generally is ducted to a cyclone for recovering the finer grades of vermiculite concentrate. The classified concentrate then is stored in bins or silos for later shipment or exfoliation. The rotary dryer is the more common dryer type used in the industry, although fluidized bed dryers also are used. Drying temperatures are 250 to 900 F, and fuel oil is the most commonly used fuel. Natural
gas and propane also areused to fuel dryers.

After being transported to the exfoliation plant, the vermiculite concentrate is stored. The ore
concentratethen is conveyedby bucket elevator or other means and is dropped continuously through a gas- or oil-fired vertical furnace. Exfoliation occurs after a residence time of less than 8 secondsin the furnace, and immediateremoval of the expanded material fkomthe bee prevents damageto the structure ofthe vermiculiteparticle. Flame temperatures of more than 1000F areused for exfoliation. Proper exfoliation requires both a high rate of heat transfer and a rapid generation of steam within the vermiculite particles. The expandedproduct fallsthrough the furnace and is air conveyed to a classifier system, which collectsthe vermiculite product and removes excessive fines. The firmace exhaust generally is ducted through a product recovery cyclone, followedby an emission control device. At some facilities, the exfoliated material is ground in a pulverizerprior to being classified. Finally, the material ispackaged and stored for shipment.

Perlite -

Perlite is a glassy volcanic rock with a pearl-like luster. It usually exhibits numerous concentric cracks that cause it to resemble an onion skin. A typical perlite sampleis composed of 71 to 75 percent silicon dioxide, 12.5 to 18.0percent alumina, 4 to 5 percent potassium oxide, 1to 4 percent sodium and calcium oxides, and trace amounts of metal oxides. Crudeperlite ore is mined, crushed, dried in a rotary dryer, ground, screened, and shipped to expansion plants. Horizontal rotary or vertical stationary expansion furnaces are used to expand the processed perlite ore. Crude perlite is mined using open-pitmethods and then is moved to the plant site where it is stockpiled. A generalized process diagram for perlite processing is included in Appendix A of this document

The first processing step is to reduce the diameter ofthe ore to approximately 0.6 inch in a primaryjaw crusher. The crude ore is then passed through a rotary dryer, which reduces the moisture content fkom between 4 and 10percent to less than 1percent. After drying, secondary grinding takes place in a closed-circuit system using screens, 

### air classifiers, hammer mills, and rod mills. Oversized material produced from the secondary circuit is returned to the primary crusher. Large quantitiesof fines, produced throughout the processing stages, 

### areremoved by air classification at designated stages. The desired size processed perlite ore is storeduntil it is shipped to an expansionplant. At the expansion plants, the processed ore is eitherpreheated or fed directly to the furnace. Preheatingthe material to approximately 800 F reduces the amount of finesproduced in the expansionprocess, which increases usable output and controls the uniformity of product density.

In the furnace, the perlite ore reaches a temperature of 1400to 1800 F, at which point it begins to soften to a plastic state where the entrapped combinedwater is released as steam. This causes the hot perlite particlesto expand 4 to 20 times their original size. A suction fan drawsthe expanded

NMhQroc TSD 6-14-02 Created on 6/18/2002 9:39:00AM 4