The cause of the leakage was said to be a broken sulphur-feeder pump and an error of measurement, according to information on the company's blog posted on Monday.
No elevated level of the leakage was found outside, and neither was evacuation carried out in the surrounding areas.
Hydrogen sulphide is used in the process of metal precipitation.
On April 8, 2013, a leak also developed in a gypsum pond of a Talvivaara nickel mine in eastern Finland, forcing the company to shut down mining operations to prevent water pollution by the leakage.
A similar leak was found in November 2012, at a metal factory of the company.
