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Blending bulk materials, maintaining particle structures and avoiding dust

Bulk material homogenization and dust binding

The ability to mix and wet powders is as old as human civilization. Druggists and colorants used mortars and crushed powder particles in crucibles to increase the surface area of the mixture particles. In this way, crushing and bulk mixing were done at the same time. From a strictly scientific point of view, many other changes have been made to the powder particles. Electron microscopic observations show how variously and individually powder particles can change. Clumpy powder deagglomerates, dry particles wet and coat themselves with finer fractions, dispersed particles agglomerate, absorbers can dehumidify powders, and under pressure and friction, dispersions can crystallize.

These phenomena are the subject matter of mechanical process engineering. This scientific discipline is part of engineering science. It was decisively founded by the German professor Hans Rumpf (University of Karlsruhe) in the seventies. Empirical individual findings in powder processing were broken down by him and investigated on the basis of physical laws. Using methods of dimensional analysis, he and his scientists developed reliable calculation methods to describe complex comminution or mixing processes.

Processes, branches and the branch-specific designations

The cube symbolizes the three dimensions: Processes, Industries and the industry-specific designations/ applications. The listed processes are of different importance for the respective industries:

  • In pharmaceutics, active ingredient synthesis and vacuum drying are of great importance.
  • In solids galenics, trituration of the active ingredients is important with the aim of large surface areas and good "bioavailability".
  • In the food industry, gentle mixing and particle preservation are important. In addition, the powders produced should be dust-free.
  • In the fine chemicals industry, reliable total flow is important. Only then does efficient heat transfer take place - an essential precondition for complete reaction processes.
  • Polymers are conditioned with additives to form high-performance plastics. These are, for example, prepared fillers, reinforcements, color pigments, stabilizers, metal soaps, self-extinguishing additives, antistatica, .....
  • In the solid-state synthesis of engineering ceramic materials (powder metals and nanoceramics), nanoparticles serve as coating substances. They coat finely dispersed metal salts and metal oxides. Nanofine particles are either reactants or act as catalysts in the calcination process at high temperatures.

The terms "solid blending, powder blending or bulk blending" are actually only headings for diverse types of powder formulation/ solids refinement. Such a preparation process can only be successful, if almost every single particle of the batch has completed the desired material change. In this respect the flow of all particles in the mixer/ reactor/ blender must be total and free of dead space.

Dust

Wherever powdery goods are processed, dusts occur. Their unpleasant property is that they can escape through the smallest of leaks. For example, from big bags, mixers, silos, pipelines or from filling plants. Dust can remain suspended in the air for a long time and settle everywhere. This happens preferentially on horizontal surfaces. In the event of a airblow, they swirl up again and float in the air. Some dusts are hygroscopic, absorbing moisture from the surrounding air and wetting all the surfaces on which they are deposited. This makes cleaning more difficult. Salt dusts, in particular, can be corrosive. Dusts can be combustible and even explosive. This is always the case if they are of organic origin; if they are very fine and dry. In the working environment, dusts are always a nuisance, they affect our well-being and irritate the respiratory tract. Escaping dusts are expensive, they represent material loss and imply disposal costs.

Powders that are end-used by consumers are usually conditioned, so that their dust content is negligible.

When particles are smaller than 0.1 µm, they are referred to as nanoparticles. Many materials exhibit completely new properties when they are nanodisperse. In medicine, powder metallurgy, electrical engineering and high-performance ceramics, nanodisperse structures pave the way for new materials and functions.

KoneSlid® powder blender

Dust formation can be avoided within the bulk material by avoiding abrasion on the particles. This is where a conflict of objectives becomes apparent: on the one hand, the mixing process should achieve ideal homogeneity, but on the other hand, the particles should not break, crush or round during mixing. This problem can only be solved if an ideal mixing quality can be achieved with minimum energy consumption. This KoneSlid® mixer from amixon® realizes the gentlest way of powder mixing. It is preferably used:

  • Where many spray-dried agglomerates are involved in the mixing process. These are, for example, milk powder derivatives for dietary nutrients such as baby food, cappuccino, .....
  • medical active ingredients
  • biological substrates 
  • enzymes 
  • cultures of bacteria 
  • instant beverages, iced tea, spices, tea, muesli, fat powder, lecithin powder, .....

KoneSlid® mixers achieve ideal mixing qualities in a very short mixing time. The discharge process is completed in just a few seconds.

Efficient powder mixing: Extremely short mixing time, minimal energy, emptying in seconds.

Efficient powder mixing: Extremely short mixing time, minimal energy, emptying in seconds.

amixon® manufactures Gyraton® mixers in many sizes upon request (sizes are graduated in 1000 liter increments).

amixon® manufactures Gyraton® mixers in many sizes upon request (sizes are graduated in 1000 liter increments).

Gyraton® mixer for large bulk material quantities

It is a great privilege when a production manager can have homogeneously mixed raw materials at his disposal. This simplifies the subsequent process steps and guarantees consistent quality of the end product. Gyraton® mixers from amixon® make a valuable contribution here. For example, when the contents of a 40-foot deep-sea container (approx. 70 m³ bulk material) are to be homogenized. The mixer spiral rotates slowly and mixes all powder particles from bottom to top. In the process, the mixer spiral glides slowly along the mixing chamber wall - similar to a gyroscope. A Gyraton mixer can also discharge the mixed materials completely. It is then equipped with ComDisc® tools.

The mixing principle of the Gyraton® mixer works universally.

The mixing principle of the Gyraton® mixer works universally. In it, high mixing qualities are achieved. Particle sizes, bulk densities, finenesses, moisture contents can differ significantly.

Gyraton® mixer for continuous bulk mixing

A large volume Gyraton® mixer can also operate as a continuous mixer. The mixing process is based on continuous boiler flow. Normally, gravimetric feeding systems would have to be used. However, in contrast to batch feeders, these are relatively expensive. But even if the mixer is filled by batch, the mixed material is discharged continuously. This is largely independent of the number of components, as long as they are of similar size. The mixed material is all the more homogeneous

  • the smaller the batches are 
  • the more constant their weight
  • the more constant the time cycles are
  • the higher the filling level is adjusted
  • the faster the mixing tool rotates

Binding dusts. Gently moisten bulk materials. Distribute extremely small liquid material quantities into large powder quantities

Dust binding by wetting can take place gently and accurately in amixon® mixers. Dust emissions can be avoided by agglomerating dust-fine particles together or by adhering them to larger particles. Even the smallest amounts of a suitable liquid can be helpful. The precondition, however, is that the liquid is distributed evenly over the entire surface of the bulk material.

Two-substance nozzles are also called atomizing nozzles.

Two-substance nozzles are also called atomizing nozzles. They can nebulize low-viscosity liquids to a microfine degree. At the same time, they create a fluidization zone in the mixture.

Build-up granulation or build-up agglomeration

In the literature, both terms are used and usually mean the same thing. In any case, the vocabulary "agglomeration" is correct. It describes the grain enlargement by adhesion of many small particles. A granulation process also describes when a solid mass (flakes, extrudates) is reduced to a crumbly structure.

In this blog, a more aggressive form of agglomeration should also be mentioned. So-called ring-layer mixing granulators can manage agglomeration processes particularly effectively and inexpensively. This ring-layer mixing machine is operated continuously. A mixer shaft is equipped with many pins. At high rotation frequency, the peripheral speed is 10 to 25 m/s. Accordingly, the powder is accelerated into a rotational motion and presses against the wall of the mixing chamber. Shear and pressure forces are effective in the powder, which are 200 times greater than the gravitational force. The particles come so close to each other that interparticle forces become effective. Agglomerates build up and grow. They are discharged down the other side of the mixing chamber. The binding mechanisms can be improved by mixing in small amounts of a liquid binder.

Wetting, compacting, enlarging, rounding, discharging powder particles.

The mixer shaft rotates at high speed. The tiller agitates the mixing goods. The faster the tiller rotates, the faster the mixing goods rotate. The faster the mixing goods rotate, the stronger the centrifugal forces. The mixing goods are thrown against the mixing chamber. There it is slowed down. The peripheral speed at the mix wall is 0 m/s. At the front of the mandrels, however, the peripheral speed is 10 to 20 m/s. The gap between the mixing chamber and the mandrel is approx. 1.5 mm. In this shear gap, mixing, wetting, shearing, compacting, deagglomeration and agglomeration take place with high intensity. The annular gap mixing granulator from amixon can agglomerate very effectively. The result is uniformly round pellets. Ideally, 100% of the dust particles are bound.

 

Important parameters for good agglomeration include

  • Particle size of the powder / dust
  • rotational speed
  • Number of pins
  • Length of the process chamber
  • Type of liquid
  • Location of the liquid addition
  • Pre-treatment and type of liquid addition
  • Temperature of the goods
  • Mass flow of powder
  • Mass flow of liquid
  • Filling level of the ring layer mixer
  • Dwell time

Testing and process development

The mixing systems described above can be tested in the amixon GmbH technical center. Here, we gain interesting insights together with our customers. amixon® permanently keeps more than 30 test apparatuses available for you. In this way, we can make almost "the entire world of powder mixing" available to our customers at one location. A day in the amixon® technical center will be rewarding for a profitable investment decision. A powder mixer usually has a long service life. Often more than 20 years, sometimes even 40 years. Many times a day, important questions repeat themselves in the daily routine:

  • Preservation of particle structures: How efficiently is mixing carried out?
  • Flexibility: How much may filling levels vary?
  • Efficiency: How well does self-cleaning and residual emptying work?
  • Production reliability: Is it enough to take a look inside the mixer?
  • Acceptance and ergonomics: How quickly can inspection openings be opened and closed?
  • Are inspection doors permanently tight?
  • Exclusion of contamination: How long does dry cleaning take?
  • Control of microbiology: How long does wet cleaning and drying take?
  • many more questions .....


We predict with high probability very good results and a high gain of knowledge.

Tests for your powder preparation in the amixon® technical center

In the course of a day of experimentation in the pilot plant, many ideas are exchanged. Even phenomena that seem incidental can have a big impact.

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