Carbon dioxide dissolution and ammonia losses in bubble columns for Precipitated Calcium Carbonate (PCC) production

The slag2PCC process concept for transforming steelmaking slag into a high quality precipitated calcium carbonate (PCC) with market value is developed towards commercial application (in Finland). This paper reports on R&D work on two features that significantly impact the overall performance of slag2PCC as a carbon capture and storage (CCU) technology and costs related to make-up chemicals. In this process, operating at (near) ambient conditions, calcium is selectively leached from steelmaking slag using ammonium salt solvent in water, followed by carbonation using a carbon dioxide (CO ₂ )-containing gas stream. Separators for removing spent slag and PCC from the solutions connect two reactors for extraction and carbonation, respectively, between which the solvent solution is cycling. Two process aspects will be reported on: One requirement is effective conversion of the CO ₂ fed to the system into PCC: since the dissolution of CO ₂ is the rate-liming process step it is essential to optimise its feeding rate so that release of unreacted CO ₂ is avoided. Mixing of the solutions enhances the mass transfer, which was analysed using high-speed video. Recordings were made around mixers located at various heights in a 2 m high, 13 cm inner diameter bubble column with mixers operated at up to 200 rpm speed, analysing CO ₂ bubble swarm dissolution. A second critical feature addressed here are losses of ammonia (NH ₃ ) from the solution. A gas outlet for unreacted feed gas presents a risk of release of NH ₃ vapour (in equilibrium with dissolved ammonia). This loss of solvent salt lowers the alkalinity of the solutions while adding costs. Multicomponent mixture mass transfer analysis showed that diffusion of NH ₃ into the gas feed may be significant at least during initial stage of dissolution and depends in a complex way on temperature, pH and dissolved chemical species concentrations. Experimental findings could be verified.