Selective removal of calcium ions from seawater or desalination brine using a modified sodium carbonate method

Brine drained from desalination plants or obtained from underground is a valuable source of minerals. However, the presence of calcium ions in the brine makes its direct application uneconomical. Sodium carbonate can be used to selectively remove calcium ions from brine but is associated with drawbacks, including low calcium removal efficiency, slow settling of the precipitate, significant magnesium loss, and high reagent cost. In this study, the influence of the operating conditions on calcium removal was systematically evaluated. The experimental results revealed that temperature was the most important factor affecting calcium removal efficiency and calcium/magnesium selectivity. The optimum operating conditions were determined to include a reaction temperature of 85°C, an equimolar dosage of sodium carbonate, and a brine salinity of >56 g kg^–1. These conditions afforded a calcium removal efficiency of up to 85.4% with a magnesium loss of <6.7%. A pilot plant with a capacity of 120 m³ d^–1, based on static mixing/reaction and membrane distillation, was operated continuously for the decalcification and concentration of brine obtained from a desalination plant. The integration of the developed decalcification method, a brine-concentrating technology and separation technology such as nanofiltration would provide an alternative technique for exploiting brine.