This is an induced-roll magnetic separator: a rotating drum carries mixed feed past a fixed magnet. Every particle riding the drum is held on by the magnet's attraction and let go the instant centrifugal force wins the tug-of-war, so each mineral's own magnetic susceptibility χ decides exactly where on the arc it flies off.
outward force(θ) = ω²R − k·B·χ − g·cos(θ)
released the instant outward force(θ) > 0
ω is the drum's angular speed, R its radius, B the field-strength setting, and θ the angle traveled since the feed point at the top. Strongly-paramagnetic rare-earth grains (high χ) need a much larger θ before centrifugal force overtakes the magnet, so they cling deep into the arc and drop close to the drum into the concentrate bin. Non-magnetic gangue (χ≈0) has almost nothing holding it on, so it flies off tangentially near the top of the drum and lands far out in the tailings bin. Moderately magnetic grains land in between.
- Field strength — scales the magnet's holding force k·B·χ. Turn it up and even weakly-magnetic grains stay on long enough to reach the concentrate gate — raising recovery but pulling gangue in with them and diluting grade.
- Drum speed — sets ω, and therefore the centrifugal term ω²R. A faster drum overwhelms the magnet sooner for every grain, shortening every release angle and shrinking the whole spread.
- Feed rate — how many grains per second are dropped onto the drum at θ=0.
- Gate sliders — move the two splitter positions that carve the landing zone into concentrate / middling / tailings, exactly like adjusting the physical splitter plates under a real roll separator.
- Recovery is the fraction of all REE-grade grains fed in that end up in the concentrate bin; grade is the purity of the concentrate bin itself — the two trade off against each other as field strength changes.