Optimization of leaching conditions for heavy metals decontamination of biomass fly ashes

Biomass (mostly wood) as CO2-neutral fuel is increasingly used for heat and combined heat and power generation. After incineration two ash fractions remain. Bottom ash (the coarse ash fraction) is usually used as soil enhancer for agricultural soils as it contains valuable nutrients (e.g. potassium) and is in general not so much contaminated by heavy metals. Fly ash (the fine ash fraction) on the other hand often contains higher amounts of heavy metals and therefore has to be disposed. In Austria the limit concentrations for arsenic, cadmium, chromium, copper, lead, nickel and zinc in the ashes for the utilization of biomass ashes on soils are given by the Austrian Ministry of Agriculture. It is known from previous studies that heavy metals are set free under acid conditions. In this study biomass fly ash from an Upper Austrian biomass incinerator has been leached with hydrochloric acid. The aim of the experiments was to reduce the heavy metals concentrations - especially cadmium, which is the element with the lowest limit and which exceeds the limits most frequently - below the limits for ash recycling on soils. After filtration of the leachate the heavy metals were precipitated with bases. For optimization of the process, different leaching conditions (pH, duration, solid to liquid ratio) and different precipitation agents were investigated. Inductively coupled plasma – optical emission spectroscopy (ICP-OES) was used for analysis as it has low detection limits (in the ppb range) and gives reproducible results. The results show that for leaching a pH-value of about 5 gives already sufficient results for the reduction of the cadmium concentration in the ash below the limit value. This could also be found for the other heavy metals.