Mineral powders are now extensively used as fillers and extenders in a whole range of applications. Many of these require coating or surface modification to enhance the functionality of the base material. Clays used in these applications, particularly kaolin, will undergo a beneficiation process leading to a filter cake that requires drying.

The various forms of calcium carbonate will generally be wet milled to an ultra-fine particle size and then require drying. High-energy mill-dryers offer the perfect solution to process these materials to a dry, classified form and where required, a coated product in one unit operation. This is much more efficient than the traditional method where they are first dried and then milled or de-agglomerated in a subsequent operation. These separate operations involve making and then breaking inter-particle bonds, which in simultaneous processing generally do not form. Therefore, simultaneous drying and de-agglomeration should increase process energy efficiency.

With their complex interactive forces and high turbulence - tip speeds up to 120m/s- advanced mill-dryers can flash dry to less than 0.1% moisture and achieve a highly uniform  surface  coating.  Fitted  with  an

integral classifier, the unit offers improved control of top particle size down to~20µm. For already fine materials fed as slurry or press cake, these units achieve close to total de-agglomeration.

A mill-dryer system can process a range of fine wet-milled mineral products, presented to the process as 65/35 solids/water suspension and to particle size specifications generally less than 10µm with a high percentage below 2µm.

The system includes an injector for  the  aqueous  suspension.  To

ensure that any residual coarse agglomerates are rejected from the dry product stream, a mill-dryer may be fitted with a high integrity classifier unit. The classifier has an inverter controlled variable speed drive to allow adjustment of cut point, and the coarse particles are rejected back into the base of the dryer.

Some dryer classifiers reject oversize externally. This gives the operator a choice of whether to reintroduce this material immediately into the feed stream, to reintroduce it further upstream, or to periodically reject it in the case of some reacted product dryers, or where there maybe a build up of abrasive impurities. External rejection may also be better in the case of heat-sensitive products such as organo-clays.

Air is drawn through a packaged gas or oil-fired heater designed to raise the air temperature from ambient to ~400ºC. Control of the dryer outlet temperature at around 90ºC maintains product moisture level. Spray injection occurs in the bottom section of the mill-dryer body.

In some mill-dryers process cells are created between the serrated body liners and the closely orientated high velocity rotor blades; the high turbulence within these cells giving perfect conditions for optimum particle de-agglomeration and for efficient flash drying with maximum exposure of product surface to hot air.

The metered surfactant is typically injected by a compressed air driven venture (pipe with constriction) into a port above the slurry entry point. After final classification, ducting carries the air and dry product from the mill-dryer to a reverse-pulse product collection filter. The duct has a sensor to monitor the temperature of the air/product mixture as part of the air heater control system, providing a consistently dry product. The product is discharged from the filter through a rotary valve and the filtered air passes to the main process fan.

George Milburn.