What are influencing factors for successful liquid distribution into the powder?
In almost all areas of the bulk solids processing industry, powders are wetted with liquids. This process seems trivial. But sometimes the wetting results are not satisfying. Lumps may form, the liquid may not be homogeneously distributed, or the bulk material may subsequently have poor flow properties.
In addition to viscosity, the surface tension of the liquid has a major influence on its distributability in the powder. However, the properties of the powders also play an important role if the liquid distribution should be successful.
How liquid-affine is the powder surface? What is the capillarity of the powder?
Typical appearances of various liquids:
a. Low surface tension
b. High surface tension
c. Refluxing liquid
d. Advancing liquid
e. Liquid injected into the turbulence region of the atomizer (single-substance nozzle)
f. Liquid material is sprayed microfine and the powder is fluidized (two-substance nozzle)
Capillarity of the powder and surface tension of the liquid
If the surface tension of a liquid is low (a), then the liquid wants to spontaneously wet the solid surface. The capillaries (5) of a particle are then also penetrated. The higher (b) the surface tension of a liquid is, the less the liquid wants to penetrate the capillaries of the particles. The same applies to the cavities of a powder mixture. With high surface tension, liquid distribution in the powder is only possible if the liquid and particles are intensively mixed and triturated together.
Homogeneous wetting of a powder is not trivial.
In dry condition (1), the powder is present as a solid-air dispersion. The cavities between the particles are irregular and constantly change during mixing. If a liquid is poured into the moving mix, then the air is displaced from the cavities. The liquid spreads as a thin liquid layer around each individual particle. This is the adsorption layer (1). The liquid adheres firmly and can only be removed thermally. If the liquid content increases during mixing, the liquid accumulates at the contact points of the particles in so-called bridges and (2) gussets. Thus, agglomeration (buil-up-granulation) begins. As the liquid content increases, larger cavities (3) between the solid particles are filled with liquid. When all capillaries formed by the particle system are filled with liquid, (4) saturation occurs. The bulk material becomes a suspension.
If the surface tension of the liquid is low and the affinity between liquid and solid is particularly high, then so-called flash absorption can take place. The available liquid is immediately absorbed by the solid. This often leads to unwanted agglomerates. But a prolonged mixing process can hardly improve the liquid distribution. In cases of high affinity between powder and liquid, liquid addition (as shown in Figure f) should be consistently slow, dosing and spraying during mixing. Spraying below the bulk level usually improves the self-cleaning of the mixer.
Question of a customer: Today, the properties of liquids and bulk solids can be conveniently analyzed. Is it still appropriate to carry out practical mixing tests in powder mixers?
Indeed, today we can quickly and accurately analyze characteristics of liquids and also powders. Unfortunately, however, disperse systems of different powders are too complex to simulate their mixing or even their wetting. For the foreseeable future, the calculation effort will be much more expensive than the practical experiment.
In the amixon® Test Center, a wide variety of wetting processes are applied on an almost daily basis. We would be pleased to demonstrate these processes using your original products and to show you impressively how your wetted powders subsequently look and flow.
Powder moistening can lead to unwanted contamination of the mixer.
Depending on the viscosity and stickiness of the liquid, unwanted buildup can occur. These can form on the wall or also on the mixing tools.
As a rule, it is advantageous if the mixing chamber has a high degree of filling. The dry powder components should be filled in first. Mixing of the powder components is the preliminary stage of wetting. The method of adding the liquid material is of great importance. High differences in concentration should be avoided. amixon® uses liquid addition lances that extend into the lower part of the mixing chamber. The volume flow of the liquid addition should be based on two parameters:
- Adsorption capacity of the powder
- Speed of the mixing tools. The faster the powder flows, the more evenly the liquid is distributed in the powder.
Deposits and buildup of liquid materials must be avoided at all costs. They should only be tolerated in exceptional cases, as they can cause major problems:
- In the end product, the very liquid components that contaminate the mixer are missing.
- Any buildup can increase with each batch. They can become detached during mixing and contaminate the mix in an uncontrolled manner.
- Adhesions increase friction during mixing. The mix is heated unintentionally.
- Heavily adhering caked material can block the mixing tools.
The best method of adding liquid during mixing can be determined by trials in the amixon® test center.
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