Engineering and Mining Journal - Whether the market is copper, gold, nickel, iron ore, lead/zinc, PGM, diamonds or other commodities, E&MJ takes the lead in projecting trends, following development and reporting on the most efficient operating pr
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Writing the introduction to a special edition of the Transactions of the U.K.'s Institution of Materials, Minerals and Mining (IOM3), published in 2011, Professor Yves Potvin, director of the Australian Centre for Geomechanics at the University of Western Australia, highlighted the three main risk areas facing companies looking to develop block cave mining operations. The first, he said, "Is the capability of the undercut alone or, assisted by pre-conditioning, to generate the conditions that will allow for the fragmentation and recovery of the entire orebody. "Once the orebody is fragmented, the potential for dilution as well as sterilization of the significant quantity of broken ore becomes a second order of risk, which can be managed by draw control." "A third order of risk associated with cave mining, more particularly the ones operating in high-stress environments, is the risk associated with seismic activity that can lead to rockbursting damage," Potvin said. Block caving offers the potential for high-volume, low-cost production usually, although not always, from low-grade deposits that could not be mined prof- itably by other methods. As a technique, its successful application relies on a geological endowment that is structurally favorable, backed by the substantial financial resources needed in order to prepare the extensive development infra- structure needed. Two commodities stand out within the relatively meager ranks of operating block cave mines: copper and diamonds, reflect- ing the method's suitability for working por- phyry and pipe-type deposits, respectively. Indeed, a rough count of current block- and panel-caves—as separate from mines using systems such as sublevel caving— suggests that there are fewer than 20 in operation worldwide. Some of those are, however, among the world's largest in terms of underground production capacity. Deepening the Pit Floor Aside from blind deposits where block cav- ing is designed as the main production sys- tem from day one, the other application has been where underground mining has replaced an earlier open pit that has reached its economic limit. Examples include several of the diamond mines and Palabora (copper) in South Africa, Argyle in Australia, and the forthcoming transition at both Grasberg in Indonesia and Chuquicamata in Chile. Other candi- dates include Bingham Canyon, where a sequence of caving operations has been under evaluation for several years, and some of Botswana's diamond mines. Nonetheless, a smooth transition is by no means guaranteed, as experience at Palabora suggests. Production from the open pit ended in early 2002, the operation having invested some $460 million in developing its replacement underground section of life. Although an output of 30,000 metric tons per day (mt/d) was scheduled, two years later that had yet to be achieved. There was clearly a learning curve involved and, as the company pointed out, "no other block-cave mine has been put into as competent an orebody." Very blocky fragmentation presents a major production constraint, requiring a significant amount of secondary breaking—both by blasting and hydraulic hammers—to keep the draw- points flowing. Meanwhile at Grasberg, Freeport McMoRan Copper & Gold (FCX) is well on its way with the development of its "com- mon infrastructure" underground haulage project, providing access to the forthcom- ing DMLZ and Grasberg block-cave mines. The DMLZ will come on stream next year, while the Grasberg block cave will replace capacity from the open pit when that clos- es in 2017. Later in the schedule, the company is also targeting the outlying Kucing Liar orebody for block caving. 32 E&MJ; • JULY 2014 www.e-mj.com B LO C K C A V I N G Block Caving: Mining Specialization While block caving is not a new concept, it needs the right combination of geology and technology to make it work successfully By Simon Walker, European Editor Copper and diamonds are the predominant commodities currently being mined by block caving. Shown here is a scan, developed using Geovia's PCBC software, of the bottom of a kimberlite pipe at the Finsch diamond mine, dis- playing selected draw points and draw cones. EMJ_pg32-37_EMJ_pg32-37 7/2/14 8:35 AM Page 32