With heterogeneous bulk materials that have differing bulk densities, particle sizes, shapes and flow properties, no single mixing tool is decisive; what matters is the coordinated interplay of mixing principle, tool geometry, dosing, mode of operation and discharge concept. It is particularly important that the mixing system repeatedly draws all product regions into an active mixing zone and that the mixture is subsequently discharged with as few drop paths, vibrations or unfavourable buffering as possible. Differences in particle size, shape and density are among the most important causes of segregation; this can occur during mixing, but occurs especially often during discharge and in downstream vessels.

Suitable mixing principles

For large differences in bulk density and particle characteristics, actively convective mixing systems are in many cases better suited than pure free-fall mixers. These include slowly or moderately operated paddle, blade, ploughshare, helical and multi-shaft mixers. They generate defined product streams and shorten the distance that individual components must travel for complete spatial distribution. Which concept is best suited, however, depends substantially on cohesiveness, abrasiveness, particle sensitivity, target mixing time and batch.

Ploughshare mixers can generate intensive mechanical fluidisation and are useful where cohesive powders, agglomerates or difficult minor components need to be distributed rapidly. However, the high mixing intensity can also promote particle breakage, abrasion, heat generation and, with excessively long processing, renewed segregation. They are therefore only sensible where the formulation allows this level of stress.

Twin-shaft paddle or blade mixers produce superimposed mixing streams and, with suitable tool geometry, can enable a short mixing time. They are suitable for heterogeneous mixtures where good three-dimensional distribution and comparatively limited particle stress are both required. With very sensitive granules or breakage-prone components, tool speed, blade angle, fill level and discharge should be checked with particular care.

Vertical conical or helical screw mixers are advantageous where gentle, convective product circulation with variable fill levels is required. They can be well suited to products whose structure should be preserved as far as possible. With strongly cohesive fine particles or firm agglomerates, however, the pure helical motion alone may not be sufficient. In this case, a time-limited intensive mixing stage or an upstream premix may be required.

Trough mixers with ribbon screws are an economical solution for free-flowing products and moderate differences between components. With extreme differences in density or particle size, however, there is an increased risk that the product circulation does not adequately reach all zones, or that the mixture separates again during discharge. Free-fall mixers such as double-cone, V or tumble mixers are particularly gentle, but are mainly suitable for free-flowing components that are largely free of agglomerates and sufficiently similar in particle properties. With marked differences in size, shape or density, they may require longer mixing times and tend toward segregation.

Tools and options

Paddles and blades with a defined angle of attack can combine axial, radial and tangential product flows. A targeted tool arrangement helps to reach the entire volume and avoid product-poor edge regions. Ploughshare tools lift the product away from the vessel wall and generate an intensive flow field. Helical and ribbon tools convey the product in a controlled manner along the wall and guide it into the central return flow. The optimal tool shape cannot be derived generically from the density ratio; it must match the particle structure, cohesiveness, abrasiveness and particle sensitivity.

Wall-near scrapers can be useful with sticky or moist products to limit product build-up on the vessel wall and base. With dry, abrasive powders, however, they must be designed so that they do not create unnecessary friction and abrasion zones. Adjustable tool angles or exchangeable tools can increase flexibility with changing formulations, but must be designed to be hygienic, mechanically stable and reproducibly positionable.

Choppers, cutting heads or knife heads are useful where cohesive agglomerates need to be specifically broken up. They should not be used permanently as a substitute for suitable base mixing kinematics. Switching them on for a limited time during a clearly defined process phase reduces the stress on the whole batch. Particularly with sensitive, coated or abrasive particles, it should be checked whether the de-agglomeration effect gained justifies the possible particle breakage or abrasion.

Precise liquid dosing can, in suitable formulations, help bind fine fractions to coarser carrier particles and thereby reduce percolation. Dosing lances, spray nozzles or two-fluid nozzles should introduce the liquid into an active mixing zone. Quantity, droplet size, dosing duration and addition timing must be controlled so that no local over-wetting, sticky pockets or unwanted agglomerates arise. For moisture-sensitive or water-reactive products, this approach is naturally not suitable.

Variable speed control via frequency inverter allows an operating mode matched to the process. For example, after charging, the batch can first be gently homogenised, then briefly processed more intensively for de-agglomeration, and finally mixed out again at low stress. The mixing time should be as short as possible and only as long as necessary. Over-mixing can worsen homogeneity again with products prone to segregation.

Dosing and discharge

With large differences in density or particle size, the addition strategy is often just as important as the mixer itself. Minor components and fine particles should not fall into stagnant edge zones, but should be dosed in a controlled manner into an active product flow. Geometric premixes can be useful where highly concentrated additives, dyes, active ingredients or very fine components need to be distributed at low dosing levels. Gravimetric dosing systems support reproducible weighing and help distinguish dosing errors from actual mixing problems.

Discharge must be regarded as part of the mixing task. A mass-flow vessel ensures that the product moves across the entire cross-section, whereas with funnel or channel flow, individual areas remain stationary for longer and fractions can be preferentially discharged. Steep wall angles, suitable wall materials, sufficiently large outlet cross-sections and a matched discharge element promote mass flow. This reduces both segregation and bridging and dead zones.

Free drop distances and strong horizontal deflections should be minimised. They promote trajectory, rolling and percolation segregation. In pneumatic conveying, gentle, slower dense-phase conveying can be advantageous, provided it is technically suitable for the product. Vibration at conveyors, vessels and packaging stations should likewise be critically examined, because it can move a fine fraction downward and allow coarse particles to rise.

Characteristic values and testing

The Froude number Fr is a helpful characteristic value particularly for rotating mixing systems. It describes the ratio of inertial to gravitational forces. At too low a rotational speed, circulation can be inadequate; at too high a speed, particles can be pressed against the vessel wall and centrifuge. A general rule such as "Fr>1 prevents segregation" is not correct. At Fr≈1, centrifuging already begins in many rotating systems, which is why significantly lower Froude numbers are usually required for actual mixing.

For continuous and batch processes, further characteristic values supplement the design: fill level, number of tool revolutions, circumferential speed, specific power input, mixing time, residence time distribution, dosing accuracy and discharge time. These values are not universal target figures but must be determined through product trials. Mixing quality should be assessed not only in the mixer, but through time-staggered samples during discharge and, where applicable, after the downstream conveying.