The Doe Run Company and its predecessors have been mining for over 130 years utilizing the room and pillar mining method. This method has left approximately 20% of the orebody in the remaining pillars. The original design and planning for the mines did not include extensive pillar extraction. This paper summarizes the introduction of the pillar extraction and backfill techniques, which have been implemented since 1991. In conjunction with these accomplishments, much experience and confidence has been gained in the use of numerical modeling as a method of predicting pillar stability as the mining of pillars progresses. These advances have led to pillar extraction becoming a major portion of the overall production and have added to the mine life of Doe Run's eight operating mines in Missouri.
1.1 Background
The Doe Run Company lead mines are located in southeast Missouri, USA. The company is the largest integrated lead producer in North America providing 85% of the total United States primary lead production. The Doe Run Company is owned by the RENCO Group, a privately held company. In Missouri the company owns and operates eight underground lead/zinc/copper mines, six mills, two primary smelters, and a secondary smelter. In addition, Doe Run owns the La Oroya smelter and Cobriza mine in Peru, lead fabrication businesses in the USA, and several greenfield properties in North America and South Africa.
The eight Missouri mines are comprised of Fletcher, Brushy Creek, Casteel, No. 28 and No. 29 mines which were previously owned by the St. Joe Minerals Corp., the Buick mine which is a combination of the old Homestake/Amax mine and the Cominco/Dresser Magmont mine, and the Sweetwater and West Fork mines which were recently purchased from Asarco.
Mining operations in Missouri take place in a flatlying, tabular orebody 150 m to 350 m below the surface. The orebody measures from 1 m to 40 m in thickness, 10 m to 600 m in width, and has a trend length of 65 km. The mine workings are located below an aquifer, separated by 30 m of impermeable shale. The ore trend traverses across both the Mark Twain National Forest and privately owned lands.
1.2 Primary mining
The primary mining method is a highly mechanized room and pillar method incorporating two boom drill jumbos, 7 to 9 tonne capacity loaders, and 27 to 45 tonne capacity haul trucks. Thicker ore zones are mined first with a pillar development pass followed by a varied combination of back, bottom, undercut, and overcut passes. Pillars range in height from 4 m to 37 m.
Until 1993, pillars were cut 9 m by 9 m with 10 m drift openings between pillars. Since 1993, larger panel pillars have been utilized where practical. These measure 11.5 m by 23 m with 10m drift openings between pillars. Current pillar reserves are estimated to be 16,000,000 tonnes, much of which is high grade.
