The composition of dregs is similar to ash but it is much more difficult to recycle because of its moist consistency and the heavy metals it contains.
As a part of UPM's Zero Solid Waste to Landfill project new, long-lasting processes of recycling green liquor dregs are being researched. The project's focus is on five of the most difficult side streams to recycle including dregs, ash, landfill waste, sandy bark and sludge. Out of the five green liquor dregs is the second largest side stream after ash.
UPM Kaukas mill in Lappeenranta for example produces almost 10 000 tonnes dregs annually. From this, half has been used in soil construction projects and the other half has been transported to landfill sites. Globally UPM produces 50 000 tonnes of dregs annually.
"The better utilisation of green liquor dregs has been researched in many projects at UPM and in the forest industry in general. Its consistency makes it a difficult side stream to store even at a landfill where it has thus far been mixed with ash or rock material for proper landfilling. Mixing dregs with ash does create a material that is possible to use in soil construction, for example as the building material in field bases," says Harri Jussila, Manager, Waste and Chemicals at UPM Environment and Responsibility team.
The consistency and substance of dregs varies depending on the production site. Because of this a uniform and long-standing recycling process is not easy to create.
Dregs can be more efficiently utilised in energy production and soil construction projects
The UPM Kymi mill integrate acts as the Zero Solid Waste to Landfill pilot site. Since 2008 dregs have been driven straight to incineration during several trial-periods at the Kymi integrate. The ashes from these incinerations, have been used in soil construction projects and found suitable for the purpose. The process now unique to Kymi is hoped to be applicable at other sites as well.
In Pietarsaari, at the partly UPM-owned Alholmens Kraft power plant, the use of dregs to neutralise acidic gases and remove sulphur dioxide instead of lime injections has been piloted. To make use of dregs in this manner on a larger scale would require for dregs to be classified as a sidestream of production and not as process waste.
"With dregs the easy options have already been covered but there is still research to be done and options to look at. There are for example efforts to isolate fractions from dregs," describes Jussila. The volume of dregs created in production can also be cut down. UPM is now doing some research on an alternative method of separating dregs, in which caustic lime mud is not used in filtering. If this is successful the amount of dregs will be cut down by the amount of caustic lime mud previously used. The caustic lime mud can then be more efficiently recycled in other ways – in farming as liming material for example.
"If we intend to habit this planet for some time more we have to be much more careful in how we use our valuable fertilizing minerals. The amount of waste to landfill should be minimised on an industrial scale and also in one's personal life," concludes Harri Jussila.
Main picture: UPM Kymi mill integrate. The recovery boiler, where green liqour dregs are created, is the tallest building on the mill site beside the pipe.