TECHNICAL NOTES 5 CRUSHERS - Mineral Tech

1 Copyright R P King 2000 TECHNICAL NOTES 5 CRUSHERS5-1 GapeCSSOSSGapeFigure Schematic diagram of a crushershowingg the open- and closed-side Jaw and Gyratory and gyratory CRUSHERS are used mostly for primary crushing. They are characterized by wide gapeand narrow discharge and are designed to handle large quantities of capacity of the crusher is determined by its size. The gape determines the maximum size of material that can be accepted. Maximum size that canbe accepted into the crusher is approximately 80% of the CRUSHERS are operated to produce a size reduction ratio between 4:1 and 9:1.

2 Gyratory crusherscan produce size reduction ratios over a somewhat larger range of 3:1 to 10 primary operating variable available on acrusher is the set and on jaw and gyratory the open-side set (OSS) is specified. This reflects the fact thatconsiderable portions of the processed material fallthrough the crusher at OSS and this determines thecharacteristics size of the product. The set of acrusher can be varied in the field and some crushersare equipped with automatically controlled actuatedfor the automatic control of the set. The open- andclosed-side sets and the gape are identified in The throw of the crusher is the distance thatmoving jaw moves in going from OSS to CSS.

3 Throw = capacity is a function of size and OSS. Manufacturers publish tables of capacity for theircrushers of various size as a function of the open-side Cone CRUSHERS Cone CRUSHERS are commonly used for secondary, tertiary and quaternary crushing duties. Twovariations are available - standard and short headThe chief difference between cone and gyratory or jaw CRUSHERS is the nearly parallel arrangementof the mantle and the cone at the discharge end in the cone crusher. This is illustrated in Figure ratios in the following ranges are common for cone CRUSHERS :6:1 - 8:1 for secondaries 4:1 - 6.

4 1 for tertiary and quaternary bowlO scillating m antleBroken productFigure Schematic view of the crushingzone of the cone particlesbrokenNippingBroken particles fall throughOpeningFeedFeedFigure The opening and nipping cyclesin the crusher on which the model is size distribution of the products tends to be determined primarily by the CSS since no particlecan fall through during a single open side period and all particles will experience at least one closedside CSS is adjusted by screwing the bowl up Impact crushersBreakage is achieved by impact using either hammeraction on the individual particles or by suddenimpact from a high velocity reduction ratios of between 20:1 and 40:1 canbe achieved with hammer type impact low reduction ratios of about 2:1 can beachieved with kinetic energy type impact Crushing mechanisms and product crushing action of a crushing machine isdescribed most usefully through the classification -breakage cycle model.

5 The operation of a crusher isperiodic with each period consisting of a nippingaction and an opening action. During the opening part of the cycle material moves downward intothe crusher and some material falls through and out. A certain amount of fresh feed is also taken is illustrated in Figure us now describe this behavior quantitatively. Itis best to work with a discrete size distribution. SodefinepiF= fraction of the feed in size class i,pi= fraction of the product in size class i,M= mass of material held in the crusher,bij= fraction of particles breaking in size classj by that end up in size class fraction of material in the crusher in sizeclass ici= c(di)= fraction of material in size class i that isretained for breakage during the next nip of5-3the Mass of total feed that is accepted during a single opening= mass of product class 1 contains the largest particles.

6 Let us follow the fortunes of material in the largest size class starting with an amount Mm1 in thecrusher. During an opening phase of the cycle:Material discharged from the crusher = (1-c1)Mm1 Material positioned for breakage in the breakage zone during next nip = c1Mm1 Accepted from feed = Wp1 FAfter the next nip the crusher must again have an amount Mm1 in the crushing zone since theoperation is at steady state:Mm1 WpF1 c1Mm1b11Mm1W pF11 c1b11( )Product discharged = Wp1 = (1-ci)Mmip1 (1 c1)Mm1W( )Now consider the next size down:During an opening phase of the cycle:Material discharged = (1-c2)Mm2 Material positioned for breakage during next nip = c2Mm2 Accepted from feed = Wp2 FAfter next nip:Mm2 WpF2 c2Mm2b22 c1Mm1b21Mm2W 11 c2b22pF2 c1Mm1Wb21( )Product discharged = Wp2 = (1-c2)Mm2p2 (1 c2)Mm2W( )The next size down can be handled in exactly the same way to give5-4Mm3 WpF3 c3Mm3b33 c2Mm2b32 c1Mm1b31Mm3W 11 c3b33pF3 c2Mm2Wb32 c1Mm1Wb31( )This procedure can be continued from size to generalMmiW 11 cibiipFi Mi 1j 1cjMmjWbij( )The series of equations ( ) can be easily solved recursively for the group Mmi/W starting from sizeclass number 1.

7 The size distribution in the product can then be calculated frompi (1 ci)MmiW 1 ci1 cibiipFi Mi 1j 1cjMmjWbij( )And the distribution of sizes in the product is completely determined from the size distribution inthe feed and a knowledge of the classification and breakage functions. The classification function is usually of the form shown in Figure d1 and d2 are parameters that are characteristic of the crusher. They are determined primarily by thesetting of the crusher. Data from operating crusher machines indicate that both d1 and d2 areproportional to the closed side setting.

8 D1 is the smallest size particle that can be retained in thecrushing zone during the opening phase of the cycle. d2 is the largest particle that can fall throughthe crushing zone during the opening phase of the useful form of the classification function is ci 1 dpi d2d1 d2nford1<dpi<d2 0fordpi d1 1fordpi d2( ) size dpClassification function c(dp)d1d2 Figure A typical internal classification function for a crusherFigure The breakage function for crushingmachines. This function has a value 1 at therepresentative size of the parent class. Compare this with the breakage function usedfor grinding both standard and short-head Symons cone CRUSHERS ,d1.

9 1 CSS( )d2 .2 CSS d ( ).1 varies from about to and .2 varies from about to n is usually approximately 2 butcan be as low as 1 and as high as 3. Higher values of n usually require higher values of .2. d* isusually set to functions of the typeB(x;y) Kxyn1 (1 K)xyn2( )are normally used to describe crusher values of bij can be obtained from thecumulative breakage function bybij B(Di 1;dpj) B(Di;dpj)( )andbjj 1 B(Dj;dpj)( )represents the fraction of material that remains in5-6size interval j after breakage. These relationships are illustrated in figure n1 is for both standard and short-head CRUSHERS and n2 is approximately for short-head and forstandard CRUSHERS .

10 The parameters in the classification and breakage functions are obviously specific to the type andsize of crusher. Unfortunately not many studies have been done to establish their values under arange of actual operating conditions. In practice it is often necessary to estimate them from measuredparticle distributions in the products from operating CRUSHERS . Once established for a particularmaterial in a particular crusher, they should be independent of the closed side set. This allows thecrusher performance to be simulated at the various based on:1. Whiten Walter and White A breakage function suitable for crusher models.