The operating-mode selection of the ball mill (sometimes called «the operating-mode of grinding media») depends on three main parameters:
- The relative frequency speed of drum is equal to the ratio between the actual speed to critical speed of mill drum.
Proxy critical rotation frequency of rotation of the drum is equivalent to a speed at which the inertia of balls rotational motion equals the gravity of this ball. In so doing, grinding media of infinitesimal size raised to the highest point are in dynamic equilibrium.
Critical speed is calculated by the formula:
D — inside diameter of ball mill drum, m.
- Ratio of grinding mill loading by grinding balls. Ratio of grinding balls volume to mill working volume is calculated by the formula:
Gн – mass of grinding balls, kg;
γ – apparent density of grinding media, kg/m3;
L – drum length, m.
Apparent density of grinding media depends on ball diameter and density of the material.
- Technological parameters of grinding: grinded material, requirement for the grinding, etc.
The mill operation modes are as follows:
- The »cascade» – the rolling the grinding balls inside the mill drum.
- Mixed mode – the rolling the grinding balls when part of the grinding balls move in a circular trajectory with further fall.
- Waterfall – the grinding balls move along a circular trajectory and change it further under gravity.
- Supercritical mode – centrifuging of the total grinding media volume.
Shredding of material in cascade (figure a) mode occurs because of crushing and abrasive effects of grinding balls. This is the most common mode used during operation of rod mills, ball mills with central unloading and pipe mills in the case of wet and dry grinding of non-corrosive materials.
Mixed operating mode is illustrated in the figure b, where the dotted line shows the position of the grinding balls in flight along the trajectory and solid lines – balls moving by rolling.
Waterfall operating mode (figure c) observed at roller speed which ensures that most layers of grinding balls pass from a circular to a parabolic trajectory. This mode is widely used in most industrial mills of different types.
Sub-critical mmode is a special case of waterfall operating mode. It operates at a drum speed close to or equal to the critical speed.