Mechanical dewatering, often to reduce the thermal load of the final drying stage, is achieved by a variety of devices, eg. Rotary vacuum filters, dewatering centrifuges, plate presses, tube presses, evaporation and sedimentation operations. Higher pressures and the use of dewatering aids helps to extend the limit of the mechanical dewatering process and for the production of slurry end products this is often sufficient on its own.

For the production of powders, other than for the conditioning of a dryer feed (adding dry back mix with the wet feed to obtain a material that can be mechanically handled and transported within the system), there is no way of achieving a dry end product without the use of thermal drying and dispersion.

Fluid bed and flash dryers are effective for thermally drying materials but the levels of dispersion required in finished products invariably mean that, in most drying operations, de-agglomeration is required. If this process can be carried out in a single stage with the added simultaneous application of any surface treatment, this is most energy efficient and normally requires lower capital investment and plant building space.

As indicated above, materials that require this type of dewatering and drying are mainly those that are recovered from a wet process such as precipitation, wet milling to achieve particular PSD's or minerals that have to be beneficiated by bleaching, flotation, etc.

Efficiency, particularly in the face of increasing energy costs, is a key word in the development and improvement of mineral drying technology. Choice of technology and identifying how a producer might work with the dryer manufacturer to improve process efficiency are becoming more commonplace.

The key capability is to obtain very low moisture levels from very fine materials. This requires efficient dispersion of the mineral in the drying and de-agglomeration stage in order to get the maximum exposure of the mineral surface to the hot air.

This is even better when combined with high turbulence to shorten the dryer residence time and to improve the efficiency of any surface coating required. This ability to dry and coat (where appropriate) with the minimum pre-preparation and conditioning of feed to the dryer also offers some advantages as the number of process and handling stages can be reduced. Therefore the technology of simultaneous drying, dispersion, coating and de-agglomeration is becoming a more and more attractive proposition for minerals producers.

Without a doubt, the requirements of end users are becoming more and more demanding. Most producers recognise that mineral fillers must now have particular functional properties in order to give the end user the performance that is required as they themselves look to improving their own products with respect to quality and consistency. In addition to this, the engineering of these mineral products can also offer enhanced processing and 'run-ability' for the customer, helping to optimise process output and stability and reduce downtime.

An example of this is in the production of polymer based products, eg. UPVC where the mineral filler, predominantly coated calcium carbonate, has a strong influence on the processing and extrusion of the compound and the end properties-strength, finish, etc.

The fine minerals required for these applications are  very  often

wet processed for the precise control of particle size distribution and refining in order to achieve a very clean top cut. The achievement of this clean top cut must not then be sacrificed during drying where the formation and presence of agglomerates or aggregates impacts very significantly on end processing.

These products are also predominantly surface treated with stearic acid in order to improve dispersion into polymer systems. Where possible, for the purpose of reducing process complexity and investment, the drying and coating of such materials in a single stage offers significant advantages. But this is feasible only if the drying and dispersion is effective.

In the polymer application industry, well dispersed surface treated mineral fillers with extremely clean top cuts contribute to increased machine output and extruded profiles with good gloss and no surface imperfections. It is also possible to achieve higher loading percentage levels of the mineral, which can help offset the cost of the more expensive polymers.

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