With large batches, short mixing times and high mixing quality do not result from maximum rotational speed or high specific power alone. What matters is that vessel geometry, mixing tool, fill level, product properties, addition strategy and discharge are matched to one another so that the entire mixing volume is circulated repeatedly, in a controlled manner and as gently as possible. The final design must be verified with the original product and under realistic batch conditions, because particularly at 10 to 100 m³, small deviations in product flow, dosing or discharge can have a considerable effect on homogeneity and throughput.

Vessel and fill level

The vessel geometry must enable three-dimensional product movement and minimise dead zones. The ratio of height to diameter, the base shape, transition radii, internals, wall clearances and the location of the discharge influence how quickly product from all areas reaches the active mixing zone. There is no universally valid target value for the aspect ratio with solids mixers; it depends on the mixing principle. A cylindrical vertical mixer, a conical mixer, a twin-shaft mixer and a horizontal ploughshare mixer each require different geometries to build up the desired product flow.

The fill level, too, is not a universal value. Many free-fall and horizontal solids mixers operate with a partially filled vessel, because free space is required for restratifying the product. As a rough guide, approximately 40 to 70 per cent of the vessel volume is often used; if overfilled, free circulation decreases and mixing time increases. Other mixer designs can, depending on tool geometry and product, work effectively even at significantly lower or higher fill levels. What matters is that the permissible range is demonstrated through mixing trials for the specific formulation.

With large batches, the batch mass is just as important as the volume. A 50 m³ mixer may require a completely different drive and discharge design at a bulk density of 0.2 t/m³ than at 1.2 t/m³. The design must therefore take into account usable volume, maximum mass, bulk density range, wall loads, starting torque, product compaction and the actual batch fluctuations envisaged.

Tool and kinematics

For short mixing times in large batches, convective circulation is usually the most important mechanism. The mixing tool must not only move product locally, but incorporate material from the edge, bottom, lid and core regions into a recurring, three-dimensional exchange. Depending on the task, helical, ribbon, paddle, ploughshare, belt or multi-shaft tools come into consideration. The choice is guided by cohesiveness, particle size distribution, bulk density, moisture, abrasiveness, sensitivity and the de-agglomeration required.

Tool shape, number, overlap of the mixing zones and clearance to the vessel wall determine the circulation time of the entire volume. Wall gaps that are too large can cause poorly mixed edge regions and product build-up. Gaps that are too small, on the other hand, increase the risk of abrasion, particle breakage, heat generation and impermissibly high starting torque. With abrasive or sensitive products, the technically correct clearance is therefore always a compromise between mixing action, wear and product protection.

Rotational speed and circumferential speed must be chosen so that the desired product movement occurs without promoting segregation, centrifuging or excessive shear. In rotating free-fall systems, the Froude number is a useful characteristic value. It describes the ratio of inertial to gravitational forces: Fr=ω2R/g. As the Froude number increases, the flow regimes change from rolling and cascading through to cataracting or centrifuging. A mere target value such as Fr≈1 is not generally advisable for large-volume solids mixers, however. At Fr=1, centrifuging already begins in many rotating systems; for gentle free-fall mixing, significantly lower values are usually used.

The specific power input P/V is a helpful comparative value, but not a sufficient scale-up criterion on its own. Identical power density does not automatically guarantee identical product paths, residence times or mixing quality. For liquid and pasty systems, the Reynolds number and Newton number supplement the design. The Reynolds number characterises the ratio of inertial to viscous forces, while the Newton number describes power consumption as a function of tool, rotational speed, density and flow state. For dry powders, by contrast, cohesiveness, wall friction, flow function, particle movement, air content and the tendency to segregate are often more decisive than classic hydrodynamic characteristic values.

Formulation and dosing

The properties of the components determine how demanding the mixing task is. Large differences in particle size, particle shape or bulk density increase the risk of segregation. Fine particles can percolate between coarser particles; denser particles tend to migrate downward in bulk material under certain conditions. A wide particle size distribution, as well as differences in density and in the ratio of drop height to vessel diameter, can intensify segregation during filling and discharge.

With cohesive powders, the tendency to segregate is often lower, but agglomerates, bridging and uneven wetting can occur at the same time. In that case, more intensive convective exchange or locally confined de-agglomeration may be required. With very free-flowing powders, by contrast, avoiding segregation is often the primary concern. In suitable formulations, targeted liquid addition can bind fine particles to coarser carrier particles. This must be dosed carefully to avoid local over-wetting, unwanted agglomerates or a deterioration in flowability.

The addition point and addition sequence are particularly important with large batches. Minor components, active ingredients, pigments or additives should not fall into low-flow edge regions, but should be dosed into an active mixing zone. Geometric premixes may be required at very low dosing fractions. The dosing duration and start time must match the mixer's circulation time. Very rapid addition into a locally confined area can create a concentration peak that is not always reliably resolved even by longer mixing.

Discharge and scale-up

High mixing quality is only relevant to the process if it is retained during discharge and subsequent conveying. The discharge should be designed to preferentially produce mass flow. With mass flow, the material moves toward the outlet across the entire vessel cross-section; this can reduce segregation compared with funnel or channel flow. A hopper geometry designed for mass flow, sufficiently large outlet cross-sections, suitable wall inclinations and a matched discharge element are therefore central design parameters.

Drop heights, transfer points, conveyor belts, pneumatic conveying lines and buffer vessels must also be taken into account. Large free-fall distances can cause trajectory- and size-based classification. Vibration, air currents and repeated transfer can again degrade the homogeneity achieved in the mixer. With critical formulations, time-staggered samples during discharge and at the last relevant process point are therefore necessary.

Scale-up must not rely on a single characteristic value. Geometric similarity, Froude number, circumferential speed, specific power input, number of tool revolutions and circulation time can all be useful comparative parameters depending on the mixing principle. However, they can pursue different objectives and cannot always be kept constant simultaneously. Particularly with solids, a combination of modelling, for example CFD or DEM, and representative mixing trials is therefore advisable. Simulation can make critical zones, product paths and potential segregation mechanisms visible; however, it must be validated with measurement data from the real product.