My analysis and understanding of the question asked:
 
    7.0 m diameter tank
    8.5 m straight side
    300 cubic meter capacity
    Milk of Lime suspension storage (not powder addition and dispersion)
 
    Two impellers, each 84" (2.13 m) diameter
    calculations done for two common turbine types: pitched blade and straight blade (pictured below)

Pitched Blade

Straight Blade
 
    Recommended speed for two pitched-blade turbines - 17 rpm (0.28 rps)
    anticipated impeller power 3.12 kW (4.11 hp), suggested motor size 4.8 kW
 
    Recommended speed for two straight-blade turbines - 16 rpm (0.27 rps)
    anticipated impeller power 5.66 kW (7.45 hp), suggested motor size 9.0 kW
 
    Other process conditions, impeller types, or additional information may result is different recommendations.