Dissolution of steel slag and recycled concrete aggregate in ammonium bisulphate for CO2 mineral carbonation

• Multi-step mineral carbonation processes are a promising technology.
• Concrete aggregate and steel slag were for the first time dissolved in NH4HSO4.
• Most of the Mg and Fe in steel slag dissolved while Ca produced crystals of CaSO4.
• Avrami kinetic model is valid for steel slag.
• Al in concrete aggregate partially dissolved and Ca precipitates as solid CaSO4.

Recent research on mineral carbonation has focused attention on multi-step processes which appear more attractive than single-step processes due to the higher purity of the final products. Recent studies on mineral carbonation have indicated that industrial wastes can require a lower degree of pre-treatment and less energy-intensive carbonation conditions, in comparison to mineral rocks. In light of this, a closed-loop, multi-step process which allows precipitation of calcium carbonate (CaCO3) from Ca-rich waste streams has been developed. The main objective of this work is to focus on mineral dissolution kinetics for steel slag and recycled concrete aggregate, to confirm the formation of CaSO4(s) for the following carbonation reaction. Dissolution studies using ammonium bisulphate (NH4HSO4) solutions were carried out at three different temperatures (25–50–90 °C). For the steel slag sample, solid crystals of CaSO4 were produced on the surface of the reacted particles of the starting material. Mg and Fe, instead, dissolved into solution and their efficiencies of dissolution, after 3 h at 90 °C, were 85% and 90% respectively. Kinetic analyses found that the Avrami model, which explains the diffusion process for systems in which crystallization occurs on the sample surface, can accurately represent the kinetics of steel slag dissolution. The calculated activation energy was 42 kJ/mol. Dissolution of recycled concrete aggregate into an NH4HSO4 solution, as for steel slag, produced precipitated CaSO4, while aluminium, the other main metal present, dissolved partially (40% at 25 °C) into solution. For the recycled concrete aggregate, the Avrami model fitted best the results from dissolution experiments. However, because of the low values of R2 at 25 °C and 90 °C, the Avrami model cannot be considered valid in terms of the kinetic of the reaction and further investigations are needed to corroborate precipitation of crystals of CaSO4 on the surface of RCA samples.