Dr Mark Board, Mr Wes Johnson, Mr Clayr Alexander
Hecla Mining Company’s Lucky Friday silver mine in Mullan, Idaho is one of North America’s deepest (3000 m) and oldest (76 years) mines. A mechanized underhand cut and fill method is employed in near-vertical veins, and working faces have reached approximately 2200 m in depth. Recently, the #4 winze (4 Shaft Project) was completed to a total depth of approximately 3000 m to provide access to deeper, high grade reserves in the primary 30 Vein and parallel ore shoots. Mining at such great depths presents numerous issues such as seismicity, high ground stress, high rock temperature, and diesel particulate matter (DPM). Use of standard drill and blast methods requires personnel exposure at the mining face as well as significant unproductive time during the clearing of blasting gasses and during the transportation of miners during shift change. For several years, Hecla has been investigating alternative mining methods to minimize personnel exposure to the issues described above. Unique characteristics of the veins at the Lucky Friday are the narrowness of the veins (typically 4 m or less), the near vertical dip, the straightness of the veins, the long mineable strike lengths, the low to moderate strength of the the fractured vein rocks, and the low abrasivity. Based on these characteristics, investigations have been conducted of a mining method based on the use of a continuous, mechanical mobile miner technology termed the RVM (Remote Vein Miner). The machine that has been designed, in partnership with Atlas Copco, Örebro, Sweden, uses a highly maneuverable road-header-like rotating cutter head based on standard TBM-style steel disc cutters. Cutter testing of vein samples at the Colorado School of Mines has confirmed the relative ease in cutting. The prototype machine is currently in the manufacturing process and will be tested in 2019. The mining method will employ an underhand longwall to minimize stress concentrations and seismicity; the machine will be powered electrically to eliminate DPM and minimize heat rejection to the working area. Ultimately, the goal of Hecla will be to operate this machine in a full teleremote mode. This paper describes the rock mass conditions at the Lucky Friday and the basis for the decision to move to continuous, mechanical mining. The basic machine specifications and performance predictions are described; the modification of the current mining method to accommodate the mobile miner is also described.