The National Union of Metalworkers of South Africa has called for the shutdown of Fortune Steel in Nigel, outside Gauteng, following workplace injuries on Saturday night.
In a media statement the trade union said that at least seven workers were injured when a furnace exploded.
Numsa spokesperson Phakamile Hlubi-Majola said one of the injured was in a critical condition at a hospital in Alberton after he sustained extensive injuries to the head, chest and arms.
Hlubi-Majola claimed that Fortune Steel had tried to downplay the severity of the incident.
Fortune Steel did not immediately reply to a request for comment around what caused the injuries, the nature of the blast and its reaction to the union's call for a shutdown. Fin24 could also not connect to the number listed on its website on Monday.
This article will be updated when Fortune Steel responds.
According to its website, Fortune Steel is part of the Fortune Group, an Indian steel producer.
"This incident occurred less than 24 hours after Numsa members marched and protested against the company on Friday. Our members are fed up with Fortune Steel because it is an abusive company which acts with impunity," said Hlubi-Majola.