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 are used to fuel dryers. 

After being transported to the exfoliation plant, the vermiculite concentrate is stored. The ore 

concentrate then is conveyed by 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 seconds in the furnace, and immediate removal of the expanded material fkom the bee 

prevents damage to the structure of the vermiculite particle. Flame temperatures of more than 

1000 F are used for exfoliation. Proper exfoliation requires both a high rate of heat transfer and a 

rapid generation of steam within the vermiculite particles. The expanded product falls through 

the furnace and is air conveyed to a classifier system, which collects the vermiculite product and 

removes excessive fines. The firmace exhaust generally is ducted through a product recovery 

cyclone, followed by an emission control device. At some facilities, the exfoliated material is 

ground in a pulverizer prior to being classified. Finally, the material is packaged 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 sample is composed of 71 to 75 

percent silicon dioxide, 12.5 to 18.0 percent alumina, 4 to 5 percent potassium oxide, 1 to 4 

percent sodium and calcium oxides, and trace amounts of metal oxides. Crude perlite 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-pit methods 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 of the ore to approximately 0.6 inch in a 

primary jaw crusher. The crude ore is then passed through a rotary dryer, which reduces the 

moisture content fkom between 4 and 10 percent to less than 1 percent. 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 quantities of fines, produced throughout the processing stages, are removed by air 

classification at designated stages. The desired size processed perlite ore is stored until it is 

shipped to an expansion plant. At the expansion plants, the processed ore is either preheated or 

fed directly to the furnace. Preheating the material to approximately 800 F reduces the amount of 

fines produced in the expansion process, which increases usable output and controls the 

uniformity of product density. 

In the furnace, the perlite ore reaches a temperature of 1400 to 1800 F, at which point it begins to 

soften to a plastic state where the entrapped combined water is released as steam. This causes the 

hot perlite particles to expand 4 to 20 times their original size. A suction fan draws the expanded 

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