Owners Hanwha Chemical Corp said the blast happened during welding work to expand a waste water disposal facility.
"Gas from the waste water, which had piled up inside the tank, was apparently ignited by the welding torch", a fire official told AFP.
An investigation into whether negligence was the cause of the explosion is under way.
The plant makes polyvinyl chloride used in wide range of products, such as cling film.
At least four people have been killed in South Korea in an explosion at a chemical plant in the city of Ulsan.