The blue building rising up next to the recovery boiler of Metsä Group's Äänekoski bioproduct mill is home to a sulphuric acid plant that is unique even on the global scale.
“This is because – apart from a pilot plant that operated in Sweden for a brief period of time in the 1990s – sulphuric acid has not been produced in a pulp mill environment before,” says Jukka Kiuru, Project Manager for the bioproduct mill's recovery line.
The installation of equipment for the bioproduct mill's sulphuric acid plant has already been completed. The electrical and instrumentation installation work and the commissioning are currently underway. The plant will be completed according to plan in mid-June.
“The design and construction have been interesting and challenging, because, for example, the degree of corrosion resistance required from the materials and solutions is exceptional,” says Kiuru.
Significant environmental benefits
Thanks to the sulphuric acid plant, the bioproduct mill's chemical cycle will be more closed than before and emissions of sulphates into the waterways, for example, can be minimised.
“We will use the sulphuric acid produced by the plant in the bioproduct mill, so the need to transport chemicals by rail or road will also decrease considerably,” says Kiuru.
Thanks to the sulphuric acid plant, the bioproduct mill's chemical cycle will be more closed than before.
While sulphuric acid is a common and relatively inexpensive chemical around the world, the considerable environmental benefits make the plant's construction adjacent to the bioproduct mill sensible.
“This is why I believe that sulphuric acid plants will become more common in conjunction with pulp mills elsewhere as well.”
Sulphuric acid from malodorous gases
The sulphuric acid is produced from the malodorous gases originating from the bioproduct mill's recovery boiler.
“In the sulphuric acid plant, the gases are burned in the malodorous gas boiler. When the sulphur compounds burn, they give off sulphur dioxide (SO2), which is converted into sulphur trioxide (SO3) with the help of a converter. If you add water to that, you get sulphuric acid (H2SO4),” says Kiuru, summarising the process.
The need to transport chemicals by rail or road will also decrease considerably.
The sulphuric acid plant will have no impact to speak of with regard to the mill's odour emissions.
“The reason for this is that the cutting-edge recovery boiler alone will already make the bioproduct mill nearly odourless.”
Three possible uses
The plant will produce approximately 50 tonnes – one truckload – of sulphuric acid a day. This will be used in the bioproduct mill primarily in processing crude tall oil, in which the tall oil is separated from the soap, which will be taken from the evaporation plant.
The plant will produce approximately 50 tonnes – one truckload – of sulphuric acid a day.
“In the initial phase, we will be making a little over half of the sulphuric acid needed by the bioproduct mill ourselves, but the plant has been dimensioned in such a way that, in the future, the bioproduct mill will be self-sufficient in terms of sulphuric acid,” says Kiuru.
Once production gets into full swing, it will probably be possible to use the sulphuric acid in the production of chlorine dioxide as well.
“The third possible use has to do with lignin. If we start to separate it in the future, the process will require sulphuric acid.”