A recent post in Analyzing Metals, A Snapshot of the World’s Rarest Metals, gave us a look at some extremely scarce metals—rhenium and tellurium—that have important applications in the aerospace, energy, and petrochemical industries. With high demand and low supply, where can we find more of these metals?
According to the USGS 2015 Rhenium Mineral Commodity Summary, most rhenium occurs with molybdenum in porphyry copper deposits. Rhenium is also associated with copper minerals in sedimentary deposits in Armenia, Kazakhstan, Poland, Russia, and Uzbekistan, where ore is processed for copper recovery, and the rhenium-bearing residues are recovered at the copper smelter.
Tellurium also is most often associated with copper and is primarily recovered from copper and copper-gold porphyry-type deposits as a byproduct of copper refining. As explained in the USGS Tellurium Fact Sheet, significant tellurium quantities are recovered as a primary ore from only two mines: the adjacent epithermal gold-tellurium vein deposits at Dashuigou and Majiagou in southwestern China, and the Kankberg deposit, Skellefte VMS district, Sweden. These deposits account for about 15 percent of annual tellurium production worldwide.
Because world supplies of both rhenium and tellurium depend on the supply of and demand for copper, let’s take a look at copper mining. According to the Advancing Mining post, Mining Copper: Meeting the Demand for the World’s Most Useful Metal, copper can be found in the earth in pure form, or in many different kinds of mineral deposits including porphyry copper deposits, which yield about two-thirds of the world’s copper. Large copper deposits of this type are found in mountainous regions of western North and South America.
Copper mining usually employs open pit mining techniques although there are also underground copper mining operations. Individual copper deposits may contain hundreds of millions of tons of copper-bearing rock. However, ore deposits are inconsistent, having high concentrations of metals in one area but much lower concentrations in other areas. The grade may be high at the surface, but diminish with depth, or vice versa. Geologists perform ore grade control to pinpoint the most profitable ore deposits and create models or maps of these locations so that they can make the most efficient and economical drilling and excavation decisions. Knowing how and where copper resources are deposited also helps target where undiscovered deposits may lie.
Field-portable x-ray fluorescence (FPXRF) instruments provide fast acquisition of geochemical data for mine mapping and ore deposit modeling. FPXRF is an established technique for easily determining elemental constituents for most natural low concentration samples, and it is now recognized as an effective analytical tool for high grade ore concentrates and grade control as well. Read Exploration & Mining of Porphyry Deposits Using Thermo Scientific Portable XRF Analyzers to learn about a case study involving the application of portable XRF in porphyry copper exploration
Once the copper deposit is identified, it must be extracted from the other minerals and rocks contained in the ore. The extraction process varies depending on the type of ore and degree of copper purity required but usually involves concentration, smelting, and refining. Copper must be completely refined and free from impurities to maintain its electrically conductive properties. When smelting copper, it is very important to understand the complex morphology of the various compounds in the raw material to improve the refining efficiency of each element. Read study results in which copper-compound raw material was evaluated by phase analysis using the multivariate statistical analysis of EDS spectral imaging data.