The blast occurred this morning at Zhejiang Hongxiang Chemical Co. Ltd, in Shangyu City, the local authorities said.
Two of the injured are in a serious condition, they said.
About 70 firefighters and 22 fire engines were rushed to put out the blaze and prevent the flames from reaching a nearby warehouse containing 50 tonnes of inflammable chemicals, including sodium chlorate.
The disaster sparked widespread outrage over alleged safety violations and fears of pollutants contaminating the air and water.
Over 170 people were killed while hundreds others were injured in the Tianjin blasts which took place in August.
Another explosion at a chemical plant in Shandong killed one person and injured nine in the same month.