Editor’s note: The Geology and Mining series, edited by Dan Wood and Jeffrey Hedenquist, is designed to introduce early-career professionals and students to a variety of topics in mineral exploration, development, and mining, in order to provide insight into the many ways in which geoscientists contribute to the mineral industry.
Abstract
Copper production by heap leaching, coupled with solvent extraction and electrowinning (SX-EW), is a well-established technology, with an annual output of about 3.7 million tonnes (Mt) of copper metal. Ores presently amenable to copper heap leaching include copper oxides and secondary copper sulfides. Most copper deposits amenable to acid sulfate heap leaching result from supergene processes within porphyry copper systems, although copper heap leaching has been applied to sandstone and shale-hosted deposits, among others. Copper heap leaching is a rate-dependent process sensitive to copper mineralogy (copper oxides > secondary sulfides > hypogene sulfides), driven by the pH of the leach solution, the activity of ferric iron (Fe3+ (aq)) dissolved in the leach solution, and temperature. Acid consumption, a principal operating cost item, depends on the pH of the leach solution; the presence of reactive gangue minerals, notably carbonates, Ca plagioclase, pyroxene, Fe-rich amphibole, and olivine; and the cumulative surface area of material in the heap.
There are three basic approaches to commercial copper heap leaching—run-of-mine, dedicated pad, and on-off pad leaching, with variables that include crushing, acid/ferric agglomeration, solution application rate, and leach solution pH. These approaches affect copper leach kinetics, overall copper recovery, acid consumption, and capital and operating costs. A successful copper heap leach evaluation program requires a systematic approach, beginning with geologic mapping, then drilling and hydraulic and metallurgical testing, and concluding with financial analysis, engineering, and permitting. As geologists are the unique party in the process, with a thorough understanding of the overall deposit geology, including ore and gangue mineralogy, the domains that comprise the deposit, and the geochemistry of leaching, they must remain fully involved in the project throughout the evaluation. At the outset, geologists must manage the drilling program and define the grade-mineral domains. Later, they must participate in the metallurgical and hydraulic testing programs, including the evaluation of test results; then, during financial modeling, they must collaborate with all of the other specialists.
