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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 higher. 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, the 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 component 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 zone during operation and also allows the crushing sequence to be 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 statistical tests, which indicated that at least 2000g of sample needed to be crushed in order to estimate the product P80 to within +0.1 ram with 95% confidence. The product was collected and riffled into ten subsamples, and a standard wet/dry sieving method was then used to determine the product size distribution.