What is trona? Trona is a relatively rare sodium-rich mineral found in the United States, Africa, China, Turkey, and Mexico. Sweetwater County, Wyoming is a major contributor to the total world production of trona, which is mined and then processed into soda ash. Soda ash is a significant economic commodity because of its applications in manufacturing glass, chemicals, paper, detergents , textiles, paper, food and conditioning water. It is an ingredient in both sodium bicarbonate (baking soda) and sodium phosphate (detergents). Soda ash has been used since ancient times. The Egyptians made glass containers from it, and the early Romans used it as an ingredient in medicines and bread.
How did it form? About fifty to sixty million years ago, a shallow, expansive body of water called Lake Gosiute covered the Green River Basin in southwestern Wyoming. The area climate would cycle from wet to dry: wet periods would deposit mud on the lake bottom, while dry periods would precipitate trona and other saline minerals. Over a period of many years, these cycles left behind approximately 100 billion tons of nearly pure trona layered between beds of sandstone, marlstone and shale. The trona beds have a nearly horizontal orientation, similar to when they were originally deposited, at depths ranging from about 800 feet to 3500 feet below the surface.
How was it discovered? Trona was discovered in Sweetwater County in 1938 during oil and gas exploration. The first mine shaft was sunk in 1946, and commercial soda ash production began in 1948. Up until that time, all soda ash in the United States was produced synthetically.
What is the extent of trona in the Green River Basin? There are 25 major trona beds that occur within the Wilkins Peak Member of the Upper Eocene Age Green River Formation. The trona deposit as a whole covers about 1000 square miles within the Green River Basin, mostly in Sweetwater County, Wyoming. Four beds are actively being mined (Beds 25, 24, 20 & 17), Bed 19 has mined in the past, and Bed 14 is proposed for mining in the future.
How is trona extracted? Mining occurs at depths ranging from 800 feet to 1600 feet below the surface in trona beds that are 8 to 14 feet thick. The trona ore is recovered utilizing dry or wet methods. Dry mining is similar to underground coal mining; the mine workings are developed using room-and-pillar and longwall mining techniques. The mining cycle includes shearing the trona from the face with either a longwall shearer or continuous miners and then loading it onto conveyor belts. The conveyor belts move the trona to ore skips that carry it to the surface through vertical shafts. The recovered trona ore is stockpiled on the surface to be used as feed for the processing plant.
Wet mining (solution mining) is done by injecting a solution from the surface into the trona deposit using a series of bore holes as injection wells. This is done in either previously unmined ground or in the mined-out areas of active operations. In both cases, the injected solution dissolves the trona ore which saturates and enriches it. Subsequently, the saturated solution is pumped to the surface through recovery wells for further processing into soda ash. Some mine operators use dry recovery methods for primary ore extraction and then wet mining methods for secondary recovery. This technique results in maximum recovery of the trona reserve, some of which was previously considered unminable.
Statistics: BLM's Rock Springs Office administers approximately 76,000 acres leased for sodium to four major operators in the Green River Basin. The annual combined output for 2004 neared 17.3 million tons of trona, which was processed into almost 10.7 million tons of soda ash. This accounts for approximately 30% of total global production and 90% of total domestic soda ash production. During 2004, the industry provided an employment base of 2,115 people for Sweetwater and Uinta Counties in Wyoming.