Extraction of heavy metals from biomass fly ashes under acid conditions

Biomass (mostly wood and wood residues) is increasingly used for the generation of heat and electricity as it is a natural product, CO2-neutral and reduces the necessary amount of mineral fuels. The remaining ashes contain on the one hand nutrients (e.g. potassium) and on the other hand heavy metals (e.g. cadmium, lead). Bottom ashes (the coarse ash fraction) are usually not so much loaded with heavy metals and can therefore be used for agricultural soils and in forests. Fly ashes (the fine ash fraction) often contain heavy metal concentrations above the limit values given by the Austrian Ministry of Agriculture and therefore have to be disposed. In this study leaching of biomass fly ashes for the reduction of heavy metal concentrations has been investigated. Fly ashes from an Upper Austrian biomass incinerator have been extracted with different acids (hydrochloric acid, nitric acid, sulfuric acid, lactic acid, acetic acid, formic acid) at different pHs in order to evaluate the optimal conditions for the extraction of heavy metals. The aim was to clean the ashes at least as far as the remaining heavy metal concentrations are below the limits for ash recycling on soils. The focus was especially set on cadmium as it is the element with the lowest concentration limit due to its toxicity. Additionally, in fly ashes the cadmium concentration quite often exceeds the limit concentration. For the analysis, inductively coupled plasma – optical emission spectroscopy (ICP-OES) was used as it has low limits of detection (in the ppb range) and gives good reproducible results. It was found that cadmium can be leached with acetic acid and hydrochloric acid at moderate pH values in the range of 5. Advantages of moderate pH values are the low consumption of acid and subsequent neutralization agent.