Impurity distribution and leaching behaviour of spent cement kiln refractories for recycling optimisation

Four spent cement kiln refractory bricks and residual materials from a recycling-oriented sorting process were analysed to identify spatial impurity distribution and leaching behaviour at natural and changing pH to optimise recycling processes. Results indicate variable contamination levels across brick cross-sections, with Cl consistently exhibiting lower concentrations near the kiln shell. The altered mineral assemblage was dominated by KCl and various sulphate phases. Element mappings indicate their preferred connection to Ca-Si-bearing phases. Leaching tests revealed high solubility of K+, Na+, Cl-, and S6+ across a wide pH range, with peak dissolution at mildly basic pH. However, while a pH of 5 could enhance the leaching of Cl-, Na+, and S6+, it would also significantly increase the leaching of valuable main components, posing a notable drawback. Conversely, the stabilisation of the pH at 11.2 during the elution with pure water effectively balanced impurity removal while preserving the integrity of the main refractory components. The findings provide insights into impurity behaviour and serve as a basis to optimise subsequent treatment and minimise the material load necessitating further treatment.