Dissolution of steel slag and recycled concrete aggregate in ammonium bisulphate for CO sub(2) mineral carbonation

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 (CaCO sub(3)) 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 CaSO sub(4) (s) for the following carbonation reaction. Dissolution studies using ammonium bisulphate (NH sub(4)HSO sub(4)) solutions were carried out at three different temperatures (25-50-90 degree C). For the steel slag sample, solid crystals of CaSO sub(4) 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 degree 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 NH sub(4)HSO sub(4) solution, as for steel slag, produced precipitated CaSO sub(4), while aluminium, the other main metal present, dissolved partially (40% at 25 degree 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 R super(2) at 25 degree C and 90 degree 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 CaS0 sub(4) on the surface of RCA samples.