density separation (sink-float separation)

Density separation sorts a mixed, shredded material stream according to the relative density (specific gravity) of its constituents. The classic form is sink-float separation: the shredded flakes are dropped into a liquid medium of a chosen density (water, or a salt/calcium-chloride solution tuned to a target density), so that fragments lighter than the medium float and heavier ones sink, and the two streams are skimmed and drained off separately. Related dry techniques — air classification (using an upward air stream to carry light material away from heavy) and shaking-table/jigging gravity separation — work on the same principle without liquid.

In recycling it is used at two points. First, in plastics recovery, density separation cleanly splits polymer types: polyolefins like polyethylene (PE) and polypropylene (PP) have density below 1.0 g/cm³ and float on water, while PVC, PET, ABS and PC are denser and sink — so a simple water float-sink does much of the polymer sorting that e-waste and packaging recyclers need. Second, in the back end of metal recovery, after magnets remove ferrous metal and eddy-current separators remove the bulk of non-ferrous metal, density and gravity separation help split the remaining mixed residue — for example separating heavy metal fines from light plastic and glass in chopped cable or shredder fines.

The attraction of density separation is that it is simple, low-energy and cheap to operate relative to sensor-based sorting — a sink-float tank is essentially a tuned bath with skimming and conveying — and it handles high throughput. Its limits are that it only separates by density, so materials of similar density (e.g. PVC and PET, both heavier than water) cannot be split by a single water bath and need either a second medium of intermediate density or a different technology (XRT, near-infrared, electrostatic). Liquid media also create a wet stream that must be drained, dried and, for salt solutions, managed as effluent.

For an Indian recycler, density separation is a workhorse first-pass sorting step that gives a lot of separation for little capital, fitting the cost structure of an Indian operation well. The practical guidance: use a water sink-float to pull the polyolefins off the denser plastics, tune medium density deliberately for the specific cut you need, pair it with magnetic and eddy-current separation upstream so it only handles non-metallics, and plan for water/effluent handling. Where density separation cannot resolve similar-density materials, that is the point to add sensor sorting (near-infrared for plastics, XRT for metals) — but density separation should do the cheap bulk of the work first.