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 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 are not necessarily used in combination with crushing plants but also show their strengths in standalone materials-processing scenarios.

Scalpers – the preparers

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 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 handle 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 foldable hopper filling aids enable the loading of feed material from the rear with wider loader buckets or prevent material spillage over the far 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 even three in the case of the MOBISCREEN MSS 802 EVO. This enables optimum material transfer, even at the low feed height of an upstream crusher.

Classifying screens – the refiners

Classifying screens are the specialists for high-quality end products or important intermediate products. The name says it all: this is where finer, usually pre-crushed, material with smaller pieces is processed. Classifying screens are essential components in multi-stage crushing and screening scenarios 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. The wide hopper opening guarantees a good material flow. As classifying screens are usually fed with finer material, they do not require as much in the way of impact or wear protection as 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 a classifying screen plant is inclined against the conveying direction of the feeding conveyor and thus deflects the material flow in the opposite direction, this deflection would lead to blockages in a scalper due to the often extremely coarse feed material. In the case of scalpers, oversize grain flows in one direction from the hopper-feed conveyor over the upper deck of the screen casing and onto the main discharge conveyor.

A perfect material flow is decisive

It determines the material throughput and the final result, which must be precisely classified. The requirements for the material flow begin with the material feeding. For instance, the speed of the feed conveyor has a direct influence on the layer height on the various decks of the screen casing. Should the layer 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 due to a complex interplay between the speed settings of the hopper-discharge conveyor and the feed conveyor, the setting of the dosing flap, the position of the feed conveyor relative to the screen casing, the setting of the impact plate, the screen-casing inclination and its amplitude, and – last but not least – the right choice of screen media. There are fewer influencing parameters in the case of a scalper, because the quality requirements for the screened material are generally 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 screen media

The choice of the right screen media for a screening plant can be compared to choosing the right tyres for your car. If the wrong tyres are chosen, the vehicle will be unable to perform at its best on the road and may fail in difficult situations such as aquaplaning. This means that even the world’s best screening plant will not deliver good screening results without screen media tailored to the application. Here, it is not only a matter of choosing the right opening cross-section for the target grain size.

The upper decks of scalpers are often subjected to high loads due to coarse, sharp-edged pieces of rock impacting on the screen media and causing enormous strain on the material. Robust screening media are needed here. This is why finger screens, slotted grates or punched plates are often used on the upper deck of a scalper, depending on the application.

The situation gets trickier as the material to be screened gets finer:

Depending on the selectivity requirements and screening capacity, dry material down 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. Let’s go into more detail here:

• Thinner wires reduce the risk of incrustations, increase the size of the screening surface of the screen casing and, as a result of this, the output of the screening plant. At the same time, the thinner the wire, the shorter the service life of the screen media. Here, a balance must be struck between the higher performance, the costs of replacement screen media and the lost time when the plant is shut down to change them.
• The pattern of wire mesh screens can also help to protect them against clogging, or, at least, keep them free for a longer period. In certain harp screen patterns, induced vibration of the meshes shakes them free of incrusted material.
• In such extreme conditions, the material also plays a decisive role. For instance, stainless steel mesh is not as prone to incrustation as conventional wire mesh.
• 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 rib between the openings due to the material used, which leads to a reduction of the screening capacity, and the generally high cost.

The screening process and selectivity

To ensure ideal selectivity, it is decisive to be able to regulate the dwell time of the material on the screen deck according to requirements. This is carried out primarily by adjusting the inclination of the screen casing. A steep inclination of the screen casing increases the screening capacity but can, under certain circumstances, lead to poorer product quality due to undersize grains in the oversize grains. Although using a shallow inclination 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 product quality.

An even broader range of screen inclination settings is required to enable classifying screens to screen typically finer grain sizes in high quality. In the case of the MOBISCREEN MSC EVO classifying screens, the range is from 20–38°.