Metals in MSWI fly ash - problems or opportunities?
Incineration of municipal solid waste is a commonly used management method to take care of our waste. However, the residues produced are a problem. They often contain large amounts of potentially toxic metal compounds and soluble salts, which can cause harm to the environment and human health if released from the ash. These ashes are therefore usually classified as hazardous materials and are deposited in specialized landfills. However, as society strives towards more sustainable material cycles, a larger fraction of the materials today classified as waste will, in the near future, be recycled. Since the ashes produced from waste incineration contain significant amounts of metals, they represent a possible source of these metals. Recovery of metals from waste combustion residues would thus give an opportunity to turn a waste into a valuable resource. This thesis focuses on the leaching and recovery of minor metals, such as Cu and Zn, and proposes a recovery procedure for Cu. The leaching of metal compounds from the ash is a very important step in the recovery process and several factors, such as leaching time, pH, leaching agent used and the liquid-to-solid ratio (L/S), affect the leaching properties. In some cases more or less all Cu was leached from the ash. Recovery of metals from ash leachates can be done using solvent extraction, and the results obtained showed that about 90% of the Cu in the leachates could be selectively recovered.
The ash matrix itself is highly affected by leaching, which generally increases the specific surface area and changes the particle size distribution. In landfill leaching tests the release of many ions from pre-leached ash was lower than that measured for the original ash, indicating a possibility to utilize the resulting ash as well.