- THE MAGAZINE
Conventional technology quickly reaches its limits in trying to meet these challenges, and manufacturers have to look to new technologies to help them keep up with industry advancements.
Ferrites, molding materials and other compounds for high performance ceramics greatly depend on the quality of raw materials used and their subsequent preparation. The polycrystalline microstructure formed during mixing, dispersing, homogenizing, granulating and cooling of these materials significantly influences the quality of the end product. Most technical applications require a high density of the pressed ceramic body.
Ceramic Preparation Under VacuumA new ceramics process technology (Evactherm® [Eirich Vacuum Thermo Technology], a registered trademark of Eirich Machines, Inc.) uses superheated steam and vacuum to combine five preparation steps in a single machine, providing the answer to many of these challenges. The system can provide improved homogeneity in the mix, especially of minor ingredients, higher product density and improved mix consistency.
MixingThe nucleus of the new system is a high-intensity mixer with an inclined, rotating mixing pan in a vacuum-capable shell (see Figures 1 and 2). A counter-rotating mixing tool (rotor) and a stationary multi-purpose device reach into the pan from above. The rotating pan carries the mix materials in a circular motion. Where this flow encounters the stationary tool, it is deflected into the rotor, affecting high-intensity mixing. The design eliminates dead zones inside the mixer, optimizes power distribution into the batch, and forces all the materials in the batch to participate in the mixing process. The result is a homogenous blend in a very short period of time.
In the preparation of a ferrite mix under vacuum, for example, the materials are first over-moistened to a wetness of approximately 20% to achieve a highly consistent, easy-to-knead mix. The high differential speeds during this preparation phase cause high shear, resulting in maximum dispersion and homogenization. This is especially true for minor ingredients, which are often difficult to uniformly moisten and disperse with conventional methods.
A prerequisite for a high quality final product, the semi-wet mixing also prevents the building of any secondary agglomerates, which are almost certain to develop in drier mixes.
Vacuum Drying and GranulatingThe moisture in the mix is removed through evaporative drying under a light vacuum. The level of vacuum and the associated thermal requirement to induce such evaporation is dictated by the thermodynamic characteristics of the liquid component to be removed. Progressive drying in combination with vigorous agitation of the mixer results in the formation of spherical, solid grains, with large granules forming first and smaller ones forming toward the end of the cycle.
The grain spectrum is both variable and reproducible by adjusting the speeds of the rotating mixing pan and rotor, choosing an agitator design appropriate for the individual process, and controlling the moisture content of the granules.
Drying under vacuum results in further improving the consistency of the mix, increasing shear forces and leading to increased homogeneity and higher product density (see Table 1).
Operating CostSpace Requirements, Material Handling and Cycle Times
While most conventional systems (see Figure 3) consist of a mixer, a separate pelletizer, possibly one or more mills, and material handling equipment to move the mix from one to the other, the new system uses a high intensity mixer to perform all five process steps-mixing, dispersing, homogenizing, granulating and drying-in a single, compact unit (see Figure 4).
Conventional methods call for dry mixing of the various powdery components to disperse ingredients, and mixing times vary greatly, depending on the type of components. The new process granulates the semi-wet bodies in approximately 30 minutes, depending on the types of raw materials used. Slurrying, which may be required in some processes, and material handling between machines is eliminated. No mills are needed since the new system produces granules in the pre-determined size range.
Since the system takes the place of two or more machines, production capacity can be increased in the same amount of space previously occupied by a conventional system.
While the new system, in addition to the mixer itself, does require the installation of a vacuum pump with peripheral controls to allow a vacuum to be drawn, it eliminates several pieces of equipment, reducing maintenance efforts drastically compared to a conventional system.
The stationary tool mentioned earlier also acts as a combination wall/bottom scraper in the mixing pan. It prevents materials from sticking to the sides of the pan and aids in the discharge of materials through the bottom discharge. This makes the unit almost self-cleaning, and also allows for fast changeover between recipes.
Large side doors provide access to the inside of the mixing pan; mixing tools can be replaced quickly and without special tools.
Energy Consumption and Batch Cycle Times
The short mixing cycles described, and the fact that all of the process steps are performed in a single machine, lead to savings in energy consumption. The system is dust tight, and any heating medium remains in a closed circuit, making the system both environmentally friendly and safe.
A small system is available for testing at the manufacturer's test lab to determine actual material response characteristics and cycle times for individual applications.