The fire brigade in the western city of Krefeld dispatched around 130 first responders and reported four people with major injuries. A spokesman for the Krefeld-Uerdingen plant spoke of two seriously injured people.
Both sources said eight other people had been slightly hurt in the blast, which released titanium dioxide, a white pigment used in toothpaste, and nitrogen into the atmosphere.
Local authorities said there was no danger to the public and that they were still working to ascertain what set off the explosion. There was no immediate indication of foul play.