CO2 mineralisation of brines with regenerative hydrotalcites in a cyclical process

Abstract

CO2 mineralisation is a process that can store the CO2 as a solid mineral permanently. Brines rich in alkaline earth metals present an opportunity to trap CO2 in the form of insoluble carbonates. Moreover, this can be accomplished in this work, using flue gas concentrations of CO2 and at near-ambient temperatures, eliminating the energy-intensive CO2 capture step. Alkali or pH-buffering substances to neutralise the acidification of brines caused by CO2 dissolution is one of the primary costs that limits the technology deployment. Hence, we herein developed a process to remove chloride anions from brines by ion-exchange, namely hydrotalcites (HT), while avoiding the addition of alkali. These layered double hydroxides release hydroxyl groups in exchange for chloride, sufficiently raising the pH of brines by dechlorination to enable the precipitation of carbonates during CO2 mineralisation. Moreover, the HT is recyclable for multiple usage by taking advantage of the ‘memory effect’ property. Gaseous CO2 and Na2CO3 solution were tested as the recharging agents, to replace the chloride anions from the spent HT interlayers and intercalate with HCO3− or CO32−, followed by a calcination process to produce the reusable calcined-HT; the approach of gaseous CO2 regeneration is newly proposed. It was found that the chloride removal efficiency remains over 70% after multiple cycles, and calcium utilisation efficiency of the brine carbonation process can surpass 90%. This unique cyclical closed-loop HT process presents a potentially more cost- and energy-effective approach to brine carbonation than our previous studies. In addition, the precipitated calcium carbonates are of sufficient quality for a variety of applications that can reduce the process cost and accelerate the progress of CO2 capture, utilisation and sequestration.