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%.
Mill-dryer coating technology is now applied by the world's leading suppliers of high quality fillers and coated minerals. Slurry injection systems play an important part in its success as it is possible to simultaneously dry, de-agglomerate and 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.
In some dryer types this 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 mill-dryer. The slurry typically enters from the injector into the lower body of the mill-dryer mixing with the incoming process hot air at around 400ºC. Any surfactant is typically fed higher up the body from a compressed air assisted venture 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 co-ordinated loss-in-weight feeding systems.
Dry coated powder production rates for a range of mill-dryers are typically as follows:
