A zig-zag / vertical air classifier separates shredded waste by density: an upward air stream is blown through a column while mixed material falls in from a mid-height feed point. Each particle has a terminal (settling) velocity in still air that depends on its density and size. If the air velocity exceeds that terminal velocity, drag wins and the particle is carried upward into the light-material stream; otherwise gravity wins and it sinks into the heavy-material stream.
v_t = sqrt( 4·g·d·(ρ_p − ρ_air) / (3·C_d·ρ_air) )
net rise velocity: v_net = v_air − v_t
v_net > 0 → carried up (light stream)
v_net < 0 → falls down (heavy stream)
g = 9.81 m/s², ρ_air = 1.2 kg/m³, C_d = 0.47 (sphere)
- Air velocity — sets the separation cut-point; too low over-sorts everything heavy, too high carries contaminants up with the light fraction.
- Feed rate — number of particles active in the column at once.
- Heavy-material mix — probability that a spawned particle is glass/aluminum vs plastic/paper.
- Particle diameter — d² term inside v_t; smaller shredded fragments need far less air velocity to lift.
Real recycling microfactories tune air velocity and feed geometry per material stream to maximise purity of both outputs while keeping throughput high — exactly the trade-off this column lets you explore.