A police officer says today's assault started at dawn with a suicide car bomber hitting the main gate of the plant in the town of Taji, about 20 kilometers (12 miles) north of Baghdad.
Then several suicide bombers and militants broke into the plant and clashed with the security forces. He added that 15 other troops were wounded.
A medical official confirmed the casualty figures. Both officials spoke on condition of anonymity as they were not authorized to release information.
The Islamic State group has recently increased their attacks far from the front lines in a campaign that Iraqi officials say is an attempt to distract from their recent battlefield losses.