Evaluation of parameters controlling calcium recovery and CO2 uptake from desalination reject brine: An optimization approach

Abstract

A multistage process scheme for extracting valuable minerals from desalination reject brine while simultaneously sequestering CO2 has been proposed. As a part of the proposed scheme, simultaneous removal of calcium ions and CO2 uptake of a magnesium free reject brine was investigated in an inert particle spouted bed reactor (IPSBR) by reacting CO2 with reject brine in presence of NaOH. Response surface methodology was applied to optimize the effect of NaOH dosage, gas flowrate and salinity on the reaction with the target of maximizing calcium removal and CO2 uptake. A second order regression model was generated for both target responses. The predicted responses for both targets were in good agreement with the obtained experimental response. Based on a multivariate optimization, the optimum conditions for NaOH, flowrate and salinity for simultaneous calcium removal and CO2 uptake were found to be 5 g/L, 2 L/min and 75 g/L, respectively. Under these conditions, 100% removal of Ca2+ ions from the brine could be achieved with a CO2 uptake of 5.4 g/L. Moreover, the solid products obtained under the optimum conditions were characterized confirming the formation of high purity calcium carbonates. Based on optimized conditions, the proposed multistage process scheme promises to produce highly pure Mg(OH)2 and CaCO3, while sequestering 5400 tons of CO2 for every million tons of brine treated. The scheme can be further improved by incorporating an electrolysis unit to produce HCl along with NaOH that can be recirculated to eliminate the need for any external addition of NaOH, thus, creating a sustainable process for reject brine management and CO2 sequestration.