Fire Chief Henri Manninen of Kainuu emergency services described the release as "serious".
"This was a really serious incident because during the brief release, concentrations of hydrogen sulphide were very high," explains Manninen.
In March 2012 a man working at the site of the Talvivaara mining complex died after inhaling hydrogen sulphide.
This time, there were no casualties.
According to the company, the leak was caused by a fault in the production process at the metals processing plant. No further details were immediately available.