Release of metals from grate-fired boiler cyclone ash at different pH values

Abstract

The concentrations of metals (Ba, Cr, Zn, Cd, Ni, Cu, As, Hg, Pb, Mo, Se and Sb) extractable from cyclone ash, obtained from a municipal district heating plant (6MW) incinerating forestry and sawmill residues, was studied as a function of pH (1.1-12.8) at a liquid-to-solid (L/S) ratio of 10 L/kg. The following main fractions were obtained: (1) metals (Ba, Cr, Zn, Cd, Ni, Cu, As, Hg, Pb and Sb) that were primarily extractable in extreme or strongly acidic conditions; (2) metals (Cr and Zn) that expressed amphoteric behaviour, i.e. they were extractable both in acidic (low pH) and alkaline (high pH) conditions, but were relatively insoluble at neutral pH values; and (3) metals (Mo and Se) which were primarily extractable only in alkaline conditions. The maximum extraction recovery (R) value (%) of the metals, which varied between 1.3 and 93.4%, indicates that most of the metals in cyclone ash are not easily soluble in the conditions normally found in nature.