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Evaluating effective mobile crushing systems

It seems logical to position the systems forth processing of natural rocks in those locations where mining takes place. However, only small amounts of waste material are generated at different places during building material recycling. It is expensive to store or transport demolition material and it requires significant logistical support. It is also very expensive to transport the demolition material to landfills or to stationary recycling yards. The distances between the location where the material is generated and the location where the material is processed lead to the ?mobile? idea.

The requirements for recycling are rarely the same as the requirements for processing of natural rock. However, Kleemann generated significant sales for wheeled systems during the 1970s and 1980s, especially for construction projects with a limited duration. This included road building projects in remote areas with the requirements to cost-effectively mine local rock deposits or large construction sites with limited space such as tunnels, bridges or dam projects. Due to the high requirements on the final product quality, mobile crushing and screening systems were originally only used as primary crushing and pre-screening systems. The refinement of the material through the use of additional crushing and screening stages followed only much later.

With few exceptions, wheeled systems can currently only be found in countries where the transport of tracked systems on flat bed trucks results in significantly higher costs. So-called portable systems are used in those cases where stationary systems cannot be used for legal approval reasons. Small to medium size systems with weights of 20 to 70 tonnes are mainly placed on skids but also on hydraulic support feet. Larger systems are disassembled for the transport of their components. This system option only makes sense if it only needs be relocated rarely, eg one to two times per year or even after several years. This is why portable systems are often found as part of a stationary recycling system or as a supplement for a natural rock processing system with limited space. The ?tracked? crushing and screening systems have by far the largest market share among the ?mobile? systems.

The new series SHB impact crushers are used for the tracked Mobirex systems. The development of this crusher was specifically aimed at a wide application range because a large number of customers use the system to process different materials. All impact crushers of this series have hydraulically adjustable impact aprons and a rotor with four blow bars that can be delivered in different material variations depending on the application. The crusher can produce different crush curves and final grain sizes for the same base material by adjusting the rotor speed in a range of approximately 30 to 40m/s and by adjusting the crusher gap of -100 to +150mm at the lower apron and 0 to 300mm at the upper apron. Drive force and centrifugal mass are very important in an impact crusher.

The size reduction of the crushed product in an impact crusher is achieved in several different ways. When the rocks first enter the impact crusher, they will be caught by the rotor?s blow bars and thrown against the first impact apron.

At first, the rocks are reduced when they are struck by the rotor blow bars, then they are further reduced in size when they strike the impact aprons. They will be thrown back into the rotor, caught again by the blow bars and afterwards again thrown against the impact apron. This process is repeated until the required grain size is achieved, which allows the rocks to pass through the adjusted gap between impact apron and blow bar. Three mechanisms contribute to crushing the rock parts:
1. The contact of the rock with the blow bar.
2. The contact of the crushed product with the impact apron.
3. The contact of the material impacting with itself.

Different drive options are used, depending on the application and size of the system. An electrical generator is always used. This produces the electricity required to drive the conveyor belts, vibrating chutes and screens. Depending on the system type, the crushing operation is normally powered hydraulically or electrically. Smaller systems are normally delivered with so-called direct drives.

Impact crushers are used for the processing of soft to medium hard rocks, such as limestone, dolomite, clay, shale and sandstone and they are also widely used for crushing demolition and construction materials such as asphalt, bricks, concrete and reinforced concrete.

Impact crushers are ideally suitable for crushing reinforced concrete because they achieve a better separation of steel and concrete when compared to squeeze crushers (eg jaw crushers, gyratory crushers). After crushing, the loosened steel is separated from the concrete by high performance electrical or permanent magnet separators. Asphalt crush products or asphalt milling products can only be processed to a usable final product (0-11, 0-22, 0-32mm) by using an impact crusher.

Most rock types can be reduced in size with the help of impact crushers. However, the laws of physics bring the impact crusher to its limits if the material hardness increases, and makes it inefficient when used for hard, tough and abrasive material. It has been demonstrated in practical applications that the impact crusher can also process medium hard to hard rocks due to improvements with respect to wear and tear. Its strengths include the high throughput and the outstanding grain form of the final product. However, the Mob fox system with its SNH hard rock impact crusher is normally used for this type of application.

A SSTR series single rocker jaw crusher is used for the tracked Mobcap systems. The single rocker jaw crusher has a hydraulically supported crush gap adjustment. The smaller Series MC 100 and MC 110, which are used for different applications, have jaw crushers with continuously adjustable hydraulic gap adjustments. The achievable final grain sizes depend on the size of the crusher. A small crusher generates smaller final grains than a large crusher because the clearance angles of the crusher jaws can only move in a small range due to physical limitations. The size reduction in the jaw crusher is performed by entering the material into the crusher opening with two profiled crusher jaws, one of the jaws is fixed and the other swinging. The oscillating swinging movement of the mobile jaw changes the gap continuously in reference to the opposite fixed jaw. The rocks are reduced in size through a squeeze breakage as the two jaws come closer to each other until they reach the smallest gap (CSS= Close Side Setting). The mobile jaw moves back into the open position during the back stroke. The material can now move forward and exit the crushing area after the appropriate size reduction.

The speed of the eccentric shaft is between 200 and 240 rpm. Kleemann crushers have comparatively large strokes and therefore deliver strong driving forces. The jaw crusher types MC 100 and MC 110 are equipped with reversible hydraulic drives. The crush gap can be opened with a few manual operations and the crusher can be reversed in case rock becomes jammed in the crusher opening. A release is therefore possible without manual emptying of the crusher.

Larger Mobicat systems such as the MC 120, the MC 140 and the MC 160, exclusively designed for quarry operations, are electrically powered.

The Mobicat is suitable for medium hard to hard rock with a compression strength of up to 320N/mm2, eg for basalt, granite gneiss,
diabase and quartzite. Single rocker jaw crushers are also mainly used for initial size reductions in multi-step limestone processing systems, because they show less wear and tear than impact crushers and they produce a smaller fine grain proportion when processing limestone.
The tracked jaw crusher system type Mobicat MC 120 Z (see illustration above) was designed in such a way that transport weight and performance were optimised. This is especially important for users who operate the equipment at different locations and therefore have to transport the equipment frequently by road. It is the objective of this design to install the largest possible jaw crusher on a tracked system without the need to disassemble the system for transportation.

All mobile jaw crushers manufactured by Kleemann are equipped with a constant material flow width, ie vibration chute, pre-screen, crusher and discharge belt all have the same system width. The material flow for the MC 120 Z, which was also designed for use in recycling by equipping it with a vibration chute, can be opened by an additional 200 mm from 1200 to 1400 mm. This design prevents constrictions and guarantees high operational safety even for inhomogeneous materials.

The design of the jaw crusher system can be adapted individually to match the respective applications. For example, it is possible to install a double-deck grizzly or a roller grate for applications in natural rock as described above. Large special systems can therefore also include apron feeders instead of vibration chutes. Smaller compact systems are also available with a grate chute instead of an active pre-screen. Alternately, a vibration chute can be installed underneath the crusher to discharge the crushed product. If the system is used to crush reinforced concrete, this protects the conveyor belt from damage by steel reinforcement.

Limestone is normally full of impurities. Organic materials such as humus, wood or clay should not contaminate the final product and must therefore be separated completely before crushing takes place. These can be eliminated with so-called heavy product screens in front of the crusher because impurities occur mainly in the fine material. Depending on the composition of the feed material, the cover of this double-deck grizzly can be selected in different versions. If required, the medium grain can be returned to the crushed product by bypassing the crusher.

More strongly contaminated feed material presents the problem that conventional vibration chutes compact the clay parts of the feed material in such a way that they adhere to the chute trough after a short time, which results in a major reduction of the feed performance. A similar effect is generated by very glutinous feed material in the pre-separator. The screen cover becomes increasingly clogged, starting at the outside walls and moving towards the middle. This reduces the effective screening area and therefore the quality of the pre-screening. Kleemann?s directly fed roller grates offer significant advantages for these applications.

The continuously rotating elliptical disks of the two-stage roller grate are always in contact with the feed material and clean it. Under the weight of the rocks, the glutinous fine material is pressed through the horizontally oscillating gap. The roller grate cannot clog even when strongly contaminated feed material is used. Adhering material is removed by the scrapers mounted underneath the shafts. The large design of the roller grate permits a feed size equivalent to the inlet geometry of the following crusher. The gap opening and therefore the grain size of the pre-screen remains unchanged. This is different to the conventional pre-screen which requires a change of the screen covers. The total pre-screen material can also be routed through the crusher bypass if a roller grate is used as a pre-screen. The speed of both roller grates can be adjusted independently of each other for continuous feeding of the crusher.

Pre-screens are part of Kleemann?s modular system concept. They can be installed in front of a Mobicat series jaw crusher or a Mobirex series impact crusher.

It is necessary to provide two different secondary and tertiary crushing system configurations to meet the process requirements. Mobifox S and Mobicone S are secondary and tertiary crushing systems that work in combination with two or three deck screens in a closed loop. The screen and the crusher are installed on a chassis.

Both systems include a large feed hopper, which can be fed separately with the help of a wheel loader. It is used as a buffer hopper during operation with a primary crushing system. A continuously adjustable vibration chute transfers the feed material onto a conveyor belt, which leads to a large three-deck screen. The screen is designed in such a way that the oversized grain is directly transferred to the secondary crusher, while the graded products from the screen are discharged to ground through conveyor belts. The crushed product is routed back into the loop from the secondary crusher into the feed hopper and from there again to the screen. Elaborate change valve systems permit different production alternatives without the need to change the screen cover. For example, if the upper deck is covered for a 56mm separation grain, the middle deck for 32mm and the lower deck for 16mm it is possible, through a simple manual operation by changing the valves, to produce either a 0-16mm, a 0-32mm or a 0-56mm as well as a 16-32mm or a 32-56mm final grain.

In contrast to the S version, Mobifox and Mobicone series systems are available without a post-screening unit. The total pre-crushed feed material is fed into the crusher and will only be classified afterwards with a separate screening machine. The systems can be operated without personnel. An appropriate sensor control system prevents an overflow of any individual machine in the system. The upstream crushing system is controlled by a sensor, ie the upstream system is switched off or the crusher feed reduced if the hopper is filled. An additional sensor at the secondary crusher inlet ensures that it will be continuously fed. The frequency of the buffer hopper feed conveyor chute will be increased to supply more material into the gyratory crusher in case of a material shortage. The feeding of material to the screen is reduced if the secondary crusher may overflow due to a high quantity of oversized grain.

A gyratory crusher is included as a secondary or tertiary crusher in the Mobicone system. The size reduction in the gyratory crusher takes place between the fixated crusher jacket and the movable crush cone. The slight slope of the crush cone axis, due to the eccentricity of the bearings, results in a stagger movement and therefore infrequently changing distances of the crush cone relative to the jacket. The size reduction principle is similar to the one of the jaw crusher. Gyratory crushers work evenly and continually because the opening and the closing phases are simultaneously offset by 180 degrees.

The Mobicone crushing system is mainly suitable for the processing of medium hard to hard rocks. It is mainly used for processing of primary raw material and it is rarely used for recycling. So-called steep gyratory crushers are mainly used for pre-crushing and intermediate crushing with low-quality requirements. Flat gyratory crushers are post-size reducers for mineral mixers and high quality chippings. Gyratory crushers are not suitable for size reductions of soft and glutinous materials (danger of clogging).

The design of Mobifox S is only slightly different from the design of Mobicone S. It has an SNH series impact crusher on the chassis instead of a gyratory crusher. The size reduction principle is the same as the principal described for the SHB series. The main difference of the impact crusher can be found in the smaller crushing chamber geometry relative to the rotor. Depending on the use in secondary or tertiary applications, the crusher can be equipped with an additional integrated milling belt with a corrugated steel profile surface. The milling belt contributes to an increase of the sand and chippings content in the final product as well as to a reduction of the non-cubic final grain content.  

An additional impact on the quality and the grain distribution can be achieved by changing the rotor speed.

The Mobifox crushing system is mainly suitable for size reductions of soft to medium-hard and brittle rocks. It is also used for hard rocks by
improving the wear-and-tear materials, where special emphasis must be put on the economy of the process.

The advance of mobile crushing and screening technologies has now also reached quarries. Complete mobile systems are installed in increasing numbers with the objective of producing concrete and asphalt aggregates in accordance with EN standards.

A classical limestone processing system can be installed with one or several crushing stages depending on the raw material and the requirements for grain composition. Mobile systems are currently installed with up to three crushing stages similar to stationary models. Single-stage crush systems with a size reduction ratio up to 1:20 in a closed loop, which produce final grains with sizes of 0-25 mm, can be found in increasing numbers. The system configuration to be used depends on many factors, including the physical and chemical characteristics of the raw material, the desired composition of the final grain, requirements for the cubic configuration, the required or permitted filler content, the limited content of parts that can be removed with water and also economic considerations such as energy use, performance, wear-and-tear cost, personnel cost and space requirements.

With the exceptions of the washing and sand processing functions, Kleemann manufactures all components required for a mobile limestone processing system.

The processing of tough and hard rocks such as granite or basalt is, in most cases, more complex.

The classical mobile hard rock processing systems always work with a jaw crusher as pre-crusher. The number of additional crushing stages required depends on the crushed ratio that can be achieved for each crushing stage and from the desired grain form. Normally two additional gyratory crushers are operated in series to split basalt, for example, to ensure that the crush ratio based on feed grain to crushed grain is not exceeded. This ensures that the final product shows the characteristics of an evenly graded grain curve and contains the required high content of grains with a cubic form required for the production of high quality chippings.

A screen machine may not be required in the secondary area (first post-crush stage) if only small amounts of fine material are included in the feed material from the primary crusher. However, this is required in the tertiary area (second post-crush stage), because a large fine content in the feed material, which, in this case comes from the upstream gyratory crusher, would significantly increase the load to the gyratory crusher and therefore would require increased power for size reductions. In addition to the increased wear and tear, this also would result in an overload for the gyratory crusher. Hard rock processing systems can also be implemented with an impact crusher as a tertiary crusher for feed material that crushes with ease. The use of so-called cubicers (vertical impact mills) is only possible with a relatively small crush ratio of 1:1.5. Very high wear-and-tear costs for all three crushers must be accepted if it is used as a tertiary machine for hard rocks because the largest possible crush ratio and therefore high load is required for all three crushing stages.

In the past, it was almost impossible to achieve adequate chippings production with mobile systems. This is now possible even for high hourly usage based on the systematic development of the mobile system technologies. Consistent production monitoring of the individual mobile equipment with level sensors and buffer hoppers ensures that sufficiently high quality chippings can be produced in accordance with requirements for grain form, screen curve, separation accuracy and cleanliness. The continuous electrical interlocking of all system components guarantees the high operational safety of the total mobile crushing and screening system.

There are numerous advantages for a mobile system, the production plant can follow the mining progress, whereas a stationary plant has to be permanently positioned, even if the mining moves on. Mobile installations are also often selected if the duration of the use has not yet been determined. A flexible reaction to a change in raw material is possible through an exchange or an addition of individual systems. Kleemann?s mobile crushing systems maintain the option to operate the systems with stationary electrical power supplies. The continuously increasing demand for mobile processing technology has over recent years resulted in the development of a large used machine market. In contrast to the stationary system, the investment risk is acceptable because mobile systems have a defined resale value even after long usage.

Thomas K?hnle is the technical director and Thomas Mossner is the mobile plant technical manager for Kleeman GmbH.

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