Acid neutralisation capacity (ANC) of biomass ashes and its potential use for phosphogypsum leachate cleaning

Abstract

The production of acidic leachates from phosphogypsum (PGL) stacks is a worldwide problem, since high concentrations of pollutants (heavy metals, metalloids, sulphates, and phosphates, among others) can be released to the environment. PGL should therefore be treated before its discharge. The aim of this study was to develop a neutralisation procedure using ashes and slags from a biomass power plant to clean PGL, thus contributing to the circular economy. To assess the acid neutralisation capacity (ANC) of ashes and slags, the neutralisation curves with nitric acid, phosphoric acid, and PGL were performed after adding each waste type. Furthermore, the removal efficiency (RE) of the cleaning treatment for major and trace elements was obtained. The biomass ash demonstrated a greater degree of pollutant removal efficiency (around 100 % for many heavy metals), exhibiting a 20 % improvement over the slags in the neutralisation process. This can be attributed to the higher quartz content and lower specific area observed in the slags, which ultimately resulted in lower ANC. For the PGL neutralisation, the biomass ash had an ANC range between 7.37 mEq/g and 9.79 mEq/g of residue, and the RE for toxic metals was generally >90 %. Finally, when diluting PGL (1/10), the residue could clean all the elemental pollutants considered by the regulations.