Are there continuous mixers that combine high throughputs and short residence times with reproducible mixing quality for battery materials?
Yes. Continuous mixing and dispersing systems can combine high throughputs, compact plants and reproducible product quality for battery materials. Whether they are suitable, however, depends substantially on whether dry powders, high-viscosity electrode slurries, binder solutions or electrolytes are being processed.
For electrode slurries, continuous intensive mixers, twin-screw extruders or rotor-stator systems are frequently used. Twin-screw extruders permit conveying, wetting, kneading and dispersing in a single continuous process. Their screw modules can be matched to the particular formulation, the solids content and the required shear loading. They are particularly suitable for high-viscosity slurries and demanding dispersing tasks, but must be designed so that particles, conductive carbon black structures or binders are not impaired by excessive energy input. The use of extruders for continuous slurry mixing is increasingly being investigated in battery cell manufacture; process control and optimisation are still regarded as an important development task.
Rotor-stator systems are suitable above all for the rapid intake, wetting and dispersing of powders in liquids. The high local shear can break up agglomerates and shorten the dispersing time considerably. The residence time in the shear gap is very short, but the overall process time required is determined by the formulation, the solids input, the dispersing task and, where necessary, recirculation loops or downstream homogenisation. A blanket statement that every slurry is fully dispersed "in real time" would therefore not be sound.
For dry electrode processes or the premixing of powdered components, continuous paddle, blade or high-speed mixers may be considered. They work with small mixing volumes and mean residence times frequently ranging from a few seconds to about a minute. In one documented example, mean residence times lie between 5 and 50 seconds; they are influenced by throughput, fill level, speed and outlet geometry.
Reproducibility depends less on the type of mixer alone than on the process as a whole. The prerequisites are gravimetrically controlled material streams that are stable over time, defined steady-state operation, a controlled residence time distribution and suitable measurement and control variables. Throughput, speed, torque, temperature and – depending on the product – density, viscosity, conductivity or spectroscopic signals are typically monitored. The residence time distribution should be determined by tracer trials, since it influences how quickly a process reaches a stable state after start-up or a change of formulation.
Static mixers can be worthwhile as a downstream homogenising stage with low to medium viscosity liquids. They do, however, require uniform metered addition and cannot compensate for dosing errors or inadequate premixing. For high-viscosity, solids-rich electrode slurries they are usually not a complete alternative to dynamic dispersing systems.
Continuous systems can reduce the space required and the quantity of material in process compared with large batch plants. Closed product paths also make it easier to handle dusty powders or solvent-containing slurries. The specific benefit must, however, be demonstrated for each chemistry: NMC, LFP, graphite and silicon systems differ markedly in solids content, tendency to agglomerate, shear sensitivity and target rheology.
AMK: throughput and residence time adjustable independently
The amixon® AMK continuous mixer permits high throughput at a short residence time with consistently high homogeneity. Throughput and residence time can be set independently of one another. The AMK range extends from the AMK 50 with a throughput of approximately 1–3 m³/h to the AMK 3000 with around 45–135 m³/h. The residence time is independent of the rotational frequency of the mixing tools; mixing intensity and throughput can be controlled separately. For simple mixing tasks the residence time can be kept short, while for complex formulations with many solids and liquids it is deliberately extended. The SinConcave®/SinConvex® mixing tool generates the same three-dimensional forced restratification as in the amixon® batch mixers. The mixing quality is reproducible and corresponds to a technically ideal random mixture with a defined, broad residence time distribution.
AMK: setting mixing intensity and residence time independently
The amixon® AMK continuous mixer is designed for continuous processes in which high throughputs, a defined mean residence time and reproducible homogeneity have to come together. Its particular advantage is that mixing intensity and residence time can be set separately: the mixing tool speed determines above all the intensity of the product movement, while fill level, discharge control and throughput influence the mean residence time.
The AMK can therefore be designed both for short mixing tasks and for complex formulations with several solids and liquids. Depending on size and process, the AMK series ranges from small throughputs of around 1 m³/h up to approximately 135 m³/h. For demanding mixing tasks, mean residence times of up to several minutes can be provided for.
Without the usual start-up losses
A particular feature of the AMK is its controlled production start. At the beginning the discharge remains closed. The gravimetric dosing units start simultaneously at a reduced mass flow and align their dosing ratios with one another. As soon as the mixer is about half full, the mixing tool begins to work; at about 80 % of the working fill level the discharge opens step by step. The fill level is then held constant and the throughput increased to the intended value.
Product can thus be discharged within the target mixing quality from the very start of production. The start-up quantities outside specification that otherwise frequently arise in continuous mixing processes can be avoided or considerably reduced. At a controlled end of production, too, the mixer can be run empty continuously.
A long residence time as a mixing buffer
The AMK uses a defined product fill level as an active mixing chamber. A sufficiently generous mean residence time supports the homogenisation of complex formulations and can attenuate short-term, unavoidable dosing fluctuations over time. This is particularly advantageous with many components, with micro-additions or when mixing in small quantities of liquid.
The actual residence time distribution is, however, always dependent on product and process. With demanding applications it should be investigated with the original product, for example by tracer trials. What is decisive is not a long residence time alone but the balanced relationship between dosing consistency, fill level, throughput, mixing intensity and discharge behaviour.
Product-protecting or intensive
The SinConcave®/SinConvex® mixing tool generates a three-dimensional product movement and supports uniform intermixing as well as very good residual discharge. The mixing intensity can be adapted from particularly gentle homogenisation through to targeted de-agglomeration. SinConcave® mixing tools achieve residual discharge rates of up to 99.997 % in suitable applications.
The AMK can process dry, moist and suspended products. Liquids can be introduced into the mixing chamber in a controlled manner; whether additional temperature control, drying or reacting is worthwhile depends on the specific machine version agreed and on the process task. The mixing chamber can be designed for Zone 20.
Testing with the original product
amixon® tests the continuous mixing process in the pilot plant with the original product. Homogeneity, dosing and discharge behaviour, the residence time required, product protection, liquid incorporation and cleanability are investigated in particular. It can thus be demonstrated before the investment whether the AMK reproducibly achieves the desired mixing quality across the intended throughput range.