The National Union of Mineworkers (NUM), which represents mine workers in South Africa, said it will ask for a double-digit increase in pay when negotiating with industry group Chamber of Mines in May, according to a Bloomberg News report.

While tensions between labor groups are intensifying, NUM Secretary-General Frans Baleni told Bloomberg, “We will try to conclude the negotiations before July 1,” he said.

Lonmin PLC, a producer of platinum group metals operating in the Bushveld Complex of South Africa, recently ended a six-week strike at Marikana last year by agreeing to pay increases for workers of 11 percent to 22 percent. Strikes in 2012 spread from platinum to gold and coal mines, costing Africa’s biggest economy 4.5 billion rand ($494 million), Bloomberg News reports.

South Africa’s NUM was founded in 1982 by workers including Cyril Ramaphosa, who went on to lead the biggest-ever strike in the country’s gold industry five years later. Now the richest black South African after Patrice Motsepe, Ramaphosa is a prominent figure in the business community.

Gold, coal and most of platinum producers’ wage agreements will expire at the end of June, Elize Strydom, senior executive for employment relations at the Johannesburg-based Chamber, told Bloomberg in an e-mailed statement.

“The AMCU indicated that it would reflect on the Chamber’s proposal and reasons given for the proposal and would revert formally,” Strydom said.

The union will be holding discussions with the Chamber on behalf of its 310,000 members, according to Baleni.