PhD Final Defense – Vikram Kumar

Transforming Waste-to-Energy Ashes into Chlorellestadite-Based Carbonatable Binders and Sources of Critical Metals

Advisor: Associate Professor Nishant Garg

Abstract

Waste-to-Energy (WTE) facilities manage ~11% of the municipal solid waste generated worldwide via incineration. After incineration, ~25% of the input waste remains as ash. This ash is largely landfilled but can be a source of development minerals (e.g., calcite, portlandite, gypsum, and quartz) and technology-critical metals (e.g., REEs). These development minerals can be used as cementitious materials. However, their use as cementitious materials is limited by the presence of chloride-bearing species and heavy metals, which present corrosion and long-term leaching risks. This thesis addresses these limitations by introducing two distinct ash treatment protocols.

The first ash treatment protocol transforms poorly soluble chloride minerals in WTE fly ash to chlorellestadite (Ca10(SiO4)3(SO4)3Cl2). This transformation reduced chloride release from WTE fly ash in an alkaline cement-like environment by more than 2 orders of magnitude (from 1500 mg/l to <10 mg/l) while significantly reducing Pb leaching (from >5 mg/l to <0.5 mg/l), likely due to the crystallo-chemical incorporation of Pb into chlorellestadite. These observations demonstrated that chlorellestadite formation reduces corrosion and leaching risks.

The second ash treatment protocol is a simplified version of the first, with reduced and optimized treatment steps to maximize chlorellestadite formation. After this treatment, CO2-cured mortar samples prepared with 40:60 blends of treated ash and Type IL cement can achieve strengths (~50 MPa) comparable to those of a 50:50 blend of synthetic chlorellestadite and Type IL cement. These findings indicate that the CO2 reactivity of chlorellestadite formed during treatment can transform WTE ashes into carbonatable binders.

The ash treatment protocols introduced in this thesis can be further fine-tuned to enable simultaneous separation of technology-critical metals during thermal treatment. To identify ash feedstocks suitable for critical metal recovery, we validated the alkaline fusion digestion method using lithium metaborate and lithium tetraborate as fluxes for digesting two standard reference materials. The results suggested that lithium metaborate is a superior flux for alkaline fusion.

Overall, the findings from this thesis enable the transformation of WTE ashes into carbonatable binders and sources of critical metals, creating a closed-loop system that valorizes ash and sequesters CO2 sourced from the flue gas of a WTE facility.