1. High speed dispersers
2. Rotor-stator mixers
3. Media mills

High speed dispersers (HSD)
These are used as the first stage processing equipment to manufacture a coating. They are the simplest in construction i.e. similar to propeller type agitators except they use a saw tooth impeller disk to ensure thorough wetting of solids and their incorporation in the surrounding liquid. Most of these dispersers have variable speed drives. Combination of speed and disk configuration allows them to wet the solids and reduce the particle size. Most of the blades are of stainless steel but in abrasive applications high density polyethylene disks of special configuration can be used.

High speed dispersers are the work horse of the coating industry. Many times the batch is completed in the tank with HSD. Other times the material processed in the HSD is processed in another processing vessel to finish the batch. It is also possible to process the material from HSD further using one of the following equipment.

Rotor-stator mixers
Rotor-stator uses a rotor within a stationary slotted stator. These mixers come in two designs. One type is used as batch mixers and the other type are used in-line. When used in tanks they have limitations when viscosity and tank volume increase. Their efficacy drops when tank sizes exceed 1000 gallons and viscosity exceeds 10,000 centipoises.

In-line mixers are typically used after a high speed disperser. This configuration is very beneficial as it allows re-circulation of the liquid back to the feed tank until the desired results are achieved. Since these are high shear machines they can at times replace expensive media mills to deliver the needed particle size. Furthermore they do not use any media and are easy to clean.

In a different configuration if an in-line rotor stator is used on a tank, solids can be fed through an eductor thereby eliminating and/or reducing dust. This configuration is very useful for dusty materials. Sometimes in-line rotor stator is also used with an agitated tank without the eductor. The material once dispersed to specification is pumped out using the in-line rotor stator for further processing.

Companies using HSD followed by in-line rotor stator have been able to significantly increase throughput at very low investment. It has been possible to eliminate the use of a media mill and reduce the process time. Cleaning is also easy as there is no media and the processing time is also short. This allows capacity increase at minimum investment.

Media Mills
Media mills are used when a particle size reduction that cannot be achieved by HSD or in-line rotor-stator is needed. Horizontal and vertical mills are the most common for the coatings industry. Performance needs dictate usage of type of media mill size and type of media and flow rate of the liquids. This is a trial and error process and people experience comes into play.

Material that has been premixed in a HSD is feed into the mill and recirculated. Care is needed in how these machines are operated. Mills, though useful, are capital intensive. It is possible to replace a media mill with an appropriate in-line rotor stator machine as mentioned earlier. This allows significant batch cycle time reduction and cleaning is easier. This alternate has to be tested for the application.

Process optimization
At times producers have excess capacity with the existing equipment. If excess capacity is available with any of the three types of dispersers at a site, many times a new product is developed keeping in mind the available excess capacity of these dispersers. This may not be the most economical method as the product performance and cost many not be the most optimum. One has to select the best equipment for the product rather than using the available equipment for the product.
It is necessary that different dispersers be evaluated as their application in any combination or alone can produce optimum product performance at the lowest manufacturing costs. Such an effort can improve production capacity at a significantly lower cost.

Back to Patents and Publications