COMMINUTION_IN_A_NON-CYLINDRICAL_ROLL_CRUSHER1.pdf

Pergamon 0892 6875 01 00159 5 MineraL En ineeril Vul 14 No 11 pp 1459 1468 2001 2001 I Isc ier Science l td All rights reserved 2 N75 11 see lront matter COMMINUTION IN A NON CYLINDRICAL ROLL CRUSHER P VELLETRI and D M WEEDON Dept of Mechanical 19 Gladstone Qld 4680 Australia Received 3 May 2001 accepted 4 September 2001 ABSTRACT Low 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 crushing INTRODUCTION Conventional 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 as cone 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 high throughput rates and also makes the crusher capable of processing wet and sticky ore Presented at Comminution 01 Brisbane Australia March 2001 1459 1400 P Vcllclri and l M Wecdon The NCRC is a novel roll crusher that has been dcveloped at the University of Western Australia in ordcr t address some of the problems associated with conventional roll crushers The new crusher incorporates two rolls 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 in the NCRC It also looks at various othcr factors that influencc the perli rmance of the NCRC and explores the effectiveness of using the NCRC for the processing of mill scats PRINCIPLE OF OPERATION The angle of nip is one of the main lectors effccting the performance of a roll crusher Smaller nip angles are beneficial since they increase tl e likelihood of parlictes bcing grabbed and crushed by lhe rolls For a given 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 angle of nip at various points during one cycle or revolution of the rolls In addition to the nip angle a number of other factors including variation m roll gap and mode of commmution were considered when selecting Ille roll profiles The final shapes of the NCRC rolls are shown in Figure I One of the rolls consists sI an alternating arrangement of plane and convex surfaces while the other is formed from an alternating arrangement of phme and concave surlaccs C Fig 1 Profiles of the non cylindrical rolls 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 CommimJlilm in a non cylindrical roll crusher 1461 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 100 90 80 70 o 50 z 4o a 30 20 10 Nip Angle 10 2 I 30 40 50 60 Roll Angle Degrees Fig 2 Variation in Nip Angle for the NCRC and a conventional cylindrical roll crusher 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 Product Fig 3 Side view of the laboratory scale prototype NCRC 1462 P Velletri and D M Weedon 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 1ram with 95 confidence The product was collected and riffled into ten sub samples 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 there were 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 parameters including roll gap roll force feed size and the effect of single and multi particle feed The roll speed was set at maximum and was not varied between trials as previous experiments had concluded that there was little effect of roll speed on product size distribution It should be noted that the roll gap settings quoted refer to the minimum roll gap Due to 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 RESULTS Feed material The performance of all comminution equipment is dependent on the type of material being crushed In this respect the NCRC is no different Softer materials crushed in the NCRC yield a lower P80 than harder materials Figure 4 shows the product size distribution obtained when several different materials were crushed under similar conditions in the NCRC It is interesting to note that apart from the prepared concrete samples the P80 values obtained from the various materials were fairly consistent These results reflect the degree of control over product size distribution that can be obtained with the NCRC Multiple feed particles Previous trials with the NCRC were conducted using only single feed particles where there was little or no interaction between particles Although very effective the low throughput rates associated with this mode of comminution makes it unsuitable for practical applications Therefore it was necessary to determine the effect that a continuous feed would have to the resulting product size distribution In these tests the NCRC was continuously supplied with feed to maintain a bed of material level with the top of the rolls Figure 5 shows the effect that continuous feed to the NCRC had on the product size distribution for the Normandy Ore These results seem to show a slight increase in P80 with continuous multi particle feed however the shift is so small as to make it statistically insignificant Similarly the product size distributions would seem to 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 in Figure 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 hence interaction between particles is smaller Comminution in a non cylindrical roll crusher 1463 1 0 0 0 1 10 1 n 0 001 I ar yOrel I I I iiill Ia l Speed 14 rpm I 0 1 I 10 Size mm Fig 4 Product size distribution for different rock samples crushed using the same equipment set up 100 00 pL 13 10 00 D 0 Single Fe Pa 4 i f b t l l t i p l e F Feed Range 19 25ram 1 00 0 01 0 1 I 10 Size mm Fi e S Product size distribution for sinele and multiple feed particles All of these tests would seem to indicate that continuous feeding has minimal effect on the performance of the NCRC However it is important to realise that the feed particles used in these trials were spread over a very small size range as evident by the feed size distribution shown in Figure 6 the feed particles in the Normandy trials were even more uniform The unilormity in feed particle size results in a large amount of free space which allow s for swelling of the broken ore in the crushing chamber thereby limiting the amount of interaction between particles True choke feeding of the NCRC with ore having a wide distribution of particle sizes especially in the smaller size range is likely to generate much larger pressures in the crushing zone Since the NCRC is not designed to act as a high pressure grinding roll a larger number of oversize particles would pass between the rolls under these circumstances 1464 100 g 13 E o p D 10 o 5 E C 1 0 01 P Velletri and D M Weedon B 0 1 1 10 Size mm lOO Fig 6 Product size distribution for single and multiple feed particles at different gap settings Roll gap As with a traditional roll crusher the roll gap setting on the NCRC has a direct influence on the product size distribution and throughput of the crusher Figure 7 shows the resulting product size distribution obtained when the Aurora Gold ore mill scats was crushed at three different roll gaps Plotting the PSO values taken from this graph against the roll gap yields the linear relationship shown in Figure 8 As explained previously the actual roll gap on the NCRC will vary over one revolution This variation accounts for the difference between the specified gap setting and product Ps0 obtained from the crushing trials Figure 8 also shows the effect of roll gap on throughput of the crusher and gives an indication of the crushing rates that can be obtained with the laboratory scale model NCRC 100 n n 0 10 0 1 1 10 100 Size ram Fig 7 Effect of roll gap setting on product size distribution Comminution in a non cylindrical roll crusher 1465 6 gs g O 3 8 o 1 0 5 b El Product Size I I A Throughput 1 1 5 2 2 5 3 3 5 4 4 5 Roll Gap mm 0 7 0 6 0 5 E 0 4 0 3 P 0 2 0 1 Fig 8 Relationship between roll gap setting and the crusher product size and throughput Roll force The NCRC is designed to operate with minimal interaction between particles such that comminution is primarily achieved by fracture of particles directly between the rolls As a consequence the roll force only needs to bc large enough to overcome the combined compressive strengths of the particles between the roll surlaces If the roll force is not large enough then the ore particles will separate the rolls allowing oversized particles to lall through Increasing the roll force reduces the tendency of the rolls to separate and therefore provides better control over product size However once a limiting roll force has been reached which is dependent on the size and type of material being crushed any further increase in roll force adds nothing to the performance of the roll crusher This is demonstrated in Figure 9 which shows that for granite feed of 25 3 Imm size a roll force of approximately 16 to 18 kN is required to control the product size Using a larger roll force has little effect on the product size although there is a rapid increase in product P80 if the roll force is reduced bek w this level 3 2 7 3 0 v 2 8 o ao 3 2 6 i D 2 4 Q 2 2 2 0 Rock Type Granite I Feed Size 25mm 31 5mm I Roll Gap 1ram Rol Speed 14 rpm 10 12 14 Roll Force kN 16 18 Fig 9 Effect of roll force on product size Feed size distribution 100 o Q lO As mentioned previously the feed size distribution has a significant effect on the pressure generated in the crushing chamber Ore that has a finer feed size distribution tends to choke the NCRC more reducing the effectiveness of the crusher However as long as the pressure generated in not excessive the NCRC maintains a relatively constant operating gap irrespective of the feed size The product size distribution will therefore also bc independent of the feed size distribution This is illustrated in Figure 10 which shows the results of two crushing trials using identical equipment settings but with feed ore having different size distributions In this example the NCRC reduced the courser ore from an Fs0 of 34mm to a Ps0 of 3 0mm reduction ratio of 11 1 while the finer ore was reduced from an Fs0 of 18mm to a Pso of 3 4mm reduction ratio of 5 1 These results suggest that the advantages of using profiled rolls diminish as the ratio of the feed size to roll size is reduced In other words to achieve higher reduction ratios the feed particles must be large enough to take advantage of the improved nip angles generated in the NCRC 1466 P Vclletri and D M Weedon 0 1 1 10 100 Size ram Fig 10 Effect of feed size distribution Mill scats Some grinding circuits employ a recycle or pebble crusher such as a cone crusher to process material which builds up in a mill and which the mill finds hard to break mill scats The mill scats often contain worn or broken grinding media which can find its way into the recycle crusher A tolerance to uncrushable material is therefore a desirable characteristic for a pebble crusher to have The NCRC seems ideally suited to such an application since one of the rolls has the ability to yield allowing the uncrushable material to pass through The product size distributions shown in Figure 1 1 were obtained from the processing of mill scats in the NCRC Identical equipment settings and feed size distributions were used for both results however one of the trials was conducted using feed ore in which the grinding media had been removed As expected the NCRC was able to process the feed ore containing grinding media without incident However since one roll was often moving in order to allow the grinding media to pass a number of oversized particles were able to fall through the gap without being broken Consequently the product size distribution for this feed o