Conceptual design of a CO₂ capture and utilisation process based on calcium and magnesium rich brines

Carbon capture and utilisation processes, CCU, are those processes aiming to produce usable products from CO₂-based materials. Although there are a large number of CCU alternatives, only a few are able to propose routes for the massive production of marketable CO₂ based products with a negative or neutral carbon footprint in the long term. In this work, a conceptual design of a process aimed to produce a cementitious construction material, based on magnesium carbonate trihydrate, also known as nesquehonite, is discussed and experimentally tested. Desalination brines are proposed as the source of aqueous magnesium. However, these brines contain a non-negligible amount of calcium, which interferes in the development of cementitious properties from nesquehonite. In order to avoid this interference, a multi-stage precipitation is proposed, where calcium carbonate precipitates in a first stage, and pure nesquehonite can be obtained in a second stage. A thermodynamic model, based on empirical observations from the precipitation system, is proposed and the technical feasibility of three process alternatives are evaluated against the outcomes from the model. The preferred alternative, from the technical and economic point of view, is to conduct a full absorption of CO₂ as aqueous carbonate, which is then split in two streams for both precipitation stages. The advantage of this alternative is the applicability to different types of brines, the probably faster and more economic absorption stage, and the easier control of the reaction conditions, as pH and temperature.