(Adds CEO comment, background) Dec 21 (Reuters) - Rio Tinto Plc...

(Adds CEO comment, background)

Dec 21 (Reuters) - Rio Tinto Plc RIO.L (RIO) said on Tuesday it would buy the Rincon lithium project in Argentina for $825 million as the global miner builds its battery materials business.

Rio's purchase comes at a time when lithium is in strong demand globally as a vital ingredient in batteries for electric cars, while another expected mining product, borate, is used in solar panels and wind turbines.

"The Rincon project holds the potential to deliver a significant new supply of battery-grade lithium carbonate, to capture the opportunity offered by the rising demand driven by the global energy transition," Rio Chief Executive Jakob Stausholm said in a statement.

The Anglo-Australian miner said Rincon, a large undeveloped lithium brine project located in the heart of the lithium triangle in the Salta Province of Argentina, is being bought from Rincon Mining, which is owned by funds managed by the private equity group Sentient Equity Partners.

The direct lithium extraction technology proposed for the project has the "potential to significantly increase lithium recoveries" compared to solar evaporation ponds, Rio said, adding that a pilot plant was currently running at the site.

Rio committed $2.4 billion in July to a lithium project in Serbia, which is attracting protests by environmentalists saying it will pollute land and water.