Evaluation of the leaching characteristics of wood ash and the influence of ash agglomeration

The release of mineral nutrients and other species from untreated and stabi lised wood ash has been investigated. Stabilisation is applied with the aim to modify the solubility of ash components and the ash particle size, i.e. to form dense ash particle agglomerates. This process induces the formatio n of several secondary minerals. The most important reaction is the transfo rmation of Ca(OH)(2) into CaCO3 which lowers the calcium leaching rate sign ificantly. A significant fraction of the alkali metals, K and Na, is presen t in salts which are rapidly released. The short-term release of these salt s, as simulated in laboratory experiments, was not reduced by the stabilisa tion methods applied. Generally. low leaching rates were observed for the i mportant plant nutrients P and ME as well as for Fe and other metals from b oth untreated and agglomerated ashes. Thermodynamic equilibrium modelling o f the hardening process showed that in addition to the transformation of Ca (OH)(2) to CaCO3, formation of the mineral ettringite is possible at a high pH. Experimental results have confirmed this. As the pH in the pore soluti on decreases during long-term leaching ettringite will be transformed into calcium carbonate and gypsum. In accordance with the experimental results, no formation of secondary solubility controlling potassium or sodium minera ls was indicated by the modelling results. (C) 1999 Elsevier Science Ltd. A ll rights reserved.