Your process description is missing a couple of details that may be important, but I have made some assumptions that are likely to be reasonable.  You have not described the purpose or intensity of mixing required, so I assumed a moderate level of mixing.  You have not specified a viscosity for your rubber solution, so I assumed 1,000 cp (mPa.s), rather than a lower viscosity.  I would recommend your 1,500 mm diameter by 1,800 mm height vessel for maximum flexibility.  You vessel needs baffles (four vertical plates about 125 mm wide and 1,800 mm high).  I did my calculations for three (3), four-blade, pitched-blade (45 deg angle) impellers.  At 60 rpm, the impellers need to be 575 mm in diameter with 115 mm wide blades.  The bottom impeller could be about 380 mm off the bottom and the other impellers 500 mm separation.  A 1.13 kW motor should be adequate, which results in a torque of 178 Nm.  Many calculations are needed for mixing intensity, power requirements, shaft design, and other factors.  For more information about calculations and design, refer to the Handbook of Industrial Mixing, John Wiley & Sons, 2004.