For crop protection products there are several ways to closely couple mixing, granulating and drying. A genuine one-pot process without product transfer is possible above all with fluid-bed granulators. In other processes, mixing, granulating and drying take place within an integrated process line, but in separate apparatus or successive process stages.

This is particularly relevant for water-dispersible granules, also called WDG or WG. These formulations typically consist of finely ground active ingredient, wetting agents, dispersants, binders and, where applicable, fillers. The granulate must flow well, be low in dust, remain stable in storage, and redisperse quickly and completely after being added to water. Established production routes for WDG include fluid-bed granulation, extrusion, spray granulation, pan granulation, mixing agglomeration and roller compaction, among others.

Fluid-bed granulators combine mixing, spray granulation and drying in the same process chamber. The powdered product is fluidised with a temperature-controlled air or inert-gas stream. A binder or active-ingredient solution is then sprayed onto the moving particle bed. The resulting agglomerates dry at the same time through the gas stream. The process is particularly suitable for free-flowing, readily fluidisable powders and for water-dispersible granules with good redispersibility. The process parameters of air temperature, air quantity, spray rate and binder concentration determine the granulate's grain size, residual moisture, strength and dispersing behaviour.

High-shear mixers and high-shear granulators are suitable for the intensive premixing and wet agglomeration of powdered active ingredients and excipients. Binder solution or water is added during mixing, producing a moist granulate. The actual drying, however, often does not take place in the high-shear granulator itself, but in a downstream fluid-bed dryer, vacuum dryer or another suitable drying stage. High-shear mixing systems are therefore usually part of an integrated granulation line and are not automatically complete one-pot systems.

Ploughshare mixers, intensive mixers and horizontally operating mixing agglomerators can also be used for premixing and wet granulation. They are particularly suitable where different bulk densities need to be homogenised, liquids mixed in evenly, or agglomerates produced in a targeted way. Vacuum equipment and heating can enable post-drying or, for suitable recipes, more intensive contact drying. For water-dispersible granules, however, a downstream drying stage is frequently chosen, because it allows a more precise setting of residual moisture and grain size distribution.

Extrusion lines are an important alternative where dense, mechanically robust granulates are required. The raw materials are first mixed and wetted with water or binder solution. The moist mass is then extruded, and the resulting strands or particles are dried and, where applicable, classified. Extrusion offers good control over the shape and strength of the granulate, but usually requires separate units for mixing, extruding, drying and sieving.

Spray granulation and spray drying are suitable above all for formulations that can be processed as a suspension or slurry. Active ingredient, wetting agent, dispersant and other recipe components are first dispersed and homogenised. The suspension is then sprayed and dried by hot process air. The process yields low-dust, free-flowing granulates, but can produce lower bulk densities than extrusion processes.

In selecting the system, the desired granulate structure, grain size distribution, bulk density, abrasion resistance, redispersibility, residual moisture and thermal stability of the active ingredient are decisive. Dust protection, containment, explosion protection, cleanability, handling of process exhaust air, and the recovery and treatment of volatile constituents are equally important.

Where as compact a process as possible – with mixing, binder application, granulate formation and drying in a single process chamber – is required, fluid-bed granulators are frequently the first choice. Where particularly dense or robust granulates are needed, mixing-extrusion-drying lines are often better suited. High-shear mixers and ploughshare mixers are suitable above all for intensive premixing and wet agglomeration; their drying function must be assessed individually depending on the product and target quality.