Carbon dioxide dissolution and ammonia losses in bubble columns for precipitated calcium carbonate (PCC) production

The slag2PCC concept aims at transforming steelmaking slag into precipitated calcium carbonate (PCC) with market value. This paper reports on R&D work on two features that impact the overall performance and costs of slag2PCC as a carbon capture and utilisation (CCU) technology. Operating near ambient conditions, calcium is selectively leached from steelmaking slag using aqueous ammonium salt solvent, followed by carbonation using a CO₂-containing stream. Separators for removing spent slag and PCC connect two reactors for extraction and carbonation, respectively, between which the solvent solution is cycling. One requirement is effective conversion of the CO₂ fed to the system: while the dissolution of CO₂ is the rate-liming step it is essential to minimise release of unreacted CO₂. Mixing the solutions enhances mass transfer. High-speed video recordings were made around mixers located at various heights in a bubble column, analysing CO₂ bubble swarm dissolution. A second feature studied are losses of ammonia (NH₃) from the solution. An outlet for unreacted gas presents a risk of NH₃ vapour release, which lowers solution alkalinity while adding costs. Multicomponent mixture mass transfer analysis showed that diffusion of NH₃ into CO₂ bubbles may be significant at least during initial stages of dissolution. Experimental findings were verified.