Selective Calcium Removal at Near-Ambient Temperature in a Multimineral Recovery Process from Seawater Reverse Osmosis Synthetic Brine and Ex Ante Life Cycle Assessment

Abstract

Potable water production from seawater generates brines that can produce stress in ecosystems, but they are also a potential source of metal and minerals. In our multi-mineral modular seawater brine mining process under development, calcium removal with minimal magnesium removal was the first stage. Even though calcium removal from reverse osmosis brine has been widely studied, there is no relevant research on its precipitation by carbonates at a near-ambient temperature (a range of 15–35 °C) and its selectivity over other minerals, as well as studies on operating conditions for selective precipitation considering the presence of antiscalants. We studied its reaction kinetics and equilibrium and conducted an ex ante life cycle assessment (LCA). The control of pH levels together with the Ostwald ripening process were very important factors to obtain a selective CaCO3 precipitation. The first-order average kinetic constant of the precipitation at 35 °C was 0.582 ± 0.141 h−1. The presence of minor ions and an antiscalant did not influence the precipitation, obtaining 85–90% on average for the %Ca2+ precipitation while the Mg2+ co-precipitation was lower than 5–7%. A lab-scale plant, tested in continuous (5 L/h synthetic brine) and in batch (15 L) modes, showed that the latter performs better and could be of interest at a larger scale. The ex ante LCA for the batch (100 L) showed that the main environmentally impactful factors were the thermostatic heating and the addition of a precipitant (Na2CO3), but these could be mitigated at the industrial level.