Design and construction of the Montana Tunnels tailings disposal facility

CIM Bulletin, Vol. 81, No. 919, 1988

J.P. HAILE and K.J. BROUWER, Knight and Piesold Ltd. Vancouver, British Columbia

Construction of the first stage of the tailings disposal facility for the Montana Tunnels Project was completed in October 1986. The project is located 37 km south of Helena, Montana, and involves mining of 42 million tons of weathered diatreme ore at a rate or 13 500 tpd to produce a gold/silver dore, as well as lead and zinc concentrates. The tailings disposal facility for the project includes a fully drained tailings impoundment for storage ofthe tailings solids, and a process water pond for storage of all decanted water prior to recyling to the mill. The over-all facility is designed to achieve the basic objectives of minimizing seepage to the environment in the short- and long-term, and achieving a fully drained stable tailings mass suitable for immediate reclamation on completion of mining. Specific features of the design include a soil bentonite seal and drainage system within the tailings basin, a free draining embankment with a design flow-through capacity of 4000 USgpm and an ultimate height of 230 ft (70 m), and a soil bentonite lined make-up water pond with a capacity of 420 acre-feet (530 000 m3). The tailings will be discharged in a rotational sequence through multiple spigot offtakes to achieve a thin-layer low-energy laminar flow. This 'sub-aerial' deposition results in higher stored densities and maximizes liquid-solid separation on the tailings beaches. The over-all design played a major role in the successful permitting of the project, and was significantly cheaper than a conventional wet disposal system, with overall unit costs for tailings disposal of less than US$0.36 per ton. This paper outlines the site characteristics and the tailings material properties and their influence on the design. Extensive use was made of naturally occurring free draining materials as well as bentonite modified soil liners within the tailings basin, embankment and retention ponds. General construction procedures for the various components are described, as well as on-going construction requirements utilizing materials from the open pit. Finally, the over-all costs and unit costs for tailings disposal are presented.
Mots Clés: Mineral processing, Tailings, Equipment design, Montana Tunnels Project.