Multistage modified Solvay process based on calcium oxide for carbon dioxide capture and reject brine desalination

Abstract

Previous studies have investigated the overall performance of the modified Solvay process based on calcium oxide (CaO). In the modified process, calcium oxide (CaO) is reacted with brine salts and carbon dioxide. The results confirmed its effectiveness in capturing CO2 and managing reject brine in a single reaction. However, more attention is needed to overcome the low sodium (Na+) removal efficiency, which does not exceed 35 %. Therefore, the main objective of this research work to develop a novel technique where reject brine is passed through a total of seven stages. In each stage, a specific metal ion is recovered. In the first stage, a 100 % recovery of magnesium ions in the form of magnesium hydroxide solid was obtained by chemical reaction with ammonia (NH3) solution. Then, in the second stage, the ammoniated brine was treated based on the traditional Solvay process, where Na+ reduction, in the form of sodium bicarbonate (NaHCO3), and CO2 uptake reached almost 32.97 % and 37 g of CO2/1000 ml of treated brine, respectively. In the third stage, the electrocoagulation process (EC) was used to recover the sulfate ions (SO42-) in the form of pure calcium sulfate solid and also for the regeneration of NH3 as ammonium hydroxide leading to a total reduction of 96.5 % for SO42- ions. In the fourth stage, CaO was added to the treated brine according to the modified Solvay process. Reductions of 37.98 % and 27.97 % in Na+ and chloride (Cl−) ions were achieved, respectively. In stage 5, more recovery of NaHCO3 was attained by adding ammonium bicarbonate, while in stage 6, the treated brine mixture was passed through a second modified Solvay process. The CO2 uptake reached up to 60 g of CO2/1000 ml. In addition, Na+, Ca2+, K+, and Cl− removal was improved with 47.22 %, 56.33 %, 72.12 % and 34.41 %, respectively. In the last stage, the treated brine was inserted into the second EC process to recover CaCl2 solution and NH3 compounds. The overall reductions of for Na+, Ca2+, K+, and Cl− were 51 %, 93.59, 79 %, and 43.63 %, respectively. Moreover, solid products from each stage were characterized using SEM, XRD, FTIR, and Raman analyses. The products of each stage have diverse industrial applications.