gravity circuit, page-3

I think gravity circuits are used to make the CIP end of the plant (the back end of the plant ) more efficient as less reagents (carbon ,cyanide) are needed for processing the Au and Ag as much of the Au and Ag is taken out by the gravity circuit.
There is probably a number of calcs to determine how much more efficent the system is but is probably based on bond index, grind size of ore and chemical nature of the host rock.