Abstract
Municipal solid waste production is expected to increase globally and especially in less developed regions of the world. One of the most common ways to deal with waste is by incineration. Fly ash is a product of this incineration that can contain high amounts of heavy metals. This presents an environmental challenge due to their toxicity, but also an opportunity in extracting valuable metals. The most common way to do this currently is by acidic leaching using HCl. While other acids have been investigated, there is currently limited knowledge of the usage of methanesulfonic acid (MSA) to extract metals from fly ash.
This master’s thesis investigated the usage of MSA for fly ash leaching and compared it to HCl, the state of the art. Experiments investigated key leaching parameters such as acid concentration, solid/liquid (S/L) ratio, temperature, and time. The addition of H2O2 was also tested since MSA is a non-oxidative acid. Five ashes underwent leaching experiments comparing MSA to HCl and citric acid, an organic acid investigated in prior work. Experiments were also conducted for water washing of the ash as well as precipitation and cementation of metals from the leaching solution. Based on the results, the economic and environmental impacts of fly ash leaching were analysed.
MSA, especially when combined with H2O2, extracted similar amounts of heavy metals compared to HCl. Using 1/20 S/L, 40◦C, 2M MSA, and 1% H2O2, 99% of the Cu, 99% of the Cd, 80% of the Zn, and 60% of the Pb were extracted from the ash at 30 minutes. After 30 minutes, little change was observed with the exception of Cu, which in some cases leached out slower. Leaching efficiency improved when increasing acid concentration up to 2M, but little benefit was observed when further increasing to 3M. The optimal S/L ratio was 1/20. Temperature had little to no effect. Fly ash leaching using either MSA, HCl, or citric acid could contribute to detoxify the ash and enable new use cases. The economic analysis indicated that the value of the metals alone extracted does not make up for the material costs when using MSA as a lixiviant, however, potential new use cases of the residue may improve the economic outlook. Cementation was observed to be effective at removing metals from the leaching solution.
