Integrated systems can combine powder mixing, liquid addition and granulation in a single process chamber. This can reduce transfer points, product losses and possible segregation between individual steps. Whether this is advantageous compared with separate apparatus depends on the product, granulate quality, cleaning requirements, throughput and process risk.

Suitable concepts

High-shear mixer-granulators combine a main mixing tool with a fast-running chopper. Powders are first mixed, the granulating liquid is sprayed in, and the agglomerates are built up and limited through shearing and chopper action. The method is suitable for fast batch-wise wet granulation and often for compact, mechanically stable granulates. However, excessive energy input can damage particles or lead to over-granulation.

Fluid-bed granulators fluidise the powder with air and spray in the granulating liquid. Granulation and drying can take place in the same plant. They are often suitable for porous, readily soluble granulates, but require precise coordination of air volume, temperature, spray rate and droplet size. Unstable fluidisation or too high a spray rate can cause caking and agglomerates.

Continuous twin-screw granulators continuously dose powder and liquid into a screw system. Conveying, mixing and kneading elements determine residence time, fill level and energy input. The concept is suitable for continuous production lines with stable dosing and process monitoring. Low hold-up quantities are advantageous but increase the demands on raw-material supply, control and start-up strategy. Downstream drying and classification are usually required.

Mechanically fluidised mixers such as ploughshare or paddle mixers can mix powder intensively and introduce liquids via nozzles. With a suitable recipe, build-up granulation is possible, particularly for larger batches. Granulate density and particle-size distribution depend strongly on liquid addition, tool speed, mixing time and product behaviour.

Vertical cone-screw mixers with spray nozzles operate in a comparatively gentle manner. They are suitable for homogenisation and liquid addition with sensitive powders. A pronounced, tightly controlled granulation is not automatically achievable with such mixers; it must be confirmed through trials for the product and the desired granulate structure.

Selection criteria

The choice depends above all on the desired granulate density, porosity, particle-size distribution, residual moisture, flowability and mechanical stability. Also relevant are batch or continuous operation, product changes, liquid viscosity, solvents, explosion protection, cleanability and available space.

The liquid should be introduced at a point where it is distributed quickly and evenly. Decisive factors are dosing accuracy, spray direction, droplet size, viscosity, temperature and the local product movement. An unsuitable nozzle or injection position can lead to over-wetting, caking, a broad particle-size distribution or incomplete wetting.

For process monitoring, torque, current draw, temperature, moisture and, with suitable calibration, near-infrared spectroscopy can be used. Near-infrared spectroscopy measures in the near-infrared range and can, for example, assess moisture or concentration. The measurement signals must be validated against product samples and quality data; a torque profile alone is not general proof of granulate quality or the granulation endpoint.

With organic solvents or combustible dusts, the requirements for explosion protection, inerting, venting and recovery must be planned at an early stage. In hygienically or regulatorily critical applications, materials, seals, the cleaning strategy and the effectiveness of cleaning must be demonstrated for the specific product.