Congo's government is banning exports of cobalt and copper concentrates to force miners to process the raw minerals and increase the value of its exports.

A letter from Mines Minister Martin Kabwelulu obtained Friday gives mining companies 90 days from April 12 to offload unprocessed stocks.

The ban is sure to be disputed. Congo's biggest mining project, Freeport-McMoRan affiliate Tenke Fungurume, said its exports of copper cathode and cobalt hydroxide are allowed under its contracts — drawn up before the ban.

Congo is by far the world's largest producer of cobalt — 60,000 of the 110,000 tons mined worldwide in 2012 according to the U.S. Geological Survey — and is the eighth largest producer of copper, with production rocketing from 5,000 to 600,000 tons in the past decade.