Starting with the fundamental idea of wetting finely dispersed powders in a large liquid surface, the revolutionary in-line disperser Ψ-MIX™ was developed by NETZSCH-Feinmahltechnik GmbH. Effective wetting of solid particles by high-quality dispersion improves quality and enables unmatched operation of the machine at high production rates. A unique characteristic of the Ψ-MIX™ is the dust- and emission-free dispersion that takes place in the enclosed process chamber. Compared to conventional rotor-stator systems, this machine uses significantly less energy for dispersion. Temperature-sensitive products and materials with a broad viscosity range can be processed. The Ψ-MIX™ is easily integrated into automated plants that are processing large batches and is especially suited for emission-free and explosion-proof operations. The design of the machine is distinguished by the fact that foreign bodies in the pigment suspension do not cause damage to the system.

The Ψ-MIX™ (Figure 1) combines a new dispersion method with an emission- and dust-free in-line process. The solid components are dispersed onto a large liquid surface created by the pre-loaded liquid in the batch tank. Since the new machine is capable of processing both low- and high-viscosity suspensions, the entire application range of dispersion technology is covered. Applications for temperature-sensitive or shear-thickening materials are easily processed within the design of the process chamber.

A significant feature of the Ψ-MIX™ that contributes to its success is the deaeration function that occurs automatically within the operating method of the new machine. This is a major improvement for processing water-based suspensions.

Batch sizes between 100 and 10,000 liters or more are processed on only one machine. Compared with conventional single- or multiple-shaft mixers, the energy input is less than half.

The machine can be used for a great variety of applications:

• Sheet-fed offset inks;
  
• Paint concentrates;
  
• Finished products for paints;
  
• Solventborne and waterborne products;
  
• Suspensions with fine powders like fumed silica; and
  
• Suspensions consisting of granulate, like beaded carbon black or extruded solids.

Current Methods for Dispersion

Current methods for dispersion are described in Table 1. These methods are based on producing wet agglomerates (Figure 2), which should be broken apart by mechanical energy input via rotation. One of the problems with conventional technology is the broad size distribution of the agglomerated particles initially produced by the wetting process, from 10 microns (µm) to 2.0 mm. This wide distribution of particles results in an obvious difference in the required specific energy input to achieve a specific degree of dispersion. Considering the processes that may follow, i.e., fine grinding (fine dispersion) using bead mills, the energetic differences inherent in the type of pre-dispersion process become even more obvious. An optimum pre-dispersion reduces the running time on a mill considerably, and possibly eliminates the need for fine grinding.

Technology of the Ψ-MIX™

The advantages of in-line processing of solids into liquids are well known. What is new on this machine is the treatment of the solids and liquids during initial wetting. A design criterion was the controlled formation of wet agglomerates, with the aim of an "optimum dispersion" (Figure 2), i.e., to achieve the required degree of dispersion. In contrast to this, one could define "ideal dispersion" where every primary particle is completely wetted (Figure 3). In most cases, the ideal dispersion is not the same as the quality specification, i.e., required degree of dispersion or fineness of grind that has been developed by conventional methods.

One problem for an effective wetting process is the quantity of micro air included in the charge of solid components. This air is inherent in the dry agglomerates. When the dry agglomerate is immersed into the liquid, the capillary path on the exterior of the particle is filled, sealing the interior of the dry agglomerate. In the core, an air pocket is created that stops the wetting process. High shearing speeds now break apart these stable, wet agglomerates (Figure 4). Sub-level vacuum feeding or the condensation principle with gas exchange can be an acceptable solution to this problem.

The Ψ-MIX™ Dispersion Technology

By connecting the individual process stages within a small space, all processes required for an optimum dispersion can be achieved within milliseconds. Figure 5 shows a schematic of the complete process system. The dispersion vehicle is loaded into the batch tank (8). The solids are fed to the rotary valve (1), by either bag dump station, silo or super-sack. The positive displacement pump, (7), feeds the vehicle into the Ψ-MIX™ dispersing head.

Figure 6 (page 59) shows details of the process. The rotary valve attached to the solids feeding station seals off the dispersion chamber from the atmosphere and serves for dosing the powder. In the feeding tunnel (2) that is under vacuum, the dry agglomerate is deaerated and fed to the atomizer (3). The vacuum is produced by a hydro cyclone effect, similar in concept to a liquid ring vacuum pump. The rotor (5) functions as a pump, transporting a liquid stream tangentially to a ring-shaped acceleration chamber where it is hydraulically accelerated into rotation. The rotation corresponds approximately to the speed of the rotor, which prevents the liquid from splashing into the solids feeding tunnel. A "free-falling liquid curtain" is created and levelled by centrifugal forces. After about 100 mm, this liquid layer merges with the rotor, forming a seal and is discharged at high speed.

The finely atomized solids falling from above are put into rotation by the rotor (5) and spun towards the outside onto the liquid layer. Wetting occurs during the immersion of the individual solids (4) and is assisted by the immediate increase in pressure (7). At the discharge of the machine, wetting is already perfect and cannot be improved by further processing with a disperser.

The necessary rotary motion of the rotor leads to slight shearing, but this is negligible due to the wide gap between the rotor and the housing. These are also the reasons for the low induced specific power input of this invention. Micro cavitation occurs in a rapid transition from vacuum vapor phase to process overpressure, further improving the dispersion process. A further critical aspect of the process for the optimum wetting is the quickly flowing liquid surface, which is available with a surface of more than two square meters per second. The high liquid surface in relation to the specific surface of the atomized solid agglomerates provides an explanation of the basic principle of the Ψ-MIX™.

The technical data for the Ψ-MIX™ are described in Table 2 and the technical details are as follows:

• Operating method within an enclosed system;

• Dosing speed adjusted by viscosity (vacuum reduction and power comparison);

• Deaeration function by a "thin film process;"

• Cooled operating range;

• Cavitation control for low-boiling liquids;

• Protection from dry-running by measuring pressure (no liquid - no vacuum);

• Overflow protection for the dry area (pressure measuring);

• Low sensitivity to foreign bodies;

• Smooth surfaces; all peripheral parts accommodated in the machine stand;

• Suspension flow in pipework accelerated by internal centrifugal pump (rotor);

• Swivelling device to facilitate maintenance;

• Mechanical seal with monitoring function;

• Automatic operation via PLC and frequency inverter for all drives;

• Error indication in plain language;

• Operation on the machine (service mode) and master via Profibus; and

• Communication with existing plant control prepared as software.

Summary

Innovative dispersion technology sets new benchmarks for modern production processes in all fields where micronized, pulverized solids have to be mixed as fine and as homogenous as possible. If the impression within the industry is that the development of dispersion technology has reached a standstill, the invention of the Ψ-MIX™ shows promise for the future. It may be difficult to fully integrate the concept of this new invention, but from a technical point of view there are no objections. This new concept may raise some conflicts with conventional mixing systems. For established dispersion systems, it may define a new era in the market for dispersing equipment, and manufacturers of mills will need to make adaptations to accommodate the higher throughput rates. This concept should prompt one to do some rethinking about the dispersion process and to look at an innovative dispersion technology.

For more information, see www.netzschusa.com.