Effect of Accelerated Carbonation on the Leaching Behavior of Cr in Municipal Solid Waste Incinerator Bottom Ash and the Carbonation Kinetics

Abstract

The accelerated carbonation process with CO 2 absorption to confirm the leaching behavior of Cr in municipal-solid-waste incinerator (MSWI) bottom ash was investigated. This investigation was performed by placing samples in a CO 2 chamber. Only the temperature of the chamber at atmospheric pressure was varied and the CO 2 concentration was kept constant at 30% while the water-to-solid ratio was held at 0.3 dm 3 /kg. The result of an XRD analysis indicated that CO 2 could combine with portlandite, ettringite and hydrocalumite to form mainly an amorphous Al-rich material and calcite. Comprehension of the Cr leaching behavior during the carbonation reaction was gained by understanding the results obtained in this study. The carbonation kinetics includes a two-step process with a phase-boundary chemical reaction and diffusion through the product layer. As the carbonation process proceeds, the rate-controlling step was switched from a chemical reaction to diffusion by means of product layer control. The experimental data showed that the increase in the reaction temperature increased the carbonation rate. The carbonation kinetics of two types of mechanisms for varying temperatures was analyzed and an equation fitted to the experimental data was formulated. The variations of the rate constant with the temperature obeyed the Arrhenius equation with activation energies of 13.82 and 21.98 kJ/mol. Approximately 34 g/kg CO 2 was sequestrated in MSWI bottom ash with a particle size of less than 0.15 mm after a carbonation time of 120 min.

Effect of Accelerated Carbonation on the Mineral Phase of Alkaline Inorganic Waste

“…10, we could confirm that Cr-fractions exist in the inner porous layer of Ca-Al compounds before carbonation reaction, but new and irregularly-shaped substances with CaCO 3 and amorphous Al 2 O 3 are wrapped around Cr-fractions after carbonation reaction. This provides evidence of the morphological changes in the particles in what is termed the encapsulation effect 21,22 .…”

Effect of Cl Removal in MSWI Bottom Ash via Carbonation with CO₂ and Decomposition Kinetics of Friedel’s Salt

“…This structure consists of columns of {Ca 6 and AsO 2À 4 . 21,22) However, the important phenomenon 23,24) is that heavy metal ions substituted into ettringite can be easily released from the particle surface to the outside because the ettringite is easily decomposed by carbonation with CO 2 . In addition, the occurrence of volume change due to the carbonation with CO 2 in the bottom ash can be also expected.…”

“…However, it couldn't reach 100% because of the existence of other insoluble chlorides except Friedel's salt, such as sodalite, which is not easily decomposed with CO 2 . According to the literatures, 23,24) the quench products are the main alkaline inorganic materials in the reaction with CO 2 . These carbonation reactions of Friedel's salt (eq.…”

“…In addition, in Step 2, the carbonation reaction may be generally controlled by the solid-liquid heterogeneous system. 23,24) Thus, it can be expressed as the shrinking core model with three steps such as surface chemical reaction, diffusion through the product layer, and fluid film diffusion control. Because no fluid film covers the unreacted bottom ash particle as the reaction proceeds, there could be only surface chemical reaction and diffusion through the product layer, as follows.…”

Recent developments and perspectives on the treatment of industrial wastes by mineral carbonation — a review

“…Um et al [75] subjected municipal solid waste incinerator bottom ash (MSWI-BA) with particle size < 150 µm to carbonation using 30 vol% CO 2 with 0.3 L/S at different temperatures (20-40 • C), for the purpose of studying the leaching behavior of Cr. XRD results showed that portlandite (Ca(OH) 2 ), ettringite (Ca 6 Al 2 (SO 4 ) 3 (OH) 12 ·26H 2 O) and hydrocalumite (Ca 8 Al 4 (OH) 12 (Cl,CO 3 ,OH) 2− ·4H 2 O) disappeared from the material's composition after carbonation experiments.…”