PVC producer Hebei Shenghua Chemical Industry Co suffered a leak of vinyl chloride, which caught fire and caused a chain explosion that burned trucks and buildings, ChemChina said.
In the statement posted on its website, ChemChina apologised for the accident and said it reflected serious problems in its safety management systems.
Zhangjiakou, about 155 km (100 miles) northwest of Beijing, is set to host the 2022 Winter Olympics along with the capital.
The National Administration for Work Safety Emergency Response said it would launch an inspection of all large-scale chemical plant plants in the wake of the last explosion, the official Xinhua news Agency reported.
Public anger over safety standards has grown in China in recent years after three decades of swift economic growth have been marred by accidents ranging from mining disasters to factory fires.
In August 2015, 165 people were killed in a chemical warehouse explosion in the port city of Tianjin. The government found that the disaster was causes by improperly or illegally stored hazardous materials.
China has vowed to improve industrial standards, but environmentalists say they fear oversight weaknesses persist, including an opaque production process for hazardous chemicals.
(Reporting by Meng Meng, Aizhu Chen and Min Zhang; Editing by Tom Hogue and Alison Williams)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)