Problem:  Difficulties were experienced at a paint manufacturing operation when pigment particles were reagglomerated during let-down.

In let-down, turbine agitators mix resin, solvent and a finely dispersed pigment concentrate (also called mill base) to the finished paint consistency.

In this particular application, the dispersion became unstable.  Pigment particles "kicked-out".  The paint had a number of undesired specks.  These specks could be removed by careful filtering; however, filtering was slow.  Filter cartridges had to be changed frequently.  Labor and cartridge costs were very high.

The specks could be redispersed by pumping the paint through a continuous sand mill.  However, sand mill operating costs were also prohibitive.  Redispersing a 1,000 gallon batch could take 8 hours or more.  Serious production delays were encountered.

Solution:  The reagglomeration difficulties were solved with an in-line dispersing grinder.  Acting as its own pump, the in-line disperser sucks the specky paint into the high shear head.  The pigment agglomerates are torn apart by teeth on the edges of the 3600 RPM, 6" diameter impeller.  The rotating teeth pass a second set of teeth jutting inward from the cylindrical body.  The intermeshing teeth smash the specks in the presence of the paint.  A grid plate, with 1/8" venturi-type openings, in the discharge zone provides positive retention time in the high shear head.

The exiting paint stream is recycled by the disperser to the let-down tank.  The disperser continues recycling the paint.  High shear mixing and extra dispersing occur each time the paint passes through the intensive dispersion zone.

Results:  On a low viscosity (below 70 Krebs-Stormer units) industrial paint, the dispersing grinder recycle rate is approximately 100 gpm.  Six to eight passes through the disperser eliminates all the reagglomerated pigment specks.

Approximately 1,000 gallons of paint can be redispersed in two hours or less.  By adding casters and a quick release fitting, the unit becomes portable.  A single disperser is easily used in several areas of the plant.

Reprinted from July 1982 Chemical Processing Magazine