A "best" mixer for this application may require some testing and evaluation.  The process you describe might need more intense mixing to draw the solids into the liquid initially and less intense mixing to bring the water to the surface where it can be driven off.
 
One option that might help solve both problems is to mount two axial-flow impellers on the same mixer shaft.  The impeller at the bottom of the shaft would pump downward to keep the solids in suspension and circulate them throughout the tank.  The impeller nearer the top of the tank, but still below the liquid surface, would pump upward.  The upward flow from the top impeller would both push solids off the surface and also bring water to the surface.  A good axial-flow impeller could be either a hydrofoil impeller or a marine propeller.  One impeller would have a left-hand pitch (probably the bottom one) and the other would have a right-hand pitch for them to pump in opposite directions.
 
Any time competing requirements or processing combinations are desired from the same mixer, compromises must be made to accomplish the most difficult task without losing the needed effect for the other process.

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