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Minerals Engineering
Volume 14, Issue 11, November 2001, Pages 1459–1468
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)
ABSTRACT
Low reduction ratios and high wear rates are the two characteristics most commonly associated with conventional roll crushers. Because of this, roll crushers are not often considered for use in mineral processing circuits, and many of their advantages are being largely overlooked. This paper describes a novel roll crusher that has been developed in order to address these issues. Referred to as the NCRC (Non-Cylindrical Roll Crusher), the new crusher incorporates two rolls comprised of an alternating arrangement of plane and convex or concave surfaces. These unique roll profiles improve the angle of nip, enabling the NCRC to achieve higher reduction ratios than conventional roll crushers. Tests with a model prototype have indicated that, even for very hard ores, reduction ratios exceeding 10:1 can be attained. In addition, since the comminution process in the NCRC combines the actions of roll and jaw crushers there is a possibility that the new profiles may lead to reduced roll wear rates.
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. 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 two rolls comprised of an alternating arrangement of plane and convex or concave www.docin.com
設計巴巴工作室www.88doc88.com 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 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.
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 www.docin.com
設計巴巴工作室www.88doc88.com
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. www.docin.com
設計巴巴工作室www.88doc88.com 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 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 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 www.docin.com
設計巴巴工作室www.88doc88.com
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. www.docin.com
設計巴巴工作室www.88doc88.com 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. www.docin.com
設計巴巴工作室www.88doc88.com 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 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. www.docin.com
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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 www.docin.com
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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. 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. www.docin.com
設計巴巴工作室www.88doc88.com 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
ore shows a shift towards the larger particle sizes, and the Ps0 value increases from 4ram to 4.7mm. In spite
of this, the NCRC was still able to achieve a reduction ratio of almost 4:1. www.docin.com
設計巴巴工作室www.88doc88.com Wear Although no specific tesls were conducted to determine the wear rates on the rolls of the NCRC, a number
of the crushing trials were recorded using a high-speed video camera in order to try and understand the
comminution mechanism. By observing particles being broken between the rolls it is possible to identify
portions of the rolls which are likely to suffer from high wear and to make some subjective conclusions as
to the effect that this wear will have on the perlbrmance of the NCRC. Not surprisingly, the region that
shows 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 actually
the transition between the fiat and convex surfaces (on the opposing roll) that results in the reduced nip
angles.
The vide() also shows that tor part of each cycle particles are comminuted between the flat surfaces of the
rolls, in much the same way as they would be in a jaw crusher. This can be clearly seen on the sequence of
images in Figure 12. The wear on the rolls during this part of the cycle is likely' to be minimal since there is
little or no relative motion between the particles and the surface of the rolls. www.docin.com
設計巴巴工作室www.88doc88.com CONCLUSIONS
The results presented have demonstrated some of the factors effecting the comminution of particles in a
non-cylindrical roll crusher. The high reduction ratios obtained from early single particle tests can still be
achieved with continuous multi-particle feed. However, as with a traditional roll crusher, the NCRC is
susceptible to choke feeding and must be starvation fed in order to operate effectively. The type of feed
material has little effect on the performance of the NCRC and, although not tested, it is anticipated that the
moisture content of the feed ore will also not adversely affect the crusher's per[Brmance. Results from the
mill scat trials are particularly promising because they demonstrate that the NCRC is able to process ore
containing metal from worn grinding media. The above factors, in combination with the flaky nature of the
product generated, indicate that the NCRC would make an exc