Tellurium: A Rare Element Behind Thin-Film Solar Cells

Tellurium (element 52, symbol Te) is one of the scarcest elements on Earth. Rocks average about 3 parts per billion, making it rarer than the rare earth elements and eight times scarcer than gold. It sometimes shows up as brittle, silvery-white grains of the native element, but more often in telluride minerals that carry varying amounts of gold, silver or platinum. It is a metalloid, with properties of both metals and nonmetals.

It was found in gold ores in Transylvania, Romania, in the late 1780s, and isolated as a distinct element fifteen years later, named from the Latin tellus, "earth". Historically it went into metallurgy — added to stainless steel and to alloys of copper, lead and iron.

Because there is so little of it, not much is known about its natural background levels or its toxicity to people and ecosystems. Exposure can cause garlic-smelling breath, nausea and, eventually, breathing problems.

What it is used for

• Solar cells. Its main use is thin films for photovoltaic cells. Alloyed with elements such as cadmium, it forms a compound with enhanced electrical conductivity, so a thin film can absorb sunlight efficiently and turn it into electricity. Cadmium telluride solar cells were first developed in the 1950s.
• Thermoelectric cooling and alloys. Added to steel, copper and lead alloys, it improves ductility and tensile strength, helps resist sulfuric acid corrosion and makes machines more efficient, especially in thermoelectric cooling.

Solar and thermoelectric uses together take more than two-thirds of world consumption. Tellurium also turns up in copying machines, as a colouring agent in ceramics and glass, as a vulcanising agent (including an additive that helps rubber withstand heat), in integrated circuits, laser diodes and medical instruments, and even in gasoline to stop engine knock.

Where it comes from

Most tellurium is mined indirectly, recovered as a byproduct from milled copper, iron and other base-metal ores that hold traces of tellurium minerals. The main sources worldwide are large, low-grade porphyry copper and copper-gold deposits, with tellurium recovered during copper refining: the anode slimes left at copper refineries typically hold 1 to 4 percent tellurium. Seafloor volcanogenic massive sulfide deposits can also yield a lot — the big iron- and copper-rich ore bodies of Russia's Ural Mountains are an important example.

Most gold deposits concentrate tellurium in telluride minerals and in sulfide minerals, and tellurides are important gold ores in giant deposits such as Cripple Creek, Colorado, and the Golden Mile in Western Australia. But the total tonnage of tellurium in gold deposits is small next to what comes from refining big base-metal deposits, so gold deposits, despite their high grades, are rarely seen as tellurium resources.

Only two mining districts in the world mine tellurium as a primary ore: the neighbouring gold-tellurium vein deposits of Dashuigou and Majiagou in southwestern China, and the Kankberg deposit in Sweden's Skellefte district. Together they supply about 15 percent of world production. Kankberg, in Västerbotten County, was first mined for copper and zinc sulfide; it now produces 10 percent of the world's tellurium, averaging 186 grams per tonne of ore. Small amounts have also come from a small gold deposit in northern Sonora, Mexico.

Supply and demand

Specific sources are hard to pin down, because refineries take in base-metal ores from mines in many countries and process them together. How much tellurium can be recovered depends on demand for its parent metal, copper. It is usually extracted during electrolytic refining of high-grade copper ores, a cost-effective process in which tellurium, gold, silver, platinum and other impurities collect on crude copper anodes and are then removed. Lower-grade ores are refined more cheaply by solvent leaching, which cannot recover tellurium — so a worldwide decline in high-grade copper ore could limit tellurium supply and make gold deposits a likelier source.

In the United States, demand is met by domestic production and imports. Nearly all domestic tellurium comes from the Asarco copper refinery in Amarillo, Texas, recovered from copper anode slimes and from skimming lead refinery ores mined in the western states. Much of the production data is proprietary, but the country is estimated to import about 50 percent of its tellurium, about 75 percent of that from China and Canada and smaller amounts from the Philippines and Belgium. The world's main producers are Sweden, Japan, Russia, China, the United States and Peru. U.S. reserves are about 15 percent of the global total of 24,000 metric tons, and on their own come to roughly eight times world annual production of 450 metric tons.

Securing future supply

Today's demand is met from anode slimes during electrolytic copper refining. Geopolitics is unlikely to threaten supply, because porphyry copper deposits are mined widely across the western United States and the world. Still, demand is expected to rise steadily over the next 15 to 20 years as solar power spreads and the world cuts its carbon footprint, and the central question is whether copper production can keep pace.

Options include:

• Better extraction from copper slimes, above today's 30 to 40 percent — especially as new copper recovery methods under consideration do not recover tellurium at all;
• Recycling solar cells, though tellurium films last a long time and have rarely been reused so far;
• Mining telluride minerals directly during gold mining, at places such as Cripple Creek, the Sierra Foothills of California and southeastern Alaska;
• The ocean floor, where tellurium is considered abundant in ferromanganese nodules, though undersea mining is still in its infancy.

Sources

• Goldfarb, R.J., 2015, Tellurium — The bright future of solar energy: U.S. Geological Survey Fact Sheet 2014–3077, USGS Mineral Resources Program. https://pubs.usgs.gov/publication/fs20143077
• Photo: U.S. Department of Energy, Western Area Power Administration, from the fact sheet's PDF. The sheet's other photographs — the CUORE experiment's crystal towers, a calaverite specimen (irocks.com) and the Kankberg mine (Boliden) — are not federal and are left out.
• Rewritten in hubnx's own words.