There was a spark from the welding of a pipe igniting a substance that led to the deaths, according to news agency New China. Twelve firefighters were taken to hospital in connection with fighting the fire but should not have received life-threatening injuries.
Deadly workplace accidents are common in China, where, among other things, corruption makes arbetsmiljölagar far from always followed.
In the last month there was an explosion at a chemical company in the province of Jiangsu, and at least 78 people were killed while hundreds were injured.