Process simulation of utilization of Co₂ and steelmaking slags to form precipitated calcium carbonate (PCC)

The increasing atmospheric carbon dioxide concentration has lead to concerns about global warming. One of the options that can contribute to the reduction of carbon dioxide emissions is the so-called CO₂ sequestration by mineral carbonation. Steel manufacturing which is one of the biggest industrial sources of CO₂ emissions could benefit from this option by utilizing its own by products, steelmaking slags to combine with CO₂. Additional benefit would be achieved if the end-product is a pure and marketable calcium carbonate. The objective of this study is to evaluate the feasiblity of the method we have recently invented: producing pure calcium carbonate from steel converter slag and to compare it to the other promising methods found in the literature. Aspen Plus simulation software was used to simulate the process. In the simulations a solid feedstock from steelmaking slag is mixed with a ammonium acetate and water to selectively extract calcium from the slag. The resulting solid-aqueous solution is pumped to a carbonation reactor where it reacts with injected CO₂ gas forming precipitated calcium carbonate CaCO₃ as a final product. If the used solvent is not recycled, 5.00 kg of steel converter slag and 7.75 kg of ammonium acetate would be needed in order to fix 1 kg of CO₂ according to model. Solvent recycling results with slight increase in slag consumption, but it is needed in order to minimize ammonium salt consumption. Heat required by the extraction reactor was found to be the most significant energy cost, but it could possibly be covered by the heat released from the carbonation reactor. Solvent recycling experiments were made using ammonium acetate, ammonium nitrate and ammonium chloride as solvents. Both calcium extraction efficiency and carbonation efficiency were somewhat smaller when recycled solvents were used. This seemed to be result of lost ammonia vapor. However, as long as electricity requirements of the separators, mixers, and ammonium recovery can be held below ~200 kWh/t CO₂ fixed, the invented method using ammonium salt as solvent should be competitive with other carbonation methods proposed.