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COMMINUTION INANON-CYLINDRICAL ROLL CRUSHER*P.VELLETRI and D.M.WEEDON Dept. of Mechanical accepted 4 September 2001)Velletri and Weedon, 2000 P. Velletri and D.M. Weedon, Preliminary investigations into a roll crusherwith non-cylindrical rolls, Proc. Minprex 2000 International Congress on Mineral Processing and Extractive Metallurgy, AIMM, Melbourne (2000), pp. 321328.ABSTRACTLow reduction ratios and high wear rates are the two characteristics most commonly associatedwith conventional roll crushers. Because of this, roll crushers are not often considered Jor use inmineral processing circuits, and many of their advantages are being largely overlooked. This paperdescribes a novel roll crusher that has been developed in order to address these issues.Referred toas the NCRC (Non-Cylindrical Roll Crusher), the new crusher incorporates two rolls comprised ofan alternating arrangement of plane and convex or concave surfaces. These unique roll profilesimprove the angle of nip, enabling the NCRC to achieve higher reduction ratios than conventional roll crushers. Tests with a model prototype have indicated thar even for very hard ores, reductionratios exceeding l0:l can be attained. In addition, since the comminution process in the NCRCcombines the actions of roll and jaw crushers there is a possibility O that the new profiles maylead to reduced roll wear rates. 2001 Elsevier Science Ltd.All rights reserved.Keywords: Comminution; crushingINTRODUCTIONConventional roll crushers suffer from several disadvantages that have led to their lack ofpopularity in mineral processing applications. In particular, their low reduction ratios (typically limited to about 3:1) and high wear rates make them unattractive when compared to other types ofcomminution equipment, such as cone crushers. There are, however, some characteristics of rollcrushers that are very desirable from a mineral processing point of view. The relatively constantoperating gap in a roll crusher gives good control over product size. The use of spring-loaded rollsmake these machines tolerant to uncrushable material (such as tramp metal). In addition, rollcrushers work by drawing material into the compression region between the rolls and do not rely on gravitational feed like cone and jaw crushers. This generates a continuous crushing cycle,which yields high throughput rates and also makes the crusher capable of processing wet andsticky ore.The NCRC is a novel roll crusher that has been developed at the University of Western Australiain order to address some of the problems associated with conventional roll crushers. The newcrusher incorporates two rolls comprised of an alternating arrangement of plane and convex orconcave surfaces. These unique roll profiles improve the angle of nip, enabling the NCRC toachieve higher reduction ratios than conventional roll crushers. Preliminary tests with a modelprototype have indicated that, even for very hard ores,reduction ratios exceeding 10:I can beattained (Vellelri and Weedon, 2000). These initial findings were obtained for single particle feed.where there is no significant interaction between particles during comminution. The current workextends the existing results by examining multi-particle comminution in the NCRC. It also looksat various other factors that influence the performance of the NCRC and explores the effectiveness of using the NCRC for the processing of mill scats.PRINCIPLE OF OPERATIONThe angle of nip is one of the main lectors effecting the performance of a roll crusher.Smaller nip angles are beneficial since they increase the likelihood of particles beinggrabbed and crushed by the rolls. For a given feed size and roll gap, the nip angle in aconventional roll crusher is limited by the size of the rolls. The NCRC attempts toovercome this limitation through the use of profiled rolls, which improve the angle ofnip at various points during one cycle (or revolution) of the rolls. In addition to thenip angle, a number of other factors including variation m roll gap and mode ofcomminution were considered when selecting the roll profiles. The final shapes of theNCRC rolls are shown in Figure I. One of the rolls consists of an alternating arrangement of plane and convex surfaces, while the other is formed from analternating arrangement of plane and concave surfaces. The shape of the rolls on the NCRC result in several unique characteristics. The most important is that, for a given particle size and roll gap, the nip angle generated m the NCRC will not remainconstant as the rolls rotate. There will be times when the nip angle is much lower than it would befor the same sized cylindrical rolls and times when it will be much higher. The actual variation innip angle over a 60 degree roll rotation is illustrated in Figure 2, which also shows the nip anglegenerated under similar conditions m a cylindrical roll crusher of comparable size. These nipangles were calculated for a 25ram diameter circular particle between roll of approximately200ram diameter set at a I mm minimum gap. This example can be used to illustrate the potentialadvantage of using non-cylindrical rolls. In order for a particle to be gripped, the angle of nipshould 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 theNCRC, however, the below 25 once as the rolls rotate by (0 degrees. This means that thenon-cylindrical rolls have a possibility of nipping the particle 6 times during one roll revolution.EXPERIMENTAL PROCEDURE The laboratory scale prototype of the NCRC (Figure 3) consists of two roll units, each comprisinga motor, gearbox and profiled roll. Both units are mounted on linear bearings, which effectivelysupport any vertical component of force while enabling horizontal motion. One roll unit ishorizontally fixed while the other is restrained via a compression spring, which allows it to resist avarying degree of horizontal load. The pre-load on the movable roll can be adjusted up to a maximum of 20kN. The two motors thatdrive the rolls are electronically synchronised through a variable speed controller, enabling the rollspeed to be continuously varied up to 14 rpm (approximately 0.14 m/s surface speed). The rollshave a centre-to-centre distance ,at zero gap setting) of I88mm and a width of 100mm. Bothdrive 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 gapbetween the rolls. Clear glass is fitted to the sides of the NCRC to facilitate viewing of thecrushing 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 scatsand concrete. The granite and diorite were obtained from separate commercial quarries; the formerhad been pre-crushed and sized, while the latter was as-blasted rock. The first of the ore sampleswas SAG mill feed obtained from Normandy Minings Golden Grove operations, while the millscats were obtained from Aurora Golds Mt Muro mine site in central Kalimantan. The mill scatsincluded metal particles of up to 18ram diameter from worn and broken grinding media. Theconcrete consisted of cylindrical samples (25mm diameter by 25ram high) that were prepared inthe laboratory in accordance with the relevant Australian Standards. Unconfined uniaxialcompression tests were performed on core samples (25mm diameter by 25mm high) taken from anumber of the ores. The results indicated strength ranging from 60 MPa for the prepared concreteup 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 onthe basis of statistical tests, which indicated that at least 2000g of sample needed to be crushed inorder to estimate the product P80 to within +0.1ram with 95% confidence. The product wascollected and riffled into ten subsamples, and a standard wet/dry sieving method was then used todetermine the product size distribution. For each trial, two of the sub-samples were initially sieved.Additional sub-samples were sieved if there were any significant differences in the resultingproduct size distributions.A number of comminution tests were conducted using the NCRC to determine the effects ofvarious parameters including roll gap, roll force, feed size, and the effect of single andmulti-particle feed. The roll speed was set at maximum and was not varied between trials asprevious experiments had concluded that there was little effect of roll speed on product sizedistribution. It should be noted that the roll gap settings quoted refer to the minimum roll gap. Dueto the non-cylindrical shape of the rolls, the actual roll gap will vary up to 1.7 mm above the minimum setting (ie: a roll gap selling of l mm actually means 1-2.7mm roll gap).