You left out what could have been an important piece of information, and that is the current impeller diameter.  However, I was able to do a sufficient comparison to give you a simple solution to the problem.  You can add baffles and replace the two 8-blade Rushton turbines with three 4-blade pitched-blade turbines of the same size as the current Rushton turbines.  These three impellers will require slightly less power than the current two impellers, even after adding baffles.
 
You are entirely correct that the strong vortex, the lack of baffles, and the radial-flow disk-style impellers could not do a much worse job of solids suspension and solids dissolution.  Unless your solids dissolve extremely slowly because of equilibrium, I would expect that you could cut in half the time required to achieve solution.

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