Johannesburg, Feb 18 : South Africa's ArcelorMittal lost three employees after an explosion in their Johannesburg premises, said the company on Thursday. ArcelorMittal said a portion of a 90-m stack at one of the operation's coke batteries failed in the early hours of the morning, and fell onto the coke battery control room in which the three employees had been working, the Xinhua news agency reported. The company sent condolence messages to the families on Thursday morning. "The safety of our employees and contractors remains our primary concern. This is an absolute tragedy, and we will do a full and thorough investigation to understand what happened so that we can avoid anything like this from happening again at ArcelorMittal South Africa," said Kobus Verster, CEO of ArcelorMittal South Africa. He said the emergency personnel is working day and night to recover the remains of the three. The company would cooperate with investigators to find out what caused the incident. The National Union of Metal Workers of South Africa (NUMSA) called for full investigations into the cause of the three workers' death. "We call on the department of employment and labor to embark on a detailed and thorough investigation into the cause of the incident," said NUMSA regional secretary Kabelo Ramokhathali. /IANS