Decarbonisation of calcium carbonate in sodium hydroxide solutions under ambient conditions: effect of residence time and mixing rates

Abstract

The decarbonisation of CaCO₃ is essential for the production of lime (Ca(OH)₂ and CaO), which is a commodity required in several large industries and the main precursor for cement production. CaCO₃ is usually decarbonised at high temperatures, generating gaseous CO₂ which will require post-process capture to minimise its release into the environment. We have developed a new process that can decarbonise CaCO₃ under ambient conditions, while sequestering the CO₂ as Na₂CO₃·H₂O or Na₂CO₃ in the same stage. Here, the effects of increasing stirring rates and residence times on reaction efficiency of the key reaction occurring between CaCO₃ and NaOH solution are studied. It is shown that the reaction is enhanced at lower stirring rates and longer residence times up to 300 seconds of contact between the reactants. The mass balance performed for Ca and CO₂ revealed that up to the 95% of the process CO₂ embodied in CaCO₃ was sequestered, with maximum capture rate assessed at nn moles CO₂ captured per second of reaction progress. A deeper insight into the precipitation of Na₂CO₃·H₂O or Na₂CO₃ under different reaction conditions was gained, and SEM-EDX analysis enabled the observation of the reaction front by detection of Na migrating towards inner regions of partially-reacted limestone chalk particles.