Mobilisation and utilisation of metals from Victorian brown coal fly ash

Abstract

Yallourn fly ash is mainly rich in iron (Fe) and magnesium (Mg) and there is a potential to utilise this waste material by the exploitation of the metallic cations within it. Four valuable major metals: Fe, Mg, Ca and Si have been separated and expressed in the form of a variety of compounds: ferrihydrite, magnetite, magnesioferrite, magnesian calcite and silica gel, all of which are potentially useful compared to the original fly ash which is a valueless waste. Another key target of this research is to utilise the magnesium derived from this fly ash for mineral carbonation which can permanently be used to capture CO2 without any leakage because the resultant carbonates are thermodynamically stable. Chemically, this process relies on the use of magnesium (Mg) and calcium (Ca) to form stable carbonate compounds. The brown coal fly ash studied here is unique as it is rich in magnesioferrite (MgFe2O4) that forms the main matrix, with iron (Fe) being the most abundant element (~ 50 wt%). The relatively high amount of Mg (~ 28 wt%) renders it worthy to investigate the possible extraction of magnesium from this fly ash by chemical leaching, which can then be used in carbonation processes. The main challenge is the co-existence of Fe with Mg, which means that Fe will be extracted out simultaneously during the leaching process. The core challenge addressed here is to effectively mobilise Fe and Mg out of the magnesioferrite matrix in this unique fly ash. As Fe and Mg were strongly chemically bound in the fly ash matrix, the initial phases of this work entailed the testing of experimental leaching parameters using hydrochloric acid that led to greater Fe and Mg mobility. A key novelty of this research was the implementation of a two-stage leaching process that resulted in greater Fe and Mg extractions compared to just a single leaching stage. It was found that any sulphur (S)-bearing species inherently present in fly ash acts as a reductant for the reduction of solid Fe3+ into aqueous Fe2+, resulting in a greater mobilisation of Fe into the aqueous phase. This hypothesis was verified by performing the leaching of fly ash with the addition of S-bearing compounds. The kinetics of fly ash leaching was also investigated, which resulted in the development of a model that took into account the varying reactivities of metals composed of different elution affinities at a definite temperature range. Eventually, the Fe2+/3+ and Mg-rich leachate produced was pH-adjusted to produce several compounds with the potential to be precursors to value-added products. Finally, the extraction of Si and Al from fly ash was found to be achievable by leaching the pre-acid leached fly ash with sodium hydroxide.