Metal recovery from municipal solid waste incinerators fly ash by sulphuric acid leaching and bioleaching

Municipal solid waste incinerators (MSWI) ashes are being investigated in order to understand recovery opportunities with the final aim of diversifying sourcing and partly overcoming raw materials shortage. Turning waste into resource in the most efficient way is challenging, especially when secondary critical raw materials are the final objectives of recovery. The advances on bio- and hydrometallurgy are crucial for enhanced metals recovery and mitigation of environmental risks directly associated with the treatment of solid waste. Here we present preliminary results of bio- and hydrometallurgical experiments for critical metals recovery from MSWI fly ash and a comparison of the performance characteristics between acid leaching and acid bioleaching. Sulphuric acid leaching showed the advantage of reduction and mobilisation of Ca, thus improving the quality of the final product. Conversely, bioleaching experiments were performed in a mixed culture of T. ferrooxidans and T. thiooxidans and their efficiency was evaluated over a period of several weeks; reagents involved are lower than those needed for acid leaching thus making the process economically feasible and more environmental friendly. The influence of the experimental parameters will be discussed for both acid leaching and acid bioleaching procedures.