Quantifying the structure and thermophysical properties of heavy metal-containing molten chloride salts derived from toxic MSWIFA for high-temperature heat storage

Abstract

Chloride salts extracted from hazardous municipal solid waste incineration fly ash (MSWIFA) are promising candidates for ultra-high-temperature heat storage with potential economic and environmental benefits. This study explores the structural transitions, transport performance, and thermophysical properties of heavy metal-containing molten chloride salts in the MSWIFA system (NaCl-KCl-CaCl₂-ZnCl₂). The results reveal that as ZnCl₂ content increases, the short-range order of molten salts decreases while the medium-range order enhances. At the long range, the disorder first increases before decreasing, governed by the interactions between ion clusters and their mixing degree. The structural transformation results in a linear increase in the average density from ∼ 1.44–1.58 g$\bullet$cm⁻³ at 1073 K. Self-diffusion coefficients for Na, K, Ca, Zn, and Cl ions are in the range of 5.94–7.70$\times$10⁻⁵, 7.26–8.35$\times$10⁻⁵, 2.31–3.45$\times$10⁻⁵, 2.45–3.28$\times$10⁻⁵, and 4.66–5.96$\times$10⁻⁵ cm²/s, respectively. The melt viscosity (∼ 1.04–6 mPa$\bullet$s) follows the N-type variation trend with turning points at 5 and 15 wt% ZnCl₂ and the thermal conductivity (∼ 0.308–0.463 W$\bullet$m⁻¹$\bullet$K⁻¹) follows the reverse N-type trend with turning points at 15 and 20 wt% ZnCl₂. Mass specific heat capacity is negatively correlated with ZnCl₂ content, ranging from 1.03 to 0.926 J$\bullet$g⁻¹$\bullet$K⁻¹, while the molar entropy is positively correlated with ZnCl₂ content, varying from 193 to 208 J$\bullet$mol⁻¹$\bullet$K⁻¹. The operating temperature window gradually narrows from around 410–303 K. These findings provide crucial insights for optimizing the design and performance of MSWIFA-based molten salt systems for recycling hazardous waste into useful resources making contributions to sustainable development.