The Expert's Corner

BACKGROUND:

Zinc-rich coatings are a very common type of coating for the prevention of corrosion of metals. This is due to the favorable sacrificial behavior of zinc, resulting from its position in the electrochemical spectrum.

PROBLEM:

A company that manufactures zinc coatings was using a high-speed disperser with a modified sawtooth blade to disperse the fine, dusty zinc powder into a resin based vehicle.

This process was time consuming due to material handling issues such as the minimization of dusting during material addition to the mixing vessel. In addition, the disperser was extremely slow to achieve the level of dispersion required by the client for the finished product. Batch times were typically two hours or more.

SOLUTION:

The client evaluated a Ross High Shear Mixer, Model HSM-105, with the Solid Liquid Injection Manifold (SLIM) arrangement.

Ross batch-type High Shear Mixers are used extensively for the dispersion of powdered raw materials into liquid vehicles. The High Shear Mixer is also very adept at handling high density powder dispersions. The High Shear Mixer is “bottom-vortexing”, enabling dense materials, which may tend to settle, to be drawn off the bottom of the mixing vessel into the rotor/stator.

The Ross Solid Liquid Injection Manifold (SLIM) system takes advantage of the strong vacuum force that occurs above the rotor of the Ross High Shear Mixer. By connecting a tube from this area to a hopper or wand, powders or liquids are sucked into the high shear zone and are immediately dispersed or emulsified by the high speed rotor/stator combination.

The SLIM utilizes a special rotor which is open, not only on the bottom, but on the top as well. As the rotor turns, it displaces liquid centrifugally, expelling it radially through the stator. This centrifugal action creates a low pressure zone within the rotor area. For standard batch-type rotor/stator High Shear Mixers, this centrifugal action and the corresponding low pressure zone provide the motivation that draws liquids up into the bottom of the rotor with a powerful flow pattern that pulls solids from the bottom of the mixing vessel into the rotor. The same low pressure zone is also the source of the vacuum that draws liquids or powders through the SLIM induction tube, into the rotor/stator zone.

PROCESSING:

In a 30 gallon vessel, 9 gallons of the resin based vehicle was added to the mixing vessel. With the HSM-105 running, various rheological additives, which were minor constituents of the batch, were then sucked into the vehicle using the SLIM system.

The zinc powder, accounting for 83% by weight of the total batch, was drawn into the vehicle using the SLIM system. Despite the high bulk density of the zinc powder (186 lbs/cu.ft) the HSM-105 with SLIM system efficiently inducted 367 lbs of the zinc powder into the vehicle in under five minutes.

The finished product was a well-dispersed zinc-rich coating with a viscosity of approximately 3,000 cps.