Mixing instead of stirring in the production of paints

“Stirring” is of crucial importance in many areas of the process industry. Stirring is a necessity to mix different components with each other, or to prevent disintegration, sedimentation or flotation of recipe components, in short, to achieve homogeneity and avoid separation.

Stirring is also required for heating and cooling the vessel content and for temperature compensation. Melting, gassing, crystallising and plunging, to a certain degree even dispersing and emulsifying are also tasks for an agitator. For all these basic tasks, special emphasis is always on homogeneous mixing of the vessel content.

There are also agitators that are to hinder homogeneous mixing in a targeted way. This is required for reaction columns, where a solid is added at the very top, or a gas at the very bottom, which then disperse slowly throughout the vessel, level by level with maximum dwell time in every single layer. There are even agitators for segregation, degassing and phase separation. In most cases however, agitators are used for homogeneous mixing – and here, the differences between traditional agitators as they are often still used for the production of paints and coatings, and innovative mixing technologies are significant. With state-of-the-art jetstream mixers, the process time compared to traditional agitators can be shortened drastically, while 100% homogeneity is achieved at the same time. 

Fig. 2: Mixing states for the jetstream mixer compared to different conventional and more recent agitator technologies

The suspension state and the homogeneous or inhomogeneous distribution can be observed very well in a glass vessel. Fig. 2 shows the mixing state in a glass vessel measuring approx. 700 litres when using conventional as well as more recent agitators compared to a jetstream mixer (1st position from the left). The second tool from the left is a marine propeller with distinctly vertical flow. The third tool is a hydrofoil impeller with very low Newton number. Both tools are favoured over simple pitch blade agitators, as they require significantly less power. The fourth agitator is a cone-type impeller, which has been in use for approx. 25 years. The agitator element on the right is a dissolver disc.

The photograph illustrates that a markedly inhomogeneous dispersion can be seen for all agitators, and the liquid in the top area appears to be almost clear, while the concentration in deeper layers is excessive. This confirms that more than 90% suspension level can hardly be achieved with agitators.

Only the jetstream mixer on the far left in Fig. 2 generates a homogeneous dispersion from the vessel base to the surface of the liquid.

Fig. 3: Stationary direction of flow in a vessel when using different agitators.

When observing the direction of flow in the vessel, another phenomenon can be seen. Other than in multiple images with lines of flow in agitator vessels, two clearly separate different flows below and above the propeller can be seen for the marine propeller and the hydrofoil propeller. At the precise level of the propeller, the direction of flow changes completely. Below the propeller, the liquid flows from bottom to top. Above the propeller, in horizontally rotates in a circle. This is shown by the arrows in Fig. 3. With the cone-type agitator, the entire liquid rotates horizontally with an enormous vortex on the surface. With the high speed dissolver, the liquid flows downwards and upwards slightly diagonally below and above the disc. A vortex also forms on the surface.

Fig. 4: Dispersion of a colour paste with different agitators.

As Fig. 4 shows, this flow also affects the homogeneous dispersion of a colour paste in the agitated vessel. The colour paste and the base fluid have approx. the same viscosity, which means mixing should be very easy. The figure shows the colour distribution when using different agitators. When using the jetstream mixer, the yellow colour paste in the blue liquid is already fully dispersed homogeneously after 12 seconds. There is another surprise with regard to the marine as well as hydrofoil impeller: the yellow colour paste only disperses homogeneously in the whole area above the agitator within 20 seconds. Below, the liquid remains grey. This means that not only the direction of flow changes at the level of the propeller, but that the liquid is even separated into two distinct zones, which are only dispersed very slowly. The figure in the middle shows the state after 2 minutes of stirring time. After 4 minutes, there is still a clear difference in colour. Only after 6 minutes of stirring time, there is no visual difference any more. In the right image, the mixing effect of the cone-type agitator becomes clear: The blue colour paste moved downwards within the vortex remains only in the area of the vessel base for a very long time. 

Fig. 5: Jetstream mixers rely on a consistently vertically mixing principle. They are usually installed laterally into the vessel wall.

Modern jetstream mixers focus on a consistently vertically mixing principle and combine a turbulent micro-mixing zone in their mixing head with an almost turbulence-free vertical macro-mixing of the entire vessel content. Other than when using a conventional agitator, the product is actually fully mixed homogeneously at the end of the mixing process – without any unmixed zones and sediments – and consistent results are achieved independent of the batch size.

With this, they achieve not only a homogeneous concentration of the recipe components, but also a homogeneous viscosity dispersion throughout the vessel. For this reason, jetstream mixers are also deployed when using paint splatter robots. Paint splatter robots respond to minimal differences in the composition and consistency of the paint to be sprayed with visible differences in the paint. The paint quality must thus remain consistent for the full and later the almost empty storage vessel. Therefore, jetstream mixers are used for homogeneous mixing for these applications. 

When using traditional agitators, the required homogeneity often results in long stirring times, repeated checking and readjusting of the product properties. As an actual homogeneous distribution is achieved in the vessel with the jetstream mixer, the control effort is much reduced. Due to the significantly shorter mixing times, the energy consumption is also between 60 and 90% lower. The entire process time and utilisation of the process systems can be reduced by 50 to 90%. This enables increases in capacity simply through improved mixing technology.

Depending on the vessel size and installation situation, jetstream mixers can be integrated into a vessel from the top, the side or the bottom. In the production of paints and coatings, lateral installation is usually preferred (s. Fig. 5), as very variable fill levels can be processed with this, and it is much easier to clean the vessel than in case of installation from the top.

Fig. 6: Flow pattern when using a jetstream mixer.

Fig. 6 shows the direction of flow when using a jetstream mixer from different perspectives: The liquid flow is directed onto the base by the rotor-stator system and guided to the surface along the vessel wall, where it returns to the mixing head. The colour progression in Fig. 6 from dark blue to red represents the respective flow rates here. The entire vessel content is in motion. No baffles are needed. The liquid on the vessel base has the highest speed. With this, media prone to sedimentation are permanently mixed and homogeneously dispersed in the entire vessel.

Fig. 7: Simulation of a mixing process for a producer of coatings and paints: After 1,200 seconds, 99.9% of the vessel content are equally distributed.

A producer of coatings and paints wanted to reduce the mixing of their paints and adjustment of hues, which previously had taken many hours, to half an hour by using new technology. For this purpose, the mixing process with a jetstream mixer was simulated in advance with addition of a black paste in a process vessel holding 30,000 litres. The different colours shown in Fig. 7 represent the respective concentration, the target value corresponds to the hue dark orange in the diagram. 

It takes approx. 60 seconds until the entire liquid in the vessel is in motion. The stream flowing to the surface is redirected and already pulls parts of the paste into the liquid in doing so. After approx. 90 seconds, the solid-liquid mixture reaches the mixing head for the first time, gets accelerated here immediately and starts to be vertically rotated. From this moment, a consistent flow is built in the entire vessel. The end concentration is already reached in some areas of the process vessel after 400 seconds, after 600 seconds, this already applies to large areas of the vessel. After 1,000 seconds, 99% of the vessel content is homogeneously distributed, after 1,200 seconds it is 99.9 %. In practical applications of the process system, the results of this simulation were not only confirmed, but exceeded. 

When planning such as system, it must be made sure that addition of the paste onto the liquid surface from the top is the least favourable location also for a jetstream mixer. The entire liquid might permanently move from one side to the other on the surface and dip down again, but this addition remains the slowest and thus least favourable option for the induction of additives, colour pastes or other recipe components. With an optimised addition below the liquid level – ideally close to the vessel base, where the flow speed is still significantly higher – the mixing time can be shortened again by approx. 50 %. 

Fig. 8: Retrofitting of two jetstream mixers in an existing process vessel.

Jetstream mixers can be easily and flexibly retrofitted in an existing process vessel. The shape of the vessel is not relevant, for example, retrofitting is also easily possible in a rectangular vessel. 

The existing agitator elements often do not need to be removed during retrofitting.  If there is no flange on the vessel wall yet, this can also be retrofitted. Different wall thicknesses – the wall thickness is often less in the cylindrical area compared to the base – also do not go against retrofitting: If the wall thickness is not sufficient, a bumper panel can be welded on, which means the jetstream mixer can be easily installed even in case of a thin outer wall. Besides, for applications in the production of coatings and paints, where high mixing performance is required due to high density and viscosity, multiple jetstream mixers can be retrofitted in one vessel (s. Fig. 8).

This text has been published in an edited version in: 
EUROPEAN COATINGS JOURNAL 03 – 2025, p. 32-36.
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