Why do some polymetallic plants still prefer staged grinding rather than a single closed-circuit?

In complex Pb-Zn-Cu ore bodies—especially those with variable hardness—single-stage grinding often leads to either overgrinding of soft minerals or undergrinding of harder sulfides.

A case example is from a Central Asian Pb-Zn concentrator (real industry pattern, non-specific) where the ore transitioned from quartz-rich zones to soft carbonates within the same bench.
Shifting to two-stage grinding (rod mill → ball mill) produced:

• More stable flotation feed size
• Lower steel consumption in the ball mill
• ~3% higher Zn recovery (due to reduced fines loss)

Xinhai Mining’s design team has implemented similar staged-grinding flowsheets in several polymetallic plants, especially when ore competency varies significantly across benches.