Are there vertical mixers that achieve complete blending even with large density differences?
Yes. Specially designed vertical mixers can achieve a very high homogeneity even with markedly differing bulk densities, provided the geometry, rotational speed, fill level and mixing time are matched to the product.
Operating principle
Vertical mixers work with a forced circulating movement of the mix. The material is conveyed from bottom to top and sinks down again in the vessel. Heavy and light constituents are thereby continuously redistributed and segregation is effectively reduced.
Constructional features
Important prerequisites are suitable screw or helix geometries, mixing zones close to the wall and a flow through the entire vessel that is as uniform as possible. With demanding products, additional choppers or high-speed tools can help to break up agglomerates and distribute fine constituents better. A variable rotational speed makes it possible to adapt the mixing intensity to the particular product.
Design and limits
The Froude number, the fill level and the mixing time are also decisive. Typical fill levels often lie in the range of 40 to 70 per cent. Very large differences in density, strongly deviating particle sizes or moist products can, however, limit mixability.
The Froude number relates to rotating free-fall, drum and throw mixers as well as to systems with a horizontally mounted mixing tool. In vertical mixers with forced restratification it is not a design or scale-up criterion. What is decisive there is that the mixing principle remains the same across all sizes and that the restratification covers the entire mixing chamber independently of the fill level; transferability is secured in the pilot plant with the original product.
Assessment in practice
The achievable homogeneity is usually assessed via the coefficient of variation. In many applications, a value below 5 per cent is regarded as a sign of technically very good blending. With difficult products, pilot plant trials are advisable in order to establish the optimum operating parameters.
How amixon® masters large differences in density and particle size when mixing
A mixing tool that controls the space instead of random movement
With large differences in density, mixing principles that rely on free bulk material movement fail: heavy particles sediment, light constituents float, and centrifugal forces can lead to undesirable classification. The amixon® SinConcave®/SinConvex® helical mixing tool, by contrast, controls the entire mixing chamber. It conveys the mix upwards at the periphery under compulsion and lets it flow downwards in the centre under gravity.
Slow flows arise in the mixing chamber in the process, so that hardly any fluidisation occurs. The upward and downward flows take place in dense phase, whereby the particles are guided uniformly. Every particle thus passes through the same forced flow – irrespective of density, size or flow behaviour. The low-speed mode of operation with a circumferential speed of approximately 0.8 to 3.5 m/s avoids centrifugal separation of fine or light constituents.
Conveying principle and scaling
The conveying principle is the same for all media and can be described approximately by the following relationship:
I_V = (π/4) · (D² − d²) · Φ · S · n · ζ
Here I_V is the conveying capacity, D the outer diameter of the helix, d the inner diameter, Φ the fill level, S the pitch of the helix, n the rotational frequency and ζ the speed coefficient. For the scale-up it is therefore not the rotational speed alone that is decisive, but above all the resulting conveying capacity and circumferential speed. The mixing quality thus remains reproducible across different scales.
For sensitive particles
Where the particles are particularly sensitive, the SinConcave®/SinConvex® helical mixing tool can be supplemented by MultiPlane® technology. The mixing intensity can thereby be matched to the product properties without losing product protection.
Evidence from practice: powder metallurgy
In powder metallurgy, amixon® twin-shaft mixers of the HM type distribute pressing aids such as zinc stearate homogeneously in the metal powder. Very high bulk densities of the metal powder meet markedly lighter additives here. Such density ratios are demanding in practice, but are reliably mastered with suitable mixing technology.
An all-rounder for heterogeneous material compositions
The Gyraton® GM too is expressly designed for heterogeneous material compositions. Coarse and fine particles, differing bulk densities, deviating flow properties and changing moisture contents can be processed reliably up to large batches. The result is a technically ideal random mixture, even at a low fill level.
Verification in the pilot plant
Whether the specific combination of density spread, particle size distribution and cohesion delivers the desired result is examined in the pilot plant with the original product. Mixing trials, sampling and a documented assessment of mixing quality form the basis for the later machine design.
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 crevices and ground smooth, and the mixing tool is mounted at the top only, so that the contamination-critical lower shaft passage is eliminated. Large Clever-Cut® inspection doors with a permanently dead-space-free OmgaSeal® seal make the product-contact surfaces readily accessible. Dead-space-free discharge devices and integrated washing lances permit validatable dry and wet cleaning in accordance with the EHEDG, FDA and 3-A specifications.
Run reproducibly, documented without gaps
Mixing programmes with mixing times, rotational frequencies, dosing and temperature profiles are stored in the PLC and run identically batch by batch. Integration into the operator's ERP system is possible; a barcode scanner can be integrated for real-time documentation. Formulation, batch and process parameters thereby remain linked without gaps.