Rock fragmentation results for a simulated underground leaching stope

In situ stope leach mining is part of a clean mining research program designed to increase the domestic supply of metals for the United States. The U. S. Bureau of Mines (USBM) is developing this innovative mineral recovery technology to help preserve the ecosystem. The USBM is applying a systems approach to develop mineral recovery methods that minimize environmental impact by selectively extracting and recovering targeted minerals. The motivation for this approach derives from the conviction that we can no longer afford the environmental or economic cost of moving large volumes of rock to recover small amounts of metal. To accomplish a clean mining system, one must maximize the solutions contact with ore minerals and also maintain control of the leach solutions. This can be achieved by performing in situ fragmentation to obtain relatively uniform rock particle size to increase rock mass permeability, and also minimize blast damage to the perimeter of the excavation. The USBM is investigating the feasibility of in situ stope leach mining, where leaching solutions are passed through rubblized ore. Rubblizing blocks of ore by blasting for subsequent underground in-stope leaching requires special applications of blasting techniques. This paper presents the results of in situ fragmentation of a simulated cylindrical stope which is 1.8 m in diameter and 6.1 m high. The blast design combined conventional and smoothwall blasting techniques. Three drop-raise and smoothwall blasts were used to fragment the simulated test stope. The average powder factor for the stope fragmentation was 2.6 kg per metric ton, compared to 1.10 kg per metric ton for routine drift development work at the mine. All the material from the first blast, 14.2 metric tons, was sieved. The resulting distribution was compared to the distribution predicted by empirical equations. The best fit was found with a USBM developed equation based on over 50 sieved, reduced-scale (1- to 2-m) high wall blasts. Comparing the broken rock distribution data for round 1 with the photo image processing data for rounds 1, 2, and 3 shows that using photo image processing is cost effective and reasonable when compared to screening and weighing.