Counter-current decantation: the principle

Counter-current decantation, CCD, is a washing circuit you can read off first principles once you see the trick. A train of thickeners is connected in series so that the solids and the wash water move through it in opposite directions — solids one way, water the other. The purpose is to recover the dissolved value held in the liquor that clings to the thickened solids, using the least wash water for the most recovery. It is the great washing flowsheet of the alumina industry and a standard across hydromet, and the counter-current arrangement is the whole of its cleverness.

A train of thickeners

Picture several thickeners in a row, each doing the gravity-thickening job of the previous topic: feed in, clear overflow off the top, dense underflow out the bottom. In CCD the underflow from one thickener is pumped to the next as its feed, so the solids march down the train from the first stage to the last, washed at each stage. The overflows run the other way. The result is a cascade in which every stage both thickens and washes, and the solids leave the last stage carrying as little dissolved value as the circuit can economically displace.

Why the wash flows opposite the solids

The counter-current arrangement is what makes the washing efficient, and it repays thinking through. Fresh wash water is added only at the last stage — where the solids are already nearly clean — and its overflow is pumped forward to wash the stage before it, and so on up the train, until the most loaded wash liquor meets the dirtiest incoming solids at the first stage and leaves as the strong product solution. So the cleanest water meets the cleanest solids and the dirtiest water meets the dirtiest solids. Each stage sees only a small concentration difference, but the cascade as a whole drives the soluble content of the underflow liquor down stage by stage — the way a counter-current heat exchanger drives temperature, a pattern a crosser will recognise at once. Running the water with the solids instead would let the first wash dilute itself against the loaded liquor and recover far less for the same water.

The washing cascade

The reason to spend the engineering on counter-current plumbing rather than one large wash is water economy. Washing the soluble value out of the underflow in a single stage would take an enormous volume of water and leave a dilute product solution; the counter-current cascade reaches the same recovery with a fraction of the water, because each increment of water is used several times as it works its way up the train. The trade is more stages and more pumping against less water and a stronger product — and where the wash water reports to the strong liquor that goes to recovery, less water also means a more concentrated, cheaper-to-process solution. The calculators below estimate the wash water a target wash ratio needs and the process water the thickeners recover; the worked thread runs the committed CCD example.