Rio Tinto is studying ways to extract lithium from waste rock at a mine it controls in California, making it the latest company trying to produce the battery metal in the United States for the fast-growing electric vehicle market.

The move by Rio comes as US politicians and regulators push to expand domestic mining of so-called strategic minerals used to make EV batteries and other high-tech equipment. China is the world's largest producer and consumer of many of these minerals.

Rio has produced borates - a group of minerals used to make soaps, cosmetics and other consumer goods - for nearly a century in the Mojave Desert, about 195 km north of Los Angeles.

That has left behind decades' worth of waste rock, known in the industry as tailings. Rio said it had been probing the tailings for gold and discovered lithium at a concentration higher than rival US projects under development, although the company declined to give the exact percentage.

"The material being used has already been mined, so this will be a low-energy option for the production of lithium," Bold Baatar, Rio's chief executive of energy and minerals, said in a statement to Reuters.

The company is spending $10-million to build a pilot plant that will extract the white metal using a heat-and-leaching process involving a kiln heated to 949 °C.

The pilot plant will only produce about 10 t annually. If that step is successful, Rio said it would consider spending $50-million to build an industrial-scale plant to make 5 000 tof lithium annually.

That would be roughly the same production capacity as a lithium brine project in Silver Peak, Nevada, controlled by, Albemarle Corp - the world's largest lithium producer.

Rio said its facility could eventually be the largest lithium producer in the United States, though it did not give any long-term output estimates.

If brought into commercial production, the Rio facility would make battery-grade lithium, the type of the metal sought by Panasonic Corp and other cathode producers, who in turn supply the battery part for use in Teslas and other electric vehicles.

Rio also controls Utah's Kennecott copper mine and a lithium deposit in Serbia that has yet to be developed.

Other companies developing US lithium projects include Lithium Americas, Standard Lithium, Texas Mineral Resources, Piedmont Lithium and ioneer, which is developing a Nevada lithium project also containing a large concentration of borates.