1728_齒輥破碎機(jī),破碎 第 1 頁 翻譯英文原文COMMINUTION IN A NON-CYLINDRICAL ROLL CRUSHER*P. VELLETRI ~ and D.M. WEEDON ~~[ Dept. of Mechanical & Materials Engineering, University of Western Australia, 35 Stirling Hwv,Crawley 6009, Australia. E-mail piero@mech.uwa.edu.au§ Faculty of Engineering and Physical Systems, Central Queensland University, PO Box 1!:;19,Gladstone, Qld. 4680, Australia(Received 3 May 2001; accepted 4 September 2001)ABSTRACTLow reduction ratios and high wear rates are the two characteristics ntost commonh" associated with conventional roll crushers. Because of this, roll crushers are not often considered Jor use in mineral processing circuits, attd many of their advantages are being largely overlooked. This paper describes a novel roll crusher that has been developed ipt order to address these issues.Relbrred to as the NCRC (Non-Cylindrical Roll Crusher), the new crusher incorporates two rolls comprised qf an alternating arrangement of platte attd convex or concave su@wes. These unique roll prqfiles improve the angle qf nip, enabling the NCRC to achieve higher reduction ratios than conventional roll crushers. Tests with a model prototype have indicated thar evell fi)r very hard ores, reduction ratios exceeding lO:l can be attained. In addition, since the comminution process in the NCRC combines the actions of roll arM jaw crushers there is a possibili O' that the new profiles may lead to reduced roll wear rates. ? 2001 Elsevier Science Ltd. All rights reserved.Keywords: Comminution; crushingINTRODUCTIONConventional roll crushers suffer from several disadvantages that have lcd to their lack of popularity 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 of comminution equipment, such ascone crushers. There are, however, some characteristics of roll crushers that are very desirable from a mineral processing point of view. The relatively constant operating gap in a roll crusher gives good control over product size. The use of spring-loaded rolls make these machines tolerant to uncrushable material (such as tramp metal). In addition, roll crushers work by drawing material into the compression region between the rolls and do not rely on gravitational feeci ~like cone and jaw crushers. This generates a continuous crushing cycle, which yields 第 2 頁 high throughput rates and also makes the crusher capable of processing wet and sticky ore. The NCRC is a novel roll crusher that has been dcveloped at the University of Western Australia in ordcr to address some of the problems associated with conventional roll crushers. The new crusher incorporates tworolls comprised of an alternating arrangement of plane and convex or concave surfaccs. Thcse unique roll profiles improve the angle of nip, enabling the NCRC to achieve higher reduction ratios than conventional roll crushers. Preliminary tests with a model prototype have indicated that, even for very hard oics,reduction ratios exceeding 10:I can be attained (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 work extends the existing results bv examining inulti-particle comminution inthe NCRC. It also looks at various othcr factors that influencc the perli~rmance of the NCRC and exploresthe effectiveness of using the NCRC for the processing of mill scats.PRINCIPLE OF OPERATIONThe angle of nip is one of the main lectors effccting the performance of a roll crusher. Smaller nip anglesare beneficial since they increase tl~e likelihood of parlictes bcing grabbed and crushed by lhe rolls. For agiven feed size and roll gap, the nip angle in a conventional rtHl crusher is limited by the size of thc rolls.The NCRC attempts to overcome this limitation through the use of profiled rolls, which improve the angleof nip at various points during one cycle (or revolution) of the rolls. In addition to the nip angle, a numberof other factors including variation m roll gap and mode of commmution were considered when selectingIlle roll profiles. The final shapes of the NCRC rolls are shown in Figure I. One of the rolls consists {sI analternating arrangement of plane and convex surfaces, while the other is formed from an alternatingarrangement of phme and concave surlaccs. 第 3 頁 The shape of the rolls on the NCRC result in several unique characteristics. Tile most important is that, lk)ra given particle size and roll gap, the nip angle generated m the NCRC will not remain constant as the rollsrotate. There will be times when the nip angle is much lower than it would be for the same sized cylindricalrolls and times when it will be much highcr. The actual variation in nip angle over a 60 degree roll rotationis illustrated in Figure 2, which also shows the nip angle generated under similar conditions m a cylindricalroll crusher of comparable size. These nip angles were calculated for a 25ram diameter circular particlebetween roll of approximately 200ram diameter set at a I mm minimum gap. This example can be used toillustrate the potential advantage of using non-cylindrical rolls. In order for a particle to be gripped, thcangle of nip should normally not exceed 25 ° . Thus, the cylindrical roll crusher would never nip thisparticle, since the actual nip angle remains constant at approximately 52 °. The nip angle generated by theNCRC, however, tidls below 25 ° once as the rolls rotate by (~0 degrees. This means that the non-cylindricalrolls have a possibility of nipping the particlc 6 times during one roll rewHution.EXPERIMENTAL PROCEDUREThe 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 verticalcomponcnt of force while enabling horizontal motion. One roll unit is horizontally fixed while the other isrestrained via a compression spring, which allows it to resist a varying degree of horizontal load. 第 4 頁 The pre-load on the movable roll can be adjusted up to a maximum of 20kN. The two motors that drive therolls are electronically synchronised through a variable speed controller, enabling the roll speed to becontinuously varied up to 14 rpm (approximately 0.14 m/s surface speed). The rolls have a centre-to-centredistance ~,at zero gap setting) of I88mm and a width of 100mm. Both drive shafts are instrumented withstrain gauges to enable the roll torque to be measured. Additional sensors are provided to measure thehorizontal force on the stationary roll and the gap between the rolls. Clear glass is fitted to the sides of theNCRC to facilitate viewing of the crushing zonc during operation and also allows the crushing sequence tobc recorded using a high-speed digital camera. 第 5 頁 Tests were performed on several types of rocks including granite, diorite, mineral ore, mill scats andconcrete. The granite and diorite were obtained from separate commercial quarries; the former had beenpre-crushed and sized, while the latter was as-blasted rock. The first of the ore samples was SAG mill feedobtained from Normandy Mining's Golden Grove operations, while the mill scats were obtained fromAurora Gold's Mt Muro mine site in central Kalimantan. The mill scats included metal particles of up to18ram 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 relevantAustralian Standards. Unconfined uniaxial compression tests were performed on core samples (25mmdiameter by 25mm high) taken from a number of the ores. The results indicated strength ranging from 60MPa 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. Theundersized ore was then sampled and sieved to determine the feed size distribution. For each trialapproximately 2500g of sample was crushed in the NCRC. This sample size was chosen on the basis ofstatistical tests, which indicated that at least 2000g of sample needed to be crushed in order to estimate theproduct P80 to within +0.1ram 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.For each trial, two of the sub-samples were initially sieved. Additional sub-samples were sieved if therewere any significant differences in the resulting product size distributions.A number of comminution tests were conducted using the NCRC to determine the effects of various 第 6 頁 parameters including roll gap, roll force, feed size, and the effect of single and multi-particle feed. The rollspeed was set at maximum and was not varied between trials as previous experiments had concluded thatthere was little effect of roll speed on product size distribution. It should be noted that the roll gap settingsquoted refer to the minimum roll gap. Due to the non-cylindrical shape of the rolls, the actual roll gap willvary up to 1.7 mm above the minimum setting (ie: a roll gap selling of l mm actually means 1-2.7mm rollgap).RESULTSFeed materialThe performance of all comminution equipment is dependent on the type of material being crushed. In thisrespect, the NCRC is no different. Softer materials crushed in the NCRC yield a lower P80 than hardermaterials. Figure 4 shows the product size distribution obtained when several different materials werecrushed under similar conditions in the NCRC. It is interesting to note that apart from the prepared concretesamples, the P80 values obtained from the various materials were fairly consistent. These results reflect thedegree of control over product size distribution that can be obtained with the NCRC.Multiple feed particlesPrevious trials with the NCRC were conducted using only single feed particles where there was little or nointeraction between particles. Although very effective, the low throughput rates associated with this modeof comminution makes it unsuitable for practical applications. Therefore it was necessary to determine theeffect that a continuous feed would have to the resulting product size distribution. In these tests, the NCRCwas continuously supplied with feed to maintain a bed of material level with the top of the rolls. Figure 5shows the effect that continuous feed to the NCRC had on the product size distribution for the NormandyOre. These results seem to show a slight increase in P80 with continuous (multi-particle) feed, however theshift is so small as to make it statistically insignificant. Similarly, the product size distributions would seemto indicate a larger proportion of fines for the continuously fed trial, but the actual difference is negligible.Similar trials were also conducted with the granite samples using two different roll gaps, as shown inFigure 6. Once again there was little variation between the single and multi-particle tests. Not surprisingly,the difference was even less significant at the larger roll gap, where the degree of comminution (and henceinteraction between particles) is smaller. 第 7 頁 All of these tests would seem to indicate that continuous feeding has minimal effect on the performance ofthe NCRC. However, it is important to realise that the feed particles used in these trials were spread over avery small size range, as evident by the feed size distribution shown in Figure 6 (the feed particles in theNormandy trials were even more uniform). The unilormity in feed particle size results in a large amount offree space, which allow:s for swelling of the broken ore in the crushing chamber, thereby limiting theamount of interaction between particles. True "choke" feeding of the NCRC with ore having a widedistribution of particle sizes (especially in the smaller size range) is likely to generate much larger pressures 第 8 頁 in the crushing zone. Since the NCRC is not designed to act as a "'high pressure grinding roll" a largernumber of oversize particles would pass between the rolls under these circumstances.Roll gapAs with a traditional roll crusher, the roll gap setting on the NCRC has a direct influence on the productsize distribution and throughput of the crusher. Figure 7 shows the resulting product size distributionobtained when the Aurora Gold ore (mill scats) was crushed at three different roll gaps. Plotting the PSOvalues taken from this graph against the roll gap yields the linear relationship shown in Figure 8. Asexplained previously, the actual roll gap on the NCRC will vary over one revolution. This variationaccounts for the difference between the specified gap setting and product Ps0 obtained from the crushingtrials. Figure 8 also shows the effect of roll gap on throughput of the crusher and gives an indication of thecrushing rates that can be obtained with the laboratory scale model NCRC. 第 9 頁 Roll forceThe NCRC is designed to operate with minimal interaction between particles, such that comminution isprimarily achieved by fracture of particles directly between the rolls. As a consequence, the roll force onlyneeds to bc large enough to overcome the combined compressive strengths of the particles between the roll 第 10 頁 surlaces. If the roll force is not large enough then the ore particles will separate the rolls allowing oversizedparticles to lall through. Increasing the roll force reduces the tendency of the rolls to separate and thereforeprovides better control over product size. However, once a limiting roll force has been reached (which isdependent on the size and type of material being crushed) any further increase in roll force adds nothing tothe performance of the roll crusher. This is demonstrated in Figure 9, which shows that for granite feed of25-3 Imm size, a roll force of approximately 16 to 18 kN is required to control the product size. Using alarger roll force has little effect on the product size, although there is a rapid increase in product P80 if theroll force is reduced bek>w this level.As mentioned previously, the feed size distribution has a significant effect on the pressure generated in thecrushing chamber. Ore that has a finer feed size distribution tends to "choke" the NCRC more, reducing theeffectiveness of the crusher. However, as long as the pressure generated in not excessive the NCRCmaintains a relatively constant operating gap irrespective of the feed size. The product size distributionwill, therefore, also bc independent of the feed size distribution. This is illustrated in Figure 10, whichshows the results of two crushing trials using identical equipment settings but with feed ore havingdifferent size distributions. In this example, the NCRC reduced the courser ore from an Fs0 of 34mm to aPs0 of 3.0mm (reduction ratio of 11:1), while the finer ore was reduced from an Fs0 of 18mm to a Pso of3.4mm (reduction ratio of 5:1). These results suggest that the advantages of using profiled rolls diminish asthe ratio of the feed size to roll size is reduced. In other words, to achieve higher reduction ratios the feedparticles must be large enough to take advantage of the improved nip angles generated in the NCRC. 第 11 頁 Mill scatsSome grinding circuits employ a recycle or pebble crusher (such as a cone crusher) to process materialwhich builds up in a mill and which the mill finds hard to break (mill scats). The mill scats often containworn or broken grinding media, which can find its way into the recycle crusher. A tolerance to uncrushablematerial is therefore a desirable characteristic for a pebble crusher to have. The NCRC seems ideally suitedto such an application, since one of the rolls has the ability to yield allowing the uncrushable material topass through.The product size distributions shown in Figure 1 1 were obtained from the processing of mill scats in theNCRC. Identical equipment settings and feed size distributions were used for both results, however one ofthe trials was conducted using feed ore in which the grinding media had been removed. As expected, theNCRC was able to process the feed ore containing grinding media without incident. However, since oneroll was often moving in order to allow the grinding media to pass, a number of oversized particles wereable to fall through the gap without being broken. Consequently, the product size distribution for this feedore shows a shift towards the larger particle sizes, and the Ps0 value increases from 4ram to 4.7mm. In spiteof this, the NCRC was still able to achieve a reduction ratio of almost 4:1. 第 12 頁 WearAlthough no specific tesls were conducted to determine the wear rates on the rolls of the NCRC, a numberof the crushing trials were recorded using a high-speed video camera in order to try and understand thecomminution mechanism. By observing particles being broken between the rolls it is possible to identifyportions of the rolls which are likely to suffer from high wear and to make some subjective conclusions asto the effect that this wear will have on the perlbrmance of the NCRC. Not surprisingly, the region thatshows up as being the prime candidate for high wcar is the transition between the flat and concave surfaces.What is surprising is that this edge does not play a significant role in generating the improved nip angles.The performance of the NCRC should not be adversely effccted by wear to this edge because it is actuallythe transition between the fiat and convex surfaces (on the opposing roll) that results in the reduced nipangles.The vide() also shows that tor part of each cycle particles are comminuted between the flat surfaces of therolls, in much the same way as they would be in a jaw crusher. This can be clearly seen on the sequence ofimages in Figure 12. The wear on the rolls during this part of the cycle is likely' to be minimal since there islittle or no relative motion between the particles and the surface of the rolls. 第 13 頁 CONCLUSIONSThe results presented have demonstrated some of the factors effecting the comminution of particles in anon-cylindrical roll crusher. The high reduction ratios obtained from early single particle tests can still beachieved with continuous multi-particle feed. However, as with a traditional roll crusher, the NCRC issusceptible to choke feeding and must be starvation fed in order to operate effectively. The type of feedmaterial has little effect on the performance of the NCRC and, although not tested, it is anticipated that themoisture content of the feed ore will also not adversely affect the crusher's per[Brmance. Results from themill scat trials are particularly promising because they demonstrate that the NCRC is able to process orecontaining metal from worn grinding media. The above factors, in combination with the flaky nature of theproduct generated, indicate that the NCRC would make an excellent recycle or pebble crusher. It wouldalso be interesting to determine whether there is any difference in the ball mill energy required to grindproduct obtained from the NCRC compared that obtained from a cone crusher.