A uniform layer thickness arises, when a sprayed suspension or solution is introduced into the mix bed of a vertical mixer, through the coordinated interplay of product recirculation, spray technology, droplet size, liquid properties and solidification. What is decisive is not the total quantity of coating material fed in alone, but that every particle passes through the spray zone sufficiently often and is thereby coated in stages, without being locally overwetted or suffering abrasion of a freshly formed layer.

Product circulation and spray rate

When coating, the spray rate must match the actual recirculation rate of the mix. The suspension or solution must not be introduced into the mix bed faster than the mixing motion can carry wetted particles out of the spray zone and blend them with the less-wetted particles of the whole batch. If the spray rate is too high, several droplets hit the same particles or the same local product area in quick succession. The result is locally wet nests, liquid bridges, wall build-up and agglomerates. At the same time, other particles remain initially uncoated.

A uniform layer arises when the liquid hits moving particles in many small sub-quantities. The wetted particles must then be carried rapidly out of the spray zone. They are repeatedly mixed with predominantly drier or less heavily coated particles and only return to the spray zone after a certain time. The mixing kinetics must therefore be faster than local overwetting.

The actual goal is: every particle should experience many controlled passes through the active spray zone over the entire dosing time. With every pass, a small additional amount of coating material is applied. The layer thereby grows in stages and statistically evenly across the entire particle population. Repeated particle contact with a defined spray zone is regarded as an essential prerequisite for uniform coating.

The spray nozzle should be installed in a zone that is permanently flushed through by the mix with a sufficiently high mass flow. A nozzle above a resting powder layer, in a poorly mixed peripheral zone, or with direct impingement on the vessel wall, tends to lead to overwetting, product build-up and uneven layer thickness. Dosing time, fill level, rotational speed, tool geometry, nozzle position and product movement must therefore be coordinated together.

Droplets and spray pattern

Droplet size must match the particle size, particle porosity, wettability and viscosity of the coating suspension or solution. Large droplets can capture several particles at once and create liquid bridges. This increases the probability of agglomeration and a wide spread of layer thickness. Smaller droplets distribute the same amount of liquid over more impact points and reduce the risk of local overdosing.

The rule of thumb that droplets should be small relative to the particles being coated is useful, but not sufficient. With very fine or cohesive powders, even small droplets can trigger agglomerates if the particle surfaces remain sticky for a long time or if the solvent evaporates too slowly. With porous, absorbent or structured particles, the liquid can partly draw into the pore structure instead of forming a surface layer exclusively. The resulting layer thickness therefore depends not only on droplet size, but also on penetration depth, spreading behaviour and solidification of the liquid.

Important control variables are droplet size, droplet count, spray angle, spray pressure, spray distance, throughput, nozzle position, and the movement of the powder at the time of liquid application. A nozzle with a good spray pattern can nonetheless deliver poor results if it sprays into a zone with insufficient product movement. Conversely, vigorous product recirculation is not sufficient if the droplets are too large or the liquid is too viscous and atomised unevenly.

The liquid properties substantially determine atomisation. Higher viscosity, surface tension or density frequently promote larger droplets. Higher viscosity can also slow spreading on the particle surface and extend the time during which liquid bridges persist between particles. Studies on contact spreading in particle coating show that viscosity and liquid transfer between particles influence the development of coating uniformity.

For highly viscous suspensions or solutions, temperature-controlled feed vessels, lines, pumps and nozzles can be necessary. The temperature should be set so that the liquid remains stably pumpable and finely atomisable, without thermally damaging active ingredients, binders or solvents. With suspension-based coatings, it is additionally important that solids content and dispersion state remain stable throughout the entire dosing. Otherwise, sedimentation in the feed vessel, phase separation or viscosity changes during the dosing time lead directly to fluctuations in layer thickness.

Solution, suspension and solidification

With a solution, the coating consists, immediately after application, of dissolved coating material and solvent. The solvent must evaporate in a controlled manner after spreading, so that the coating material remains behind as a coherent layer. With a suspension, suspended solids are additionally present. The liquid phase spreads over the particle while the solvent evaporates or transfers into the particle bed and the solids fraction forms a layer or film. The stability of the suspension is therefore decisive: sedimentation, flocculation or concentration fluctuations lead to locally varying solids loading and thus to variable layer thickness.

Temperature control and solidification determine whether a droplet turns into a defined layer or an agglomerate. If a solution dries too early, the liquid cannot spread sufficiently on the particle surface. The layer then becomes rough, patchy, discontinuous or uneven. If, on the other hand, the droplet remains liquid or sticky for too long, particles can adhere to one another via liquid bridges. Agglomeration is particularly favoured by high spray rates, large droplets and inadequate evaporation or drying performance.

The optimal product temperature is therefore not a single fixed set point, but a process window. It depends on the solvent, viscosity, solids content of the suspension, evaporation behaviour, particle temperature, vessel temperature, fill level, gas or air routing, and mixing intensity. With temperature-sensitive particles, it must additionally be limited how high the average product temperature and local surface temperatures are allowed to rise.

When a suspension is applied, it must additionally be prevented that the spray nozzle changes its spray pattern due to solids deposits or drying-out at the nozzle tip. Stable liquid delivery, suitable filtration, controlled agitation in the feed vessel and, where applicable, nozzle flushing cycles are important prerequisites for a constant application throughout the entire batch duration.

Choosing the right mixing intensity

A high tool speed can accelerate the distribution of the liquid, but is not automatically advantageous. More mixing intensity increases the frequency and energy of particle contacts. This can cause freshly coated particles to collide more forcefully, generate abrasion, damage the forming layer, create fines, or heat the product through friction. Crystals, granules, spray agglomerates, biologically active products, or particles with a soft coating not yet fully solidified, are particularly sensitive.

Too low a mixing intensity is likewise problematic. Then the product passes through the spray zone too rarely, the liquid does not distribute quickly enough, and local overwetting increases. Wet nests, agglomerates and a wide spread of layer thickness result. The optimal setting therefore lies between these extremes: product recirculation must be fast enough to distribute the liquid application over the entire batch, but gentle enough not to strip off the growing layer again.

Mixing intensity should not be assessed by rotational speed alone. Decisive factors are tool geometry, fill level, circumferential speed, actual product circulation, mixing time, nozzle position, spray rate, liquid quantity and the mechanical sensitivity of the particles. Where high-shear or deagglomeration tools are used, they should be applied in a targeted way, for a limited time, and only where they are needed for distributing highly viscous liquids or for breaking up unwanted agglomerates. A permanently high energy input can damage the coating and increase the proportion of fines.

Assessing layer thickness

A uniform total quantity of coating material in the batch does not yet prove a uniform layer thickness on every particle. Quality assessment should therefore cover several levels. First, it is checked whether the mean quantity of coating applied corresponds to the target quantity. Then the spread of the coating quantity in representative individual samples is assessed, for example via the coefficient of variation or RSD of a suitable marker component.

For direct assessment of layer thickness, imaging and particle-based examinations are useful. Depending on the product, light microscopy, scanning electron microscopy, particle image analysis, cross-sections, gravimetric determination, chemical extraction, colour measurement, moisture determination, dissolution or release testing, and sieve analysis can be used. Sieve analysis in particular helps to reveal unwanted agglomerates as well as fines formation caused by mechanical stress.

Sampling must represent the entire process. Samples from several zones of the mixing chamber are useful, but are not sufficient on their own. In addition, time-staggered discharge samples from the beginning, middle and end of emptying should be examined. This makes it possible to assess whether the coating is uniform within the mixing chamber and whether abrasion, segregation or further changes occur during discharge, conveying or filling.

Practical approach

Process development begins with characterising the core material. To be captured are particle size distribution, particle shape, bulk density, specific surface area, porosity, moisture, flowability, cohesion, surface energy, wettability, thermal stability and, where applicable, abrasiveness. For the coating suspension or solution, viscosity, surface tension, density, solids content, particle size of the suspended solids, sedimentation stability, solvent fraction, evaporation behaviour, drying mechanism and chemical compatibility with the core material are important.

A trial plan is then built up. First, dry product recirculation should be examined: which mixing speed, fill level and tool geometry produce stable and gentle product movement? Only after that are spray rate, droplet size, spray pressure, spray angle, nozzle position, liquid temperature, product temperature, solids content, mixing speed, fill level and post-mixing time systematically varied.

Not every influencing factor needs to be examined individually. A structured DoE approach can reveal interactions and reduce the number of trials required. For example, a higher spray rate can work when the product temperature is simultaneously higher, while the same spray rate at a lower temperature leads to agglomeration. Likewise, a higher solids content in the suspension can ease drying but worsen atomisation and thus the spray pattern.

As a first goal, a process window is defined in which no visible agglomerates, no excessive wall build-up, no impermissible heating and no discernible layer damage occur. Mean coating quantity, spread of coating quantity, layer thickness, particle size distribution, abrasion, moisture, discharge stability and reproducibility are then assessed. The final setting is not chosen solely on the basis of the visually best sample, but on its tolerance to raw-material fluctuations, cleanability, product protection, stability during discharge, and reliable repeatability at production scale.