(91f) Elucidation of Change in Oxidation State for Metal Recovery from WTE Ash in Reactive Environments

Nearly half of the 2800 million tons per year of municipal solid waste (MSW) that is produced in globally is sent to landfills. Waste-to-Energy (WtE) facilities are the only viable alternative to this that matches the scale of waste produced, producing electricity and reducing the waste volume. The declining cost of electricity due to increased natural gas and renewable electricity generation means the WtE facilities need to go beyond energy generation. MSW contains valuable materials that must be recovered and incorporated back into the economy.

This work focuses on treatment of WtE ash with other waste streams like gypsum wallboard and spent Fluidized Catalytic Cracking catalyst (FCC) in high temperature and oxidizing environments to understand the change in the oxidation state of metals and their formation into more extractible forms. Currently spent FCC catalysts are classified as non-hazardous and amount to nearly 400,000 tons annually which are currently sent to landfills. Gypsum waste is estimated at 13 million tons annually of waste produced annually with only 2% recycled.

Therefore, the aim is to combine these waste streams in different proportions with the MSW generated in the WtE plants and treat it thermally and process it to recover valuable materials which are landfilled right now. The type of reactions that takes place are solid-solid and gas-solid between the ash and the additives at high temperatures forming metal oxides and different metal complexes. This leads to changes in the oxidation states of the metals and depending on the oxidation state, metal can be easy or difficult to extract.

The addition of gypsum into MSW ash has been shown to convert metals like Al, Mn, Mg, and Fe into more extractible phases. The phase change of Aluminum from crystalline to the amorphous, reduces the evolution of hydrogen gas from the ash-additive mixtures in an inert environment. This helps to reduce the deleterious effects of hydrogen gas caused in the landfills. Addition of spent FCC catalyst to the ash can lead to formation of various alumino-silicates compounds causing phase change of metals in ash and reduce slagging in the WtE plants. The characterization techniques utilized to examine the effect of additive to the ash are X-ray diffractometer, Energy dispersive X-ray, thermogravimetric and evolved gas analyzers.

This presentation will show comparative results for several conditions and discuss possible reaction mechanisms leading to the change in the oxidation state of the metals and the evolution of gases.