New Underground Technology Mines One of World’s Oldest Chemicals

A Joy longwall system at FMC’s Westvaco trona mine in Wyoming, USA.
Soda ash, now derived principally from trona, is one of the world’s oldest
industrial chemicals, with use dating back five centuries.
In 1791, French physician Nicolas Leblanc developed a process for extracting soda ash from trona, using sulphur, salt, and coal gas; and in 1860, Ernest Solvay modified the process to achieve a better soda ash using brine, ammonia, salt, coal gas and carbon dioxide.

In 1938, trona was found in a core sample while drilling for oil at the John Hay No. 1 Well south of Westvaco, Wyoming. Trona was first deep-mined in 1947 by Westvaco Chlorine. Shortly after that, FMC Corp. acquired Westvaco’s chemical business, including the mine, and then added the Granger mines. Today, Philadelphiabased FMC is the world’s largest natural soda ash producer. The company, founded in 1883, maintains a leading position in three chemical markets: agriculture, specialty and industrial.

In the Green River Basin, 25 of the thickest beds contain 127 billion tons (115 billion mt) of trona with 40 billion tons considered recoverable—sufficient for 2,350 years of extraction at today’s rate of mining. Bed 1 is 3,500 ft (1,070 m) beneath the surface and Bed 25 is 800 ft (245 m) down.

Typically, trona is processed into soda ash (sodium carbonate, Na2CO3) at a recovery rate of 1 ton of soda ash for every 1.7 tons of trona. Industry-wide, Wyoming trona mines employ more than 2,225 people, annually mining more than 17 million tons (15 million mt) of trona to produce more than 10 million tons of soda ash, shipped from the mine by rail bagged or in bulk.

About 44% of the trona in Wyoming is produced from federal owned lands and some 56% from state and private lands, all of it deep-mined using boring machines with continuous haulage, continuous miners with continuous haulage and shuttle cars, and, at the Solvay and FMC mines, longwalls.

The most successful application of longwall mining has been at the FMC Westvaco mine. “Here, we have 225 people involved in the mining some 4.6 million tons of clean soda ash,” said Tim Davis, senior mine engineer and longwall project manager. “We run 10 hour shifts, 24 hours a day, 7 days a week, usually 11 production shifts a week, and six or seven maintenance shifts.

“Presently, we have two borer miners on longwall development work and a third machine working on our bleeders, all working in tandem with Joy 10SC-32 shuttle cars,” said Davis. “The shuttle cars dump onto conveyor belts that transport the trona to skip hoists for transfer to the surface and the processing plant.

“Unlike in coal mining, in addition to the headgate and tailgate, we drive a third entry up the center of the longwall panel, about 125 feet from the headgate, then mine through those pillars as we advance the longwall,” said Davis.

Westvaco’s longwalls typically measure 750 ft (230 m) across the face by 9,400 ft (2,865 m), with 1,600 ft of cover. According to Davis, two problems in mining trona can be excess water and the size of the material as it comes off the longwall face.

“For one thing, trona is water soluble. When we get it outside, the first thing we do is calcify it, drive off the water, so the less water the better. Also, when trona and water do mix, it runs into places you don’t want it to run and then sets up like concrete,” Davis said. “Fortunately, while extracting trona does generate dust, it is a nuisance dust, not a hazardous dust, so we don’t have to use as much water for dust suppression, as they do in coal mining.

“The second problem we encounter is the size of the slabs of material that comes off the longwall face,” Davis said. “While we continue to work on that problem, making adjustments to the shearer, the shield operators still have to, at times, break up large slabs using jackhammers.”

As explained by Davis, FMC’s Westvaco mine is 1,550 ft (475 m) below ground and has more than 2,500 miles (4,000 km) of tunnels stretched out over 36 mi2. Entries are 16 ft (5 m) wide, 9 ft (3 m) high and the mine operates at 1,000 t/h, with an additional 100 t/h per hour coming from a unique solution mining process involving the injection of water in another area of the mine.

According to Scott Lewis, Joy sales engineer, “Allied Chemical (now General) pioneered longwall mining of trona in 1973. FMC began longwall mining in 1981. Longwall mining was halted briefly in 1987 for retooling. In 1993, the mine replaced the longwall system with a Joy longwall system, adding a Joy 6LS5 shearer in 1996. Now equipped with a Joy 7LS5 shearer, the current longwall is recovering 50% more of the in-seam trona than the mine had been realizing in borer mining.”

Along with the Joy 7LS5 shearer, the FMC Westvaco longwall installed this past January consists of 131 Joy 2 x 870-ton shields, 2 x 1,000-hp Joy armored face conveyor and 42 x 146 flat link chain, and a 450-hp stage loader with a 42-mm Joy Broadband low-profile chain. The longwall system also features Joy’s Faceboss control platform.

According to Lewis, “With a combination of assistance tools, Joy’s Faceboss control platform provides the operator with automated cut sequence settings, advanced diagnostics with on-board graphic displays, and machine performance monitoring and analysis. Combined, this system ensures the operator can achieve the optimal balance of production rate and cost.

“Since its introduction in 2006, the Faceboss control platform has proven so successful that today it is standard on all Joy 7LS shearers, as it is on all continuous miners, flexible conveyor trains, and armored face conveyors,” said Lewis.

As featured in Womp 2009 Vol 07 -