Air classifier (air separation)

An air classifier separates a granulated feed into light and heavy fractions using a controlled upward air stream. In e-waste and plastic recycling, it is the workhorse for stripping plastic, foam, paper and dust from metals — typically positioned after granulation and before eddy current separator work. The separation is by aerodynamic settling velocity, which depends on particle density, shape and size, not by chemistry; that makes it cheap, dry, and tolerant of dirty feed.

Three configurations are common. A zig-zag classifier uses a vertical S-shaped duct: feed enters mid-column, air rises against gravity, lights are entrained upward into a cyclone, heavies fall through the zig-zag. The path forces repeated direction changes that liberate plastic from clinging metal flakes, giving cleaner separation than a straight column. A cross-flow (air table) design uses a fluidised perforated deck with a horizontal air sweep — lights are blown off the discharge edge, heavies walk along the deck. Air knives, the simplest variant, blast a focused jet across a belt or chute, deflecting lights into a separate hopper.

Performance hinges on tuning air velocity against feed density distribution. For mixed PCB granulate (1.3-1.5 g/cc plastic, 2.7 g/cc aluminium, 8.9 g/cc copper), classifier velocity is held at 4-7 m/s — fast enough to lift plastic, slow enough to settle metal. Feed must be pre-screened to a narrow size band (e.g. 5-15 mm); otherwise small heavies report with lights and large lights drop with heavies. Throughput ranges 1-8 TPH for industrial units, power 15-55 kW, footprint 8-25 m².

Limits are honest. Air classifiers cannot separate two materials of similar density (PP from PE, aluminium from glass at finer sizes), so they are paired with downstream density (sink-float), electrostatic, or eddy current stages. They generate dust — a baghouse on the cyclone vent is mandatory, especially for printed circuit board granulate that may carry brominated dust requiring HEPA filtration and dedicated hazardous-waste disposal.