Which mixing systems are recommended for ceramic powders with large differences in density and grain size?
For ceramic powders with large differences in density and grain size, mixing systems are needed that generate intensive three-dimensional restratification while also limiting segregation during mixing, discharge and subsequent conveying. Fine and coarse, as well as light and heavy, particles can separate again through percolation, vibration or falling motion. The mixer type alone is therefore not decisive; the entire process management, including dosing, mixing time, discharge and conveying technology, is decisive as well.
For demanding dry mixtures, intensive mixers with a rotating mixing vessel and a separately driven mixing tool are often particularly suitable. They combine large-scale circulation with precisely adjustable shear action. This allows fine additives to be distributed and existing agglomerates to be broken down where necessary. Such systems are particularly suitable for ceramic raw material mixtures with large density differences, small addition quantities or a pronounced tendency to agglomerate. Intensive mixers can also be used to mix and granulate in a single process step.
Vertical single- and twin-shaft mixers are an alternative where high mixing quality needs to be combined with comparatively gentle product treatment. They generate a three-dimensional material flow and are suitable for press compounds, spray-dried granulates, and mixtures with sintering aids, pigments or functional additives. Mixing-tool geometry, speed and mixing time must be chosen so that the desired homogeneity is achieved without unnecessarily destroying sensitive granulates.
Ploughshare and paddle mixers can also be useful for medium to large batches. They generate an intensive movement of the product bed and can be equipped with separately switchable choppers for de-agglomeration. For abrasive ceramic powders, a wear-resistant design of the product-contact surfaces is important. The intensive tools should be used only as strongly and for as long as the recipe requires.
Conical screw mixers are particularly suitable for spray-dried or breakage-sensitive ceramic granulates. They operate with lower mechanical stress and can enable good homogenisation. With extreme density differences or very fine, agglomeration-prone additives, however, their dispersing action may not be sufficient. In that case, an upstream de-agglomeration step or a more intensive mixing principle is preferable.
Planetary mixers are suitable above all for ceramic slips, pastes, highly filled suspensions and CIM feedstocks. They enable controlled blending of highly viscous systems with binders, dispersants and ceramic solids. For pure dry powder mixtures, on the other hand, they are usually not the first choice.
Tumble, drum and V-mixers operate very gently and cause only minor abrasion. With large differences in density, grain size or particle shape, however, they need to be assessed critically. The gravity-dominated movement can homogenise, but it can equally encourage segregation. They are therefore better suited to smaller batches and mixtures with similar particle properties.
For particularly critical recipes, wet mixing followed by spray drying is often the most robust solution. The components are first distributed homogeneously in a suspension. Spray drying then produces a defined, free-flowing press granulate. This process is used for ceramic and metallic suspensions and enables the production of press granulates with precisely adjustable properties.
The order of addition, mixing duration and moisture control are decisive. Fine additives should be dosed so that they can be distributed reliably. Excessively long mixing times, in turn, can promote segregation or abrasion. With abrasive powders, materials and surfaces must be chosen so that wear and the ingress of foreign particles remain limited. Homogeneity should be checked with representative sampling and product-specific analysis.
For dry ceramic powders with strong differences in density and grain size, intensive mixers or high-performance vertical mixers are usually the first choice. Where the recipe is particularly critical or demands the highest distribution uniformity, wet mixing followed by spray drying often offers the most reliable process route.
How amixon® masters large density and particle-size differences in mixing
amixon® supports the homogenisation of ceramic raw materials with large differences in bulk density, grain size and flow behaviour. At the centre is a controlled, three-dimensional product movement. It enables an even distribution of the components and limits the tendency to segregate ahead of subsequent process steps.
The SinConvex® helical mixing tool conveys the mix upward at the periphery of the vessel. At the centre it flows back down under gravity. This continuous restratification operates at low rotational frequency and is suitable for differing bulk densities, particle sizes and flow properties. SinConvex® and SinConcave® can also support very good residual discharge and make cleaning easier.
For large batches, amixon® offers the Gyraton® mixing silo. It combines mixing, buffering and discharging in a single apparatus. The vertical helical mixing tool rotates while simultaneously moving orbitally along the wall of the mixing chamber. This deliberately captures large areas of the mixing chamber. Gyraton® mixing silos are designed for large, free-flowing bulk-material quantities and, depending on the design and product, can homogenise batches of up to around 100 m³.
The low energy input of the Gyraton® mixing silo is particularly advantageous for sensitive granulates, spray-dried press compounds and abrasive raw materials. Gentle homogenisation can be achieved even with large batches. This is relevant for ceramic process routes because as uniform a starting mixture as possible supports the reproducibility of subsequent steps such as granulating, pressing, sintering or coating.
For abrasive ceramic powders, amixon® can execute the mixing chamber and mixing tools with wear protection matched to the specific product. Oxide-ceramic coatings can significantly reduce metallic abrasion on product-contact surfaces. The suitable choice of materials depends on abrasiveness, purity requirements, grain size and the permissible contamination in the end product.
The final design is verified in the amixon® pilot plant with the original raw materials. There, mixing quality, mixing time, particle protection, residual discharge, abrasion behaviour and reproducibility can be assessed under realistic conditions. High-temperature reactors and vacuum mixer-dryers are also available for thermal and reactive process steps. They enable trials with original products at temperatures up to 350°C, under vacuum, or at pressures of up to 25 bar. This creates a robust basis for a reliable process route – from raw material homogenisation through to reproducible further processing.