Energy and chemicals company Sasol said a gas leak during routine maintenance caused the blast at its ethylene plant at Secunda, east of Johannesburg, Wednesday morning.
"Sasol regrets to announce that five people lost their lives and many people were injured," Terry Bates, managing director of Sasol Polymers, told a news conference.
Police Captain Sibongile Nkosi and company officials said a sixth worker died later in hospital from injuries.
Of the more than 100 people injured, 32 were hospitalized, said Sasol spokesman Johann van Rheede. A roll call of the 500 workers at the ethylene plant had been taken, and he said it appeared some were unaccounted for, but he believed those were not involved in the blast.
Staff at Secunda's Highvelt Medi-Clinic, where the sixth victim died, said the 43 workers admitted to the unit suffered injuries from burns to fractures. Many were later discharged, but five were in a critical condition in intensive care.
Blast Heard in Town
"It sounded like an underground explosion; it could be heard in town five kilometers (three miles) away," another official said.
The sprawling complex, with 12,000 employees, is the world's biggest coal-to-synthetic fuel production facility.
Bates said the impact on polymer production was not yet known, but the incident would not affect energy operations.
"The ethylene plant is part of Sasol Polymers chemicals production and does not affect fuel production," he said.
There was just one explosion with no continuing fires, Van Rheede said. "Everything is totally under control."
Reporters were taken into Sasol's huge site but were not allowed near the accident scene.
The 5.5-square-mile site at Secunda produces olefins, alcohols, polymers, solvents, tar, nitrates, and methane gas. Ethylene, a colorless flammable hydrocarbon found in coal gas, can be used to promote the ripening of fruit.
Sasol is the world's biggest producer of synthetic fuel from coal. It markets chemicals and liquid fuels with 50 operations in the Americas, Australasia, Europe, and Africa.