Everything you need to know about Crushers in Cement industry

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CRUSHERS

SUMMARY

The roller crushers are used for moist and sticky materials. Generally the machines have to be rigid enough to crush also hard inclusions. As the reduction ratio is only about 5:1 a 2-stage crushing is required in most applications.

Fast running type crushers like hammer and impact crushers are the simplest and cheapest solution, whenever the raw material characteristics allow to applying them.

The choice between stationary or mobile primary crushing plant depends mainly on the quarry layout and the development of the faces.  Usually mobile crushing plants only are feasible if a higher number of trucks can be eliminated. Each case has to be investigated separately. Because of the different traveling mechanisms the walking pads are the most frequently used and also involve the lowest investment costs.

INTRODUCTION

Materials Preparation Technology

The purpose of the preparatory processing of the raw materials is to convert theses chemically and mineralogical different materials, usually supplied to the plant in coarse lumps, into raw meal of homogeneous composition. This has to be accomplished with suitably chosen machinery and methods, and lowest possible cost, in order this to fulfill the basic conditions for an economical burning process.

Primary Reduction

Reduction the raw materials to a fine powder – conventionally called “ meal ” – is necessary in order to produce a homogeneous mixture which will quickly be converted in the kiln into a homogeneous clinker containing no free lime  . As a rule, size reduction (comminution) raw materials are effected in at least two main stages: crushing (primary reduction) and grinding (fine reduction).

Generally speaking, crushing denotes the size suitable as feed for the next main stage, i.e, grinding.

In present – day cement manufacture, with due regard to the possibilities of the reduction machinery employed, crushing is taken to mean reducing the particle size between about 80 and 20 mm. This crushed product is further reduced by grinding to fineness below about 0.2 mm size, in which condition it is called raw meal and is ready for feeding to the kiln.

The capacity of the crusher, reduction ratio, hardness, fragment size, moisture content, plasticity and abrasiveness of the material are important factors affecting the choice of the size reduction machines and methods for dealing with it.

The stickiness characteristics of the material fed to the crusher are the most difficult to evaluate, prevent and remedy as they relate to several physical characteristics such as the structure of the material and its content of water.

Water content below 10-12% seldom causes problems of stickiness in the types of crushers used within the cement industry.

Hard materials causing severe abrasive wear are reduced with slow running machines such as jaw crushers and gyratory crushers, which function by developing mainly a compressive action. For medium – hard to hard materials impact crushers and hammer crushers are more suitable; they achieve size reduction mainly by impact.

In average the energy consumption for crushing is 2.5 kWh /ton.

CRUSHERS TYPES

• ¨ Hammer crushers.
• ¨ Impact crushers.
• ¨ Jaw crushers.
• ¨ Gyratory crushers (cone crusher).
• ¨ Roller crushers.

Hammer Crushers

The hammer crusher is the most widely used machine for the primary reduction of the cement industry.

The main feature is the rotor, which carries a series of pivoted hammers. When the rotor is running, the centrifugal forces cause the hammers to point radially outwards. In the upper crushing chamber the feed material is subjected to a combination of impact and percussive action by the hammers and by repeated collision with the breaker plate, together with “autogenous” action by fragments of rock colliding with one another. The finer reduction is accomplished in the gap between the hammers and the breaker plate in the single – rotor hammer crusher.

The width of the product outlets between the bottom grid bars determines the maximum product particle size. As a rule the process engineering requirement of obtaining the finest possible mill feed in the single crushing pass is fulfilled by the hammer crushers.

Hammer crushers are built as single – rotor and twin – rotor machines. The rotor may consists of a series of discs mounted on a square shaft or may alternatively take the form of rollers. If hammers with forged – on individual pivot stubs are employed, the rotor discs must be axially movable for changing the hammers when they have become worn.

The forged or cast hammers range is from 30kg to 200 kg in weight, according to the size of the crusher. The discharge grids enclose the rotors through an angle of between 120° and 180° . The forged grid bars are of triangular or trapezoidal cross-section.

The grid openings of primary crushers operating as single -stage machines which supply feed for tube mills are usually 25 mm. However widths of 40-50 mm are employed in crushers which are fed with raw materials containing plastic components and above 6-8% moisture, the greater width being necessary to avoid choking of the grids.

Single – rotor hammer crushers are built for throughputs to up to about 2000t/hour. For example, a well known machine of this capacity has a rotor of 3300 mm width and a hammer circle diameter of 33500mm, equipped with 112 hammers weighing 150kg each.

The circumferential velocity of the rotors is between about 28 and 33m / second.

Single-Shaft Hammer Crusher with Grates

1 Hammers                            5 Lower Casing

2 Rotor                                   6 Grates

3 Feed Plate                           7 Feed

4 Upper Casing                     8 Product

Hammer crusher with feed roller

The hammer crusher with feed rollers (F.L.S) is a special form of construction to avoid complicated, multi-stage crushing, a hammer has been developed, which does not require preceding primary crushing.

This crusher reduces rock lumps from 2 m size down 25 mm. The crusher uses a single hammer rotor with peripheral speed of about 40 meters per second.

No inlet grating is necessary as the crusher is fitted with rollers to absorb the shock of the large rocks in the feed. The crusher is suited to handle materials containing a certain proportion of the sticky material.

Rocks are conveyed to the crusher by separate feeding equipment which distributes the material across the whole width of the crusher inlet. Upon enerting the crusher, the rocks drop on to two shocks – absorbing rollers, one of which is smooth while the other has projections. They run at different speeds to prevent the rocks from wedging. Some of the fines run through the gap between the rollers and are separated from the main input at this stage.

The rollers feed the rocks to the hammers where they are broken up and hurled against the lining plates in the upper part of the crusher for further fragmentation.

The heavy, vertical chain curtain at the crusher inlet prevents the material from being thrown back to the feeder.

Hammer crushers highlights

Function                    Impact and percussive action

Action                        Continuos

Reduction ratio         50:1

Capacity                    2000 T/H for the double rotor

Application               Primary reduction of medium hard to hard material

Hammer crusher with feed roller (F.L.S)

• feeder
• chain curtain
• fed rollers
• hammer rotor
• adjustable impact wall

Auxiliary equipment

With the evolution crushers to large and larger throughput ratings the dimensions and weights of their wearing parts are correspondingly increasing. Removal and renewal of worn parts without the aid of suitable lifting appliances is awkward and time -consuming. Several of suppliers have developed special auxiliary equipment to facilitate the operations of changing the wearing parts of their machines and this substantially reduce the repair downtime periods.

Thus, hydraulic pull – out systems for extracting the breaker plates or bars are provided. Furthermore, hydraulic rams mounted on the crusher casing enable the breaker wall and certain parts of the casing to be swung open on impact crushers and hammer crushers, without the aid of other auxiliary devices. Also various solutions for changing the bottom discharge grids of hammer crushers have been devised.

Design features on the upper part of the machine enable sections of casing which are situated beside the rotor shaft to be removed, without having to dismantle the upper casing, for taking out and refitting the rotors.

Auxiliary equipment for changing worn parts (O&K Mammut crusher), a hammer lifting device, b hammer spindle extracting device, C discharge grid extracting device

Particle Size distribution for hammer crusher product for quarry feed

IMPACT CRUSHERS

Single -rotor impact crusher

The impact crusher is best suited for dealing with brittle hard to medium -hard material with natural cleavage planes. It cannot cope very well with soft plastic and moist material.

In the impact crusher the feed material entering the crushing chamber encounters the impactor bars immovably mounted on the rotor and revolving with it at a circumferential velocity of 30-45m/second. The fragments are flung against the upper breaker plate, rebound into the crushing chamber, are again subjected to the action of the impactor bars, and so until they have been sufficiently reduced to pass through the upper gap onto the space between the two breaker plates.  Here the process is repeated until the material is fine enough to pass through the second gap. Besides the impact of the rock fragments with the bars and plates there is also an “autogenous” reduction effect due to the rock fragments colliding with one another.

Depending on the hardness and size of the feed, coarse impact crushers reduce the material to a product size 150 and 200 mm and attain reduction ratios material to a product size between 6:1 and 20:1. The circumferential velocity of the rotor is a major factor : low velocity results in a coarse product ; with higher velocity the size reduction energy is greater and the material is broken up into correspondingly smaller fragments, but the rate of wear on the bars and plates is of course higher ( it increases proportionally to the square of the velocity ) . The optimum size range of 0-25mm cannot be achieved in a single pass through the coarse impact crusher. A more efficient method is to use a secondary crushing stage, e.g. in the form of a second impact crusher operating with higher rotor circumferentialy velocity.

Compound impact crusher

In the compound impact crusher the two stages – primary and secondary – are combined in a single machine this is a dual – rotor crusher in which the primary rotor runs at 35 m/ second and the secondary rotor (mounted below and to one side of the primary) runs at about 45m/ second circumferential velocity. The maximum product particle size is determined by the bottom gap formed by an additional ridged comminuting anvil plate. Compound crushers can accept feed lumps up about 1.5 m size reducing it to a product in which 95% is smaller than 25mm, corresponding to a reduction ratio of 60:1 achieved in a single pass. The upper rotor is fitted with fixed impactor bars, while the lower rotor has impactor bars or movably mounted hammers, depending on the nature of the feed material and the required product fineness.

Impact crusher highlights:

Function                    impact surface

Action                        continuos

Reduction ratio         single crusher 6:1 and 20:1

Compound crusher 60:1

Capacity                    1500T/h

Application               Suited for brittle hard and medium hard material,

The single rotor used as primary crusher only.

Jaw crushers

Jaw crushers are used for the primary reduction of very hard and abrasive admixtures for cement manufacture.

The reciprocating motion of the crushing jaw of the double – toggle (or Black type) jaw crusher subjects the material to a mainly compressive action. This machine is especially suitable for crushing very hard material fed in coarse lumps.

In the single-toggle jaw crusher the jaw moves not only backwards and forwards but also up and down, so that there is attrition as well as compressive crushing action. Crushers of this type are more suitable for the reduction of hard to medium hard material fed in smaller lumps.

Jaw crushers are sensitive to moist and plastic feed material and tend to choke if there is  a substantial proportion of fine particles in the feed. The attainable reduction ratio is between about 6:1 and 8:1. For obtaining a product of favorable size for feeding to grinding mills it is generally necessary to apply secondary crushing in another type crusher.

The particle size distribution of the jaw crusher product is considerably affected by the loading of the machine; a crusher operating substantially below capacity will yield coarse product with high proportion of oversize.

Jaw crusher highlights

Function                    Compressive action

Action                        Intermittent

Reduction ratio         6:1 to 8:1

Capacity                    Low capacity » 300 T/H

Application               Primary crusher for very hard abrasive materials of low moisture

Content, single toggle crusher is better for inclusion of stick

Materials but, higher wear than the double toggle.

Gyratory Crushers (Cone crusher)

The gyratory crusher uses mainly medium – hard to hard and not very abrasive limestone. Its size reduction is achieved mainly by compressive action between the fixed conical bowl and the oscillating cone-shaped crushing head, which functions somewhat like a pestle in a mortar. The lower end of the shaft carrying the crushing head is mounted in an eccentric which perform a horizontal rotary motion, which the upper end is mounted in a fixed ball – and socket type bearing. As in the jaw crusher, the width of the crushing gap continually varies between a maximum and a minimum.

Raising or lowering the crushing head, an adjustment that takes only a few minutes to perform and is effect, can alter the width of the gap mechanically or hydraulically.

The ratio between the radial width A of the feed opening and the maximum discharge gap width C in large primary crushers is between 6:1 and 9:1 which corresponds to the attainable reduction ratio for predominantly cubic material. If the machine is fed with material of a more irregular shape, this ratio referred to the precise may be as high as 12:1 to 15:1.

The largest gyratory crushers in current use attain throughputs of over 6000t/hour and have feed openings 1500mm x 4400mm in size (A x B), while the discharge graps range in width from 150 to 250mm.

A jaw crusher designed for a certain throughput rate can accept larger pieces of rock than the normal gyratory crusher. In order to cope with equally large – sized feed, the gyratory crusher has to be over – designed in terms of capacity.

A general advantage of the gyratory crusher is that it is unaffected by overloading. It requires no special feeding device; the fragment rock coming from the quarry or stockpile in heavy trucks can be tipped straight into the feed opening. Uniform size distribution in the crushed product can be, however, obtained only if a controlled rate of feed is maintained.

Like the jaw crusher, the gyratory crusher is sensitive to moist and plastic feed material and it tends to choke if the material has a high fines content

Gyratory crusher highlights

Function                    compressive action

Action                        intermittent

Reduction                  12:1 to 15:1

Capacity                    6000 T/H

Application               Primary crusher for very hard abrasive material of low moisture

Roll Crushers

Roll crushers are used for the primary reduction of medium -hard moist and abrasive materials such as marl, shale and clay. The feed is subjected to compressive and shearing action between a pair of counter – rotating rolls, which may be either smooth or corrugated or provided with tooth – like projections. The teeth give better bite to the feed and concentrate the action of the crushing force, enabling large and compact pieces of rock to be split.

For primary reduction the width of t the rolls is approximately equal to their diameter, the ratio of these dimensions usually being within the range of 0.8 to 1.2. The attainable size reduction ratio is fairly low only from about 3:1 to 5:1. Circumferential velocities of the rolls are 5-9m/ second.

Double -roll ( or twin – roll crushers with 1800mm roll diameter and approximately equal effective roll width attain throughputs of 1000-1200t/hour for a gap width of about 250 mm between the rolls and can accept feed material up to 1000 mm in size .

In some machines the two bearings of one crushing roll are fixed to the frame of the crusher, while those of the other roll are mounted on slide rails.

As a rule, the rolls are driven separately, each through a V-belt. The specific power consumption is in the range from 0.2 to 0.3kWh/t.

Cutting Rollers

The cutting, designed for soft chalk and clay, is fitted with toothed rollers discs for tearing large blocks of material before they are crushed in a subsequent process.

The two rotors formed by the teethed roller discs rotate against each other with different speeds. The rotor with forward pointing teeth rotates faster, while the rotor with backward pointing teeth works more slowly and retains the material. The discs operate in such a way that the teeth of one disc pass the space between two discs on the other rotor. The machine is mostly used for soft and sticky, stone -free materials in order to prepare these for further to the next process step.

Empirical and comminution ratios

Main factors in crusher performance

Crusher type                                                 Main factors

Jaw                                                     Jaw width.

Shaft r.p.m

Cone                                                   Discharge slot diameter

Oscillation amplitude

r.p.m

Rollers                                               Roller length

Roller diameter

Roller r.p.m

Separation between rollers

Hammers                                           Hammers diameter vs. Crusher internal diameter

Rotor length

r.p.m

Impacts                                              R.P.M.

Diameter

Separation between breaker plates.

Particle size and shapes comparison according to the crusher type

Crusher type                                     Particle size and shape

Jaw                                                     Few fines

Tendency to agglomerate the particles

Cones                                                 Few fines

Particles with cubical shape

Hammers                                           Many fines

Particles with cubical shape

Impacts                                              Many fines

Particles with cubical shape

Compound                                         Great quantify of fines

Size regulation by adjust the second rotor speed.

Rollers                                               Suitable for wet material

Crushing Plants

Stationary crushing plants

In the cement industry, which uses chiefly medium-hard to hard limestone as its principal raw materials, single-stage crushing plants equipped with hammer crushers are the commonly preferred type.

Feed Hoppers

The feed hopper, feeding equipment, crusher and product removal conveyors are the main component units of the plant. The feed hopper should have a capacity equal to at least twice that of the largest vehicles supplying rock to the crusher in batches of 20 to 50 tons or more.

Caking of moist and sticky feed material can be minimized by using a well designed hopper, with rounded transitions from the end walls to the side walls. If the hopper is of concrete, it should be lined with steel plates or, preferably, with steel rails, which give much better protection against wear.

Robustly constructed apron conveyors have proved most suitable for feeding. They fulfill all requirements applicable to a feeding system in order to obtain optimum utilization of the crusher: control of the handling rate within a certain range, controllability in response to the loading condition of the crusher, feed over the full working width of the crusher, ability to start under load, feed can be stopped instantly.

The hopper can be equipped with a hydraulically operated arm with tools for stone splitting, a grab or a scraping arrangement as appropriate, to match the materials it has to handle.

Feeders

The feeder supplies the crusher with an even flow of material from the feed hopper for optimum utilization of the crusher, irrespective of irregularities in the supply of material to the feed hopper.

In some cases, cone crusher and jaw crushers can work without feeders if they are placed directly under the feed hopper. Most other types of crushers need a feeder for securing a regular rate of feed.

Apron Feeders

The apron feeder is widely used for coarse materials, i.e. primary crushing of rocky materials, an inclined apron feeder with 20° inclination is normally used, as this will automatically assure a suitable, stable layer on the apron.

The apron feeder is furnished with driving station with variable speed.

Gyratory crushers, which are used as first-stage machines when very hard and coarse feed material has to be reduced, can receive the material direct from the truck, without the interposition of a feed hopper. As the preliminary crusher delivers a product in the 300 – 500 mm size range, the second-stage crusher can function under less severe operating conditions than if the size reduction had to be performed all in one stage.

While gyratory crushers can often be employed also as second-stage machines, high-speed machines such as impact crushers or hammer crushers are more advantageous for obtaining a finer product.

Preliminary Screening

Separation of the finer particles from the raw stone before it is fed to the crusher is not standard practice in the cement industry. In exceptional cases, however, the material may first be passed over a grizzly or a reciprocating grid screen. Preliminary separation of the coarser from the finer material can serve to relieve the crusher or to improve the quality of the raw material by raising the concentration of certain desirable constituents. As a rule, it makes for better performance of the crusher, too.

The preliminary screening devices used before primary crushers are various types of stationary grizzlies or moving grid-type screens (with bars or with rollers, either round or elliptical), reciprocating separators, vibrating grates or heavy eccentric-weight-driven shaking screens. A relatively recent development is the Mogensen sizer, which is especially suitable for the separation of moist fine material that is difficult to remove by screening from the crusher feed.

Protection against foreign bodies

In the quarrying and loading of raw materials it inevitably occurs that metallic foreign bodies – excavator bucket teeth, broken drill rods, drill bits, pieces of rail, chains, etc. – turn up in the feed material supplied to the primary crusher. If the crusher is fed direct by excavators or dump trucks.

As a rule, foreign bodies can be more effectively removed from the material after it has been pre-crushed (first-stage crushing). Magnetic separators and metal detectors are used for the purpose and help more particularly to protect the high-speed second-stage crushers.

For dealing with material containing a substantial amount of tramp iron, belt-type suspended magnetic separators may be used. A device of this kind is equipped with a continuous rubber belt which carries the pieces of iron out of the magnetic field, so that the magnet pole face itself remains clear.

Favourable mounting positions are the points of feed onto, or discharge from, the conveyor, because at these points the material is loosened up and the extraction of tramp iron thus made easier.

Metal detectors

Metal detectors are used for revealing the presence of tramp metal which is not magnetically responsive. The equipment generally comprises one or two detecting coils installed over and / or under the belt conveyor or enclosing it.

The presence of a piece of metal in the otherwise constant magnetic field of a coil causes an electric pulse which can be utilized for switching off the conveyor or causing a certain length of the layer of material on the belt containing the metal, to be diverted from the main conveying path.  Obviously, there should be no moving metal parts in the vicinity of the detecting coil. Static metal parts do not disturb the detection, but are liable to weaken its sensitivity.

Hygroscopic materials which, when moist, become electrically conductive may cause false alarms due to variations in moisture content (and therefore in conductivity) on passing the metal detector. The most reliable protection against tramp metal is provided by the combination: metal detector– magnetic drum – metal detector. In this arrangement the first metal detector operates the switch-on/switch-off of the drum separator. Whose magnetic field therefore is activated only when metal is detected on the conveyor. Any non-magnetic metal that passes the drum will produce a response from tile second metal detector.

Heating of Crusher

The only heating systems applied to primary crushers are intended not for drying the material, but preventing the caking of moist sticky materials which might otherwise cause clogging.

Heating the bottom plate of the feed chute, the side walls and the breaker plates in impact crusher to surface temperature of 180° 200° C is done with the aid of heat transfer circulated at approximately 300°C through a system of pipes
Heat input ranges are in the region of 20000 kcal per hour and per square meter of heated surface.

Determining the crusher capacity

The nominal capacity or rating of the crusher will be governed by the required raw material throughput and the possible working time of the crushing plant. The quarrying operations, of which the crushing plant usually forms part, are in most cases conducted on a single – shift basis with five or six working days per week. For an 8 hour shift the effective crushing time per shift can be put at 7 or at most 7.5 hours. The crusher should therefore, in an effective time of 35 to 45 hours, be able to produce sufficient raw material to feed the kiln plant for a whole (7 – day) week.

The requisite crusher throughput capacity can be calculated from the following formula

If the crusher is designed for single – shift working, it will have sufficient capacity even if the kiln plant capacity is subsequently doubled: in that case the quarry and crusher will have to work double shifts, leaving the week ends available for repairs and maintenance.

SELECTION CRITERIA

General

Although crushers themselves are relatively old machines, the same basic problems are continually encountered when selecting a crushing system. Firstly, the geological information on the raw material to be processed is frequently inadequate and secondly there are too few test procedures to measure and assess those raw material properties which are important for crushing and grinding. The selection of the preparation machines becomes more difficult if one has not experience with the material in quest and has only data from core samples to go by. The detailed prospecting of all raw material components is therefore, of great importance also for the selection of the equipment. In spite of all, the uncertainties when selecting a crushing concept for known material, usually a poor choice for a crusher can be attributed to unpredictable changes of the chemical composition and the physical properties of the material to be processed.

When selecting a crusher type or a crushing concept, there are usually many arguments both for and against any particular concept. A lot of the points brought up seem to be without substance and it is questionable whether the selection of a crusher is actually as complicated as it is made out to be. In fact, uncertainties arise because many of these unfounded arguments cannot be refuted by clear selection criteria and also because the properties of the raw material to be crushed are not known in detail.

Some criteria can be qualified, for others; however, a subjective judgement cannot be avoided. The problem becomes more complex when one considers that even for rock with known chemical, mineralogical and mechanical properties; no unequivocal method exists to select a crusher system, which will comminute properly under optimal conditions. Thus, practical experience is the principal guide when choosing a crushing system and its machines.

The following material properties are considered to be the influencing factors:

• abrasiveness
• stickiness
• crushability
• composition of rock pile
• grain size distribution of feed
• product specifications

Abrasiveness

When speaking of the abrasiveness of a material one refers to the material’s effect on the crushing tools and not the rock’s resistance to abrasion.

The content of free silica gives a valuable indication of abrasiveness of raw materials suitable for the cement industry. Thus, the free-silica content has become one of the most important selection criteria for crushers.

Stickiness

The stickiness or plastically of a material is influenced by the amount of expanding clay minerals and by the moisture content. Since a quantitative method to determine the stickiness of a material is not available, its moisture content is taken as an indication. Other such indications are the proportion of expanding clay minerals and the water absorption capability of the raw material.

Crushability

The crushability of rock is probably the most difficult one of the selection criteria to quantify. Many attempts have already been made to narrow down the range of problems and to develop meaningful and informative test procedures, but the outcome has been unsatisfactory and thus the design of a crushing system remains a matter of experience.

The crushability serves above all to determine the dimensions of the crusher and is only of secondary importance for the selection of the crusher type.

The following tests and criteria give indications of the crushability: compressive strength/ Mohs hardness, “Bond impact crushing test” (2) and the “Los Angles Abrasion Test” (3).

A further property which must be considered when evaluating the crushability is the structure of the rocks as regards stratification, thickness of layers and breaking characteristics.

Composition of the Rock Pile

If rock piles with different properties must be crushed together, it is obvious that one must always accept a compromise solution. Whether the chosen solution actually proves successful depends extensively upon whether the composition of the quarried material varies greatly and whether the proportions of the components are known when designing the crushing system. The latter point is probably the cause of most operational problems in crushing installations.

Grain Size Distribution of Feed

The granulometric composition of the material fed to the crusher determines the required “crushing performance” to obtain the desired product and also serves as criterion for a prescreening. In this connection, mention must be made of the largest allowable rock size in the material feed for a primary crusher. This point is often included in the list of selection criteria. However, the dimensions of the largest blocks of material, which can still be fed into the crusher, can only be used as criterion for the choice of a certain type of construction among the same type of crusher. There is little sense in including this information on the list of criteria for types of machines of crushing systems.

Product Specifications

In the cement industry there are few demands on the crushed raw material as regards grain shape. The crushed product should have the largest possible fine fraction and few oversized grains.

Thus, when selecting a type of crusher, usually only the maximum grain size tolerated for the following comminution equipment is important.

Grain size distribution is a measure of the crushing performance and may influence the concept of the comminution equipment which follows the crusher. When choosing a crushing system, it has proven helpful to use the maximum grain size (with max. 5% oversized grain) as a guiding factor.

The maximum grain size of the material fed into the grinding installation is fixed depending on the process selected for the grinding installation. Therefore, the crushed products can be classified into fractions of different maximum grain sizes. The three fractions < 400 mm, < 100 mm and < 30 mm can be taken as a rough classification for the different grinding processes.

CRUSHING SYSTEMS AND APPLICATION

General

For process technological, cost reasons and insofar as allow by the properties of the raw material. One should try to accomplish all crushing activities in one machine, a hammer crusher or impact crusher. These types of crushers allow the highest reduction ratio of up to 50: 1 in one stage. If one of these crushers cannot be used and another crushing system must be chosen, it is due to the abrasiveness or the stickiness of the material. However, by selecting another crushing concept at least two stages of crushing are required. Crushing systems can be classified, with some compromises, according to the two main criteria, wear and danger of clogging: The fast running hammer or impact crushers are in the middle range of such a classification. Generally, it can be said that an attempt must be made to accomplish the comminution in crushers which have no more than two stages by selecting machines which are compatible with each other. This is particularly true for crushing systems using gyratory crushers. In the following sections a short description will be given of the different concepts.

System 1: Single Stage Crushing in Hammer or Impact Crushers

The fast-running hammer or impact crusher is nowadays the most frequently selected machine to crush the limestone component of cement raw materials. The maximum product grain size is limited by grates or other size controlling construction elements as, for example, a grinding path for a double-shaft impact crusher. Impact crushers alone, which screen the product by operating in a closed circuit with screening station, require an elaborate installation thus considerably increasing the investment. When choosing among the large selection of impact crushers and single or double shaft hammer crushers, the process technological aspects recede into the background. Decisive are selection criteria such as proper construction, time needed for maintenance and, last but not least, the investment and operational costs.

However, limits are set for the single-stage crushers which have strict grain size limits, due to wear and tear on the crushing tools and the danger of clogging with the associated increased wear and tear when processing sticky material. The limiting values are in the order of 6% free silica and 10% moisture content.

System 2 Single-Stage Crushing in Hammer crushers without Grate

Hammer crushers without grates can be used only if a relatively coarse product is required (approx. < 100 mm). Of course a certain compromise must be made as regards maximum grain size and oversized grains, but at the same time the danger of clogging and wear are reduced. The limits of single-stage crushing can thus be stretched – the tolerable limiting values as regards moisture and quartz content can be increased by about 20% in comparison to crushers with grates. If clogging is the main criterion, one can also consider heating the critical wall sections of the crusher (see crushing system 5).

System 3: Gyratory Crushers and Secondary Impact or hammer Crusher

The crushing system with primary gyratory crusher and secondary impact crusher is particularly suitable for compact hard and also abrasive rock. With the preceding coarse reduction, it is possible to use crushing tools made of highly wear-resisting cast-steel in the secondary crusher (austenitic high alloyed cast iron). In this way, the wear and tear is considerably reduced when compared to the tougher material to be used in primary crushers.

A relatively fine crushed product can be obtained due to the fast-running crusher. Depending on the abrasiveness of the rock and the type of secondary crusher used, this system can be either equipped with a construction element limiting grain size or operated in a close circuit with a screening station.

The limiting factors of this system are on one hand the danger of clogging in the gyratory crusher and on the other hand, the maximum tolerable abrasion rate in the secondary crusher.

System 4: Double-Stage Crushing in Gyratory Crushers

With very hard and abrasive rock it is an absolute necessity to install gyratory crushers also in the second crushing stage. In most cases, two or more secondary crushers are necessary to obtain the same throughput as the primary crusher.

To lower wear and tear and energy consumption in the secondary crushers, the product of the primary crusher should be screened and only the fraction coarser than the final grain size should be fed to the secondary crusher.

An intermediary storage of the raw material is necessary since the material flow has to be divided up after the primary crusher and the secondary crushers should be loaded.

The jaw crusher should be mentioned as alternative to the primary gyratory crusher. However, its use for cement raw materials is advantageous only under very special conditions.

System 5: Primary Hammer or Impact Crusher with Roller Crusher or Impact Drying for the Secondary Reduction

As already indicated under System 2, if the critical wall and impact sections are heated, relatively sticky material can be crushed in the fast- running crushers which are not equipped with grates.

The roller crusher is one possibility as secondary crushers another is the simultaneous drying (or partial drying) and reduction in an impact or hammer crusher.

The displayed crushing system is mainly used for mixtures of limestone with large portions of sticky clay.

System 6: Primary and Secondary Roller Crushers

The roller crusher is the only machine suitable for handling pure clay and clay with hard inclusions. The reduction ratio of this type of crusher is only about 5:1. Depending on the feed size and desired final grain size, it may be necessary to install three crushing stages. If a grain size of < 30 mm is desired, the feed size of the primary crusher with two-stage reduction is limited to 500 to 600 mm. Of course, this limitation applies only to the hard rock inclusions.

MOBILE CRUSHING PLANTS

General

Transport between the mobile loading device in the quarry and a fixed crushing plant installation takes place almost exclusively by trucks in surface quarries. It is often considered a disadvantage that the connection between the almost continuously operating loading device and the continuously operating crushing plant consists in the discontinuously operating truck transport. Failure of the trucks often leads to interruptions in operation. Moreover, it must be recognized that the distance between the crushing plant and the recovery site increases with progressing operations. For this reason, a larger number of trucks are required for transport of the raw materials to the primary crusher. The site of the crushing plant can thus not always remain optimum, even with the best possible planning. In addition, the choice of the site for the new plant is influenced by the terrain conditions, particularly in case of an extension of   stationary crushing plants. With regard to the problem of costs it must be said that the costs for truck transport are very much influenced by fluctuations in wages and have thus been subject to increases according to the wage development in the course of the last few years.

In the circumstances, it is not surprising that already for many years mobile crushing plants have been built by different firms so that the raw material can be crushed immediately at the recovery site and be transported by belt conveyors for further processing.

The investment cost for a mobile crusher, conveyor belt and loading equipment is usually higher than for a fixed crushing installation with loaders and trucks. However, increased output from the excavator/ loader, reduced manpower requirements, and the non-existence of high operating and depreciation costs for trucks, means that the mobile crushing units can often produce materials at the lowest overall cost/ton when the raw material deposit allows its use.

Of course, it is only necessary to have the first crushing stage mobile. Secondary. And tertiary crushing units, if required, are always fixed installations. All of the crushers previously mentioned have been installed as mobile units.

A mobile crushing system is schematically illustrated in the following fig.

Traveling Mechanism

Three methods of support/traveling mechanisms are commonly in use and selection of the most suitable type is essentially related to quarry floor characteristics. For example, the wheel mounted machine requires a very well maintained quarry floor of good load bearing quality thus allowing maximum mobility for this machine type. Table 1 summarises the characteristics and application for each mechanism.

Mobile Crushing Plants

Mobile crushing plant type

• Wheel mounted ( rubber tyred) mobile crushing plant O&K
• Crawler – mounted mobile crushing plant
• Rail – mounted mobile crushing plant comprising two sections
• Mobile crushing plant with hydraulic walking mechanism
• Crushing plant moved with the aid of vehicles