Adding value to ultrafine minerals can be achieved by coating the product and NETZSCH Ecutec knows how to do it.
COATING SYSTEMS FOR FUNCTIONAL FILLERS Whether it is calcium carbonate in plastic applications, barite in drilling muds, glass or silica in rubber products, many fine powders have to be coated with certain additives to make them into functional fillers. Since the new technologies can produce much finer
product, the coating systems face a more difficult task to efficiently de-agglomerate these ultra-fine particles; while at the same time applying a thin layer of additives on each particle.
CALCIUM CARBONATE CASE STUDY
Coating calcium carbonate with stearic acid to prepare it for certain plastic extrusion processes provides an example to demonstrate how new technology developed by NETZSCH Ecutec can improve the coating process. Having been involved in the calcium carbonate business for many years, NETZSCH Ecutec was able to pinpoint key areas for optimization when designing a new process solution.
These were: ●
the need to fully de-agglomerate the ultrafine filler before applying any coating agent;
● use as little as possible of the costly coating agent; ● aim to achieve a perfect single layer of coating; ● avoid creating agglomerates after the coating process.
ECUTEC
FULL DE-AGGLOMERATION BEFORE APPLICATION OF COATING AGENT As a rule, the finer the particles to be coated, the bigger the Van der Waals forces acting on them will be, and the more energy is needed for de-agglomerating these particles. Typical particle sizes for such coating processes are in the range of d98 3µm to d98 15µm. The particles of such fine powders can only be separated by high shear forces, which can be generated by devices such as counter rotating pin mills. These mills contain two opposite pin discs, which run at a speed of up to 120 meters/second, resulting in shear velocities of about 240 meters/second. Only once the particles have been separated can the coating agent be applied; if the agent was added earlier, the agglomerates would be coated rather than the individual particles.
USING AS LITTLE OF THE COATING AGENT AS POSSIBLE In processing technology, everything is about economics. It is therefore important not to waste costly stearic acid, but try to distribute it as efficiently as possible onto the ultrafine particles. For different end-product finenesses, different levels of coating grades have to be used due to diverse surface areas. To achieve the right coating grade for each individual fineness, the mass flow of calcium carbonate, as well as stearic acid, has to monitored and controlled.
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