Semi-mobile crushers for oversize comminution

1‌ Introduction

In the past, quarries and gravel pits were designed primarily for the production of mass products for the construction industry. The priority goal was the effective production of standard grades for asphalt and concrete mixes.

This led frequently to following developments:

The final grades to be produced are becoming increasingly smaller

The requirements for material properties, e.g. in respect of fractured surfaces, cubicity and others are constantly increasing

In certain regions there is natural sand containing a relatively small proportion of fines smaller than 0.25 mm. Consequently, this product is difficult to sell

Often this off-spec material is only produced in relatively small quantities in the individual plants

That means that equipment is needed that can be quickly moved from one plant to another. To enable aggregate producers to react temporarily and effectively to these situations, SBM Mineral Processing has developed two semi-mobile impact crushers [1].

2 Process engineering approach and implementation

in the machine design

Typical off-spec material ranges between 5 – 70 mm in particle size. As this rock material has usually already undergone several comminution stages, the microstructure of the particles only has a few faults to enable effective recrushing.

That means that for the further comminution of this material, crushers are needed that have a high energy input and a corresponding loading frequency to realize the required comminution effect. For such applications, the vertical impact crusher and the reversible oversize impact crusher (Fig. 1) are particularly suitable. The vertical impact crusher can be seen on the left. Typical for this crusher is its vertical shaft. Usually, with these crushers, comminution is effected in a bed of autogenous material.

The principle is different for the horizontal oversize impact crusher. While the material also enters the crushing chamber vertically, it is then taken up by the impact bars in the horizontally arranged rotor and flung against several stepped impact aprons. The rock particles are stressed several times between the impact bars and impact elements.

2.1 Design and operating principle of the vertical

impact crusher

With the vertical impact crusher, the feed material is fed via an intake hopper vertically into the rotor. At the feed cone of the rotor, the particles are deflected into a horizontal direction. After acceleration in the rotor, the particles impact against a bed of material or armoured elements (Fig. 2).

Impact against the armoured elements achieves a greater comminution effect than impact in the material bed (Fig. 10). The particle shape of the crushed product, however, is not as good as particles impacted in the autogenous material bed. The wear costs are higher for the configuration with impact elements than for crushing in the material bed.

The crushing behaviour of vertical impact crushers depends to a considerable extent on the design features of the crusher and especially the design of the rotor (Fig. 3). This has been described in detail in the dissertation by Dipl.-Ing. Rafal Rychel [2].

Fig. 3a shows the structure of the rotor with unprotected chamber walls. Here the feed material can exit the rotor radially. This design is used primarily for relatively large-size feed material. The most effective comminution is achieved in the upper size range of the feed particles. For wear-intensive material, especially for small-size feed material, rotors are used in which material pockets are formed in the exit channels (Fig. 3b).

On account of the rotor geometry, the material exits the rotor tangentially. As a result, the rocks travel a long way through the material bed. This leads to the production of a high sand fraction and an excellent product particle shape. Fig. 4 shows a rotor with stable material bed.

For the sand content and particle shape formed, it is decisive whether the material accelerated in the rotor is comminuted at the impact elements or in the natural material bed (Fig. 5). While at the annular armoured lining (Fig. 5a) the energy is often sufficient to enable full comminution of the particles, in the material bed (Fig. 5b) usually only the corners and edges of the feed particles are chipped off. This results in a product with a high content of fines and excellent particle shape. However, there repeatedly remain particles of near-feed size that cannot be comminuted with this process.

A key practical aid

for the realization of stable working conditions,

for reduction of the rotor and bearing load,

for reliable wear protection and

for dust reduction

is the load-driven hydraulic intake control system available from SBM Mineral Processing GmbH.

Starting point for this control is the dependence of the crusher throughput rate on the motor current. In addition, the filling level of the feed material in the hopper is measured. If the actual motor current is lower than specified, the intake cross-section in the crusher is enlarged and the motor current increases. This leads to faster emptying of the hopper. If a minimum filling level is reached, the intake cross-section is made smaller. The filling level in the hopper now rises. When the maximum is reached, the intake gate opens. If the filling level increases further and the filling level max_max is reached, the upstream feed equipment is switched off or its speed reduced. The filling level falls, and the control loop starts over again.

2.2 Design and operating principle of the reversible

horizontal impact crusher

For the comminution of off-spec material in the size range between around 16 and 150 mm, a reversible horizontal impact crusher (Fig. 7) is particularly suitable.

In contrast to classical impact crushers with shoulder feed, the reversible impact crusher is structured symmetrically. As a result, small-size feed material can penetrate deep into the crushing circuit and is comminuted effectively. Left and right beside the rotor, the impact aprons are arranged mirror-inverted. On each impact apron, seven impact steps are arranged in a cascade, the crushing gap tapering towards the outlet. This cascade arrangement leads to circulation of the material during the crushing process. As a result, an excellent particle shape and high number of fractured surfaces result.

2.3 Special aspects of use

In the case of relatively small feed sizes, the use of vertical impact crushers and reversible oversize crushers can overlap. Such an application is shown in Fig. 8. Objective here is the recrushing of round oversize in the size 16/32.

Basically, this can be effected with both types of crusher. If you work with a vertical impact crusher, then the comminuted product represented by the black line is formed. Typical here are the following characteristics:

The flat curve

The high filler content

The relatively low comminution in the upper feed size range. That is if you feed 6/32 material, a 0/32 crushed product is obtained

The obtained percentage of chippings contained in the 2/8 fraction is 27 %

In contrast, the reversible oversize crusher demonstrates different crushing behaviour, represented by the blue line:

Here the feed and product grading curves lie well apart from each other. That is 100 % of the final product particles are smaller than 16 mm

The curve is steeper overall, and a higher chippings content is obtained (2/8 content at 37 %)

At the same time, the crushed material has many more fractured surfaces

The choice of the low-wear vertical impact crusher or the more effective and robust reversible oversize crusher often depends on the expected wear costs.

For the above-described application, these costs reach around 0.25 €/t for the vertical impact crusher. With the reversible oversize impact crusher, they work out at around 0.75 €/t crushed product. It is important that the assessment of the crushing costs is not detached from the crushed product. Besides the yield of product, the fractured surfaces and the particle shape are also important criteria for assessment of crusher performance.

3 Plant realization

The semi-mobile crushers developed by SBM Mineral Processing GmbH have the following features:

Throughput rates between 70 and 80 t/h to economically crush batch quantities between 10 000 and 30 000 t at one site

Flexible with regard to different feed sizes and materials and production of a wide range of product grades

Easy transport and short set-up times

High availability and maintenance friendly

Compatibility with standard mobile screens

Fig. 9 shows a semi-mobile plant equipped with a reversible impact crusher being prepared for transport.

4 Typical applications

4.1 Vertical impact crusher

A typical application for the vertical-shaft impact crusher is the crushing of small-size off-spec material for the production of sand (Fig. 10). The 8/11 feed material (magenta-coloured line) is to be comminuted to yield a maximized content < 4 mm. With an increase in speed from 72 to 78 m/s, and operating mode with material bed, the content < 4 mm can be increased from 26 to 32 %.

With an impact ring, the content of sand increases to 54 %. Feed and product screening curve lie far apart from each other. That means that more effective comminution takes place here. At the same time, the impact ring causes at least 30-% higher wear costs, and associated with this, higher service costs and lower availability. Ultimately the market situation decides what requirements for the crushed products must be taken into consideration and what revenues can be obtained from the product.

An interesting example of the economic application of vertical impact crushers is the concentration of fine sand [1]. In deposits in which, on account of the geological conditions, there is a lack of fines in the 0/2 sand, these crushers are used to selectively increase these fines. With an increase in the peripheral speed, the content < 0.25 mm is increased from approx. 2.8 % in the feed material to 19 % at 62 m/s and 29 % at 80 m/s (Fig. 11).

Fig. 12 shows how the sand changes in the individual particle size classes, which ultimately leads to a concentration of the sand in the lower range of the feed screening curve. If the screen curves of feed material and final product are analysed based on determination of the percentage of the respective particle class for each material and the difference between product and feed material is then calculated, the percentages shown in the diagram result.

Up to a particle size class of 0.5 mm, an increase results, i.e. here, as a result of comminution, new fractions of small particles are formed. These fractions are achieved by abrasion of the particles, primarily in the range between 0.5 and 1 mm. For the particles between 1 and approx. 2 mm, hardly any comminution is effected.

4.2 Reversible oversize impact crusher

Fig. 13 shows a typical application of this crusher in river gravel. A 5/90 mm feed is crushed to obtain a maximized amount of crushed product in the 2/8 fraction [2]. For this purpose, the speed was increased from 37 m/s to 45 m/s. For these tests, the gap width was kept constant at s = 40 mm. As expected, with the increase in speed, the comminution efficiency is improved.

Table 1 shows that the percentage of 2/8 content initially increases with faster speed. At the same time, the crusher throughput rate is reduced. The cause for this is that with higher speed, higher idle power is also necessary. As a result, less energy is available for comminution and the crusher throughput rate decreases. In respect of the yield of product particles, an optimum is indicated at 41 m/s.

5 Conclusion

The semi-mobile crushers with a throughput rate of around 70 to 80 t/h are an interesting and flexible option to selectively process off-spec material in the aggregates industry. They are particularly suitable for the production of high-quality sands, products with a high percentage of crushed surfaces and cubicity, and defined individual fractions. The semi-mobile design enables the machines to be quickly ready for action without long set-up times. On account of the optimized transport dimensions and crusher weights below 25 t, transport is simple, incurring only low costs. That makes processing viable even for smaller batch sizes. Maintenance friendliness and high availability were priorities in the design of the crusher.