Tellurium – solar lights metalloid future

Extremely rare, yet a vital ingredient to emerging solar panel technologies, tellurium is the epitome of what it means to be a critical metalloid, an element that possesses the properties of both a metal and non-metal.

"Most rocks contain an average of about 3 parts per billion tellurium, making it rarer than the rare earth elements and eight times less abundant than gold," the United States Geological Survey wrote in a 2015 report on this critical metalloid. "Grains of native tellurium appear in rocks as a brittle, silvery-white material, but tellurium more commonly occurs in telluride minerals that include varied quantities of gold, silver, or platinum."

Though this scarce element is seldom found in economic concentrations, one deposit in Alaska that also has abundant copper and rhenium hosts "an enormous tellurium endowment" and could provide a future source of this solar panel ingredient.

Solar drives growing demand

Traditionally, the small amounts of tellurium produced were used as an additive to improve the strength and pliability of steel, copper and lead alloys. This has changed with the advent of the cadmium telluride (CdTe) thin-film solar panels, which are much less expensive to produce than traditional silicon panels.

According to the USGS, roughly 40 percent of the tellurium used last year was made into cadmium telluride, a stable crystalline compound for these thin-film solar cells.

"CdTe solar cells are the second most common photovoltaic technology in the world marketplace after crystalline silicon, currently representing 5 percent of the world market," according to the U.S. Department of Energy. "CdTe thin-film solar cells can be manufactured quickly and inexpensively, providing a lower-cost alternative to conventional silicon-based technologies."

Due to the low cost of cadmium telluride solar cells, coupled with being less affected by dust, shading and high temperatures, this emerging technology is nabbing larger shares of the solar market.

Driven by the solar market, U.S. imports of tellurium increased 41 percent to 230 metric tons during 2018, accounting for more than half of all known world production.

As a result of the increased demand, tellurium prices nearly doubled during the first half of 2018, from about US$52 per kilogram at the onset to around US$100/kg by mid-year. The price has since settled, and tellurium was selling for around US$65/kg in early April of this year.

U.S. tellurium imports during 2018 came from Canada (66 percent), China (27 percent) and Germany (3 percent).

While U.S. tellurium production data is proprietary, it is estimated another roughly 60 metric tons was produced as a byproduct at one copper refinery in Texas.

In fact, almost all of the tellurium currently produced comes from copper refining and the USGS said this helps to reduce the geopolitical risks of sourcing this solar metalloid.

"Regardless, demands for tellurium, particularly as the global carbon footprint is reduced and solar energy technology becomes more widespread, are predicted to increase steadily over the next 15 to 20 years," USGS penned in its tellurium brochure. "The main concern surrounding tellurium supply is the question of whether or not global copper production can meet the growth in tellurium demand."

Enormous quantities at Pebble

If developed into a mine, the Pebble deposit in Southwest Alaska has the potential to provide an enormous domestic supply of tellurium.

While there has never been an official calculation of how much tellurium is contained within the Pebble deposit, there are indications that it is sizable. Work completed in 2013 estimates that 2.5 to 3 percent of the more than 100 million ounces of gold identified at Pebble is hosted in telluride minerals. This gold carrying telluride, in turn is hooked to chalcopyrite, which is the main copper ore mineral at the Pebble deposit.

"The majority of what is conjectured to be an enormous tellurium endowment at the Pebble deposit is potentially hosted in the structure of chalcopyrite and pyrite," USGS wrote in a 2017 report on minerals critical to the United States.

If these sulfide minerals contain telluride concentrations similar to other porphyry deposits, which provide much of the byproduct tellurium produced in the United States and the world, USGS said this could add up to a very large amount of this increasingly important metalloid within the Pebble deposit.

As such, whatever facility refines the copper concentrates from a future mine at Pebble could likely also produce tellurium.

Besides porphyry copper deposits such as Pebble, tellurium is found almost anywhere you find gold in Alaska.

In fact, tellurides are among the suite of minerals geologists look for when exploring for the large intrusion-related gold deposits Alaska's famed Tintina Gold Belt has become renowned for.

For the most part, these large deposits, such as Fort Knox near Fairbanks and Donlin Gold in Southwest Alaska, do not have economically viable concentrations of this critical metalloid.

High-grade orogenic gold deposits in Southcentral and Southeast Alaska, such as those found in the Willow Creek District north of Anchorage and Kensington Mine north of Juneau contain abundant telluride minerals.

Tellurium, though, is seldom considered an economic byproduct from this type of mine globally and only two mines on the planet produce this critical metalloid as primary commodity.

So, the world will likely lean on porphyry copper mines such as the one Northern Dynasty Minerals Ltd. hopes to develop at its Pebble deposit in Southwest Alaska for futures supplies of the tellurium that is expected to be needed to produce solar electricity in the coming years.