Which process parameters specifically control agglomeration towards a defined particle-size distribution?
The particle-size distribution in agglomeration results from the interplay of nucleation, particle growth, consolidation, breakage and attrition. The aim is not necessarily the narrowest possible distribution, but a granulate suited to the downstream process, for example with regard to flowability, low dust generation, solubility, dosability, bulk density or mechanical strength. A desired mean particle size and distribution width therefore cannot be set via a single target value, but only within a coordinated process window.
The liquid or binder content is a central influencing variable in wet agglomeration processes. The ratio of liquid to solid, the dosing speed and the addition profile over time determine how many liquid bridges form between the particles and how long they remain stable. If the liquid quantity is too low, too few or too weak nuclei are formed; the fine fraction rises and the agglomerates may not reach the required strength. If the liquid quantity is too high, the risk increases of local over-wetting, uncontrolled coalescence, lump formation and an increased coarse fraction. The liquid-to-solid ratio is therefore one of the most important control variables for particle size and particle growth in many wet granulation processes.
Alongside the quantity, the formulation of the binder also influences agglomeration. Viscosity, surface tension, wettability, solids content, concentration and drying behaviour are decisive. These properties determine how the liquid is distributed on the particle surface, penetrates into pores and forms load-bearing liquid, and later solid, bridges. A higher binder content can promote particle growth and granulate strength, but can equally increase the risk of uncontrolled coalescence. The type and concentration of binder must therefore always be matched to the starting powder, the target particle size, the apparatus energy input and the subsequent drying process.
The spray characteristics determine whether the binder liquid is introduced spatially evenly into the powder bed. Important variables are droplet-size distribution, spray angle, nozzle position, atomisation energy, spray rate and the size of the spray zone. Evenly distributed droplets matched to the powder promote homogeneous nucleation and reduce local over-wetting. Very fine droplets, however, are not always advantageous: they can partially evaporate before contact with the powder, be carried away by the process air or deposit on apparatus surfaces. Excessively large droplets, on the other hand, favour local wet areas and unwanted coarse particles. Droplet size, binder flow rate and spray-zone geometry are therefore among the fundamental parameters of binder addition.
The mechanical energy input controls the ratio between particle build-up and particle destruction. Rotational speed, tool geometry, circumferential speed, shear and impact zones, fill level and, in fluid-bed processes, the air velocity influence the collision frequency, consolidation and breakage probability of the agglomerates. A higher mechanical load can limit large, weak agglomerates and promote more compact granulates. At the same time it can generate abrasion, increase fine fractions and broaden the particle-size distribution. A lower load frequently favours stronger particle growth but can lead to more porous and less robust agglomerates. The effect of the energy input is therefore always product- and apparatus-specific.
The residence time determines how long the particles are exposed to the mechanisms of wetting, nucleation, consolidation, growth and destruction. In batch processes it is determined above all by the mixing and granulation time. In continuous processes, throughput, fill level, apparatus volume, tool speed and geometry influence the mean residence time. A longer residence time can support particle growth and consolidation but also increases the probability of oversize particles and attrition. If the residence time is too short, the desired particle size cannot be achieved. In continuous processes, the width of the residence-time distribution is additionally relevant: different dwell times lead to differently developed particles and can broaden the distribution.
Temperature, product moisture and drying conditions determine the speed at which the liquid evaporates and the binder bridges solidify. In fluid-bed, spray and mixer granulation processes, inlet air temperature, air volume, relative humidity or dew point, product temperature and exhaust air conditions in particular interact. If the liquid evaporates too quickly, inadequately stable agglomerates can form or disintegrate again. With evaporation that is too slow, or too high a spray rate, the product can become over-wetted and grow in an uncontrolled way. A stable process therefore establishes a balance between liquid addition and evaporation capacity.
The properties of the starting materials influence the entire process behaviour. Important factors are particle-size distribution, specific surface area, porosity, surface energy, bulk density, moisture content, hygroscopicity, solubility and flow behaviour. Fine or highly porous powders often require a different liquid quantity and different spray conditions than coarse, dense or poorly wettable powders. Fluctuations in incoming moisture, particle size or bulk density can shift the developed process window and should therefore be monitored.
Classification using sieves, air classifiers or similar separation stages can further secure the particle-size distribution. Undersize can, where acceptable for the product, be recycled. Oversize can be reduced in size, reclassified or likewise returned to the process in a controlled manner. Recycling, however, changes the mass balance and the processing history of individual particles. It must be designed so that neither fine fractions nor over-processed or critical constituents accumulate in the recycle loop.
For a reproducible particle-size distribution, the critical process parameters are determined through preliminary trials and statistical design of experiments, for example Design of Experiments. Relevant quality variables include d10, d50, d90, span, the proportion of fine and oversize particles, bulk density, residual moisture, granulate strength and flowability. Suitable process signals are mass flow, moisture, temperature, torque, power consumption and, where available, inline or atline particle-size measurements. The PAT approach describes that a process endpoint can be secured not solely through a fixed processing time, but through understanding and measurement of the relevant process and quality variables.
How amixon® controls the particle-size distribution in batch-wise agglomeration
The particle-size distribution of an agglomerate results from the interplay of wetting, nucleation, particle growth, consolidation, and attrition and breakage. At amixon®, this balance is generally controlled batch by batch: in the vertical and cone mixing systems VM, HM and AM, as well as in the heatable and coolable mixing dryers AMT and VMT, each batch can be set specifically to a defined target particle-size distribution via liquid or binder addition, tool speed, mixing time, fill level and temperature control. Unlike in a continuous process, the entire batch is thereby subject to the same conditions, which can be observed and readjusted as the process proceeds — a feature that is particularly advantageous for small to medium lot sizes, changing recipes and products with a narrow target particle-size band.
Liquid and binder addition as the most important control variable
Particle formation begins with the wetting of the powder particles. Decisive factors are the liquid-to-solid ratio, binder type and concentration, viscosity, addition speed and droplet size or spray pattern of the addition nozzle. amixon® can introduce the liquid via spray lances, ring lines or product-matched nozzles specifically into the moving powder layer; the position and addition rate are matched to the mixing geometry and tool movement so that the liquid is distributed as evenly as possible rather than concentrated locally. As a rough guide, typical liquid-to-solid ratios for build-up granulation often lie in the range of a few percent up to approximately 30 percent of the solids mass; the value actually required depends strongly on the absorbency, porosity, particle size and binder of the specific product and is determined through trials. Too little liquid produces too few or too weak nuclei, while too much liquid favours local over-wetting, lump formation and a broader particle-size distribution.
Tool speed and mixing kinematics
The SinConvex® forced restratification carries the product upward in a controlled manner near the wall and lets it flow back downward centrally under gravity. This three-dimensional restratification ensures that wetted and unwetted particles repeatedly come into contact, which supports nucleation and particle growth. For agglomeration tasks, amixon® mixers are typically operated at tool circumferential speeds of approximately 0.8 to 3.5 m/s, depending on design and objective; higher speeds increase the mechanical energy input, support consolidation and, through controlled breakage, limit oversize agglomerates, but can also increase abrasion and fine fraction if the stress is too high. Where cutting rotors are engaged in a targeted way, the particle-size distribution can be further fine-tuned through local de-agglomeration without changing the movement of the rest of the batch.
Batch duration, post-mixing time and temperature control
The batch duration determines how long the product is exposed to the mechanisms of wetting, growth, consolidation and breakage, and — unlike a continuous residence time — can be read directly from the running process and adjusted. It is common to structure the batch into an addition phase, a main mixing time and a short post-mixing time without further liquid addition, which gives the particle-size distribution time to consolidate. With temperature-sensitive binders or exothermic reactions, the mixing chamber of the AMT and VMT mixing dryers can be heated or cooled to keep binder viscosity and product temperature within the desired window; subsequent drying in the same apparatus is possible and can specifically stop particle growth once the target distribution has been reached.
Fill level and size selection
With the VM, HM, AM and AMT/VMT series, amixon® covers a batch range from small pilot-plant sizes up to project-specific large plants of approximately 50,000 litres usable volume; for smaller batches, the EM in the range of approximately 5 to 200 litres and the container mixer COM in the range of approximately 100 to 4,000 litres are additionally available. Depending on the product and design, fill levels of approximately 10 to 100 percent of the usable volume can be run. Because heat and liquid input, mixing kinematics and the effective residence time of the batch in the mixer vary with the fill level, the size and fill-level range suitable for the target particle-size distribution are confirmed in a pilot-plant trial with the original product before being transferred to the production plant.
Process verification in the pilot plant
The particle-size distribution actually achievable is determined at amixon® with the original product in the pilot plant. More than 30 test units of various sizes are available for this at the Paderborn headquarters, supplemented by pilot plants in Japan, India, Thailand, China, South Korea and the USA. Among other things, particle-size distribution, fine and oversize fractions, bulk density, residual moisture, granulate strength and flowability are assessed; the results serve as a basis for the recipe, batch programme and the design of the target size. For later series production, the target values validated in the pilot plant for liquid addition, tool speed, mixing time, temperature and fill level can be stored as a batch recipe in the PLC, so that the particle-size distribution determined in the trial can be reproduced reliably in production.
Where an application explicitly requires continuous rather than batch-wise operation — for example with very high throughputs or an uninterrupted material flow — amixon® offers the ring-layer mixing granulator RMG as a standalone product line specifically designed for continuous agglomeration. The RMG follows its own operating principle with continuous solids and liquid dosing and is the subject of a separate design process; the process parameters and sizes relevant to it are described there.