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CASE STUDIES: MIXING, SIZE REDUCTION & AGGLOMERATION
Disperser Cuts Processing Time 80%, Produces Smoother Flowing Product
(Reprinted from CHEMICAL PROCESSING, July 1989)

ROBERT WHITLOCK
Plant Engineer. Imperial Adhesives Co.
ALAN E. HODEL
Senior Associate Editor

Problem: Imperial Adhesives Co., Cincinnati, Ohio, manufactures spray-grade adhesives as one of its product lines. The process involves dissolving and dispersing rubber and other solids into solvents. One raw material, pieces of raw rubber (1 x 2 x 1/4"), must be processed through a two-roll mill before it can be dissolved in a batch tank with normal high-speed mixers. The process takes eight hours for milling and size reduction and almost eight additional hours for mixing to make a 1300 gal batch.

Manufacturing the adhesives was taking too long. A more efficient mixing system was needed. Production personnel wanted a single piece of equipment that could be run continuously on a 24 hour basis for five, sometimes six, days a week.

Imperial's engineers reviewed the options, made trips to trade shows, examined equipment, and invited several manufacturers to quote alternatives. The engineers talked with users of some of the equipment and discussed the problems experienced. The most frequently reported negatives were that the type of high-speed dispersion equipment needed is generally poor in pumping, expensive, overpowered, inflexible and difficult to maintain.

Solution: A manufacturer that had already supplied several pieces of equipment to Imperial was selected. A two-speed, two-motor, coaxial disperser with a rotor-stator rubber cutting head was purchased. It includes an anchor or sweep blade that rotates around the inner circumference of the tank at a very low rpm.

Before installation, test batches of adhesives were run on a prototype at the manufacturer's plant. The unit was customized to fit Imperial's specific application and further designed to minimize the need for maintenance.

The stator (the outside part) of the disperser rotates at the same speed (and direction) as the anchor. The rotor, or cutting tool, is a high-shear device that rotates at a very high speed in the center. All the bearings are protected by seals. A large bronze bushing at the base of the disperser shaft requires changing about every six months. Product lubricates the bushing.

The unit has two speeds that provide versatility as a tool in the formulation process to account for variations in batch size, vortexing, and raw material characteristics. Although competitive rotor stators are generally poor in pumping, by varying the speeds and the use of the two drives, the equipment can be employed effectively to produce low- to high-density products.

Results: The two-speed, dual-drive, coaxial disperser with a rotor-stator rubber cutting head runs virtually without maintenance for six months. There is a minimal amount of downtime. The unit is very dependable.

Imperial is able to process a batch in about 21/2 hr compared to 14 to 16 hr with the standard high-speed mixer system. The spray-grade adhesive product is better dispersed and smoother flowing.

Shorter processing time permits the process to operate at a lower temperature. Heat that previously evaporated solvent has been eliminated. The amount of solvent that has to be recovered and reprocessed has been reduced. There is no longer a need for milling of the raw rubber pieces.

Two-speed, dual-drive, coaxial disperser with a rotor-stator rubber cutting head was manufactured by Schold Machine Company, Southern Division, 10590 Oak Street N.E., St. Petersburg, FL 33702.

The two-speed, dual-drive, coaxial disperser with a rotor-stator rubber cutting head is shown mounted on rest stand before shipment to Imperial Adhesives Co. The stator (the outside part) of the disperser rotates at the same speed (and direction) as the anchor blade of the mixer The rotor, or cutting tool, is a high shear device that rotates at a very, high speed in the center

A detail of the rotor-stator rubber cutting head.