“We used the Äänekoski and Kemi bioproduct mills as the basis for our planning. In addition, we examined solutions in the mining industry, as the two industries create similar water fractions,” says Matti Toivonen, VP, Production Technology at Metsä Fibre.
Among other things, the project has looked for ways to reduce the salt discharge from production. The recovery boiler creates fly ash that contains salt, in this case sodium sulphate. Traditionally, fly ash is dissolved in water or condensate water and the resulting solution is transferred to the wastewater purification plant.
“We have carried out laboratory experiments and pilot- scale tests to figure out how to reduce the amount of salt load that ends up in the wastewater,” explains Toivonen.
According to Toivonen, a wastewater-free pulp mill is an ambitious goal. And the closer to the goal you get, the more challenging it is to achieve.
“It is interesting to concentrate on a task that at first seems impossible but may end up being achievable after all.”
The right water in the right place
Process water quality directly impacts pulp quality, for example in terms of its brightness and purity. Before being introduced into the process, raw water is screened through a rough sieve. The screening prevents fish or other objects from ending up in the plant’s water circulation.
The screened water is mechanically purified by filtering it through a layer of sand. Mechanically purified water can be used by the debarking department.
The mechanically purified water is chemically purified by adding a flocculation substance to clump the organic matter together. Chemically purified water is used mainly as process water in, for example, pulp washing.
Boiler water is water that has been treated in a desalination process. It is used in the recovery boiler and bark boiler, which have the strictest water quality requirements. The absence of salt reduces the risk of boiler corrosion.
This article was originally published in Fibre Magazine issue 2025.
