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arrangement of phme and concave surlaccs.
The shape of the rolls on the NCRC result in several unique characteristics. Tile most important is that, lk)r
a given particle size and roll gap, the nip angle generated m the NCRC will not remain constant as the rolls
rotate. There will be times when the nip angle is much lower than it would be for the same sized cylindrical
rolls and times when it will be much highcr. The actual variation in nip angle over a 60 degree roll rotation
is illustrated in Figure 2, which also shows the nip angle generated under similar conditions m a cylindrical
roll crusher of comparable size. These nip angles were calculated for a 25ram diameter circular particle
between roll of approximately 200ram diameter set at a I mm minimum gap. This example can be used to
illustrate the potential advantage of using non-cylindrical rolls. In order for a particle to be gripped, thc
angle of nip should normally not exceed 25 ° . Thus, the cylindrical roll crusher would never nip this
particle, since the actual nip angle remains constant at approximately 52 °. The nip angle generated by the
NCRC, however, tidls below 25 ° once as the rolls rotate by (~0 degrees. This means that the non-cylindrical
rolls have a possibility of nipping the particlc 6 times during one roll rewHution.
EXPERIMENTAL PROCEDURE
The laboratory scale prototype of the NCRC (Figure 3) consists of two roll units, each comprising a motor,
gearbox and profiled roll. Both units are mounted on linear bearings, which effectively support any vertical
componcnt of force while enabling horizontal motion. One roll unit is horizontally fixed while the other is
restrained via a compression spring, which allows it to resist a varying degree of horizontal load.
The pre-load on the movable roll can be adjusted up to a maximum of 20kN. The two motors that drive the
rolls are electronically synchronised through a variable speed controller, enabling the roll speed to be
continuously varied up to 14 rpm (approximately 0.14 m/s surface speed). The rolls have a centre-to-centre
distance ~,at zero gap setting) of I88mm and a width of 100mm. Both drive shafts are instrumented with
strain gauges to enable the roll torque to be measured. Additional sensors are provided to measure the
horizontal force on the stationary roll and the gap between the rolls. Clear glass is fitted to the sides of the
NCRC to facilitate viewing of the crushing zonc during operation and also allows the crushing sequence to
bc recorded using a high-speed digital camera.
Tests were performed on several types of rocks including granite, diorite, mineral ore, mill scats and
concrete. The granite and diorite were obtained from separate commercial quarries; the former had been
pre-crushed and sized, while the latter was as-blasted rock. The first of the ore samples was SAG mill feed
obtained from Normandy Mining's Golden Grove operations, while the mill scats were obtained from
Aurora Gold's Mt Muro mine site in central Kalimantan. The mill scats included metal particles of up to
18ram diameter from worn and broken grinding media. The concrete consisted of cylindrical samples
(25mm diameter by 25ram high) that were prepared in the laboratory in accordance with the relevant
Australian Standards. Unconfined uniaxial compression tests were performed on core samples (25mm
diameter by 25mm high) taken from a number of the ores. The results indicated strength ranging from 60
MPa for the prepared concrete up to 260 MPa for the Golden Grove ore samples.
All of the samples were initially passed through a 37.5mm sieve to remove any oversized particles. The
undersized ore was then sampled and sieved to determine the feed size distribution. For each trial
approximately 2500g of sample was crushed in the NCRC. This sample size was chosen on the basis of