Accelerated carbonation of waste paper fly ash by liquid process (NaHCO3) for stabilization of Ba and Pb

Abstract

The increase of energy valorization of paper sludge waste and biomass due to their high calorific value by incineration leads to an increase of waste paper fly ash (WPFA) quantities, which are in most cases considered as hazardous, and must be dealt with accordingly. The Alternative options for the management of WPFA, such as stabilization/solidification, vitrification or sintering, are not fully developed or either costly. In the present study, accelerated carbonation technology was used to reduce the hazardous nature of WPFA by improving the stabilization of metallic and metalloids trace elements (MMTE), especially Barium (Ba) and Lead (Pb) leaching. The accelerated carbonation of WPFA was found to be optimal at a water/solid ratio of 0.3 L/kg under controlled temperature and humidity conditions. The use of water saturated with 25% sodium bicarbonate (NaHCO3) improved the carbonation of WPFA and the stabilization of Ba and Pb. Batch leaching tests based on thermodynamic equilibrium was used to evaluate the solubility of the MMTE as a function of pH and at the natural pH of non‑carbonated and carbonated WPFA. Thermogravimetry, X-ray diffraction, scanning electron microscopes and energy dispersive spectroscopy analyses were performed to investigate the evolution of carbonation over time and its effect on the chemical and mineralogical properties of WPFA. After 30 days of accelerated carbonation, the leaching concentration of Ba and Pb was below the French legal limit. The leaching concentration of Ba and Pb from carbonated samples decreased 99% and 99.5% respectively. The leaching and release potential behavior of carbonated WPFA were further evaluated using the four-step sequential extraction procedure proposed by the Commission of the European Communities Bureau of Reference (BCR). The speciation of MMTE underwent significant transformation, shifting predominantly from the soluble fraction to the carbonated fraction as a result of the carbonation reaction.