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Another possible mechanism is based on fine particles being entrained in the wake behind

coarser particles, as shown in Figure 6 (Neesse et al., 2004; Kraipech et al., 2002). This

is supported by the observation that when particles of different sizes are settling together,

the settling rates of the finer particles is increased compared to the settling rate that they

have when no coarse particles are present. This effect can be quantified, and models

based on it can show a quite pronounced fine inflection. It is reported that this effectbecomes experimentally noticeable at particle sizes less than 3.5 m for lime particles,

glass beads, and dusts suspended in water (Kraipech et al., 2002). In this situation, the

quantity of fines that could be carried to the underflow would depend on the volume of

coarse particle wakes, and would therefore increase as the number of coarse particles

increased. It would also be expected that there would be relatively little dependence on

the coarse particle properties, and the effect would depend only on the size and settling

velocity of the coarse particles.

Fine particlestrapped in wake ofcoarse particle

Coarse

Particle

Figure 6: Fine particles trapped in the wake of coarse particle, and being carried along byit to the hydrocyclone underflow.

It is normally expected that there is a tradeoff between overgrinding and the presence of

locked particles. If the circuit product size is made coarser to reduce the overgrinding,this typically leads to an increase in the top size of the product as well, with the top size

particles being poorly liberated. This reduces the grade that can be produced in the

processing that follows comminution. In order to reduce this trade-off, it is necessary to

determine how to change the grinding circuit to produce a narrower size distribution,

which will reduce overgrinding while simultaneously controlling the quantity of coarse

locked particles leaving the circuit.