Critical Minerals Alaska – Rhenium
Enormous copper-gold deposit in Alaska contains enough of this superalloy metal to supply global needs for 40 years
Last updated 11/16/2018 at 5:16am
With a melting point of 5,756 degrees Fahrenheit and a heat-stable crystalline structure, rhenium is extremely resistant to both heat and wear. This durability makes it a vital element in superalloys used in jet and industrial gas turbine engines.
"The high-temperature properties of rhenium allow turbine engines to be designed with finer tolerances and operate at temperatures higher than those of engines constructed with other materials," the United States Geological Survey penned in an informational sheet on rhenium. "These properties allow prolonged engine life, increased engine performance, and enhanced operating efficiency."
While these performance enhancing properties make rhenium a vital ingredient to modern transportation, its extreme scarcity helps boost it onto the list of 35 elements critical to the United States.
According to the USGS, the average rhenium abundance in the earths crust is less than one part per billion, making it one of the rarest elements on the periodic table.
While extremely rare, economically viable concentrations of rhenium are often found associated with porphyry copper-molybdenum deposits and one such deposit in Alaska encompasses more rhenium than has been used globally for the past four decades.
Turbines drive demand
Roughly 52,000 kilograms, or 52 metric tons, of rhenium was produced worldwide during 2017. The United States is by far the largest consumer of this critical mineral, accounting for 42,600 metric tons, or nearly 82 percent, of last year's global production.
Nearly 80 percent of the rhenium consumed in the United States is used in superalloys vital to highly efficient jet turbine engines.
"The high-temperature properties of rhenium allow turbine engines to be designed with closer tolerances, thus enabling increased thrust and higher operating efficiency," the USGS explains.
The turbine engines in older generation U.S. military aircrafts, such as the F-15 and F-16 fighters, were made from a nickel-based superalloy containing 3 percent rhenium. The turbines in newer generation fighters, such as the F-22 and F-35, contain 6 percent rhenium.
The higher rhenium content alloys and limited supply pushed the price of rhenium to a high of US$10,600 per kilogram before the markets crashed in 2009.
The same restrictions on supply that drove prices skyward also prompted manufacturers to develop newer alloys that use less rhenium.
"The major aerospace companies ... were expected to continue testing superalloys that contain one-half the quantity of rhenium used in engine blades as currently designed, as well as testing rhenium-free alloys for other engine components," the USGS penned in its annual report, Mineral Commodity Summaries 2018.
The second most popular use for rhenium, accounting for about 10 percent usage, is in the production of high-octane, lead-free gasoline.
Catalysts made from platinum and rhenium improve the efficiency of refineries that make gasoline with higher octane.
Rhenium's high melting point and stability outside of a vacuum makes it a great ingredient for filaments used in open air. Camera flashes and mass spectrometers are among the common uses of rhenium filaments.
This high melting point – coupled with good wear resistance and ability to withstand corrosion – also makes rhenium a good metal for electric contacts and heating elements.
New lower rhenium content alloys have pushed the price of this critical metal down to around US$2,200/kg.
USGS, however, sees growing demand for rhenium in the United States.
"Consumption of catalyst-grade APR (ammonium perrhenate) by the petroleum industry was expected to remain at high levels. Demand for rhenium in the aerospace industry, although more unpredictable, was expected to continue to increase," USGS inked in its 2018 minerals report.
To sate its appetite for rhenium, the U.S. produced around 8,500 metric tons of this critical mineral domestically during 2017. Most of the balance was imported from Canada, Chile, Germany, Kazakhstan, and the Republic of Korea.
Four decades of rhenium
Most of the world's rhenium is produced from porphyry copper-molybdenum deposits. Molybdenite, which commonly contains between 100 and 3,000 parts per million rhenium, is the principal source of rhenium in these types of deposits.
While the overall rhenium concentrations in porphyry deposits tends to be low, usually less than 0.5 grams per metric ton, the large tonnage mined from this type of deposit makes it possible to recover economically viable quantities of the critical mineral.
The Pebble deposit in Alaska holds a particularly large store of rhenium on U.S. soil.
The Pebble deposit hosts 6.46 billion metric tons of measured and indicated resource averaging 0.4 percent (56.9 billion pounds) copper, 0.34 grams per metric ton (70.6 million ounces) gold, 240 parts per million (3.4 billion lb) molybdenum and 1.7 g/t (344.6 million oz) silver.
Calculations completed in 2011 estimates the measured and indicated resource contains roughly 0.45 g/t rhenium, which equates to around 2.9 million kilograms, or roughly US$6.4 billion, of the critical superalloy metal.
This is enough rhenium to supply the world's needs for more than four decades at 2017 consumption levels and does not account for the rhenium contained in the 4.45 billion metric tons of inferred resource outlined at Pebble.
USGS believes rich stores of rhenium at Pebble could be indicative of Alaska's larger potential for this superalloy metal.
"(R)ecent delineation of the giant Pebble porphyry copper-molybdenum-gold deposit in Alaska, which has estimated rhenium resources that represent more than 40 years of production at the current level of worldwide mine production, suggests that there is the potential for significant rhenium resources in undiscovered porphyry copper deposits in Alaska," the USGS penned in a 2018 rhenium report.
It takes a special process at the refinery, however, to recover a usable form of rhenium from the molybdenite.
"Rhenium is recovered from gases released during the roasting of molybdenite concentrates from porphyry copper deposits and of copper sulfide ores from sediment-hosted stratabound copper deposits," the USGS explains. "During the roasting process, rhenium is oxidized and passed up a flue stack with sulfur gases. Scrubbing of the flue dusts and gases produces sulfuric acid and other fluids that contain dissolved rhenium."
Most of the rhenium sold is in the form of ammonium perrhenate, a white powder that is precipitated from the solution scrubbed from the refinery flues.
Currently, there is only one refinery in the United States set up to recover rhenium. This roaster, which is located at Freeport-McMoran Copper & Gold's Sierrita facility in Arizona, processes byproduct molybdenite concentrates from the companies own mines as well as third party concentrates.
A second refinery capable of recovering rhenium is being developed. Combined, however, these facilities would only produce around 30 percent of domestic demand.
For the United States to take advantage of the rich deposits of rhenium found at Pebble and other domestic deposits, additional refineries capable of recovering this critical metal will need to be developed.
"The future supply of rhenium is likely to depend largely on the capacity of the specialized processing facilities needed to recover rhenium from molybdenite concentrates," according to the USGS.
Editor's Note: "Critical Minerals Alaska – Rhenium" is the tenth of a series of articles being published in North of 60 Mining News that investigates Alaska's potential as a domestic source of minerals deemed critical to the United States. At least 29 of the 35 critical minerals and metals identified by the U.S. Geological Survey – antimony, arsenic, barite, beryllium, bismuth, chromium, cobalt, fluorspar, gallium, germanium, graphite, hafnium, indium, magnesium, manganese, niobium, platinum group metals, rare earth elements, rhenium, rubidium, scandium, tantalum, tellurium, tin, titanium, tungsten, uranium, vanadium and zirconium – are found in Alaska.