THORby CI
TechnologyBall Mill

The technology inside the Ball Mill

Attrition · media — how it works, the equations we size it with, and what can be adjusted.

01How it works

Ball Mill

A horizontal shell rotates on trunnion bearings, lifting a charge of steel or ceramic media up the wall until it cascades back onto the material bed.

  1. 01Feed and media share the shell; rotation carries both up the rising side.
  2. 02Media rise until gravity overcomes friction, then cascade or cataract down.
  3. 03Falling media crush by impact; rolling media grind by attrition between surfaces.
  4. 04Product discharges through the trunnion or a grate — wet as slurry or dry with air sweep.
02Theory and equations

Everything hangs on critical speed

Critical speed is the rotation at which centrifugal force pins the outermost media to the shell wall. At that point they stop falling and no grinding happens at all — so a ball mill is always run at a fraction of it.

We normally operate between 65 and 80 % of critical. Lower in that band favours attrition and a finer, tighter product; higher favours impact and throughput on coarse, hard feeds.

  • Critical speedNc = 42.3 / √(D − d)
    Nc
    critical speed (rpm)
    D
    internal shell diameter (m)
    d
    media diameter (m)

    Operating speed is set at 65 – 80 % of Nc depending on the duty.

  • Media charge volumeJ = V_media / V_mill
    J
    charge filling fraction — 30 to 45 % typical
    V_media
    bulk volume of the media charge (m³)
    V_mill
    internal mill volume (m³)

    Under-filling wastes power; over-filling cushions the impacts and slows grinding.

03Features

Tuned to your material

  • AdjustMedia size and gradingLarge media for coarse feed, graded charges for a finer product — recharged on a schedule from wear rate.
  • AdjustShell speedA VFD moves the mill between attrition-dominant and impact-dominant operation.
  • AdjustWet or dry dutyThe same shell runs as a slurry mill or with an air sweep — the discharge arrangement changes, not the mill.
  • UpgradeLifter-bar linersProfiled liners set the lift height and control whether the charge cascades or cataracts.
  • UpgradeCeramic-lined shellAlumina or rubber lining with ceramic media for contamination-free grinding.
  • UpgradeClosed circuit with classifierPairing the mill with an air classifier returns oversize and lifts throughput by 20 – 40 %.
04See the product
THOR ball mill drum with its belt drive, gear reducer and motor mounted on a common base frame
See the machineBall Mill20 – 200 µm · 0.65 – 80 TPH · 18.5 – 400 kW

Other technology