Dave and solar panels, Kennecott operations

Turning slime into solar panels

Our Kennecott copper mine is extracting a rare, valuable metal from waste


Last updated: 11 May 2022

Tellurium is one of the rarest elements on Earth.

And it’s an increasingly important one, because it’s used in many emerging technologies – like cadmium telluride (CdTe) solar panels, a highly efficient, rapidly manufactured, and lower-cost alternative to conventional silicon-based panels.

Tellurium is usually found in small, sparse rock deposits, making it difficult to mine at scale. But at our Kennecott copper operations near Salt Lake City, Utah, we’ve discovered a way to extract tellurium from an unlikely source – slime.

Extracting rare value from tailing streams

Copper smelting creates an anode-slime waste material that’s rich in other metals like silver, gold, selenium, and tellurium. Once these slimes are processed, the solution is treated to remove any remaining impurities, which are disposed of as tailings.

As early as 1990, our Kennecott team first tested the idea of extracting these precious metals and minerals from the slimes generated by copper refining. The idea was resurrected in 2012, as demand for renewable energy gained momentum. Then in 2019, when our team measured high enough concentrations of tellurium in the ore, we began building the tellurium circuit. More than 90% of tellurium is produced as a byproduct of copper smelting and refining, and Kennecott is one of two primary copper smelters left in the U.S.

Before the tellurium circuit was built, the waste from electrorefining copper would be treated with reagents and sent out to be disposed of. Now, after the copper ore is smelted and refined, we push the extracted waste slimes through a tellurium circuit which uses copper chips, high temperatures and agitation to produce copper telluride as a byproduct.

And while we know we have more to do to eliminate waste completely, Kennecott’s tellurium plant is the latest example of work we’re doing globally to minimise our waste by finding a use for every material we dig from the ground or creating new products from the waste itself. Some of our latest breakthroughs include producing lithium from waste rock at our Boron operations, scandium from titanium dioxide production, and anhydrite and Alextra from our aluminium operations.

The minerals and metals we produce are essential to accelerate the transition to renewable energy. We’re committed to using innovation to reduce waste in our production process and extract as much value as possible from the material that we mine and process.”

– Gaby Poirier, Kennecott’s Managing Director

Scaling up supply of future renewable materials

Given its natural scarcity, it’s no surprise that the global tellurium industry is still relatively small.

In 2021, just 580 tonnes of tellurium were produced globally, mostly in China – by comparison, 21 million tonnes of copper were produced in the same year. It is estimated that Kennecott will become the 6th largest producer of tellurium globally.

But scaling up production of tellurium is critical. While it’s used primarily to produce thin-film solar cells, it’s also used in a diverse range of other products like solar thermoelectric devices for both cooling and energy generation, copy machines, pigment in the glass and ceramics market, integrated circuits, blasting caps, laser diodes and medical instruments like x-ray machines. As an additive, tellurium improves machining characteristics, conductivity and improves heat resistance in steel, copper alloys, lead alloys and rubbers.

Kennecott’s tellurium circuit is designed to produce roughly 20 tonnes of tellurium as copper telluride a year – contributing around 3% of global supply, and a new North American supply chain for this critical mineral. Now that Kennecott can make a copper telluride product, we are investigating how we can recovery more tellurium from our ores.

Tellurium, Kennecott Operations
A copper telluride filter cake – the finished product– at the Kennecott Tellurium Plant. 

Looking towards the future, today

The importance of tellurium is only growing. From its improved extraction process to its impact on the economy and national security, the many applications of tellurium render it a critical element and important for us to invest in responsibly, as we work with producers of American solar. In partnering with 5N Plus and First Solar, our goals of sustainability are set to expand and improve.

We’re not stopping with tellurium. Rio Tinto is working with the U.S. Department of Energy’s Critical Materials Institute (CMI) and has partnered closely with CMI experts to discover further ways to economically recover critical mineral byproducts such as rhenium, and lithium. The company is also investing in new facilities to extract battery-grade lithium from waste materials at its Boron, California mine site and high-quality scandium oxide from waste streams at its metallurgical complex in Sorel-Tracy, Quebec.

Kennecott from the air

Creating more sustainable copper operations

Extracting tellurium from our copper operations’ waste is one way we’re helping to reduce our environmental impact while producing materials that are vital for a low-carbon future.

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But it’s important that we don’t make a bigger footprint during production than our products help offset when they’re turned into renewable energy assets like solar panels.

So we’ve set some ambitious targets to decarbonise our operations by 2050, and we know we have a lot of work to do to meet them.

At Kennecott, we continue to look for new ways to improve energy efficiency and increase our on-site-generated power supply. The $10 million, 6.2 MW combined heat and power (CHP) system – large enough to power 6,000 average-sized homes – is one example of how we are doing this: Generating enough power to supply up to 50% of the electrical needs of the refinery.

In 2019, after 75 years of operation, Kennecott retired its coal-fired power plant in Magna, Utah. Power for the operation will come from renewable energy certificates purchased from Rocky Mountain Power – primarily from wind and solar resources.

Closing the plant and acquiring renewable energy certificates will remove more than one million tons of carbon dioxide every year from Kennecott’s Wasatch Front operations, reducing its annual carbon footprint by as much as 65%.

The renewable energy certificate program is Green-e Energy certified and meets the environmental and consumer-protection standards set by the Center for Resource Solutions.