Hydrometallurgy

CCD Wash Ratio Explained

What wash ratio means in a counter-current decantation (CCD) circuit at a practical, preliminary level — wash water versus underflow liquor, why underflow solids concentration matters, and why more wash water does not by itself make the design correct. Explains the difference between a wash-water estimate and a soluble-recovery model.

TypeEngineering guide — concept explainer

Definition

Counter-current decantation (CCD) is a train of thickeners used to wash soluble values out of a leached solids stream. The solids move forward from thickener to thickener as underflow, while wash water is added at the last stage and flows back counter-current as overflow, displacing dissolved metal from the liquor that travels with the solids. Wash ratio, in the simple form used by the ProcessConvert calculator, is the wash-water volumetric flow divided by the underflow liquor volumetric flow: wash ratio = Q_wash / Q_underflow liquor. It is a flow ratio that scopes how much wash water is being applied relative to the liquor carried with the underflow — not a measure of how much soluble value is actually recovered.

Why it matters

Wash water is one of the biggest operating-cost and water-balance levers on a CCD circuit: too little and soluble metal is lost with the tailings; too much and the product solution is diluted and the downstream circuit and water balance suffer. The wash ratio is the first-pass way to size that wash-water addition. But it has to be read against the underflow solids concentration, because that sets how much liquor — and therefore how much dissolved value — travels forward with the solids. A thicker underflow carries less liquor per tonne of solids, so the same wash ratio displaces a different amount of soluble. Treating wash ratio as if it were recovery is the classic mistake: actual soluble recovery depends on the number of stages, the mixing and stage efficiency, entrainment and bypass, and the thickener performance — none of which a wash ratio captures.

Formula

Underflow slurry mass

ṁ_slurry = ṁ_solids / (wt% / 100)

Underflow liquor volume

Q_liquor = (ṁ_slurry − ṁ_solids) × 1000 / ρ_liquid

Wash ratio (definition used here)

wash ratio = Q_wash / Q_liquor

Required wash water

Q_wash = wash ratio × Q_liquor

Units involved

•ṁ_solids — dry solids feed rate in t/h
•wt% — underflow solids concentration by mass
•ρ_liquid — liquor density in kg/m³
•Q_liquor, Q_wash — volumetric flows in m³/h
•Wash ratio — dimensionless (a flow ratio)

Concept diagram

Worked example

A CCD circuit treats 100 t/h dry solids at 55 wt% underflow solids with a liquor density of 1000 kg/m³. What wash water does a target wash ratio of 2.0 need, and what is the actual ratio at 160 m³/h?

01Underflow slurry mass: 100 / (55/100) = 181.82 t/h
02Underflow liquor mass: 181.82 − 100 = 81.82 t/h → 81.82 m³/h at 1000 kg/m³
03Required wash water at ratio 2.0: 2.0 × 81.82 = 163.64 m³/h
04Actual wash ratio at 160 m³/h: 160 / 81.82 = 1.96
05Read these as flow numbers — confirm recovery with a soluble balance and testwork

Result

A wash ratio of 2.0 needs about 164 m³/h of wash water; an existing 160 m³/h gives an actual wash ratio of about 1.96. These are wash-water flows, not a soluble-recovery prediction.

Common mistakes

•Reading wash ratio as soluble recovery — it is a flow ratio only.
•Ignoring underflow solids concentration, which sets the liquor carried forward.
•Assuming more wash water always improves the design rather than diluting the product.
•Using the wrong liquor density, which distorts the underflow liquor volume.
•Forgetting that stage count and stage efficiency drive recovery, not wash ratio alone.

When to use the calculator

Use the CCD wash-water calculator to size the wash-water flow for a target wash ratio and to check the actual ratio of an existing flow, and the thickener underflow density and percent-solids calculators to fix the underflow basis the wash ratio depends on. For recovery, you need a soluble balance and testwork — not this calculator.

CCD Wash Water Calculator →Thickener Underflow Density Calculator →Slurry Mass Balance Calculator →Percent Solids Mass ↔ Volume Calculator →Slurry Density Calculator →Two Product Formula Calculator →

FAQ

Is wash ratio the same as soluble recovery?

No. Wash ratio is the wash-water flow divided by the underflow liquor flow — a flow ratio. Soluble recovery is the fraction of dissolved value actually recovered, which depends on stage count, stage efficiency, entrainment, and thickener performance. A high wash ratio helps but does not equal high recovery.

Why does underflow solids concentration matter so much?

It sets how much liquor travels forward with the solids. A denser underflow carries less liquor per tonne of solids, so less dissolved value is lost and the wash water has less liquor to displace. The same wash ratio behaves differently at a different underflow density.

Does doubling the wash water double the recovery?

No. Recovery responds to wash ratio with diminishing returns and is bounded by the number of stages and their efficiency. Beyond a point, extra wash water mostly dilutes the product solution and burdens the water balance. The optimum comes from a soluble balance, not from maximising wash water.

What does this calculator deliberately not do?

It does not compute soluble recovery, stage efficiency, wash loss, or solution tenor, and it is not a SysCAD or stage-by-stage CCD model. It estimates the wash-water flow for a target wash ratio and the liquor carried with the underflow — a preliminary number to be confirmed by modelling and testwork.