9 Impact Comminution in Jet Mills
331
Fig. 14 a Relative impact velocity normal to the particle surface. b Number-weighted sum
distributions of the impact events. c Stress numbers SN % and SN stress (see Eq. 9 and 10)
in Fig. 14. Obviously, in all cases, the stress numbers are consistently high (Fig. 14c),
SN stress lies between 15 and 25). When increasing the pressure to 2 bar, the number of
non-stressed probe particles is reduced (Fig. 14b). Increasing the pressure further to
3 bar, a higher number of unstressed particles is observed. Simultaneously, particles
with a higher number of contacts appear in the sample. Köninger et al. recently
showed for fluidized beds with secondary gas injection, that the solid concentration
within the jets decreases upon increasing the gas pressure [34]. Figure 14a shows
the influence of the grinding pressure on the relative impact velocity distribution
calculated from the measured contact diameters. Between a pressure of 1 and 2 bar
only slight differences are visible. At the investigated maximum pressure of 3 bar, the
distribution shifts to higher impact velocities. Nonetheless, even at 3 bar maximum
impact velocities of 50 m s
−1 were found only in few cases. However, all median
impact velocities (Q 0 = 0.5) were smaller than 10 m s
−1 , a surprising result.
331
Fig. 14 a Relative impact velocity normal to the particle surface. b Number-weighted sum
distributions of the impact events. c Stress numbers SN % and SN stress (see Eq. 9 and 10)
in Fig. 14. Obviously, in all cases, the stress numbers are consistently high (Fig. 14c),
SN stress lies between 15 and 25). When increasing the pressure to 2 bar, the number of
non-stressed probe particles is reduced (Fig. 14b). Increasing the pressure further to
3 bar, a higher number of unstressed particles is observed. Simultaneously, particles
with a higher number of contacts appear in the sample. Köninger et al. recently
showed for fluidized beds with secondary gas injection, that the solid concentration
within the jets decreases upon increasing the gas pressure [34]. Figure 14a shows
the influence of the grinding pressure on the relative impact velocity distribution
calculated from the measured contact diameters. Between a pressure of 1 and 2 bar
only slight differences are visible. At the investigated maximum pressure of 3 bar, the
distribution shifts to higher impact velocities. Nonetheless, even at 3 bar maximum
impact velocities of 50 m s
−1 were found only in few cases. However, all median
impact velocities (Q 0 = 0.5) were smaller than 10 m s
−1 , a surprising result.