch tower (20) comprises an aggregate sizing screen (30) which receives and segregates the aggregate by size, and several underlying aggregate storage bins (32-35), with each aggregate bin receiving aggregate of predetermined average size from the sizing screen.