At least five persons were killed and another was severely injured in an explosion on Thursday at an illegal chemical processing plant in China's Hebei province, authorities said.The explosion occurred around 11 a.m. in an orchard in the Donglizhuang Township in the city of Jinzhou, Xinhua news agency reported.Zhang Chaoyi, the head of the plant, remains at large, police said.The initial investigation revealed that the plant was producing dye intermediate, after traces of benzene, methylbenzene, naphthaline and anthracene were found, which were likely to be used as raw materials.Dye intermediate is used by the pharmacy, pesticide, and material industries.