Scalpers and Classifying Screens

For a long time, the focus in mobile processing of quarried stone and recycling material 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 salable 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 are not necessarily used in combination with crushers but also show their strengths in standalone materials-processing scenarios.

The Scalper – the Preparer

The main task of scalpers, also frequently called 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 coarse material and “scalp” off the oversize content. The material from 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 frequently fed with material either from the side or from the rear with a wheel loader. The optionally available, separately foldable 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 versatility of the scalpers allows them to 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 is foldable to at least two different heights and, in the case of the MOBISCREENMSS 802 EVO, three. This enables optimum material transfer, even at the low feed height of an upstream crusher.

Classifying Screens – for the Detail Work

The true specialists when it comes to high-quality final products or important intermediate products are the classifying screens. The name says it all: these plants process small-sized, usually pre-crushed, feed materials. Classifying screens are essential components of multistage crushing and screening stages and are used just as often as standalone screening plants

The MOBISCREEN MSC EVO line classifying screens from Kleemann feature extremely high capacity feed hoppers. The feed material is loaded into them by wheel loaders, excavators or upstream crushing or screening plants The opening hopper width guarantees a good material flow. As classifying screens are generally fed with finer material, the plants feature less in the way of impact and wear protection features than scalpers.

Comparison of material flow in the two plant types

The material flow in the two screening plant types differs on account of the screening principle. While the screen casing of the classifying screen is inclined against the conveying direction of the feed conveyor and thus diverts the material flow in the opposite direction, such a diversion in a scalper would lead to blockages due to the often extremely coarse material In the case of scalpers, the oversize grain flows in one direction from the hopper feed conveyor to the main discharge conveyor via the upper deck of the screen casing.

Perfect material flow is decisive

It determines the material throughput and the final result, which should be precisely classified. The demands on the material flow start as soon as material is fed to the plant: For instance, the speed of the feed conveyor and other factors directly influence the layer height on the various decks of the screen casing. If the layer height is too high on one of the screen decks, this has immediate negative effects on the quality of the screened material At the end of the day, an excellent screening result from a classifying screen is the consequence of a complex interaction between the speed setting of the feed hopper discharge conveyor and the feed belt, the setting of the dosing flap, the positioning of the feed conveyor in relation to the screen casing, the setting of the impact plate, the inclination of the screen casing and its amplitude – and last but not least – the choice of the right screen media. There are fewer influencing factors to consider in the case of a scalper, as the quality requirements with regard to the screened material are generally much lower than for a classifying screen. In the case of these plants, the screening result can be influenced by the speed of the feed conveyor, the inclination of the screen casing and its amplitude, and the choice of the right screen media.

The screen casing and the right equipmentscreen

The choice of the right media for a screening plant can be compared with the choice of the right tires for your car. If you choose the wrong tires, your car will not perform as well as it could, and may even fail you altogether in critical situations like aquaplaning. In the same vein, the world’s best screening plant will never be able to deliver good screening results when the screen media are wrongly chosen for the application at hand. It’s not just the correct choice of the appropriate opening cross-section for the desired grain size that matters.

As the feed material often contains extremely coarse and sometimes sharp-edged pieces of rock that drop heavily onto the screen media, the upper deck of a scalper is constantly subjected to heavy impact stresses. So what is needed here are robust screen media. And that is why screen media such as finger screens, slotted grates, or punched plates are frequently used on the upper deck of scalpers.

The finer the material to be screened, the trickier it gets:

Depending on the desired particle size and the screening capacity required, dry material with grain sizes down to 4 mm can be screened with conventional rectangular sor long mesh screens or piano wire harp screens. It all becomes more challenging when even finer fractions are required, or when the material is damp, sticky, or fibrous. This is where we go into more detail:

• Thinner wire thicknesses reduce the risk of incrustation and increase the open screening surface of the screen casing, and thus the performance of the screening plant. At the same time, the smaller the wire thickness, 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 while replacing the worn media.
• The form of the wire mesh can also help to protect screen media against clogging, or at least keep them free for a longer period: in the case of certain harp patterns, induced oscillation of the meshes shakes off adhesions.
• In these 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.
• Then again, the movement of rubber screen media induced by the oscillation of the screen casing keeps them free longer than wire mesh media. The disadvantages here are the broader stage between the openings, the consequent reduction in screening capacity, and the usually high price of the screen media.

Screening process and desired particle size

For an idealc desired particle size, it is decisive to be able to influence the dwell-time of the material on the screen deck. This is effected primarily by the inclination of the screen casing. Although a steep setting increases the screening capacity, it can, under certain circumstances, reduce the final product quality through the presence of undersize grain in the oversize grain. A shallow setting of the screen casing increases the selectivity with regard to grain sizes, but in turn reduces the screening capacity. Here, it is essential to search for the best compromise between high screening capacity and achievement of the desired final product quality.

An even larger setting range for the screen inclination is required if classifying screens are to be used for screening primarily finer grain sizes in high quality. In the case of the MOBISCREEN MSC EVO classifying screens, this is from 20 – 38°.