The ultra-fine screening of dry mineral powders

A new approach to the ultra-fine screening of dry mineral powders is offered by Allgaier Process Technology , whose UK and Ireland agent is Orthos (Engineering)

The fine and ultra-fine screening, defined as the application of cut points in the range 30um - 200um, of powders even when dry is one of the most difficult tasks in the field of minerals processing.

It is essential to achieve an efficient recovery at an economic throughput rate of required products in the form of sharply defined fractions, as the products themselves are very often of high value and specification, and are widely used as constituents in pharmaceutical and cosmetic preparations, and as high-grade fillers in paints, plastics etc.

The difficulties arise, because the free aperture area of very fine meshes is a relatively low proportion of the whole mesh area, and because the powder particles are subject to adhesive surface forces, which cause them both to form loose agglomerates and to stick to the mesh surfaces.

The unaided action of vibratory screens and low frequency sifters is neither sufficient to shake the meshes free of a coating of powder, nor to break up the agglomerates efficiently.

The results are a low throughput rate, a high degree of contamination of the coarser fraction or fractions with fine material and inefficient product recovery.

Various screening aids have been devised in an attempt to deal with this problem.

Of these, bouncing balls offer a method frequently used in the screening of fine powders in flat-deck vibratory screens, plan sifters and round tumbler screens.

The balls are contained in a compartmented deck placed below the screen deck and impinge on the under surface of the mesh.

This method helps to keep the mesh apertures clear but does little to break up the agglomerates mentioned above.

Further disadvantages of this method are that damage to fragile mesh panels may occur, and material may be abraded from the balls and bring about product contamination.

Other methods used in circular tumbler screens are:.

* rotating, perforated arms, which direct a flow of air at the underside of the screen mesh to dislodge trapped particles: (this is coupled with air extraction from the lower section of the screen casing, and a filter system).

* rotating brushes placed beneath the screening surface.

* a combination of the above two.

* ultrasound.

The new approach to the ultra-fine screening of powders mentioned in the first paragraph introduces the idea of combining in one machine the two screening aids, air jets and ultra-sound.

This innovation together with the tumbler screen features described below leads to a much higher product yield without any loss of sharpness of fractionation.

The air flows, which, because of the positioning of the air extraction outlet, pass both up through the mesh and then back down, work to keep the mesh apertures clear; the ultra-sound, which can apply particle accelerations of up to 15,000g, breaks up agglomerates and also helps to keep the meshes free of powder build-up.

Recent screening tests carried out on various fine powders show that the two methods combined are much more effective than either on its own.

The analysis of trial results in the case of a powder, in which the required product was the 0 - 63um fraction, illustrate this.

Using ultra-sound only the recovery of the 0-63um fraction was 50.3% at a feed rate of 360kg/h; with air cleaning on its own product recovery was 58.3% at a feed rate of 277kg/h.

The two used together gave a product recovery of 83.8% at a feed rate of 515kg/h.

The product contained only 1% of misplaced material.

Circular tumbler screens are, of course, widely used in the screening of minerals and other materials, not only in the ultra-fine range, but also in feed particle size ranges up to ca10mm.

The feed is introduced centrally on to the top deck and, under the influence of the radial and tangential forces at play, follows a spiral path on the mesh.

In the centre of the machine there is virtually no vertical vibration of the mesh, and the machine acts on the powder like a plan sifter and removes a proportion of the finer particles.

As the radius of the spiral increases, however, the degree of vertical vibration becomes greater and more effective in the separation of the coarser particles, i.e it affords the coarser material greater opportunity to reach the mesh surface.

The spiral path followed by the feed material ensures that it is exposed to a long screening process within a machine occupying a relatively small area.

Machines are available in 11 diameter sizes ranging from 600mm to 2900mm, as are multi-deck and special configurations designed to deal with special screening requirements.

Extra deck modules can be fitted easily to existing machines, and ultra-sound cleaning can be installed without any problem in machines, which are already equipped with air-cleaning.

The Allgaier "MultiBalance" drive system is reported to neutralise ca96 - 97% of all dynamic forces generated by a machine.

It is adjustable and can, therefore, be matched to changes, which affect the weight of the machine.

Where several units are installed together, they can be switched in sequence such that small residual dynamic forces cancel each other out.

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