Brittle ceramic granulates, for example spray-dried press granulates made of alumina, zirconia, silicon carbide, ferrites or other technical ceramics, frequently need to be subsequently mixed with binders, lubricants, pigments, additives or dopants. This involves a conflict of objectives: the additives need to be distributed homogeneously across the entire batch, while the granulate particles must retain, as far as possible, the size, shape, porosity and flowability important for the shaping process.

Particle integrity is essential for the subsequent pressing process. Spray-dried granulates are used because their largely near-spherical shape and defined particle-size distribution support flowability and the reproducible filling of press tools. Changes in the particle-size distribution, in particular an increase in the fine fraction through abrasion or breakage, can change bulk density, flow behaviour, mould filling and compaction behaviour. The granulate size and its distribution in turn influence the green density, density gradients and behaviour during sintering.

For such tasks, mixing systems with low mechanical stress are fundamentally advantageous. Container, tumble, V-shell or double-cone mixers can be suitable where the components have similar particle properties and sufficient time is available for a diffusion-dominated mixture. These systems work mainly through rolling, restratifying and repeated splitting of the product layers. They avoid fast-running tools within the product space, but offer only limited options for distributing very small dosing quantities or introducing sticky liquids.

Vertical cone-screw mixers can be an alternative where gentle, convective circulation is required. A wall-hugging screw lifts the product while it falls back under gravity. With a suitable rotational speed and tool geometry, the resulting stress can be kept low. Slow-running twin-shaft paddle mixers can also be suitable for larger batches if their mixing intensity is matched to the breaking strength of the granulate. In general: a mixing system must not be assessed on the basis of its design type alone. Rotational speed, fill level, tool geometry, mixing duration, feed conditions and discharge substantially influence the actual particle stress.

High-intensity systems with fast-running size-reduction tools, cutting rotors or pronounced shear and impact zones are usually not the first choice for already finished, brittle press granulates. They can be sensible where agglomerates need to be specifically broken up or liquids need to be incorporated. For the pure final mixing of brittle granulates, however, the risk of abrasion and particle breakage rises with high mixing intensity. Whether a ploughshare mixer or another intensive mixing system can nevertheless be used depends on the granulate strength, the required mixing time and the permissible change in the fine fraction.

The process parameters must be set so that the desired homogeneity is achieved with minimal mechanical stress. Low rotational speeds and controlled product movement limit impact and friction energy. With rotating vessel mixers, the Froude number can serve as guidance for the state of motion. Rolling or gently cascading operation is generally gentler than throwing or centrifugal operation. However, a universally applicable target Froude number cannot be given, because vessel geometry, fill level, granulate properties and internals strongly influence the motion behaviour.

The fill level must suit the particular mixer. A fill level that is too low can cause large drop heights and increased particle velocity. A fill level that is too high reduces the effective restratification and can lead to longer mixing times. Instead of a generally valid fill-level range, the working range envisaged by the manufacturer should therefore be used as a starting point and then optimised with the original granulate.

The mixing time must be limited to the actual homogeneity requirement. Once the required mixing quality has been reached, additional processing time frequently only leads to further abrasion. This so-called over-mixing is determined through trials: representative samples are taken at various points in time, and both the distribution of a marker component and the particle-size distribution are assessed. The coefficient of variation of a suitable tracer or active component can be used for mixing quality. A blanket limit, such as a coefficient of variation below 5 percent, however, is not appropriate for all ceramic recipes. The permissible value depends on dosing quantity, sampling, analytical method and the quality requirements of the end product.

Particle integrity is assessed before and after the mixing process on the basis of the particle-size distribution. With press granulates, sieve analysis or imaging methods are often particularly informative; laser diffraction can be used in addition, provided the measurement method does not already destroy the granulate particles during sample preparation. At a minimum, fine fraction, oversize, d10, d50, d90 and the distribution width should be recorded. In addition, bulk density, tapped density, Hausner ratio, flow time and, where applicable, the breaking strength of individual granulates can be assessed. An increase in the fine fraction, changes in the Hausner ratio or an altered flow time can indicate attrition or structural changes.

Alongside the mixer itself, the entire periphery influences granulate abrasion. Critical points are feed locations, screw conveyors, pneumatic conveying sections, rotary valves, transfer points, discharge elements and drop heights. The granulate should be introduced into the mixer as evenly as possible and with a low drop height, and discharged gently after mixing. Baffle plates can reduce fall energy but can also cause additional friction or caking. The design must therefore be chosen on a product-specific basis. Gently guided chutes, matched transfer chutes and short free-fall distances are frequently more favourable than uncontrolled product drops.

With very small addition quantities, a premix can be sensible. The additive substance is first mixed with a small partial quantity of the carrier material; this premix is then combined with the main batch. This can reduce the mixing time required for the sensitive overall batch. Whether the method is suitable depends on whether the carrier fraction has the same particle structure and whether the premix itself can be produced without relevant abrasion.

The recipe also influences the robustness of the granulates. Binder content, residual moisture, granulation conditions and the drying profile determine the strength and deformability of the particles. An increase in granulate strength can reduce abrasion, but must not impair the pressing and sintering behaviour. Spray-dried granulates are frequently formulated deliberately so that they flow and transport well on the one hand, while breaking or deforming in a controlled way during pressing on the other. The optimal strength must therefore be designed for the entire process path — processing, mixing, transport, mould filling and compaction.