XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3 185
expected, that the materials with higher tensile strength are
comminuted less in comparison to the materials with lower
tensile strength.
The model of a critical impact speed acc. to eq. (1)
is based on certain assumptions. An important one is the
right-angle impact of the particle against a wall of com-
parably high stiffness and inertia. Moreover, the particle is
stressed just once before it leaves the process space. This
assumption can hardly be met in industrial applications.
The closest approximation may be a Vertical Shaft Impact
crusher (VSI) where the particle is theoretically just stressed
once, before it leaves the process zone, if particle-particle
collisions are ignored. A right-angle impact, however, is
only achieved with a fresh anvil ring.
With horizontal shaft impact crushers, the comminu-
tion process is more complex. Particles are usually stressed a
few times, the first time when hitting the impact bar, then
when they or their progenies hit the impact wall. There may
be further collisions with other particles in the crushing
chamber, a second or third contact with the impact bar etc.
Applying the laws of conservation of energy and impulse,
it can be shown, that a particle would be thrown away with
twice the rotor speed after a fully elastic impact (without
breakage). Furthermore, the residence time of a particle in
the process space of the impact crusher may be influenced
by the crushing gap between rotor outer radius and lower
end of the impact wall. The closer the gap is, the longer is
the residence time, the more often is the particle stressed,
yet not with a predefined impact speed. In consequence,
the particle sizes will vary substantially, at least after a num-
ber of collisions.
Nevertheless, also with horizontal shaft impact crush-
ers excellent results in industrial application of Selective
Comminution are documented. Puffe reported about a
lead mine in Germany with still a 1 Mio t of lead in the
ground but at a grade not considered economic any more
(Puffe 1955, 1960). By adding an impact crushing and a
screening stage (8 mm cut size), a preconcentrate could be
produced, containing 87% of lead (galenite mineral) in the
material reporting to the underflow, which was just about
one third of the overall material. Hence, the question was
raised whether for the ore of Hämmerlein similar results
could be achieved and what parameters would be appro-
priate. A simple test program was set up to investigate the
susceptibility of the ore to Selective comminution. A single
size fraction was stressed at a fixed speed just varying the
crushing gap from 15 mm to 60 mm, see Table 3. It needs
to be emphasized that the larger crushing gap was wider
Figure 4. Minimum impact speed for various constituents of the Hämmerlein ore
expected, that the materials with higher tensile strength are
comminuted less in comparison to the materials with lower
tensile strength.
The model of a critical impact speed acc. to eq. (1)
is based on certain assumptions. An important one is the
right-angle impact of the particle against a wall of com-
parably high stiffness and inertia. Moreover, the particle is
stressed just once before it leaves the process space. This
assumption can hardly be met in industrial applications.
The closest approximation may be a Vertical Shaft Impact
crusher (VSI) where the particle is theoretically just stressed
once, before it leaves the process zone, if particle-particle
collisions are ignored. A right-angle impact, however, is
only achieved with a fresh anvil ring.
With horizontal shaft impact crushers, the comminu-
tion process is more complex. Particles are usually stressed a
few times, the first time when hitting the impact bar, then
when they or their progenies hit the impact wall. There may
be further collisions with other particles in the crushing
chamber, a second or third contact with the impact bar etc.
Applying the laws of conservation of energy and impulse,
it can be shown, that a particle would be thrown away with
twice the rotor speed after a fully elastic impact (without
breakage). Furthermore, the residence time of a particle in
the process space of the impact crusher may be influenced
by the crushing gap between rotor outer radius and lower
end of the impact wall. The closer the gap is, the longer is
the residence time, the more often is the particle stressed,
yet not with a predefined impact speed. In consequence,
the particle sizes will vary substantially, at least after a num-
ber of collisions.
Nevertheless, also with horizontal shaft impact crush-
ers excellent results in industrial application of Selective
Comminution are documented. Puffe reported about a
lead mine in Germany with still a 1 Mio t of lead in the
ground but at a grade not considered economic any more
(Puffe 1955, 1960). By adding an impact crushing and a
screening stage (8 mm cut size), a preconcentrate could be
produced, containing 87% of lead (galenite mineral) in the
material reporting to the underflow, which was just about
one third of the overall material. Hence, the question was
raised whether for the ore of Hämmerlein similar results
could be achieved and what parameters would be appro-
priate. A simple test program was set up to investigate the
susceptibility of the ore to Selective comminution. A single
size fraction was stressed at a fixed speed just varying the
crushing gap from 15 mm to 60 mm, see Table 3. It needs
to be emphasized that the larger crushing gap was wider
Figure 4. Minimum impact speed for various constituents of the Hämmerlein ore