Senior officials rushed to the spot and are overseeing the relief and rescue operations.
According to Bijnor Superintendent of Police (SP) Umesh Kumar Singh, the incident happened at Mohit Petro Chemical Factory situated on the Dehat Marg in the morning.
Singh said six labourers died on the spot while injured were rushed to the district hospital where their condition is said to be critical. He said efforts are on trace the missing people.
As per information, the chemical tank had been leaking for the past few days. Today, while it was being repaired, there was a blast in the boiler while welding work was going on, Singh said.
Those killed were identified as Kamalveer, Lokendra, Ravi, Chetram, Vikrant and Bal Govind, an official said.