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Concentrate is assayed and paid on a dry basis, so wet tonnage must be converted first. Smelters then apply a unit deduction — commonly 1 unit for copper concentrate, 8-9 units for zinc — before paying, so the payable grade is always below the assayed grade.
Concentrate grade
Dry tonnes = wet tonnes x (1 - moisture); grade % = metal x 100 / dry tonnes
Payable metal
Payable metal = dry tonnes x (grade - unit deduction) / 100
It represents unrecoverable metal in slag and a commercial risk-sharing convention. Combined with treatment and refining charges, deductions determine the net smelter return the mine actually receives.
Usually — it reduces freight, treatment charges and deduction losses per unit of metal. But pushing grade too high normally costs recovery, and the optimum is a grade-recovery trade-off.