Filtration and centrifugation

Thickeners are not the only way to take solids out of a liquid, and they are not always the best. When the duty is a sharp final dewatering, a clean cut between solids and liquor, or a high-value solid that cannot be lost, filters and centrifuges beat gravity thickening — and a reader of flowsheets should know the family and the choices in it. This is device-selection territory: a concept page rather than a single calculation.

Where filters and centrifuges beat thickeners

A thickener leaves its product as a pumpable slurry — a dense underflow, but still wet. Filtration and centrifugation go further, driving the moisture down to a damp cake, and they do it on a footprint and a cut a thickener cannot match. Three duties call for them. Final dewatering: when the product or the residue has to be shipped, stacked dry, or fed to a kiln, a filter cake at a low moisture beats a slurry. A sharp cut: when the overflow must be genuinely clear or the solids genuinely clean of liquor, a filter separates and washes more completely than settling. And high-value solids: when the solid is the product — a precipitated metal, a fine concentrate — the near-total solids recovery of a filter is worth its cost. Thickeners win on cheap bulk separation at scale; filters and centrifuges win on dryness, sharpness and value.

The family of devices

The devices form a family graded by how they apply the separating force. Pressure and vacuum filters drive liquor through a filter medium that holds the solids back as a cake — plate-and-frame and filter presses, vacuum belt and drum filters, pressure filters — chosen by the cake’s behaviour and the moisture and wash the duty needs. Centrifuges spin the slurry so the denser solids are flung outward and the liquor decants — decanter and disc-stack machines — chosen where the particles are fine or the separation has to be fast and continuous. Across the family the trade is the same: more dewatering and a sharper cut against more capital and more energy than a gravity thickener. Which device, and at what duty, is a selection made on cake-formation and settling testwork and vendor data for the specific slurry — a judgement this page frames rather than a number it computes.