Sasol reported a gas leak and related fire at its Sasolburg plant yesterday morning, denying allegations of an explosion.
But Dirk Hermann, a spokesperson for the Solidarity union, said: "There were people 7km away who heard an explosion.
"But whether it was an explosion or a fire is immaterial. The fact is it was a serious incident shortly after a huge fire at the Secunda plant on Saturday," said Hermann.
Sasol spokesperson Johan van der Rheede said the fire at Secunda had been caused by damaged electrical cabling and no people were injured.
Sasol has come under strong criticism for a series of explosions in the past year, which led to 14 workers dying. The worst was at the polymers ethylene plant in September, which killed 10 workers and injured 366.