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Applied Technology
Air Classifier Helps Silica Manufacturer Maintain Particle Size Distribution Standards

When evaluating air classifier systems for its new Silica flour operation in Mooresburg, Tenn., Short Mountain Silica made control of particle size distribution a top priority. This manufacturer of numerous silica products, including fine silica for fiberglass, ceramic and other industries, needed equipment that would meet its rigid quality assurance program and stand up to demanding performance requirements.

Discussions with Pat Brown of C. B. Brown Engineering led Short Mountain Silica's general manager, Darrell Trigg, P.E., to Sturtevant, a supplier with many years of experience and numerous installations in related industries.

PRODUCT REQUIREMENTS

The new air classifier needed to versatile enough to produce a range of products with a fineness of 200 to 325 mesh with quick external adjustments. It also needed to maintain a consistent product quality by limiting oversize particles, in spite of variations in the particle size distribution of the ballmilled silica.

To meet these project requirements Sturtevant recommended the Side Draft (SD) high efficiency classifier. The SD offers the following features

• It uses a multipin rejecter cage instead of Whizzer blades. This configuration results in better control of maximum particle sizes and more efficient particle classification.
• It uses a combination of an air inlet volute that distributes airflow horizontally 360 around the rejecter cage, and baffles that distribute the airflow vertically in front of the rejecter cage. This design minimizes turbulence and improves the classifying efficiency of the rejecter cage.
• The airflow is generated by a fan that is independent of the SD's main drive, and a variable-frequency motor controller is used to change the speed of the rejecter cage. This feature allows the user to quickly adjust external settings according to the fineness requirements of multiple products.
• Cyclones are used to separate the classified product, and the airflow is recycled back to the classifier. This eliminates the capital cost and permitting requirements associated with a large process dust collector. The system requires only a small amount of venting for nuisance dust control.
• The classifier's standard supply includes alumina oxide ceramic liners instead of steel plate liners to provide many years of wear life. This also maintains the silica's brightness quality due to minimal iron contamination.

PERFORMANCE EVALUATION

After evaluating the features and specifications of the equipment, a decision was made to purchase the recommended unit in April 1996 and place it into operation. As a quality control check, Short Mountain Silica set up a procedure to calculate the air classifier's efficiency with mass balance (an accounting of specific particle sizes in the classifier feed from the ball mill, the finished product and the coarse material that is returned to the ball mill). Tracking the location of particles before and after classification enables a quantifiable analysis of classifier performance.

According to Trigg, Short Mountain Silica was "extremely pleased with the consistent particle size distribution of the air classified silica." Customer response was also positive. "Two of our biggest customers believe that we are supplying some of the bestquality silica that they have ever used. Oversize particles in silica flour result in lower product efficiencies when it is used for processing fiberglass. The Sturtevant SD Air Classifier gives us better control of the oversize and more consistency in our quality." Trigg was also pleased with the company's technical service and customer support.

Based on both internal testing and customer response, Short Mountain Silica purchased a duplicate system in February 1997.

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