Thermodynamic model of MSWI flue gas cooling path: Effect of flue gas composition on heavy metal binding forms.

In the context of circular economy and heavy metal (HM) recovery from municipal solid waste incineration (MSWI) fly ash (FA), detailed knowledge of HM binding forms is required for achieving higher extraction rates. The FA mineralogy is still poorly understood due to its low grain size and low metal concentration. To investigate the HM binding forms, a sophisticated thermodynamic reactive transport model was developed to simulate ash-forming processes. The stability of different binding forms was investigated at different flue gas conditions (varying ratios of HCl, SO2, O2) by simulating the gas cooling path in closed system and dynamic open system, where the gas composition is changing upon cooling due to precipitation of solids. The simulations predict that at flue gas conditions of molar ratio S/Cl < 1, Cu and Zn precipitate as oxides (and Zn silicates) at approximately 650°C. At temperatures <300°C, Zn, Cu, Pb and Cd are predicted to precipitate as easily soluble chlorides. In flue gas with molar ratio S/Cl > 1, the HM precipitate as less soluble sulphates. The results indicate that the less soluble HM fraction in the electrostatic precipitator ash represent oxides and silicates that formed in the boiler section but were transported to the electrostatic precipitator. The model provides insight into the physical-chemical processes controlling the metal accumulation in the flue gas and FA during the cooling of the flue gas. The obtained data serve as valuable basis for improving metal recovery from MSWI FA.

Speciation of iron(II/III) at the iron-cement interface: a review.

Steel is used as reinforcement in construction materials and it is also an important component of cement-stabilized waste materials to be disposed of in deep geological repositories for radioactive waste. Steel corrosion releases dissolved Fe(II/III) species that can form corrosion products on the steel surface or interact with cementitious materials at the iron-cement interface. The thermodynamically stable Fe species in the given conditions may diffuse further into the adjacent, porous cement matrix and react with individual cement phases. Thus, the retention of Fe(II/III) by the hydrate assemblage of cement paste is an important process affecting the diffusive transport of the aqueous species into the cementitious materials. The diffusion of aqueous Fe(II/III) species from the steel surface into the adjacent cementitious material coupled with the kinetically controlled formation of iron corrosion products, such as by Fe(II) oxidation, decisively determines the extension of the corrosion front. This review summarises the state-of-the art knowledge on the interaction of ferrous and ferric iron with cement phases based on a literature survey and provides new insights and proper perspectives for future study on interaction systems of iron and cement.

Porewater compositions of Portland cement with and without silica fume calculated using the fine-tuned CASH+NK solid solution model.

The CASH+ sublattice solid solution model of C-S-H aims to predict the composition of C-S-H and its ability to take up alkalis. It was originally developed for dilute systems with high water-solid ratios, and thus in this paper further optimized and benchmarked against measured pore solution compositions of hydrated Portland cement (PC) and PC blended with silica fume (SF) at realistic water-binder ratios. To get an improved agreement with the pore solution data, the stability of two CASH+ model endmembers, TCKh and TCNh, has been fine-tuned with standard Gibbs energy corrections of + 7.0 and + 5.0 kJ·mol-1, respectively (at 1 bar, 25 °C). The agreement was maintained with the experiments used to originally parameterize the CASH+ model for the uptake of K and Na in dilute systems. The K and Na concentrations predicted using the fine-tuned CASH+NK model are in a good agreement with the measured values for PC and PC + SF system at different water to binder ratios, silica fume additions, and at temperatures up to 80 °C. Supplementary Information: The online version contains supplementary material available at 10.1617/s11527-022-02045-0.