Closing the kraft mill process brings about enrichment of non-process elements (NPE) in the liquor cycle. The main NPE are chlorine and potassium and they enter the mill through the wood, water and chemical intake. Chlorine and potassium cause corrosion, cracking and plugging problems mainly in the recovery boiler.
Traditionally, the problems have been solved by purging recovery boiler ash which, however, presents both environmental and economic considerations. To control the chlorine and potassium levels Valmet offers two different ash treatment technologies - ash leaching and ash crystallizing. Ash crystallisation often has better efficiency compared to ash leaching plants, but depending on the ash composition, this is not always the case. When treating large ash volumes, as at the OKI mill, the ash crystallization process is the preferred technology.
The ash crystallization is fully energy-integrated in the evaporation plant, optimizing the steam balances. The ash crystallizer plant delivered to the OKI mill was the first ever delivered by Valmet.
"We have long experience in ash treatment chemistry in pulp mills, but maybe it was somewhat brave to build the world's largest ash crystallization plant as our first delivery. Although we have many years of ash treatment experience through ash leaching, we made the strategic choice to design our crystallization process together with KBR, who has extensive experience with their Ecoplanning crystallization technology and many references from other industries," tells Hanna Karlsson, Manager for the Ash and Bio Technology team at Valmet.
"We have been able to prove our concept and keep the chlorine and potassium amounts at the right levels. We had some challenges along the way, as expected when doing something for the first time. We needed to adjust both the evaporation and ash crystallization processes but with our technical experts, working closely together with OKI's operations staff, we were able to solve the issues," Kai Lüder, Ash Treatment Product Manager, concludes.