What are best practices for recipe development in instantising processes with binder addition?

Question

Accepted Answer

How amixon® implements binder addition and granulation in the batch mixer

Integrated liquid dosing as a feature

On request, amixon® vertical and conical mixers (VM, HM, AM) are equipped with liquid feed lances which introduce the binder liquid directly into the moving bed of mix; alternatively or in addition via a two-fluid nozzle, whose air and liquid flow generates a particularly fine atomisation for even wetting. Addition within the action range of the cutting rotor prevents local over-wetting, the main cause of uncontrolled lump formation. Viscous binders too are thereby introduced without dead spaces and without soiling the mixer; a temperature-controllable double jacket keeps temperature-critical binders reliably low in viscosity.

Division of labour in the mixing chamber

The SinConvex® mixing tool circulates the entire volume three-dimensionally and continuously conveys fresh powder into the wetting zone; the cutting rotor distributes the liquid microfinely and breaks up coarse agglomerates. The physical basis is the targeted formation of liquid bridges: with controlled wetting, rounded, stable agglomerates with even porosity build up, readily wettable, readily sinking, quickly soluble.

Adjustable intensity: from instantising to granulating

The processing modes are operating states, not a decision of design: moistening with low energy introduction (gentle build-up agglomeration, instantising), moistening with high energy introduction, deagglomerating, granulating. Torque and temperature curves from the PLC support the determination of the end point; dosing profiles with intermediate mixing phases can be stored as a mixing program and are therefore reproducible.

From batch to continuous granulation

For continuous tasks, the ring-layer mixing granulator RMG complements the range: round, low-dust granules of similar size in a stable equilibrium between grain build-up and grain destruction. Which variant is more economical is clarified by amixon® in granulation trials with the original product in the pilot plant.

Verification in the amixon® pilot plant

Evidence comes before the investment: the amixon® pilot plant at the Paderborn headquarters has 35 test units of various sizes available, complemented by pilot plants in Japan, India, Thailand, China, South Korea and the USA. The trials run with the original product, with real fill levels and batch sizes, within the intended temperature and pressure range. Mixing quality, product protection, energy input, cleanability and reproducibility in later series operation are assessed; the results are evaluated and documented together with amixon® experts, as a robust basis for decision-making that removes technical and economic risks before purchase.

Answer

Best practices for recipe development in instantisation processes with binder addition for powders aim to produce agglomerates with defined wettability, dispersibility, flowability and high process stability.

Starting materials and choice of binder

Robust recipe development begins with the characterisation of the powders and binders:

Particle size distribution (in particular the fines fraction below 100 µm), bulk and tapped density, residual moisture, water activity and hygroscopicity.
Wettability, solubility, sinking and dispersing behaviour as central target variables of the instant properties.

The choice of binder depends on the product matrix and the desired bonding strength:

Aqueous binders (water, sugar solutions, hydrocolloids) for hydrophilic systems.
Polymer binders (for example maltodextrins, starches, PVP, HPMC) to set the mechanical strength.
Lipid-based binders (for example lecithin, fats) for hydrophobic or fat-rich systems.

Best practice: check compatibility (polarity, solubility), binder-to-solids ratio and glass transition temperature (Tg) in order to avoid sticky phases and unwanted partial dissolution.

Viscosity, droplet size and binder application

The viscosity of the binder solution governs the spray pattern and droplet size distribution:

Choose concentration and temperature so that sufficient adhesive force is present while the solution remains readily atomisable.
Excessive viscosity leads to coarse droplets, inhomogeneities and clogged nozzles.
Rule of thumb: droplet diameter ≤ mean primary particle size, controlled via atomising pressure, nozzle geometry and spray position. Spray angle and droplet size distribution (for example D50) should be validated at an early stage.

Binder addition:

Liquid addition (spraying on binder solution) for homogeneous distribution and low-dosed binders.
Dry addition (binder as powder) for high binder quantities, limited by dissolution kinetics and activation behaviour.

Typical sources of error: over-wetting (lumps, wall build-up), under-wetting (high fines content) and inhomogeneous binder distribution.

Process parameters, quality attributes and thermal window

In the sense of quality by design, critical process parameters (CPPs) and critical quality attributes (CQAs) are defined early:

Typical CPPs: binder addition rate, spray pressure, spray time, nozzle geometry and position, inlet air temperature and humidity, fluidisation velocity, product temperature.
Typical CQAs: wettability, sinking behaviour, dispersion time, agglomerate size distribution, mechanical stability, freedom from dust, residual moisture, bulk density and flowability (for example Hausner ratio, Carr index).

A suitable thermal process window requires the coordinated combination of inlet air temperature, spray rate and product temperature, taking account of the Tg and the softening range of the binder, in order to avoid over-moistening, overheating and wall adhesion.

Scaling, validation and documentation

For a reliable transfer from the laboratory into production:

Step-by-step scaling from laboratory through pilot scale to the production plant, supported by dimensionless characteristic values (for example specific spray rate per mass flow).
Reproducibility studies across several batches to secure process robustness.
Systematic documentation of recipe parameters, process conditions, raw material batches and analytical results; use of statistical or multivariate evaluation to identify critical influencing variables.