Glencore held stocks of 12,797 tonnes of cobalt at the end of 2019
Its cobalt stocks at the end of 2018 stood at 3,769 tonnes.
"These stocks have been hanging over the market and held down prices," a cobalt trader said, adding that a lot of cobalt material destined for top consumer China was stuck in ports because people were in quarantine due to the coronavirus.
Glencore and Katanga declined to comment.
Sliding demand for cobalt from Chinese consumers in the electric vehicle battery supply chain saw prices on the London Metal Exchange crash more than 70% to $26,000 a tonne between May 2018 and August 2019. They are now at $33,300 a tonne.
Katanga's mining assets producing copper and byproduct cobalt are in the Democratic Republic of Congo, where Glencore's Mutanda mine is also located.
Glencore shut Mutanda last year. It produced 25,000 tonnes of metal last year or about 20% of the global total and is expected to be under care and maintenance for two years.
The Swiss-based commodity trader is due to report its financial results next Tuesday.
By Pratima Desai