What advantages does a vertical twin-shaft mixer offer when mixing and wetting cohesive powders?
Vertical twin-shaft mixers are also advantageous where cohesive powders are to be moistened, homogenised, or brought into a dough-like state. The changing flow behaviour of the bulk material plays a role here. This is significantly influenced by cohesion, adhesion, moisture content, particle-size distribution and porosity. The cohesiveness of a powder and its flow function can, for example, be determined with a Jenike shear tester. This measures the internal friction and wall friction under defined consolidation stresses. The results, in the form of flow loci, are relevant, among other things, for assessing bridging, rat-holing and discharge behaviour.
The more deeply the two mixing tools intermesh, the more efficiently a vertical twin-shaft mixer works. Helical mixing tools are the most efficient in this respect. The mixed material is conveyed upward from the near-wall regions while flowing downward in the central region. At the same time, there is strong cross-flow between the two vessel sections. This continuously redistributes powder particles, moisture zones, liquid additions and any existing agglomerates. This design is suitable for dry powders, moist suspensions, and pasty and dough-like products.
Fast and uniform liquid distribution is particularly decisive when moistening absorbent powders. Without sufficient mixing intensity, locally over-wetted zones, sticky nests and agglomerates can form while other product zones remain dry. The superimposed product movement of a twin-shaft mixer supports uniform incorporation of liquids, provided dosing quantity, dosing location, droplet size or spray pattern, addition rate and mixing duration are matched to the formulation. For products particularly prone to agglomeration, additional size-reduction or intensive mixing tools can be worthwhile, to break up any lumps that form and distribute the liquid more finely within the powder.
With suitable tool geometry and correspondingly small clearances to the vessel wall and base, the overlapping mixing zones can produce an effective scraping and self-cleaning effect. This is particularly relevant for moist and sticky products, as it limits material build-up on tools, wall surfaces and in the base area. Less build-up leads to consistent mixing conditions, reduces product residue after discharge, and can lower cleaning and downtime effort. However, the actual effect depends on product adhesion, moisture, temperature, surface finish, tool shape and operating parameters, and should be tested for the specific formulation.
The vertical design also supports product discharge by gravity. This is a design advantage for cohesive, moist or adhesive mixtures, provided the discharge element, tool geometry and vessel base are designed so that as few stagnant product zones as possible form. A low residual quantity is particularly important for frequent product changeovers, valuable ingredients, allergen-relevant formulations, or hygienically sensitive applications. Whether near-complete emptying is achievable, however, must be assessed on the basis of the actual product properties.
In combination with a temperature-control jacket, intensive circulation can support heat exchange. The product is repeatedly carried from the jacket surfaces into the vessel core and back, which reduces temperature differences within the batch. Depending on the configuration, vacuum or pressure operation and liquid-addition equipment can additionally be integrated. This allows moistening, mixing, conditioning, granulating, temperature control and, in certain applications, drying and reaction steps to be combined in a single enclosed process apparatus. Comparable vertical twin-shaft mixers are available for dry, moist and pasty products. They can be combined with heat transfer, vacuum technology and cleaning-friendly designs.
However, the achievable mixing quality does not depend on the twin-shaft design alone. Tool geometry, rotational speed, fill level, the order of solids addition, liquid dosing, residence time, moisture development and the product's cohesiveness are, among other things, decisive. Particularly for cohesive powders, the flow function determined with a Jenike shear tester can provide a robust basis for designing storage, dosing and discharge components. It describes the ratio of the major consolidation stress to the unconfined yield strength and allows the flow behaviour to be classified from very cohesive to free-flowing. Nevertheless, mixing trials with the original product remain important to confirm homogeneity, moisture distribution, agglomerate formation, temperature rise, dischargeability and batch reproducibility under realistic operating conditions.
The amixon® HM vertical twin-shaft mixer is designed for fast, uniform and, at the same time, particularly gentle homogenisation of powders, moist bulk materials, pasty products and moistened mixtures. Two helical mixing tools rotating in the same direction generate a pronounced three-dimensional overall flow: the mixed material is conveyed upward in a spiral in the outer region and flows back down in the centre under the effect of gravity. Intensive exchange takes place in the overlap zones of the two product streams. In this way, components with different bulk densities, particle sizes, moisture contents and flow properties are distributed across the entire batch volume. Vertical twin-shaft mixers of the HM series are designed for dry powders, moist suspensions, pastes and dough-like products.
This three-dimensional flow offers substantial advantages for moistening cohesive powders. Liquids can be dosed via suitable feed points into the moving powder bed and are rapidly distributed throughout the entire product volume. This helps to limit local over-wetting, persistent dry pockets and the formation of large agglomerates. The specific design of the liquid addition depends on the formulation, in particular on cohesiveness, particle structure, initial moisture, wettability, dosing quantity, addition rate and the desired end state. Where targeted deagglomeration is required, cutting rotors can be integrated. They support the mixing process by breaking up agglomerates and improving the distribution of the liquid phase within the powder.
The mixing intensity can be adapted to the task. The same mixer can be used for intensive mixing and deagglomeration as well as for very gentle homogenisation tasks. This is important, for example, when spray-dried or agglomerated instant powders, encapsulated ingredients, fragile particles or sensitive nutrient components are to be mixed. The aim is then to achieve the desired homogeneity without unnecessarily altering the agglomerate structure, solubility, flow behaviour or visual product quality. The circumferential speed of the mixing tools can be set comparatively low for this purpose; the actual setting is determined on the basis of the formulation, batch size and mixing objective.
The amixon® SinConvex® and SinConcave® technologies extend the possibilities for product flow guidance. These helical mixing tools are designed for improved spatial circulation and support uniform mixing throughout the entire vessel space. At the same time, they promote discharge, because product is guided from near-wall and near-base regions toward the outlet. For free-flowing bulk materials, near-complete emptying can be achieved. This reduces product losses, facilitates product changeovers, and contributes to resource conservation, since less raw-material residue, cleaning water and wastewater are generated.
MultiPane® is available for particularly sensitive applications. The technology enables particularly gentle product movement and is therefore suitable for mixing tasks where particle structure and agglomerates are to be preserved as far as possible. This can be relevant for dietary supplement products, dietetic mixtures, instant powders, microorganisms, sensitive granulates or encapsulated ingredients. The combination of low mechanical stress and intensive three-dimensional distribution helps resolve the conflict between high homogeneity and product protection.
ComDisc® supports very extensive residue-free emptying of the mixer. This is as important for valuable, cohesive or allergen-relevant products as it is for frequent formulation changes. A low residual quantity reduces material losses and lowers the risk of carry-over into the next batch. At the same time, it reduces cleaning effort and supports economical, sustainable production. However, how completely a specific product can be emptied always depends on its cohesiveness, moisture, adhesive tendency, particle structure, and the specific apparatus design.
With DosiFlap®, discharge can not only be opened and closed but also controlled in a dosing manner. The valve can close against the running product stream, enabling controlled filling into big bags, IBCs or other containers. Combined with load cells, defined fill quantities can be reproducibly achieved. For large mixers, several DosiFlap® valves can be provided, allowing several big bags or containers to be filled in parallel. This shortens filling time, increases line flexibility, and decouples the mixing process from downstream packaging or filling steps.
WaterDragon® extends the mixer with an automated wet-cleaning and drying function. The system works with programmable targeted-jet cleaning: extendable cleaning lances with rotating wash heads can apply cleaning fluid in a targeted manner to product-contact surfaces. After the cleaning cycle, a large volume of dry warm air can be introduced into the mixing chamber to quickly dry the vessel, mixing tool, discharge area and cleaning equipment. This shortens the time between product changeovers, supports hygienic process operation, and helps avoid unwanted moisture carry-over into the next batch.
For hygienically demanding applications, the HM can be configured with top-mounted and top-driven mixing tools, large inspection doors, and low-dead-space discharge valves. This avoids product-contact bearing points within the mixing chamber. Depending on the project, gas-tight, vacuum-rated or pressure-rated designs can be provided. This can be required, for example, for pneumatic charging, inerting, or the processing of oxidation-sensitive products. A slight inert-gas overpressure can prevent ambient air from entering the process space; the specific pressure and tightness design must be matched to the apparatus, seals, valves and process conditions in this case.
The HM series can also be used for large batches. For formulations with many components, a large batch mixer can offer logistical and economic advantages, because fewer individual batches need to be weighed, mixed, sampled and analysed. This is relevant, for example, for dietetic products, infant formula or other quality-critical powder mixtures. What remains decisive is that the sampling plan, mixing time, rotational speed, fill level and homogeneity assessment are validated for each formulation. A single mixing sample can only be considered representative if this has been demonstrated through a suitable validation concept and a robust sampling strategy.
On request, amixon® twin-shaft mixers can be executed to hygienic and regulatory requirements, for example to EHEDG and, depending on the project-specific design, to 3-A Sanitary Standards, FDA, GMP or ASME. A mixing trial with the original product is advisable before investing. This allows not only mixing quality and mixing time to be evaluated, but also moisture distribution, the suitability of liquid dosing, agglomerate formation, product damage, residual discharge, cleanability and the possible use of SinConvex®, SinConcave®, MultiPane®, ComDisc®, DosiFlap® and WaterDragon® to be assessed under realistic conditions.