Glencore declined to say how much acid was spilled from the tank, but said KCC immediately contained the spill and conducted follow-up monitoring of the environment, and there were no injuries to the workforce or contractors.

"At approximately 7pm on 16 March, KCC experienced a limited release of acid from Tank Farm 1 during maintenance work. There was no explosion," Glencore said in a statement.

"Follow-up with the community was conducted to advise of the event and our community officers have not registered any complaints nor concerns from their engagement with the surrounding communities."

Glencore's statement came after Congolese non-governmental organisation AFREWATCH said a tank containing sulphuric acid had exploded at KCC, causing acid to spill into nearby rivers, and called for an investigation.

Mines minister Willy Kitobo Samsoni told Reuters investigations are underway.

Glencore said KCC continues to engage with local authorities and communities regarding the incident, and hosted a site visit by the Department of the Environment last month.

KCC, which is 75% owned by Glencore subsidiary Katanga Mining with the remainder held by Congo's state mining company Gecamines, produces copper and cobalt near Kolwezi, the capital of Congo's Lualaba province.

KCC produced 23,900 tonnes of cobalt in 2020, up 40% in the previous year.

(Reporting by Hereward Holland and Helen Reid; Editing by Jonathan Oatis)