Soluble loss: what poor washing costs

Every washing circuit loses some dissolved value, because the solids that leave it carry a film of liquor and that liquor holds metal in solution. Soluble loss is the name for the value that walks out of the plant dissolved in the underflow liquor of the last washing stage, and it is a direct line on the revenue statement — not a rounding error but, in a poorly washed circuit, a number large enough to decide whether the operation makes money. This topic is about quantifying it and seeing why it matters so directly.

The metal lost in the washed solids

When the final underflow leaves a CCD train it is a thickened slurry: solids plus the liquor clinging to them. The solids themselves may be barren residue, but the liquor is not — it is process solution, and it carries dissolved metal at whatever tenor the circuit runs. That liquor, and the value dissolved in it, leaves with the residue to the tailings. The loss is not the solids; it is the soluble value in the liquor the solids carry. Incomplete washing — too few stages, too low a wash ratio, channelling in a thickener bed — leaves that carried liquor more loaded than it needs to be, and the difference is value the plant dissolved and then discarded.

How it is quantified

Soluble loss is quantified as a volume times a tenor: the volume of liquor leaving with the final underflow, multiplied by the concentration of dissolved value in it. The carried-liquor volume comes straight from the underflow percent solids — the same figure the thickener and CCD calculators compute — and the tenor comes from the soluble balance across the wash train, which is where stage count and wash ratio do their work. Improving the wash drives the tenor of that carried liquor down; concentrating the underflow drives its volume down; both cut the loss. The worked thread below takes the committed CCD underflow liquor and shows the volume that any soluble loss is charged against.

Why it is a direct revenue line

What makes soluble loss sting is that it is recovered value discarded, not value never won. The leach already paid to dissolve that metal; the loss throws it away at the very last step, after every reagent and every kilowatt-hour of the upstream circuit has been spent on it. So a percentage point of soluble loss is a percentage point off saleable production at the full upstream cost, which is why washing efficiency is watched as closely as recovery and why the wash-ratio-versus-dilution trade of the previous topic is an economic decision rather than a housekeeping one. The CCD and dilution calculators below give the carried-liquor and make-up figures the loss is computed from; the tenor and the balance come from testwork on the circuit.