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HEAP LEACH 40 E&MJ; • MAY 2017 www.e-mj.com Heap leaching is typical of most mineral extraction methods in that it is easy to grasp in concept and difficult to perfect in practice. At first glance, percolating a solution through a pile of crushed rock to capture metal values sounds like the eas- iest possible way to recover gold, copper and other amenable metals, surpassed in simplicity only by in situ mining. However, in the same vein as the mil- itary maxim that "the enemy always gets a vote" in a battle's outcome, a number of complicating factors often have votes in the outcome of a heap leaching campaign. These include local weather conditions, ore composition variability, leach pad design and maintenance, and the hard- to-figure settlement and segregation be- havior of rock fragments and particles, to name just a few. Regulatory and societal concerns involving the use of cyanide or sulphuric acid in large quantities as well as the potential for environmental damage by fluid leakage from pad linings or piping can also affect the operational 'ballot.' Yet, these risk factors generally don't deter companies from planning and de- veloping heap-leach operations where the mineralogy, topography and applicable technology are appropriate. Heap leach- ing often offers smaller or less-capital- ized companies an affordable, relatively quick path to production. For companies that own an oxide ore-capped sulphide deposit, heap leaching of the oxides can generate revenue needed to buy and build the facilities needed to later pro- cess the sulphide ore. Partly because of its relative economy of capital and quick startup capabilities, heap leaching technology is successfully being adapted to cope with increasing- ly riskier environments such as extend- ed frigid weather conditions, enabling leach operations to function effectively in high-altitude locales or in Arctic or sub-Arctic regions. In a paper presented at the Heap Leach Solutions 2015 confer- ence held in Reno, Nevada, USA, authors Krishna P. Sinha and Mark E. Smith noted that there are a surprisingly large number (>70) of cold-climate heap leach projects either being planned or currently operat- ing at production rates ranging from less than 1 million mt/y to 30 million mt/y. Although cold-climate heap leach mines face operational challenges, they Leach Operators Get a Lift from Tech Improvements The pursuit of higher metal recovery enters the digital age — where even time-tested, conventional leach-optimization techniques stand to benefit from emerging automation and analysis capabilities By Russell A. Carter, Contributing Editor Kinross Gold's Ft. Knox gold mine in Alaska produces about 40% of its annual output from the Walter Creek valley fill heap leach operation, pictured here. At full development, maximum heap height from toe to crest will be approximately 1,100 ft and maximum ore thickness in the pad about 500 ft. Construction began in 2007 with the project scheduled to be built in seven stages extending to 2021. (Photo: Knight Piésold)