I am not sure from the description of the process why you want to use an anchor impeller, but it can be done.  An anchor impeller does not do a good job of suspending solids, so the solids you have must settle very slowly and remain suspended for perhaps as long as 10 minutes.
Making some estimates for the slurry properties, especially the viscosity, an impeller diameter of 900 mm should work.  A typical blade width would be 90 mm with two vertical anchor blades.  At 60 rpm a 1.5 kW motor and a 25 mm diameter shaft should be adequate.