Which mixing systems support the dispersion of agglomerates in ceramic powders before the shaping process?
De-agglomeration of ceramic powders is an important step before the shaping process. Soft agglomerates can arise from Van der Waals forces, moisture, electrostatic interactions or incomplete wetting. If they remain in the system, they can lead to inhomogeneities, pores, density gradients or defects in the green and sintered body. Hard aggregates that have already formed during synthesis or calcination, by contrast, cannot always be broken up without undesirably reducing the size of the primary particles.
The suitable technique depends on whether the ceramic material is processed as a low-viscosity slip, a highly concentrated suspension, a plastic mass or a dry powder. Also decisive are agglomerate strength, solids content, target particle size, permissible foreign abrasion and the requirements of the subsequent shaping process. Complete de-agglomeration is not the goal in every case: with spray-dried press granulates, a defined granulate structure can be explicitly desired. In that case the aim is above all to remove disruptive coarse particles or hard agglomerates without unnecessarily destroying the pressable granulate structure.
For low- to medium-viscosity slips and glazes, rotor-stator systems are frequently used. The rotor accelerates the suspension while the material is guided through the narrow gap to the stator. This creates high local shear forces and turbulence, which support wetting, incorporation and the breaking up of soft agglomerates. Rotor-stator mixers are particularly suitable as a fast batch or inline step where powder needs to be drawn into a liquid and distributed evenly. Their effect, however, depends on gap geometry, rotational speed, number of passes, solids content and dispersing agent. With very hard agglomerates, or where a narrow ultra-fine particle-size distribution is required, they are often insufficient on their own.
Stirred bead mills, pearl mills and attritors are among the most effective systems for fine and ultra-fine dispersion of ceramic suspensions. Grinding media made of wear-resistant materials transmit impact, shear and friction forces to agglomerates. This can significantly reduce even firmer agglomerates. Such systems are used, for example, for alumina, zirconia, silicon carbide or other high-performance ceramic suspensions where a very homogeneous distribution of fine particles is required. The grinding-media material must be matched to the ceramic, since media abrasion can lead to contamination. Likewise, the grinding time must be limited and monitored so that primary particles are not unnecessarily damaged, unwanted phase changes are not triggered and too much fine fraction is not generated. Studies on titanium dioxide nanoparticles show, for example, that both media mills and ultrasound can be used for de-agglomeration, but their effect depends strongly on particle size and solids content.
For highly viscous ceramic masses and feedstocks, for example in ceramic injection moulding, planetary mixers, double-planetary mixers or kneaders come into consideration. They combine large-volume mixing with high local shear and kneading stress. This allows binder, plasticiser and ceramic powder to be distributed evenly and existing powder agglomerates to be broken up. With fine, agglomeration-prone powders, batch processes with planetary or sigma-blade kneaders are frequently used for producing ceramic feedstocks. Even binder distribution is essential here, because it can influence the flow behaviour of the feedstock and the shrinkage behaviour after sintering.
Intensive mixers working on the counter-current principle can be suitable for dry, moist or plastic ceramic masses. A rotating mixing vessel and a separately driven mixing tool generate relative movements with impact, friction and shear components. This allows components to be mixed in, moisture or binder to be distributed and soft agglomerates to be broken up. With spray-dried granulates for dry pressing, however, caution is required: too high an intensity can destroy the deliberately generated granulate structure, increase the fine fraction and thereby change the filling and pressing behaviour. In such applications, mixing intensity and residence time must be limited to the required product conditioning.
Jet mills can reduce dry powders further through particle-particle and particle-impact-surface collisions, or break up agglomerates. They are particularly suitable where no liquid is to be used and a narrow particle-size distribution is required. However, jet mills are primarily size-reduction units and not merely mixers. They can therefore also alter desired primary particles or press granulates. Their use is above all sensible for dry primary powders, not without further consideration for finished press granulate.
Ultrasound can support redispersion with fine ceramic suspensions and nanopowders. Cavitation generates locally high forces that can break up soft secondary agglomerates. This technique is used particularly in the laboratory, with smaller batches or as a downstream inline step. The achievable effect depends strongly on sonotrode geometry, power, treatment duration, solids content, temperature and the chemical stabilisation of the suspension. Excessively long or intensive ultrasound treatment is not automatically advantageous, because particles can re-agglomerate or the suspension can become undesirably heated. For zirconia and other oxide nanopowders, ultrasound has been described as an effective method for reducing secondary particle size.
Mechanical stress alone does not yet secure a permanently stable dispersion. Dispersing aids lower the interfacial tension, improve wetting and, through electrostatic or steric stabilisation, can limit re-agglomeration. The choice of dispersing agent, pH value, ionic strength and order of addition is at least as important as the choice of mixing or milling unit. For stable ceramic suspensions, these chemical parameters must be matched to solids content, particle surface and the subsequent shaping process. With zirconia and alumina suspensions, stabilisation is often achieved, for example, via adsorbed polyelectrolytes and a targeted adjustment of the interactions between particles.
Process control should not be based solely on mixing time or motor power. Meaningful quality characteristics are particle-size distribution, the proportion of coarse agglomerates, viscosity or flow curve, sedimentation behaviour, zeta potential in colloidal systems, green density and defect rate after sintering. Only the combination of suitable dispersing chemistry, appropriate energy input and analytical testing shows whether the desired conditioning has actually been achieved.
How amixon® supports the de-agglomeration of ceramic powders
In ceramic processing, agglomerates must be treated in a targeted way. A distinction must be made between soft agglomerates arising from storage, moisture or incomplete wetting, and hard aggregates. Soft agglomerates can often be broken up through a dosed mechanical energy input. Hard aggregates may require more intensive treatment or a separate milling process. At the same time, a desired granulate structure, for example with spray-dried press granulates, must not be unnecessarily destroyed.
amixon® therefore separates the tasks of homogenising and de-agglomerating. The SinConvex® or SinConcave® helical mixing tool takes on the three-dimensional restratification and macroscopic homogenisation of the entire batch. The circumferential speed and mixing duration are set product-specifically in order to limit the energy input and preserve the granulate structure as far as possible. A complete exclusion of heating or particle stress cannot be promised generally; both variables must be determined and monitored for the specific ceramic powder.
Where targeted de-agglomeration is required, separately driven cutting rotors can be used. These work locally within a limited processing zone and generate shear, impact and friction stresses there. Their rotational speed, tooling and duration of use can be set independently of the rotational speed of the main mixing tool. This allows the process to be guided from gentle breaking up of soft agglomerates through to more intensive de-agglomeration. Whether hard agglomerates can be broken up without altering the primary particles depends on the material, solids content, moisture, agglomerate strength and target particle size. amixon® describes cutting rotors as an equipment feature that can be used, among other things, for fine distribution and for breaking up lumps down to primary particles.
Where an existing agglomerate or granulate structure is to be deliberately preserved, cutting rotors are dispensed with or their use is kept to a minimum. In that case a low mixing intensity, a gentle tool geometry and an adapted mixing time take priority. For sensitive powders and granulates, vertical single-shaft mixers, cone mixers and the KoneSlid® mixer are suitable, depending on batch size. For large-volume batches of ceramic masses, the Gyraton® mixer can be used. amixon® states typical batch sizes of approximately 10 to 100 m³ for the Gyraton® mixer.
Ceramic powders can be highly abrasive. amixon® therefore offers wear-protected designs depending on the application. These include wear-resistant base materials, hardfacing welds with tungsten or chromium carbides, replaceable protective elements, ceramic coatings and hard-metal or high-performance ceramic plates. The suitable design depends on hardness, particle size, throughput, mixing intensity and the permissible contamination of the ceramic product. Particularly with high-purity technical ceramics, it must be checked whether materials, coatings and possible abrasion are compatible with the required product specification.
Wet processing is relevant for amixon® above all where wetting, conditioning or reaction is followed by a drying step. Vacuum mixing dryers and mixing reactors, for example the VMT or AMT series, can be used for this. These apparatus process powders, suspensions, pastes and doughs, and combine three-dimensional mixing with temperature control and vacuum. Cutting rotors can support de-agglomeration during drying and thereby influence heat and mass transfer. However, they do not generally replace wet milling or a stirred bead mill where a tightly defined ultra-fine dispersion down to the sub-micron range is required.
A trial with the original product is recommended before the design is finalised. In the pilot plant, mixing tool, cutting rotor, rotational speed, processing time, fill level and, where applicable, temperature or vacuum control can be varied. Alongside mixing quality, the assessment should include the particle-size distribution before and after treatment, the proportion of coarse agglomerates, abrasion, flowability, green density and the effects on the subsequent shaping and sintering process. This makes it possible to determine whether the de-agglomeration is sufficient without excessively altering the desired granulate or particle structure.