Limestone Crushing Solution

Question: Is there a process that will preferentially crush limestone and not granite. Currently all material is crushed down to -45 micron and floated. Is there a energy saving solution here?
They have primary Jaw crushing, secondary impact crushing and Ball Milling. -45 micron material is subject to flotation.
What about the option of elimination of secondary crushing and using the Ball mills as autogenous milling with the option of including a pebble crusher in closed circuit of autogenous milling. Pebble crusher can be HPGR or water flush cone crushers.
Pebble crusher feed material subjected to optical sorting.

Reply: The ball mill work index of granite varies between 13 and 17 kWh/t while limestone can vary between 10 and 13 kWh/t. This is not enough difference to be able to make a differential grind in a ball mill.
Eliminating secondary crushing and using the ball mill as an autogenous mill has problems. Since the Bond Work Index for the two materials is close, what the ball mill will produce is a lot of round pebbles out the discharge. The pebbles will be both granite and limestone.
One parameter of SAG mill design is the A x b number, which is indicative of how the material will behave in a large mill. Both the granite and limestone should be tested to see if there is enough of a difference to see if they break down differently. Should that be the case then screening the SAG mill discharge could separate the limestone and the granite to some degree. I do not think it will be a very good separation but until the test is performed there is no way of knowing.
The use of an HPGR should be tested. There are known instances where rock will break along grain boundaries giving a distinct color difference in the products that can be used for optical sorting.
Optical sorting is a good idea on the secondary crusher product depending on how many tons need to be sorted and of course depending on the color of the rock and the limestone.