Mobile screening plants show their particular strengths in both line-coupled and standalone materials-processing scenarios.
For a long time, the focus in mobile processing of quarried stone and recycling materials was firmly set on the use of crushing plants. Although these were able to produce aggregates that could be influenced within certain limits, these were only saleable to a limited extent without classification by downstream screening plants. In contrast, scalpers and classifying screens can separate materials into several final products and produce qualified final products. These plants must not necessarily be used only in combination with crushing plants, but also show their strengths in standalone materials-processing scenarios. A distinction is made between two groups of screening plants: scalpers and classifying screens.
Scalpers – The Preparers
The main task of scalpers, also frequently described as scalping screens or heavy-duty screens, is the sorting of the coarse feed material before it is passed to the first crushing stage. As the name suggests, they are fed with massive pieces of coarse feed material and ‘scalp’ off the oversize content. The material loaded into the feed hopper is passed directly to the upper screen deck. The oversize grains are passed over the screen deck in the material flow direction and onto the main discharge conveyor. This enables the scalper to cope with even the largest chunks of material.
The plants are often fed with material either from the side or from the rear with a wheel loader. The optionally available, separately folding hopper filling aids enable the loading of feed material from the rear with wider loader buckets or prevent material spillage over the opposite side of the hopper when loading from the side.
The versatile scalpers can also be used as a classifying screen in a plant train, where they can screen out up to three fractions from the crushed stone. To enable this, the rear hopper wall of the Kleemann scalpers can be folded to at least two different heights, and even three heights in the case of the MOBISCREEN MSS 802 EVO. This enables optimum material transfer, even at the low feed height of an upstream crusher.
For special applications: Equipment for increased durability.
Under certain conditions, the factory-fitted belt of the feeding conveyor of Kleemann scalpers can reach its limits. For example, extremely coarse and sharp-edged feed material can cause damage to the conveyor belt. In the case of flaky, clay-bonded rocks, the smooth belt can slip underneath the feed material, particularly when it is extremely moist. To avoid these complications, scalpers can be fitted with a robust apron feeder. In addition, material brakes and/or chain curtains can be installed on the feed and discharge sides of the screen casing. This prevents coarse rock from striking the main discharge conveyor belt which could damage or reduce the service life of the discharge conveyor belt.
Classifying Screens – The Refiners
Classifying screens are the specialists for the production of high-quality final products or important intermediate products. They are where finer, usually pre-crushed, materials with smaller pieces are processed. Classifying screens are essential components in multi-stage crushing and screening stages, but are used just as often as standalone screening plants.
The MOBISCREEN MSC EVO line of classifying screens from Kleemann feature extremely high-capacity feed hoppers. The feed material is loaded into them by wheel loaders, excavators, or an upstream crusher or screening plant. With its wider opening at the top, the hopper ensures a good material flow. As classifying screens are generally fed with finer material, the plants do not require as much in the way of impact or wear protection as scalpers.
A Perfect Material Flow Is Decisive
The material flow determines the throughput and the final result, which must be precisely classified. The demands on the material flow begin as soon as material is fed to the plant: For instance, the speed of the feeding conveyor and other factors directly influence the bed height on the various decks of the screen casing. Should the bed height on one of the screen decks become too high, the quality of the screened material begins to deteriorate immediately. Achieving excellent screening results from a classifying screen is ultimately the result of a complex interplay between the speed settings of the hopper discharge conveyor and the feeding conveyor, the setting of the dosing flap, the position of the feeding conveyor relative to the screen casing, the setting of the impact plate, the angle and amplitude of the screen casing, and the right choice of screen media. There are fewer influencing parameters in the case of a scalper, as the quality requirements for the screened material are generally less critical than for a classifying screen. In the case of these plants, the screening result can be influenced by the speed of the feeding conveyor, the angle of the screen casing and its amplitude, and the choice of the right screen media.
The Screen Casing and Screen Media
The choice of the right screen media for a screening plant is decisive for achieving the desired screening result. The screen media must be chosen to meet the precise needs of the application. It’s not just the correct choice of the opening cross-section for the desired final grain size that matters. The upper decks of scalpers, for example, are subjected to high loads, because large, sometimes sharp-edged rock fragments frequently strike the screen media. This is where particularly robust and resilient screen media such as finger screens, slotted grates, or punched plates are often used, depending on the application.
Finer Screening
Depending on the separation cut and screening capacity requirements, dry material up to a grain size of 4 mm can be screened with conventional square or rectangular screen meshes, or piano-wire harp screens. The situation becomes more challenging when even finer screening is required and/or the material to be screened is moist, sticky, or fibrous. This is where we go into more detail: thinner wires reduce the risk of incrustations, increase the open screening surface of the screen casing and, in turn, the output of the screening plant. At the same time, the thinner the wire, the shorter the service life of the screen media. Here, we have to weigh the advantage of higher performance against the costs for replacement media and the downtime of the plant when replacing worn media. The pattern of the wire mesh can also help to protect a screen against clogging, or, at least, keep it free for a longer period. When using certain harp screen patterns, induced vibration in the meshes shakes them free of adhering material. In such boundary conditions, the material also plays a decisive role. For instance, the tendency for material to adhere to the mesh is lower for stainless steel meshes than for conventional wire meshes. In contrast, rubber screen media remain free for longer periods than wire mesh due to the inherent movement of the material induced by the vibration of the screen casing. Negative aspects of these screen media are the broader ribs required between the openings due to the material used, which leads to a reduction of the screening capacity, and the generally high cost of the media.
Screening Procedure and Separation Cuts
To ensure an ideal separation cut, it is decisive to be able to influence the dwell-time of the material on the screen deck. This is effected primarily by the angle of the screen casing. A steep setting increases the screening capacity, but can, under certain circumstances, lead to poorer final product quality due to the presence of undersize grains in the oversize grains. Although using a shallow angle of the screen casing leads to higher selectivity, it in turn also means a lower screening capacity. What is important here is the determination of the best compromise between high output and the desired final product quality.
In the case of scalpers, the particularly wide setting range lies between 15.4 ° and 20 ° (MSS 502/802 EVO) and 12.5 °–17 °(MSS 1102 PRO). The lower deck of these plants corresponds to the shape of a so-called banana screen: The feed side has a steeper angle than the discharge side. In this way, a high flow speed and a rapid distribution of the material are initially achieved, after which the material flows more slowly in the direction of the discharge side and remains on the screen deck for a longer period in order to achieve high selectivity. This shape of the screen deck increases the screening efficiency by up to 10%.
Since classifying screens are typically used to screen finer grain sizes in high quality, a wider range of screen angle settings is required. In the case of the MOBISCREEN MSC EVO classifying screens, this ranges from 20–38 °.
Screen Media for Ideal Final Products
At the end of the day, a carefully considered choice of the plant and an ideal configuration for the feed material to be processed enables not only the optimization of the quality of the final product but also the volume discharged. Assuring minimal wear keeps material costs low – which means the production of a high-quality result with low production costs per ton. www.wirtgen-group.com
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