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Lucky,
A 10% increase in concentrate grade will generally only mean a drop in freight cost. As a general rule Concentrate grade is inversely proportional to recovery. If Mt Weld achieves 44% conc grade at design recovery then it's likely they will drop the conc grade to increase recovery. Recovery ( assuming conc grade is at acceptable levels ) is everything in most concentrators. It costs the same to dig it up and treat it, it's all about getting as much valuable metal as you can per tonne mined. Increased conc grade may also have undesirable effects in the LAMP leaching circuit.

There will be a grade/recovery curve that the finance and metallurgical staff will have on hand for various basket prices but usually recovery is king.