Mineral powders are coated with a variety of materials as a surface modifier, in most cases to improve the dispersion performance of the mineral when used as a filler in products such as PVC extrusions. Stearic acid is commonly used with carbonates and amines are applied to kaolin. Silanes and titanates are used for application specific duties.

Methods vary according to both the degree of coating accuracy required and the application. In some cases the accuracy of application must be within + 1% error; in others it may be as low as + 10% of the weight of coating agent.

The coating method in each case is designed to suit the application. Coating levels can vary from as low 0.5% to over 2%.

The Atritor Cell Mill-Dryer-Coating technology is now applied by the world's leading suppliers of high quality fillers and coated minerals. With Atritor's patented slurry injection system it is possible to simultaneously dry, de-agglomerate & coat the mineral, fed as a slurry from the wet milling stage, the dry powder typically being all less than 35µm with high efficiency de-agglomeration of sub 2µm particles.

This process involves a 2-fluid injector system with high pressure steam or hot compressed air as an atomising medium to spray the wet-milled mineral slurry, pumped from a buffer vessel, into the Cell Mill. The slurry enters from the injector into the lower body of the Cell Mill-Dryer mixing with the incoming process hot air at around 400ºC. The surfactant is typically fed higher up the Mill-Dryer body from a compressed air assisted venturi system. The feeding method is as sophisticated as required by the level of accuracy demanded for the application and can vary from simple volumetric feeders to coordinated loss-in-weight feeding systems.

Dry coated powder production rates for the various sizes of Atritor Cell Mill-Dryers are typically as follows, the first column based upon a 35% moisture feedstock:

The air heater output varies over this range of drying duties from 500kW to 3MW.

Specific energy for the plant other than the heater is ~70kWh/t for simultaneous drying & coating from 35% moisture and ~25kWh/t for coating only of dry material.

For the simultaneous drying & coating plant, the air heater specific energy is ~500kWh/t. For coating only it is ~50kWh/t.

Total specific energy for simultaneous drying and coating: ~570kWh/t
Total specific energy for coating only: ~75kWh/t

Slurries vary from 28 - 50% moisture. The above drying & coating specific energies are based on 35% moisture. Higher power motors can be fitted along with an integral classifier if the unit is to be used for primary milling. In some cases the classifier (-IC) option is chosen for drying & de-agglomeration applications where quality assurance demands an absolute control of top size.