Chemical kinetics modeling and process parameter sensitivity for precipitated calcium carbonate production from steelmaking slags

A novel process for precipitated calcium carbonate production and carbon dioxide mineralization is studied. The process consists of two steps; firstly calcium is selectively extracted from steelmaking slags with an ammonium salt solution, secondly the dissolved calcium is allowed to react with carbon dioxide and is thus precipitated as calcium carbonate. Temperatures between 20 and 70 degrees C and atmospheric pressure are used. To gain information of the effects of different process parameters on the reaction rates in both process steps a kinetic modeling study is performed and validated with experimental results. Calcium extraction rate is found to mainly depend on the particle size and chemical properties of the slag. Concentration of the ammonium salt solvent has an effect as well, while the extraction temperature hardly affects the kinetics. Rate of carbonate precipitation depends mainly on carbon dioxide flow through the reactor, mixing of the solution, size of gas bubbles and also strongly on the concentration of ammonia in the solution. Since the steelmaking slag is a heterogeneous feedstock and it will require continuous adjustments on the process, the kinetic model is used to preliminary predict the reaction rates in extraction and carbonation steps in continuously working reactors. It is found that depending on the slag properties residence times starting from 1 h can be used in extraction. For carbonation, the model does not include changes in particle morphology, but regarding the overall calcium conversion 30 min would be a long enough residence time in the studied cases. Dividing the extraction in two or more stages would also shorten the total processing time.