Which conical mixers allow almost complete residual discharge, and how is this achieved by design?

Question

Accepted Answer

How amixon® solves residual discharge and accurately dosed discharge by design

Geometry and mixing tool work together

In the AM conical single-shaft mixer, the conical geometry of the mixing chamber directs the product reliably to the central outlet. The SinConvex® mixing tool provides not only excellent mixing qualities but also very good residual discharge. Free-flowing materials can be discharged to 99.997 % and better.

In the KoneSlid® mixer KS, the lowerable closure system opens the entire cross-section. The batch flows out within a few seconds, without a cone of repose and without segregation.

ComDisc® for the final phase

In vertical mixers such as VM and HM and in the SpherHelics® SH, ComDisc® elements take over residual discharge at the bottom in the final discharge phase. The last product residues are thereby discharged without segregation. In suitable applications, residual discharge rates of 99.997 % and better are achievable.

The Gyraton® silo mixer also uses an effective discharge concept in order to discharge even poorly flowing products as completely as possible. This markedly reduces the residual quantity before cleaning.

Dead-space-free discharge devices with a dosing function

The discharge devices available are dead-space-free standard connections with a discharge flap, vacuum-resistant and pressure-resistant ball segment valves and the dead-space-free DosiFlap® fitting. With DosiFlap® and KoneSlid®, accurately metered filling of big bags and containers is possible directly from the mixer, without an intermediate vessel and without a segregation path.

The discharge device closes flush with the inner wall of the mixing chamber. No dead spaces arise above the fitting in which unmetered residual quantities could remain.

The economic effect

High residual discharge means less product loss per batch, lower risks of cross-contamination during formulation changes and shorter cleaning times. Good residual discharge saves time and money every day; with high-value formulations this design advantage pays for itself particularly quickly.

Manufacture in Paderborn as the quality foundation

amixon® develops and manufactures exclusively at the Paderborn works with a high depth of manufacture and all components from Germany. As a certified welding company with European, Japanese, Korean and American qualifications, amixon® designs every apparatus as a one-off on the basis of the operator's URS. Quality control remains entirely in-house without gaps, and every specification is verifiable down to component level.

Qualifiable and documented

For regulated environments, amixon® supplies the plant ready for qualification: every apparatus is a one-off to the URS from in-house manufacture with quality control without gaps. amixon® assists on request with DQ, IQ and OQ; documentation and execution follow, on request, EU GMP and FDA 21 CFR Part 11. On request, the mixers meet the relevant standards and are integrated into the operator's validation concept from the URS through to commissioning.

Verification in the amixon® pilot plant

Whether the specific formulation delivers the expected results is verified by amixon® in advance at the pilot plant with 35 test units in Paderborn alone. The trials take place with the original product under real conditions, are carried out, evaluated and documented jointly and form a robust basis for decision-making before the investment. Further pilot plants are available in Japan, India, Thailand, China, South Korea and the USA.

Answer

Almost complete residual discharge is achieved above all with vertical conical mixers with centrally or eccentrically arranged mixing screws whose screw extends into the lower area of the cone and close to the discharge opening. Residual quantities in the per mille range (below approximately 0.1 % of the working volume) are technically achievable with a suitable design and appropriate product properties.

Types of conical mixer with a high rate of residual discharge

Vertical conical screw mixers

Mixing screws rotating close to the wall with a planetary movement cover the entire vessel volume and actively convey the product to the lowest point of the cone at the end of the mixing process.

Conical paddle and helical mixers

Specially shaped paddles or helical mixing tools provide intensive mixing and, in combination with gravity, very good gravitational discharge even from areas close to the wall.

Single-shaft and multi-shaft conical mixers

High-performance mixers in which the mixing tools push the product deliberately towards the discharge opening during discharge – advantageous for cohesive or poorly flowing powders.

Double cone mixers

The symmetrical double cone geometry and central discharge at the lower cone allow almost complete gravimetric discharge where the flank angles are sufficiently steep.

Design features for almost complete residual discharge

Vessel geometry

A conical lower vessel section with steep wall inclination angles above the angle of repose of the bulk material (typically approximately 15°–20° from the vertical), so that gravity overcomes static friction and the product slides down reliably.
Dead-space-free, rounded transitions without edges, niches or undercuts in the area of the cone and the discharge cone.
A central outlet with minimal residual volume in the discharge area.

Mixing tool design

Mixing screws or paddles taken to immediately above the bottom outlet, so that no product zones remain unmoved.
Orbiting screws with a planetary movement that cover both central zones and zones close to the wall and mechanically release build-up.
Narrow clearances between mixing tool and vessel wall, where necessary with scrapers, in order to convey adhering product layers towards the outlet.
Overhung mixing shafts (cantilever design) without a lower guide bearing in the product space, so that no bearings, seals or structural parts remain in the discharge area as points of deposition.

Discharge device and fittings

Shut-off devices integrated flush into the vessel contour (e.g. ball segment valves, flaps, slide gates or conical closures) that continue the internal geometry of the cone without projections when closed.
Consistent avoidance of design niches in the area of the outlet seal, in order to prevent dead spaces and product deposits.
Outlet diameter and shape matched to the flow behaviour, particle size and cohesion of the product, in order to minimise bridging and caking.

Surfaces and additional measures

High surface quality of the product-contact components (e.g. roughness values down to Ra ≤ 0.8 µm), supplemented by electropolishing or suitable coatings to reduce adhesion forces.
Optional use of vibration or knocking devices, pneumatic residual discharge or heated vessel walls, particularly with moist, sticky or strongly cohesive products.
With critical bulk materials, additional wall scrapers or specially shaped mixing tools to convey adhering layers deliberately into the discharge opening.

Influence of the product properties

Even with an optimal design, the achievable rate of residual discharge depends on the flow properties of the bulk material:

With free-flowing, dry products, residual quantities in the per mille range are usual.
With cohesive, moist or caking materials, additional design measures and where appropriate process-side aids are required in order to achieve a comparably low residual quantity.

These design and process measures allow almost complete residual discharge in modern conical mixers while at the same time ensuring high cleanability and reproducible process conditions.