The decarbonisation of calcium carbonate (CaCO3) to obtain lime (CaO) is a crucial step for a wide range of major industrial processes, most noticeably in Portland Cement (PC) production; from 1 tonne of CaCO3 approximately 0.44 tonnes of embodied CO2 is released to produce ~560 kg of CaO. With a global use of ~3 billion tonnes per year of calcined calcium carbonate, this makes the decarbonisation of CaCO3 one of the major contributors to global CO2 emissions. The CO2 released from CaCO3 alone accounts for ~6 % of global CO2 emissions.
In the conventional calcination process, the decarbonisation of CaCO3 occurs at temperatures above 950⁰C, resulting in the CO2 emissions both from CaCO3 and the combustion of the fuels necessary to provide the required energy. The CO2 released from CaCO3 represents the main challenge for the industrial sectors using CaO, as it is not replaceable. The thermal calcination of CaCO3 is currently considered unavoidable, and therefore, the release of CO2 from feed CaCO3 could not be avoided.
To address this, this project proposes a decarbonisation of CaCO3 at ambient conditions without combustion but by reaction with concentrated NaOH solutions. While producing Ca(OH)2 which may be used, for example to produce cement clinkers, the CO2 from CaCO3 is sequestrated within the stable by-product Na2CO3.xH2O (x=0, 1). Therefore, it would avoid both the combustion and process CO2 from the decarbonisation of feed CaCO3. The reduction of the process CO2 is particularly meaningful as it has not been achieved by the conventional high temperature decarbonisation of CaCO3. The by-product Na2CO3 can offer a safer alternative for CO2 storage compared with the geological CO2 storage. Alternatively, it could be either used to regenerate NaOH or as another commodity.
Firstly, reagent grade CaCO3 was used as reactant to identify the optimal conditions allowing for a maximised yield of products, Ca(OH)2 and Na2CO3 or Na2CO3·H2O.
CaCO_3+ 2NaOH + x H2O → Ca(OH)_2 +Na_2 CO_(3 ) xH2O
A wide range consisting of 71 starting compositions (CaCO3 + NaOH + H2O) was investigated, and the maximum yield of Ca(OH)2 with 96 % CaCO3 conversion was achieved in the system with 8.1 wt.% CaCO3, 37.2 wt.% NaOH and 54.7 wt.% H2O. Higher NaOH concentrations were generally enhancing the conversion of CaCO3, for a specific water-to-solid ratio.
Longer reaction times were found to increase the conversion of CaCO3 up to five minutes, beyond which the system indicated little improvement in production of Ca(OH)2. It was also discovered that, in batch conditions, the reaction was enhanced at a slower stirring speed.
The computational investigation also suggested that the activity of water in the system, highly dependent on the concentration of NaOH, decide the formation of either Na2CO3 or Na2CO3·H2O. This information helps to establish the extent of the reaction based on the balance between Na2CO3 or Na2CO3·H2O in the reaction products, in addition to providing essential knowledge for the subsequent separation process of the reaction products.
At short residence times, the reaction was in accordance with Arrhenius equation, reflecting its enhanced efficiencies at higher processing temperatures, within the mild range of 45 – 80 ⁰C. For a prolonged contact between the reactants, a higher temperature did not lead to significant improvements in terms of decarbonisation of CaCO3.
In the third part of the project, the reaction was tested using industrial grade materials with different characteristics, to assess the feasibility of the developed technology on a wider range of materials used in the real industry.
Limestone (96 wt.% CaCO3) and chalk (74 wt.% CaCO3), provided by the industrial collaborator of the project, were tested, and maximum CaCO3 conversion yields of 49 % and 79 % were recorded, respectively. This is likely linked to the smaller average particle size of the chalk compared with the limestone, resulting in an enhanced contact surface. The effects of impurities, such as silica within the chalk, were also investigated.
The results o demonstrated promising prospects of the process developed, since very mild conditions are required for the reaction to occur, i.e., short residence times, slow stirring speeds, and ambient conditions. The implementation of this technology could prevent the lime and cement industry from completely relying on the development of CCS technologies, offering a valid alternative to achieve a sustainable industry.
I strongly believe that the alternative route for CaCO3 decarbonisation proposed can have a great impact on the cement and concrete research community and industry worldwide, inspiring further investigation and eventual industrial application. The outcomes of my project suggest that, with a totally different approach to the issue known worldwide, the process CO2 could be avoided, as well as the technological complications linked to CO2 capture in the conventional processes
