Modeling the formation of the quench product in municipal solid waste incineration (MSWI) bottom ash

• Formation of the quench product in MSWI bottom ash was investigated.
• Quench products are bound to bottom ash particles during discharging.
• Fine bottom ash particles controlled the formation of the quench product.
• The quench product consisted of a calcium-silicate-hydrate phase.
• Friedel’s salt and hydrocalumite were the main hydrate mineral phases of quenching process.

This study investigated changes in bottom ash morphology and mineralogy under lab-scale quenching conditions. The main purpose was to clarify the mechanisms behind the formation of the quench product/layer around bottom ash particles. In the experiments, the unquenched bottom ashes were heated to 300 °C for 1 h, and were quenched by warm water (65 °C) with different simulated conditions. After having filtered and dried, the ashes were analyzed by a combination of methodologies namely, particle size distribution analysis, intact particle and thin-section observation, X-ray diffractometry, and scanning electron microscope with energy dispersive X-ray spectroscopy. The results indicated that after quenching, the morphology and mineralogy of the bottom ash changed significantly. The freshly quenched bottom ash was dominated by a quench product that was characterized by amorphous and microcrystalline calcium-silicate-hydrate (CSH) phases. This product also enclosed tiny minerals, glasses, ceramics, metals, and organic materials. The dominant mineral phases produced by quenching process and detected by XRD were calcite, Friedel’s salt, hydrocalumite and portlandite. The formation of quench product was controlled by the fine fraction of the bottom ash (particle size <0.425 mm). From the observations, a conceptual model of the ash-water reactions and formation of the quench product in the bottom ash was proposed.