Settling the Dust in San Manuel

July 1, 2000

Curbing fugitive dust has become a priority for many companies and municipalities as the public becomes increasingly aware of dust-related air-quality and health concerns. Wind erosion raises dust at construction sites, on unpaved roads, at remote airstrips, and around mining operations. Often, though, the problem can be dealt with in small pieces. When BHP Copper Inc. in San Manuel, AZ, found that an unused area of its tailings pond was turning into a 3,000-ac. dust bowl, however, the company needed a large-scale solution – fast. It found one – in the air.

BHP Copper, owned by Broken Hill Proprietary Company Ltd. of Melbourne, Australia, operates one of the world’s largest underground copper mines at San Manuel, about 35 mi. north of Tucson. When the company shut down the part of its operation that discharged water and silt into the tailings pond, wind began picking up dust from the increasingly dry surface. To quell community air-quality concerns, BHP turned to South Western Sealcoating Inc. of Murrieta, CA.

One of the largest dust-management distributors and contractors in the western US, South Western Sealcoating uses a product called Dust-Off to dampen and penetrate dirt surfaces so they remain moist and stabilized even in high temperatures and low humidity. The product’s main ingredient is magnesium chloride, a nontoxic, environmentally safe, colorless, odorless mineral that absorbs several times its weight in atmospheric moisture and resists further evaporation. Dust-Off also contains an anticorrosive additive. The product has been approved by and used on projects for many county and state highway departments, the US Forest Service, the California Department of Fish and Game, and several mining, farming, and forest-products companies.

The challenge in BHP’s case was the sheer size of the pond. “The San Manuel project challenged us with some logistical problems,” says South Western Sealcoating’s John Church. “The large area required substantial amounts of our product, and time was of the essence.”

South Western Sealcoating has helped pioneer the use of spreader trucks equipped with the Bearcat Computerized Rate Control system, and the company uses the trucks for most standard ground-based dust-control applications. But using spreader trucks is sometimes unfeasible because of location, possible environmental damage, or – as in San Manuel – the size of the area.

For these situations, South Western Sealcoating, in conjunction with Gilbert Aviation of Visalia, CA, has developed and engineered a product loading and delivery system for an Air Tractor AT-802 aircraft. The plane can typically load up to 700 gal. within three minutes and dump in a precision pattern in just nine seconds. In lighter-concentration applications, the Air Tractor can cover up to 1 mi. in just 18 seconds.

The specially equipped aircraft is ideally suited for the application of magnesium chloride and has the capacity, strength, and maneuverability to deliver a consistent 30-ft.-wide pattern efficiently and precisely on target with its Global Positioning System satellite-controlled navigation unit. Knowing exactly what was covered in the previous pass helps the pilot avoid crossover spraying and product waste.

Although other companies might use crop dusters to deliver dust-control products, Church says, the FAA-certified dump system designed for the AT-802 – one of the largest crop dusters manufactured – is what distinguishes this system. “Normally crop dusters as such dispense 5-10 gallons an acre – chemicals, pesticides, fertilizers, whatever they’re putting down. We have to apply anywhere from 600 to 1,200 gallons an acre, so you’ve got a lot of product coming out fast. We had to develop a special dump system that would not only give us a good spray pattern, but that would not interfere with the aerodynamics of the aircraft.”

A light application of Dust-Off in December solved BHP’s immediate problem. “We applied 4,826 tons of product in less than 20 days,” says Church. “Other than a couple of windy days and some minor mechanical delays, the project went off without a hitch.” A second, heavier application is planned, which will take 40-45 days and will use 100 railcars of the product. Church explains that the rate of application and amount of penetration determines product life. One application of Dust-Off has been shown to last up to a year.

“A contained operation well managed,” Gerry Brunskill, manager of concentrator operations at BHP, calls the project. “They mobilized very quickly.” The San Manuel project is believed to be the largest of its type carried out anywhere in the world. 
About the Author

Janice Kaspersen

Janice Kaspersen is the former editor of Erosion Control and Stormwater magazines.