Virtually every industrial fine powder/liquid mixer in use today is basically a giant version of a soda fountain "milk-shake" mixer - a motor-driven propeller in a cup. Besides being inefficient at mixing and very time consuming, these mixers require a ponderously sequential process. Recently, however, a completely new industrial mixing technology has been developed that addresses the forces at play during the mixing process at the micron level.

The problem of mixing fine powders in liquids is something most everyone experiences regularly. Even if you're simply trying to mix cocoa in milk or powdered soup in boiling water, the problems quickly become clear-cold or hot, powders don't disperse easily in liquids, especially when the process is sped up with agitation. Impatiently dunk the powder under the surface with a spoon, and the result is a mess of lumps (agglomerates) that just don't want to go away, regardless of the amount of shearing force that is applied.

Speeding up the industrial process of adding a large quantity of powder into a liquid to make a homogenous product isn't really that much different-the action just takes place on a significantly larger scale. Because of the limited wetting surface and high shear in traditional batch-type industrial mixers, it takes a lot of patience and energy over a long time to overcome the inevitable agglomerate problem.

Virtually every industrial fine powder/liquid mixer in use today is basically a giant version of a soda fountain "milk-shake" mixer-a motor-driven propeller in a cup. These high-shear "propeller-in-a-tank" mixers are found in just about every plant that mixes powders with liquids to produce homogenous ceramic slips, specialty chemicals, paints, coatings, inks, foods and thousands of other items. Besides being inefficient at mixing and very time consuming, these mixers require a ponderously sequential process.

Recently, however, a completely new industrial mixing technology* has been developed that addresses the forces at play during the mixing process at the micron level. The new technology also brings with it a new look. Gone is the familiar high shear agitation "propeller" in its hundreds of forms, as well as the painfully slow mixing operation, the wasted floor space and the high machinery costs. This new mixing technology speeds the production of hundreds of applications to 1/10th the usual time, using 1/10th the production floor space and 1/10th the power-improvements that can result in capital equipment savings in the range of hundreds of thousands of dollars.

Fine Powder Wetting Challenges

Ideal dispersion is achieved when fine powders come into contact with a large liquid surface under low shear. Figure 1 portrays the general principle. Other attempts at improving industrial mixers have been self-defeating because they fail to satisfy these ideal conditions. Manufacturers that mix large (500 gal or more) batches are familiar with the persistent problems that prevent fast homogenous dispersions. To avoid exceeding the absorption rate of the very limited liquid wetting surface, the dosage rate of the powders fed into the mixer must be greatly reduced.

Additionally, surface forces in dry solid powders of less than 10 microns form extremely cohesive agglomerates that resist the required capillary action (see Figure 2). Ideally, these agglomerates are broken up before being presented to the liquid. Not doing so requires the generation of high shear forces over a long period of time to achieve results that even approach acceptable, and high temperature gains, wasted powder consumption, and slow (costly) production often occur.

The new technology takes the opposite approach. Instead of slowing the powder loading process, it pre-disperses the dry powder particles prior to wetting in the vacuum chamber (see Figure 3). Additionally, it creates a significant wetting surface, 2 m²/sec, which is key to the technology's success. Particles come in contact with the wetted surface instantly instead of being allowed to cake up in chunks. By pre-dispersing the powder particles in the dry state and increasing the availability of a liquid surface area through a vacuum eductor system, the rapid loading of powders can be sustained in an enclosed, environmentally safe process. The system's production capacity is typically 7 tons (10 cubic meters) of dry solids per hour.

Wetted agglomerates are often difficult to disperse because of the pressure of trapped air between particles, which prevents further hydraulic penetration (see Figure 4). Within the unit's chamber, a controlled negative-to-positive pressure gradient forces any trapped air from small agglomerates to break them up into a homogenous mixture.

An Innovative Approach

The new mixing technology is a safe, clean, dust-free and totally enclosed process. It can handle batch sizes of 55 to 5000 gal, or it can work in a continuous process to produce homogenous products with a mix viscosity as high as toothpaste. It can also be installed into existing systems to speed up current processes, reduce temperature rises and enclose the process to reduce hazardous emissions. The system's pre-wetting dry powder dispersion, virtually infinite wetting surface and simple vacuum-to-pressure gradient reduces mixing time by up to 90% compared to traditional mixing methods in use today. As a result, manufacturing requires far fewer machines, less plant space and less energy consumption, which translates into a more cost-efficient manufacturing process.

Figure 5 illustrates the fully automated in-line operating process, which can be divided into two stages: (1) starting/dosing and (2) dispersing and emptying.

Starting/Dosing Stage. Prepared liquid fills the batch storage tank. Fine powder solids are pre-weighed and positioned in the feed hopper above the system's controlled powder-feed rotary valve. Alternatively, solids are fed into the tank from a silo, containers or bags. On startup, liquid is fed at high speed from the storage tank through four nozzles to the unit's contact zone, where it creates a virtually infinite wetting wall for powder absorption. A suction vacuum or venturi effect is formed above the rotor.

The feeding of the fine powders, which are pre-dispersed by a rotor before wetting, can begin once the vacuum has been established. By coalescing the air pockets on the solids, the system shifts a three-phase (solid-air-liquid) system into a two-phase (solid-liquid) system. The solid particles are wetted in a cyclone and dispersed in an acceleration zone within a closed housing. The system pressure controls the dosing rate, and overdosing is prevented automatically based on an ambient pressure measurement within the unit.

The tremendous suction results in rapid powder incorporation and is the key to the dramatically faster mixing speed. The liquid mix then flows directly back to the batch tank under pressure. An innovative vacuum-to-pressure gradient through the wetting zone results in an additional wetting acceleration. The output pressure ranges from 1.5 to 3 bar, depending on the pressure drop in the return line, and the whole process is completed with the product temperature not varying more than 5ºC through the mixing process.

Dispersing and Emptying Stage. This stage starts when the solids feeding is complete. Depending on the characteristics of the product requirements, the suspension can be homogenized or de-aerated for a specifically determined period of time. A large liquid surface is formed by the high throughput, and the mixing and further dispersion of the particles occurs in a long compression zone. Finally, after the batch tank has been emptied, the product is quickly flushed out of the small mixing zone, which is then ready for another batch from the same or another holding tank. One in-line mixer can be configured with multiple holding tanks to multiply productivity up to 10 times. The system also offers dust- and emission-free dispersion within a completely enclosed process chamber, making it an ideal alternative to vacuum feeding.

The new system is especially well suited for products with a high solids content, products with a low solids content in large liquid batches, and products with difficult-to-wet solids. The liquid and powder input and output are independent from the system. In a batch process layout, the system will handle multiple feed hoppers and multiple holding tanks for almost continuous batch operation (see Figure 6). During the in-line process mode, inputs and outputs are fed continuously to and from holding areas.

Process Flexibility

The fundamental concept of pre-conditioning powders in a dry state prior to feeding is well known; however, the idea of performing this procedure in a totally enclosed area immediately preceding wetting is revolutionary. The subsequent problem of how to feed a large, dry surface area into a liquid is addressed by increasing the amount of liquid surface area available through an increased the flow rate. The novel methods used to approach this problem create a high-velocity liquid tube, using a multi-port liquid injection, which results in a vacuum effect to de-aerate the system and draw the powder immediately into the liquid, preventing splashing and powder buildup.

The system reduces long-term capital costs when production expansion is desired because an additional holding tank is all that is required, as opposed to a more expensive tank combined with cooling and a high-shear disperser. Overall operating costs are reduced through an automated control system, which is included with the machine and ensures repeatable product quality.

For more information, contact Netzsch Fine Particle Technology, LLC, 125 Pickering Way, Exton, PA 19341-1393; (610) 363-8010; fax (610) 280-1299; e-mail hway@netzschusa.com; or visit www.netzschusa.com.

Links

• Harry Way's e-mail
• Netzsch Fine Particle Technology website