What options are there for integrating dosing systems into powder mixers?
Dosing systems can be integrated directly at the mixer, via an intermediate vessel, or as part of a continuous process. The technically best solution depends on product behaviour, required dosing accuracy, throughput, batch or continuous operation, hygiene, containment and available installation height. There is no blanket "best" architecture.
With direct charging, dosing devices such as screw feeders, rotary valves or vibratory troughs are connected directly to the inlet of the mixer. This reduces transfer points, free drop heights and material losses. The solution is suited to compact plants, but requires reliable timing coordination between dosing and the mixer. Volumetric feeders are comparatively simple, but react sensitively to changes in bulk density, flow behaviour or fill level. They are therefore mainly useful with stable, readily flowable powders.
For batch processes, an upstream gravimetric weighing stage is frequently used. Here, components are dosed by mass in a weighing vessel or directly on a scale and then transferred to the mixer batch by batch. This principle is often referred to as gain-in-weight: the increasing weight in the weighing system is measured. It enables clear recipe assignment and a high level of accuracy, though this always depends on the product, quantity range and plant. Load cells must be decoupled from vibration, pipework forces and mechanical side loads.
For continuous mixing processes, gravimetric loss-in-weight feeders are frequently used. Loss in weight means that the decreasing mass of a storage vessel is continuously recorded; the speed of the dosing device is controlled so as to maintain a predefined mass flow. This method can compensate for fluctuations in bulk density and material flow better than purely volumetric dosing. The refill phases, however, remain a critical point: during refilling, direct gravimetric control is limited or interrupted, which is why vessel size, refill rate, control strategy and, where applicable, buffering must be carefully designed.
Dosing accuracy should not be stated with fixed percentage values across the board. It depends on the product, dosing range, dosing time, weighing resolution, flow behaviour, discharge device, refilling process, ambient disturbances and the definition of accuracy used. Particularly with small quantities and cohesive powders, the achievable accuracy can deviate considerably from nominal figures. It should therefore be verified with the original material and across the real operating range.
Cone valves can be used as discharge and shut-off devices on mixers, silos or vessels. Raising a cone creates an annular discharge gap. With a suitable vessel geometry and suitable bulk material, this can favour mass flow: the product moves across the entire vessel cross-section rather than only in a central flow channel. Mass flow can reduce segregation, dead zones and uneven residence times. It is not, however, guaranteed by the valve alone. Hopper angle, outlet size, wall friction, product moisture, cohesion, particle size distribution and aeration must be considered together.
A variable lift position of a cone valve can influence the product stream and, in certain applications, support coarse or fine dosing. For precise gravimetric filling, however, the valve alone generally does not replace suitable weighing and control technology. With cohesive or poorly flowing powders, vibration, aeration or discharge aids may additionally be necessary. These measures must be designed carefully, since vibration or uncontrolled air movement can promote segregation in sensitive mixtures.
The process control system must logically interlock the feeder, mixer and discharge with one another. Typical conditions are: the mixer is ready for charging, the correct recipe is released, the discharge valve is closed, the weighing lies within the permissible tolerance, and the downstream plant can receive product. Limit switches, position feedback, weighing signals, level values and fault messages should be clearly documented and integrated into the control system. A programmable logic controller, abbreviated PLC, can execute these sequences; for batch documentation, recipe, raw-material and process data can be passed on to higher-level systems.
The constructional design depends on the application. For food and pharmaceuticals, geometries with minimal dead space that are readily cleanable, suitable product-contact materials and, where applicable, cleaning in place, abbreviated CIP, or sterilization in place, abbreviated SIP, are relevant. CIP refers to cleaning installed plant components without extensive dismantling; SIP refers to sterilisation in the installed state. With explosive dusts or gases, zone classification, ignition-source assessment and suitable protective measures must be taken into account. Abrasive products may require wear-resistant materials or replaceable wear parts. For reactive or oxygen-sensitive products, closed systems, inerting and defined pressure or vacuum conditions may be necessary.
How amixon® can integrate dosing, discharge and peripherals
Integrating dosing systems into a powder mixer must be planned as an overall task comprising material supply, weighing, charging, mixing, discharge, conveying and control. What matters is the properties of the bulk materials, the required dosing accuracy, the mode of operation – batch or continuous –, the required cleanability, the containment requirements, and the interfaces to the upstream and downstream plant. amixon® can design the mixing technology and the associated interfaces on a project-specific basis using the user requirement specification, abbreviated URS. The URS is the operator's documented requirement specification.
On the discharge side, the DosiFlap® fitting can support controlled product delivery in certain applications. It can be arranged directly beneath the mixer and enable transfer into big bags, containers, conveying technology or downstream process stages. The advantage of direct delivery is that additional intermediate vessels and potential transfer points can be avoided. Whether a sufficiently accurate dosing is achieved with this, however, depends on flow behaviour, bulk density, dosing quantity, discharge tolerance, valve characteristics, pressure conditions and the available weighing concept. For gravimetrically precise filling, matched weighing technology is generally required; the discharge device alone does not replace it.
Depending on the process requirement, standard connections with outlet flaps or vacuum- and pressure-resistant ball-segment valves can also be used. The suitable discharge device must match the product, the vessel geometry, possible pressure or vacuum conditions, the cleaning strategy and the downstream plant. A flush, low-dead-space design of the outlet can reduce product residues and cleaning effort. It is not, however, general proof that no residue remains above the fitting. Adhesive, moist, cohesive or electrostatically chargeable powders can form build-up regardless of the geometry. Discharge and cleaning performance should therefore be verified with the actual product.
ComDisc® technology can support discharge by moving residual product quantities to the outlet at the end of the mixing process. According to amixon®, high discharge rates of up to approximately 99.99 percent are possible depending on product and application. As complete a discharge as possible can increase product yield, simplify product changes and reduce residual quantities. The actual achievable discharge, however, depends on product adhesion, moisture, particle size, bulk density, machine geometry and mode of operation. A segregation-free discharge can only be assessed for the specific recipe and the actual discharge path.
For continuous processes, the amixon® continuous mixer type AMK shows how dosing, mixing volume and discharge can be coordinated with one another. At the start of production, discharge initially remains closed. The gravimetric feeders start simultaneously with a low mass flow and are coordinated with one another. Once the vessel is approximately half full, mixing operation begins. At a fill level of around 80 percent, discharge opens progressively; the fill level is then held at this level via the dosing streams. At the end of production, the dosing streams are reduced in a controlled manner and the mixer is emptied. This concept can reduce start-up and run-out losses and promote stable operation. Whether this actually results in no product losses or no off-spec product, however, depends on dosing accuracy, raw-material fluctuations, process control, the mixing task and the defined quality criteria.
Similar principles apply to batch plants. Automatic recipe weighing can capture components according to predefined target quantities and transfer them to the mixer in a defined sequence. For this, dosing devices, load cells, storage vessels, conveying technology, mixer and discharge must be linked via clear control and interlock logic. For example, the mixer should only be charged once the recipe is released, the discharge device is closed, sufficient vessel volume is available and the process is in a safe operating state. After the mixing phase, the downstream plant or filling system must be ready before discharge is released.
amixon® manufactures the mixing technology; feeders, screening machines, conveying technology and other peripherals can be integrated within the scope of the project. For this, mechanical connection dimensions, transfer heights, conveying capacities, throughputs, buffer volumes, material flows, signal handovers and responsibilities must be defined at an early stage. Compact vertical mixers can make integration easier, because raw materials are frequently fed in from above and products discharged downward. However, the gravity principle only works if drop heights, hopper angles, flow behaviour, bridging risks, dust routing and available installation height are adequately taken into account.
The control system links weighing, dosing, the mixing process and filling. Recipes can be stored in a programmable logic controller, abbreviated PLC, and run for each batch. A PLC is an industrial computer for controlling machines and processes. A project-related connection to an enterprise resource planning system, abbreviated ERP system, and barcode-based material identification can support batch traceability. An ERP system serves the planning and management of orders, materials and resources. For recipe, raw-material batch, target and actual quantities, process parameters and discharge to be correctly linked with one another, the data model, time stamps, recipe version, user permissions, communication behaviour and the handling of deviations must be clearly established.
The design of a new dosing or discharge solution should not be based on experience values alone. Trials with the original product can be carried out in the amixon® pilot plant. According to amixon®, more than 30 test units in different sizes are available in Paderborn; additional test centres exist in Japan, India, Thailand, China, South Korea and the USA. Depending on the task, mixing quality, dosability, liquid distribution, product protection, energy input, dischargeability and cleanability can be assessed. The results can support the design and the definition of starting parameters. They reduce technical risks before an investment, but do not replace the performance acceptance test and, where applicable, qualification of the complete production line at the operator's site.
The hygienic design of the mixing plant can support clean and controlled integration. For certain designs, amixon® describes product-contact areas with minimal joints that are ground smooth, a mixing tool supported at the top only, and Clever-Cut® inspection doors with OmgaSeal® seals. These features can improve accessibility and cleanability and reduce product-contact sealing points. Washing lances as well as cleaning in place, abbreviated CIP, or wet in place, abbreviated WIP, can be provided on a project-specific basis. CIP refers to cleaning in the installed state without extensive dismantling. WIP describes wet cleaning in the installed state. The actual cleaning effect must be demonstrated for the product, soiling and cleaning method; a washing lance or an automated cleaning cycle is not automatically validated cleaning.