In the continuous production of detergent powders and granulates, liquid components such as non-ionic surfactants, perfume oils, defoamers, polymer solutions or binders are incorporated into a solid mixture. Enzymes, bleaching agents, optical brighteners and fragrances are often dosed only after energy- or temperature-intensive main steps, because moisture, heat and mechanical stress can impair their stability. Enzymes are therefore often used in detergents as protected granulates.

The choice of mixing system depends above all on the liquid quantity and viscosity, the bulk density and flow behaviour of the base powder, the desired granulate structure, the sensitivity of the ingredients, throughput, cleaning requirements and permissible energy input. Also decisive is whether the liquid addition is only intended to achieve even wetting, or whether targeted agglomeration, granulation, coating, drying or cooling is planned.

Continuous ring-layer mixers and ring-layer granulators are particularly suitable for intensive, rapid wetting and agglomeration. The powder is guided by rotating tools along the mixing-chamber wall as a dynamic ring layer; liquid components are sprayed into this moving layer. The high relative movement supports the distribution of small liquid quantities and the formation of granulate. With excessive liquid dosing or excessive mechanical load, however, oversize particles, caking, abrasion or unwanted heating can occur. Ring-layer systems are therefore particularly sensible where targeted granulation or densification is to take place alongside mixing.

Continuous ploughshare mixers generate a mechanically fluidised product bed. Liquids can be introduced into the moving powder zone via dosing lances or spray nozzles. They are versatile where a detergent base powder needs to be wetted with liquid surfactants, perfume components or other additives. Additional size-reduction tools can limit local over-wetting and lump formation, but increase the energy input and the mechanical load on the product. They should therefore only be used where the recipe and the desired particle structure require it.

Twin-shaft paddle mixers are suitable for continuous processes with moderate energy input and high circulation performance. Counter-rotating paddle shafts generate an overlapping mixing zone in which dry components and liquid additives can be distributed evenly. They are particularly interesting where sensitive granulates, such as coated enzymes, bleach granulates or fragrance carriers, need to be mixed in as gently as possible. Figures for maximum liquid fractions or throughputs, however, are not generally transferable. They depend on the recipe, viscosity, solids surface, apparatus size and desired product structure and must be checked through trials.

Continuous fluid-bed plants are suitable where liquid addition is to be combined with granulation, drying, cooling or coating. In a fluid bed, particles are fluidised by process air while liquids are applied via nozzles. This allows low-dust, free-flowing granulates to be built up or sensitive components to be given protective coatings. The technology is used, for example, where detergent granulates need to be specifically dried, cooled or coated. The energy demand as well as the requirements for exhaust-air treatment, dust separation and process control are higher than for a pure through-flow mixer.

Vertical continuous mixers or cone-screw mixers can be sensible with very shear-sensitive, dusting or fracture-sensitive components. Their strength lies in gentle homogenisation. For highly viscous liquids, larger liquid quantities or targeted agglomeration, however, they are generally less suitable than more intensive ring-layer, ploughshare or twin-shaft systems. They are therefore more of a specialist solution for final mixtures with a low liquid fraction or for mixing in sensitive dry additives.

For liquid dosing, gravimetric solids feeders and mass-flow-controlled liquid dosing systems are appropriate. With viscous additives, positive-displacement pumps, temperature-controlled lines and heatable nozzles are frequently used. Coriolis instruments can be used to capture and control the liquid mass flow; their selection, installation, calibration and suitability for the respective viscosity must be checked. The spray technology must be designed so that droplet size, spray pattern, throughput and nozzle position match the product stream. Addition that is too coarse or locally concentrated promotes lumps and deposits, while atomisation that is too fine can lead to losses or exhaust-air loading with volatile components.

With fragrances and enzymes, late, controlled post-dosing is often sensible. Perfume oils can be applied to a finished, sufficiently cool base granulate. Enzymes are usually used as granulates or coated granulates so that dust release and activity losses are reduced. Moisture and oxidising bleach chemicals can impair enzyme stability; the dosing sequence, contact time, temperature and water content of the formulation must therefore be specifically defined.

A narrow residence-time distribution helps to better control the homogeneity and contact time of sensitive additives. Its actual width, however, is determined by mixer geometry, fill level, throughput, back-mixing and product properties; it cannot be attributed to a particular mixer type as a matter of course. With frequent recipe or fragrance changes, readily accessible product areas, short residual-discharge times and a matched cleaning procedure are also important in order to limit cross-contamination and downtime.