Pin Mill
Technology
The Pin Mill operates on the principle of high-velocity, unrestrained impact Size reduction is achieved through violent collisions between the feed material and precisely spaced concentric rows of rotating pins — fracture probability rising directly with the relative impact velocity between particle and rotor pin
- Mechanism
- Unrestrainedimpact
- Typical fineness
- 50 – 300µm
- Primary control
- Tipspeed
- Residence
- < 1 ssingle pass
Kinetic energy transfer through concentric pin rows
Material enters at the centre and is flung radially outward, striking row after row of intermeshing pins Each successive ring sits at a larger diameter — so relative velocity, and therefore impact energy, climbs with every collision until the particle is fine enough to leave with the air stream
Fracture probability is governed by velocity
Two equations describe nearly everything a pin-mill operator controls The first sets how fast the pin tips travel; the second sets how much energy each collision delivers
Peripheral velocity of the outermost pin row — the single most influential process parameter
- V
- Peripheral velocity — m/s
- D
- Rotor diameter — m
- N
- Rotational speed — RPM
Because velocity is squared, kinetic energy — and therefore fineness — rises exponentially with rotor speed
- Eₖ
- Kinetic energy available to fracture the particle
- m
- Particle mass
- v
- Relative impact velocity (particle vs pin)
The path of a particle
The trajectory inside a pin mill is complex As material enters the central feed zone it is engaged by the innermost row of pins, then driven outward through the active grinding zone in a fraction of a second
Initial acceleration
The particle is accelerated both tangentially — in the direction of rotation — and radially outward under centrifugal force
Successive impacts
It travels through concentric rows of interlocking pins — a rotor and stator, or counter-rotating discs Relative velocity increases with each row as the diameter grows
Residence time
Time in the grinding zone is extremely brief — fractions of a second This limits heat buildup, but demands high impact probability to hit the target PSD in a single pass
Key factors affecting grinding performance
Optimising a pin mill means balancing several interdependent variables — push one and the others respond
Rotor speed (tip speed)
The primary control variable Higher speed yields finer particles — but increases energy consumption and heat
Feed rate
Increasing feed mass reduces the specific energy available per particle — giving a coarser product and risking motor overload
Pin density & configuration
More pins or a narrower gap raises impact probability and fineness — at the cost of airflow resistance and heat generation
Material characteristics
Hardness (Mohs), brittleness and moisture dictate grindability High moisture cushions impact — slashing efficiency and blinding the pins