During the production and subsequent processing of natural and crushed sand, a fine dust is created by abrasion in the crusher, in the screening equipment and during conveyor transport. This fraction of fine dust with a grain size below 90 µm is designated as filler. As a rule, the proportion of filler content created in this way lies between 15 and 25%. Because many applications require filler content of less than 10% and as low as 5%, the proportion must be reduced to meet the desired value. In principle, there are two different processes that can be applied to this task: wet or dry processing.

In wet processing plants, water is already added to the screening machines and the filler is removed in a washer. The advantages of this system lie in the fact that the filler fraction can be reduced to virtually 0% and that the filler material is precipitated in sludge form, which makes it easier to dispose of at landfills. In this multi-stage process, however, investment costs are high and the cost of the plant design with respect to issues such as wear and corrosion is correspondingly high. If the final material is required in dry form, it must be dried using special drying equipment, whereby investment and operating costs are again incurred.

In contrast, dry processes are inherently simpler and must be dedusted using a filtration plant. The filler can be reduced using special screening machines or a cascaded classifier. Because the screening machines require a higher capital investment than the classifier and because the screens must be replaced at regular intervals as the result of wear-and-tear, the cascaded classifier is the more economical solution. An additional advantage of the cascaded classifier is that it can be integrated in the plant’s dedusting equipment with little additional expense.

The Problem of Sand Classification:

The difficulty here lies in the break-up of the agglomerates, i.e., the separation of fine fractions from coarse fractions and the achievement of a high degree of selectivity in the airflow with respect to coarse and fine material. Dimensioning of the airflow is determined by the so-called critical particle size in that the upward velocity of the airflow is adjusted to exactly match the settling speed of particles with this critical size. Particles with this critical size are those particles with a grain size that is precipitated 50% of the time.

An important prerequisite for optimal performance when removing filler material is that the moisture content of the sand is less than 1.5%. As the moisture level increases, so too does the fraction of grains that adhere to each other, which requires higher shearing forces to break the bonds between these grains. To ensure efficient separation and economical operation, one must strive for a distribution of material and air that is as homogeneous as possible across the entire width of the classifier.

Scheuch Sand Classifier

Based on years of experience in classifying many kinds of materials (e.g., MDF fibres, sawmill chips, construction rubble, gravel, waste material), Scheuch developed a special cascading classifier for the removal of filler material in the gravel and crushed stone industries.

As a rule, a conveyor (1) and a specially designed pendulum flap (2) feed the material from above and into the classifier (3). The pendulum flap ensures that as little leak air as possible enters the classifier, thus preventing problems with selectivity. The material flows from top to bottom over several cascades. The air required for the classification process is drawn in from below and is then blown cross-flow and at high speeds through the material by the cascades, which function here as nozzles. This creates high shearing forces that separate ultra-fine grains from coarse grains.

It is important for subsequent gravity separation to achieve a uniform up-current flow. In this process, coarse-grained material falls down and out of the classifier via a pendulum flap (5). Ultra-fine material is carried upward by the air stream and fed to a dedusting plant (6) before being discharged via a rotary valve (7).

The special advantage of Scheuch’s new development is the outstanding separation performance with variably adjustable removal of filler material from, for example, 25% filler content to 10% or, given the appropriate conditions, to even lower levels of about 5%.

The classification result can be further optimised by using forced ventilation, an option in which an additional fan connected to the air inlet openings (4) compensates for the low pressure in the classifier. This minimises the disruptive influence of leak air on material intake and discharge and results in a more homogeneous airflow, which improves selectivity. Removal of filler material takes place with a smaller fraction of oversized grains.

During commissioning, the service technician optimises the classifier settings with respect to the desired degree of filler removal, the settling speed of the critical particle size and the moisture of the material. If the composition of the initial material changes or different degrees of filler removal or critical particle size are required, the operator can fine-tune these settings later by adjusting the overall air volume.

During development of this classifier, special attention was paid to anti-wear protection and thus a long service life for the equipment. For example, optimised air guidance in the classifier casing minimises velocity spikes near the walls and wear-resistant materials are used on surfaces endangered by wear. Cascaded classifiers from Scheuch are capable of handling maximum material amounts of 85 t/h.

Practical experience

The first classifier using this system began operation in 2000 at an Austrian gravel plant operated by Schotterwerk Grandenberg GmbH. The plant processes a mass flow of crushed sand with a grain size of 0/2 at a rate of circa 70 t/h. The classifier has no difficulty in reducing the filler content of the final product from roughly 25 – 28% to a maximum fraction between 10 and 11%. The classifier was integrated in the dedusting unit for the plant’s screening equipment and secondary crusher.

ALAS International AG of Ohlsdorf/Austria commissioned Scheuch to supply dust filtration equipment for the crusher, conveyor lines and screeners operating at a hard rock processing plant in Okucani, Croatia. Scheuch installed an IMPULSE bag filter dimensioned for an air volume of 40,000 m3 /h with dust levels in the range 30 – 50 g/m3. Scheuch also installed the corresponding discharge equipment and a pneumatic transport system leading from the filter to the silo.

The separator is loaded hourly with a portion of the material (0 – 4 mm), up to a maximum amount of 10 tons, and the filler content (<90 µm) is reduced from 20% to less than 10%.

The cascaded classifier can also be used in applications involving special materials with different densities, grain sizes and critical particle sizes of different diameters. As an example, a plant was constructed in Bahrain to minimise the fine fractions in petroleum coke to < 0.5 mm.

For more information, please visit:

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Illustrations:

Fig. 1Grain with adhering filler material

Fig. 2Grain without filler material

Fig. 3Process design: classification and dedusting

Fig. 4Operating principle of Scheuch’s cascaded classifier

Fig. 5Classification plant for removing filler from limestone

Fig. 6Classification plant for removing filler from hard rock

Fig. 1Grain with adhering filler material

Attachments

scheuch_1_28.9.04 (JPG)

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