Are you saying you are in open circuit now?  Are you now in a single stage ball mill circuit? Do you have a SAG mill in front? What is your flowsheet? What is the plant tonnage? What are the dimensions of the ball mill? Motor Horsepower? Current power draw. Any ore hardness data? 

Of your 2300 HP (1700 kW) motor, how much are you drawing now? Is this already installed or are you in design?

If you want help you need to learn to read and answer the questions I ask you!

Are you trying to design this with the ball mill in open circuit?

Simplest answer: Increased specific power consumption is how you reduce the product size.  Smaller particles consume more energy.  Open circuit ball mills are not desirable for a number of reasons, and the best grinding efficiency is always achieved with high circulating loads (plan for 400%).

You have the ball mill work index, use that to calculate how much energy you need to perform the grind at each particle size.  Rowland's EF2 factor says that if you are controlling to the 80% passing size (P80), then open circuit grinding requires 1.2 times more energy than closed circuit.  This is a round number, you can do McIvor's Functional Performance calculation and get a better prediction of the performance cost of not using a classifier.

There are some aspects you should consider in your design. Your ore is very soft, it means that the non-sulphide gangue minerals would be reported in the copper rougher concentrate. For that reason, it is important to perform kinetics tests to evaluate the effect of particle size on copper recovery. The mineralogy provides information, but it is important to keep in mind the results from the flotation tests.

Other thing to analyze is the blasting parameters. The particle size distribution of the ROM material has an impact on the SAG Mill and Ball Mill performance. You have to make sure the feed size is appropriate.