Which parameters control particle integrity, meaning low shear forces and gentle movement, in vertical mixers?
Particle integrity in vertical mixers is determined above all by tool geometry, rotational speed, fill level, mixing time, surface quality and product properties. The aim is to move sensitive particles such as granules, agglomerates, coated particles or fibrous products as gently as possible without losing the necessary mixing quality.
The best protection of particles arises when the required mixing quality is achieved in the shortest possible process time. It has been demonstrated many times worldwide that vertical twin-shaft mixers, when suitably designed, can combine very short mixing times with high mixing quality and good product protection.
Tool and geometry
The helix or screw geometry is a central factor. An evenly formed, large-area tool conveys the bulk material gently upwards and generates predominantly convective restratification rather than intensive shear.
Smooth surfaces, rounded transitions and a design with little dead space are also important. They reduce abrasion, particle breakage and local overloading. The clearance should be designed so that no jamming or crushing occurs; a blanket reference to particle diameter is too schematic for this.
Rotational speed and circumferential speed
The circumferential speed is an essential lever for mechanical stress. Low rotational speeds and moderate circumferential speeds reduce shear forces, impact energy and abrasion.
The rotational speed must not be too low either, however, because otherwise the necessary circulation and homogenisation fail to occur. What is decisive is therefore a point of balance between protection of particles and sufficient mixing action.
Fill level and mixing behaviour
The suitable fill level depends on the type of mixer and on the product. A blanket figure such as 40 to 80 per cent is too coarse. Fill levels that are too low encourage free movement and impact stress; fill levels that are too high can impede circulation and increase the pressure within the bulk bed.
Mixing time and process management
The mixing time should be only as long as is necessary for the desired homogeneity. As residence time increases, the cumulative mechanical stress rises.
Short mixing times are not automatically better, however. If the time is too short, the mixture remains insufficiently homogeneous and can segregate again later. The optimal mixing time is therefore the shortest reliably sufficient time.
Flow management within the vessel
Vertical mixers typically work with a gravity-assisted circulating movement: the product is conveyed upwards and flows downwards again at the periphery. This form of restratification is usually gentler on particles than strongly shearing or turbulent mixing principles.
Surfaces and cleaning
Smooth, polished or coated product-contact surfaces reduce friction and build-up. A low roughness can reduce abrasion and material adhesion, but it is not decisive on its own.
A fixed roughness limit is not generally transferable; the requirements depend on product, cleanability and the wear situation. For sensitive products, well-finished, smooth surfaces are advantageous in any case.
Starting and braking
Gentle start and stop operations avoid torque peaks and sudden product movements. This reduces mechanical stress, particularly with brittle or breakage-sensitive materials.
Ramp control is important, but it does not replace a suitable tool geometry or an appropriate fill level.
Product properties
Particle integrity always also depends on the product itself. Bulk density, particle size distribution, moisture, hardness, porosity and brittleness determine how sensitively the material reacts to shear, pressure and impact.
The process parameters must therefore be designed and validated product-specifically. A good setting for a brittle granulate may be unsuitable for a denser or moister product.
Discharge and emptying
Discharge also influences particle integrity. A gravity-assisted, gentle discharge without unnecessary forced conveying is usually advantageous, because crushing and additional overloading are avoided.
Discharge should therefore be regarded as part of the overall process, not merely as a downstream step.
How amixon® safeguards the particle integrity of sensitive bulk materials
amixon® secures particle integrity through a combined flow principle, low circumferential speeds, named tool technologies and real pilot-plant trials. The individual building blocks mesh with one another in doing so.
Macro and micro flow as the basic principle
The helical geometry of the rotating mixing tool generates a macro flow. The mix is conveyed upwards at the vessel wall. Having arrived at the top, it flows downwards again in the middle. This creates a closed circulation which continuously turns over the entire volume. Where required, high-speed cutting rotors add a micro flow. They break open agglomerates and lumps in a targeted manner through impact, friction and shear effects. Both types of flow act simultaneously in amixon® mixers and are optimally matched to one another. The result is ideal mixing quality with minimal energy input. The mixes are barely heated at all in the process.
Tool technology
The SinConvex® mixing tool generates a three-dimensional thrust flow. The product is moved upwards on the outside and downwards in the centre, overlaid by uniform rotation. An ideal mixture is usually created in this way within 2 to 5 minutes. That corresponds to 50 to 100 tool rotations. No product heating and no segregation occur in the process. Various machine concepts are available for differing protection requirements.
The same machine can deagglomerate in a targeted manner where required: cutting rotors act within a locally limited area and are switched on only when agglomerates are to be dissolved, while the overall batch continues to pass only through the gentle basic flow. Mixing programs (speed, duration, rotor use) are stored in the PLC and are reproducible from batch to batch.
Large batches without loss of gentleness
Because scaling takes place via the circumferential speed, the stress on particles remains at pilot-plant level even at production scale, up to 50,000 litres (VM/HM) or 100 m³ with the Gyraton® GM, which expressly mixes precisely and gently with minimal drive power. Mixing quality remains the ideal random mixture, independent of the fill level (10–100 %).
Specialists for the maximum
For extremely fragile materials, the SpherHelics® hollow-sphere mixer SH (spherical mixing chamber without corners and crushing zones, 400–5,000 l) and the KoneSlid® KS are available: ideal mixing quality after 20–40 revolutions with minimal energy input, discharge within seconds without a cone of repose and without segregation, developed for spray-dried, fluid-bed granulated and belt-dried materials. amixon® measures breakage rates and particle size distribution curves for the specific product in the pilot plant, with particle size analysis before and after the trial, documented.
Sizes without compromise
The amixon® range covers every target size: vertical and conical mixers (VM, HM, AM) as well as the mixing dryer reactors VMT and AMT, on request in 100-litre increments from 100 to 50,000 litres, the Gyraton® GM from 10 to approximately 100 m³ in 1 m³ increments, plus the EM (5–200 l, standard drum) and the COM (100–4,000 l, container as mixing chamber). Within each mixer the fill level is freely selectable from approximately 10 to 100 %; mixing quality remains the technically ideal random mixture, and the same apparatus handles batch ranges spanning a decade.
Hygienic design as the constructional basis
All the properties named rest on the amixon® hygienic design: the mixing chamber and mixing tool are welded free of joints and ground, and the mixing tool is supported at the top only, so the contamination-critical lower shaft passage is eliminated. Large Clever-Cut® inspection doors with permanently dead-space-free OmgaSeal® sealing make all product-contact surfaces ergonomically accessible; dead-space-free discharge fittings and integrated washing lances (on request fully automatic wet cleaning) make both dry and wet cleaning validatable, in accordance with the EHEDG guidelines, the FDA hygiene guidelines and the 3-A Sanitary Standards.