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Coming of Age of Fines Handling

Appeared in Aggregates Manager , September 2000

".. Dry air classifying is another option when considering wash screws, wet screening and even belt presses as alternatives.

"One advantage to using an air classifier system is the minimal land requirements. With a dry air classifier, there isn't a need for a settling pond. It also doesn't encompass much territory around it in order for it to operate," said Joe Muscolino, project manager for Sturtevant.

Another advantage is that the fines removed from the crushed stone are dry and can be sold for aglime or filler for various markets, Muscolino said.

Typically, air classifiers are used to remove minus 200 mesh dry fines from a top size of 3/15 in. to 8 mesh. But, ½- to 1- ½- in. material can be fed and will not affect performance, though it will cause increased wear, said Muscolino.

The machines are also capable of removing fines as small as 325 mesh and as large as 100 mesh. The Sturtevant Whirlwind line of air classifiers are available in 14 sizes, ranging from a feed rate of 1 tph at 5 hp to 800 tph at 800 hp.

The classifiers typically can handle up to 1 to 2 percent moisture to work effectively, said Muscolino. Efficiency is reduced at 3 to 4 percent moisture, but the units can still be used as a pre-separator to remove a quantity of dry fines prior to wash screens or settling ponds.

This approach reduces water consumption and extends the life of the settling pond, Muscolino said.

Sizes of air classifiers are determined by the diameter of the upper casing and the air flow generated by the internal fan. The whirlwind model recycles nearly all of the working airflow and does not require a cyclone or baghouse to recover the fines, Muscolino said.

In the aggregates industry, the most common sizes are 14 ft. diameter, which produces 70 to 80 tph; 16 ft. diameter, which produces 100 to 125 tph; 20 ft. diameter, which produces 300 to 350 tph; and 18 ft., which produces 200 to 250 tph.

And, given the simplicity of its design, Muscolino said, air classifiers can be made portable to meet the increasing demand for mobile plants.

Above is a Sturtevant 20-ft. whirlwind air classifier, which handles 200-300 tph. Feed enters at the top from a belt conveyor. Dedusted stone exits at the bottom right onto a belt conveyor to a clean pile. Minus 200 mesh fines drop out the bottom cone onto another conveyor.

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