Wash ratio and wash efficiency

If counter-current decantation is the machine, the wash ratio is the dial you set it on. Wash ratio, the number of stages, and the efficiency of soluble recovery are the three quantities a washing circuit is read and designed by, and they trade against one another in ways worth holding clearly. This topic is the quantitative companion to the CCD principle.

Wash ratio

Wash ratio is the volume of wash water added relative to the volume of liquor carried with the underflow solids. A wash ratio of two means twice as much wash water as the liquor clinging to the solids; a ratio of one means an equal volume. It is the primary lever on how much soluble value the circuit recovers, because it sets how much displacing water is available to push the loaded liquor back up the train. The CCD calculator below works the wash water a target ratio needs from the committed underflow figures.

Stages and the efficiency of recovery

Wash ratio is only half the picture; the number of stages is the other half. For a given wash ratio, adding stages raises the fraction of soluble value recovered, because each stage gives the counter-current cascade another step to drive the underflow liquor cleaner. The two compound: a high wash ratio over few stages and a modest wash ratio over many stages can reach the same recovery, and the design picks the combination that costs least across pumping, thickener count and water. The efficiency of soluble recovery — the fraction of dissolved value the wash recovers rather than losing with the solids — rises with both, with diminishing returns, so there is an economic optimum rather than a recover-everything answer.

The wash-water-versus-dilution trade

The countervailing cost is dilution. Every cubic metre of wash water that recovers soluble value also ends up in the product solution, so more washing means a larger, weaker product stream that costs more to process downstream — more solution to pump, to heat, to treat in the recovery step. So the wash ratio is bounded on one side by recovery and on the other by dilution: too little water loses value with the solids; too much dilutes the product and inflates the downstream duty. The slurry-dilution-water calculator below sizes the water a percent-solids change takes, the same mass-balance currency the dilution trade is paid in. The honest design number — how many stages at what wash ratio — comes from a soluble balance and testwork on the actual circuit, which the wash ratio here frames rather than replaces.