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Turning Dust Into Dollars
A Kentucky quarry's solution for removing ultrafine particles from Superpave material also provides a salable byproduct.

by Russell A. Carter

Rock Products, Jan 1, 2002

When a quarry can't provide a specific product needed by a major customer, the alternatives are usually clear: Either find the resources and equipment to produce that product, or risk losing a sizable chunk of business.

That was the situation facing Cemex/Southdown's CSR Rinker Materials' Bowling Green North quarry in 2000 when one of Kentucky's biggest highway construction companies needed a large amount of Superpave. "Up to that point, we'd never produced Superpave at Bowling Green North - we were too heavy on the fines," says Robert Forbes, operations manager for the Construction Aggregates Group.

The 40-year-old quarry is a 1 million-tpy operation located five miles north of Bowling Green. It is one of two Cemex/Southdown quarries in the area that produce a combined 2 million tpy of limestone asphalt aggregate, concrete aggregate, and road base for the regional market. Production at the North quarry is divided roughly into 50% asphalt paving materials, 30% base and 20% concrete aggregate.

Rinker purchased the North quarry from Cemex/Southdown last year. The ownership change became official at the start of this year.

Getting to the stone

Mining at Bowling Green North begins with the removal of 10 to 30 ft of overburden. A special crew handles North's stripping, as well as all the company's regional operations - about 1.3 million cu yd of overburden annually. Production at the North quarry requires removal of about 250,000 cu yd per year.

The uncovered limestone is drilled and blasted, using an Ingersoll-Rand T-4 truck-mounted drill to sink 5½-in.-diameter blast holes. Each hole is loaded with Austin Powder Co.'s HEET ANFO/emulsion blend. Blasting typically occurs once per week, producing 35,000 to 45,000 tons of shot material. The quarry currently is working an 80-ft bench interval.

A Caterpillar 988 wheel loader loads the shot rock into a Hitachi-Euclid R35 or R60 haul truck for transport to the primary crushing station. Mining and processing are conducted on a two-shift-per-day basis, with each 10-hr shift producing about 3,000 tons. The mine currently employs about 24 workers, with 12 workers required per shift.

The quarry upgraded its primary crushing equipment early last year, installing a 4248 Hewitt Robins jaw crusher and Cedarapids feeder. Crusher output reports to a 5- ¥ 14-ft Simplicity scalping screen. Oversize product from the screen is sent to a Symons standard cone crusher for secondary reduction.

Feed from the secondary crusher's surge pile is conveyed to the plant's three-screen finishing tower. This is where screen oversize is reduced in another Symons standard cone, a Shorthead cone, or a Cedarapids 4033 hammer mill.

The key element in producing Superpave material at the North quarry involved dedusting the minus ¼-in. material passing through the finishing screens. The nearby South quarry employed a conventional wet wash screw to remove its fines, and management's initial intent was to follow a similar route at the North operation. But Forbes first wanted to study the available alternatives for removing fines without water. He visited a nearby installation in Hopkinsville, Ky., that used a Sturtevant air classifier.

Following the visit, he sent a drum of the North quarry's crushed limestone to Sturtevant's facility in Hanover, Mass., for testing. The test batch contained 10% to 12% minus 200-mesh fines; in order to comply with Superpave specifications, it would be necessary to reduce the minus 200-mesh fines content to 3.5%.

The Sturtevant Whirlwind air classifier technology successfully accomplished the task and Cemex/Southdown purchased a 14-ft unit for the North quarry. The installation, which took a crew of a half-dozen about four days to accomplish, has enabled Bowling Green North to remove ultrafine particles from its Superpave material and produce a salable byproduct - aglime - in the process.

The Whirlwind is a mechanical, centrifugal air classifier that uses a single motor to drive three rotating components: a distributor plate, selector blades, and fan blades. Airflow through the classifier is self-contained and recycled by return air vanes. Unlike gravitational inertial classifiers, it doesn't require cyclones, dedicated baghouses, airlocks, external fans or ductwork to operate properly, according to the company. Sturtevant also claims the units, with proper maintenance, can provide a 40- to 50-year service life.

Finding a usable byproduct

According to Joe Muscolino, Sturtevant's product manager, an air classifier with a dynamic mechanical rotor, such as the Whirlwind design, provides two advantages over gravitational inertial classifiers and wash screws. First, very little aggregate is lost during the fines removal process, assuring an optimum supply of dedusted stone. Second, the fines are recovered in the form of a dry powder with few oversize particles, providing an instant byproduct. Bowling Green North sells the fines as aglime, but they can be used in a variety of applications ranging from plastic filler to cement.

At Bowling Green North, limestone screenings enter the air classifier by gravity and are dispersed into the classification zone by the distributor plate. An internal fan lifts the minus 200-mesh fines out of the feed. Selector blades within the machine control the amount of fines to be removed; fewer selector blades allow a greater quantity of fines to be removed from the crushed stone feed, resulting in manufactured sand with very little minus 200-mesh content.

Once the optimum number of selector blades is established for a specific application, fine-tuning can be accomplished using external fineness control valves around the circumference of the classifier. The external adjusters are connected by rods to moveable plates inside the machine that baffle the internal airflow and provide additional control over the quantity of fines removed.

Dedusted sand exits by gravity through the side of the classifier's lower cone and is conveyed to the main product stockpile. Fines leave through the bottom of the cone and report to a dry secondary product pile.

Most of the quarry's manufactured sand goes into asphalt paving products, with the rest purchased by a local cement block manufacturer. The fines are eventually added to the quarry's aglime product.

"From a sales standpoint, that is a big plus," says Jeremy Goad, the quarry's territory manager. "Instead of being saddled with wet fines that have to be recovered from a pond and disposed of, we have a dry product that is immediately usable."

According to Julian McLellan, quarry manager for both the North and South operations, the cost of the air classifier was about one-third less than a sand screw. "But beyond the cost of the unit itself, we were able to avoid the problems associated with settling ponds, such as the need for a dragline to clean the pond periodically and the expense of drying the fines and storing them on site."

"The physical footprint of both the air classifier and a sand screw are similar," McLellan says, "but the space required to support them differs a lot. The classifier is pretty much a stand-alone unit, but we would need room for pumps, ponds and a drying area if we had gone with a screw."

According to Sturtevant's Muscolino, Whirlwind air classifiers are sized for a specific application on the basis of 1 hp of motor power per tph of feed rate. The North quarry's unit is designed to handle a nominal feed rate of 70 tph using a 75-hp motor, but throughput has reached as high as 125 tph during dry weather when moisture content in the feed material is extremely low.

"That is one clear advantage that wash screws have over air classifiers," McLellan says. "You can produce sand in a wash screw come rain or shine, but air classifier performance can be affected by weather and moisture."

Nevertheless, the machine has enabled the quarry to provide Superpave material to its major customer in the required volume.

"We're providing Superpave material for three major road projects out of this quarry right now," Goad says. "Our Superpave product volume has gone from zero to approximately 60% of sales over the past year."

And given Superpave's popularity, as well as the new-found salable byproducts, Bowling Green North has turned its eye to capturing - not losing - future business.

MAJOR EQUIPMENT
CSR Rinker's Bowling Green North Quarry Mobile:
1 Caterpillar 988F Loader
1 Caterpillar 980F Loader
1 Volvo L-190 Loader
1 Volvo L-320 Loader
1 Deere 675-B Loader
1 Euclid-Hitachi R60 Haul Truck
2 Euclid-Hitachi R35 Haul Truck
1 Euclid-Hitachi 95FD Stockpile Truck
1 Mack Tandem Stockpile Truck
1 International 4900 Stockpile Truck
1 Freightliner Tandem Stockpile Truck
1 Ingersoll-Rand T-4 Drill
1 Mack Water Truck

Crushing:
1 Hewitt-Robins 42- ¥ 48-in. Jaw Crusher
1 Cedarapids VGF Feeder
1 Nordberg Symons 4¼-ft Standard Cone Crusher
1 Nordberg Symons 4-ft Standard Cone Crusher
1 Nordberg Symons 4-ft SH Cone Crusher
1 Cedarapids 4033 Hammer Mill

Screens:
Simplicity 4- ¥ 12-ft Double Deck
Simplicity 5- ¥ 14-ft Double Deck
Simplicity 5- ¥ 14-ft Triple Deck
Nordberg 6- ¥ 14-ft Triple Deck
2 Stearns & Rogers Electromagnets

Air Classifier:
1 Sturtevant 14-ft Whirlwind

© 2002, PRIMEDIA Business Magazines & Media Inc. All rights reserved. This article is protected by United States copyright and other intellectual property laws and may not be reproduced, rewritten, distributed, redisseminated, transmitted, displayed, published or broadcast, directly or indirectly, in any medium without the prior written permission of PRIMEDIA Business Magazines & Media Inc.

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