Utilisation aspects of ashes and green liquor dregs from an integrated semichemical pulp and board mill

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Kati Manskinen Name of the doctoral dissertation Utilisation aspects of ashes and green liquor dregs from an integrated semichemical pulp and board mill Publisher School of Chemical Technology Unit Department of Forest Products Technology Series Aalto University publication series DOCTORAL DISSERTATIONS 102/2013 Field of research Environmental Technology within Process Industry Manuscript submitted 31 January 2013 Date of the defence 19 June 2013 Permission to publish granted (date) 14 May 2013 Language English Monograph Article dissertation (summary + original articles) Abstract This thesis investigated the properties of bottom and fly ashes originating from a bubbling fluidised bed boiler (120 MW) using two different fuel mixtures (i.e. Fuel mixture A: coal, wood and peat; and B: wood and peat) and of the green liquor dregs originating from the associated semichemical pulp and board mill in relation to the potential utilisation of these residues from various aspects. The total concentrations of As, Cd, Cr, Cu, Ni, Pb, Zn and Hg in the bottom ashes were lower than the maximum allowable concentrations for these elements in forest fertilisers. The total Ca concentrations in bottom ashes A (2.4%; d.w.) and B (3.4%; d.w.) were lower than the legal requirement of 6.0% (d.w.) for ash used as a forest fertiliser. The total Ca concentrations in fly ashes A (6.4%; d.w.) and B (11.0%; d.w.) were higher than the minimum limit value of 6.0% (d.w.), but the concentration of As in fly ashes A (46.9 mg/kg d.w.) and B (41.3 mg/kg; d.w.) exceeded the maximum limit value of 40 mg/kg (d.w.). Only bottom ash B could be used as a forest fertiliser, provided some additional Ca is used. The bottom ashes both fulfilled the Finnish regulations on waste recovery in earth construction. Due to the elevated total concentration of PAH (23 mg/kg; d.w.) and extractable concentrations of Mo (3.9 mg/kg; d.w.) and Se (0.2 mg/kg; d.w.) in fly ash A, this residue cannot be used in covered structures. Due to the elevated concentration of PAH (90 mg/kg; d.w.) in fly ash B, this residue cannot be used in covered and paved structures. However, the utilisation of these residues as an earth construction agent is still possible, but an environmental permit would be required. According to the sequential extraction studies, extractable concentrations of most of the elements in the fly ash A were higher than those in the bottom ash A. The extractability of various elements, both in the bottom and fly ashes A, varied widely. Most of the elements did not occur as readily soluble and mobile forms in these ashes, which supports their utilisation instead of final disposal. The green liquor dregs were strongly alkaline (pH 11.7) and had a high neutralising capacity (34.2% Ca equivalents; d.w.). These results support the utilisation of this residue e.g. as a soil conditioning agent or pH buffer for acidic soils. The relatively low total concentrations of non-process elements in the green liquor dregs and low partitioning of these elements in the sequential extraction procedure support the re-use of this residue. However, if green liquor dregs are to be utilised, an environmental permit is needed. In terms of human health aspects, the careful handling of fly ash and green liquor dregs is recommended to prevent material dusting problems and the penetration of material particles across the human gastrointestinal tract. In order to promote the utilisation of ashes and green liquor dregs, efforts to develop Finnish legislation on waste utilisation is needed.