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Wet and dry dilemmas

Anyone who has listened to key note speeches at recent IM conferences will recognise the mantra of "added value" stressed above all else as the key to future success in the processing and marketing of industrial minerals.

To maintain leading edge products, the processes used to make them are regularly reviewed to address precise customer demands and justify the use of adjectives such as "speciality", "high performance", "super" and others, often used by suppliers to describe    or    differentiate   the
* An installed high energy dryer-pulveriser-coating unit for stearate coated GCC
suitability and application of their products.

These demands can be seen especially in the unit operation of drying on which most other stages of mineral production exert an influence and on which the quality assurance of the finished product can so highly depend.

For example, if the mineral is wet milled, the particle size distribution (PSD) will affect the de-watering process, and this in turn will decide how much moisture has to be driven off in the drying stage. Similarly, this wet milled material should preferably be de-agglomerated to as near the PSD in the dry stage as was measured in liquid suspension.


* Kaolin beneficiation by froth flotation. Courtesy of Imerys

This may be easier to achieve if drying and de-agglomeration are carried out simultaneously in a dryer-pulveriser or mill-dryer, rather than drying the material into lumps which will then need even more energy to disintegrate in a separate milling and classification process. To make the material feedable to the drying stage it maybe necessary to back-mix with recycled dry product,     which    can    create
problems if this dry product has already been coated with a dispersant aid.

To avoid this latter problem it may be more economical to simultaneously dry, de-agglomerate and coat the material by spraying the wet-milled slurry directly into the dryer/de-agglomerator along with the dispersant agent. The methods used to achieve this are described later in the article.

 Assisting mechanical dewatering

Generally speaking, mechanical dewatering techniques are more cost efficient than thermal drying for the removal of moisture. Extremely fine minerals can be efficiently dewatered using mechanical means (centrifuges, pressure membranes, presses, etc) but these fine PSD materials have a limit in respect to the amount of water that can be removed in this way. The solids content after dewatering can be as low as ~35% with  very  fine  minerals.

This therefore requires thermal drying to complete the process from various points in the dewatering process. To save thermal energy, it is desirable to have the feed material as dewatered as possible, which may also influence the feeding of the material to the thermal dryer. If it is too sticky or sloppy, it may block the dryer feed system causing regular stoppages in production. Therefore, often the mechanical process has to be "assisted" with chemicals that aid the dewatering.

These flocculating  or  coagulating
* CM 1500 Cell Mill - Dryer. Under construction, Atritor Ltd, Coventry. Showing stearic acid injection parts
agents can in some cases impact on the properties of the dried material, particularly in respect to the ease of de-agglomeration and dispersion in dryers/dispersers that are not highly efficient or low in energy intensity. The finer the minerals, generally the higher the level of dewatering aid chemicals required.

With requirements to maintain environ-mental standards, dewatering circuits are generally closed to avoid unnecessary discharge of chemical laden effluent. With the need to recycle more of this water, the levels of these chemicals have to be controlled in order to avoid their build up.

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