Three-step precipitation process for calcium and magnesium recovery from brackish groundwater desalination concentrate

Abstract

Extensive studies have focused on precipitative recovery of magnesium from seawater desalination concentrate, but much less from brackish groundwater desalination concentrate (BWDC). The current work investigated the feasibility of a new three step process for complete Ca and Mg removal from BWDC, to enhance water recovery, while producing pure CaCO₃ and Mg(OH)₂ to offset treatment costs. Step 1 involved BWDC aeration softening, which was tested in a continuous crystallizer at different water and air flowrates, using intrinsic crystallization seed. Subsequent steps (2 and 3) involved NaOH or Na₂CO₃ addition to 24 h aerated BWDC under batch mode and precipitates recovery. Aeration energy estimation in step 1 indicated an exponential decrease from 2.25 to 0.25 kWh m⁻³_BWDC, and respective Ca removal decrease from 97 % to 60 % with respect to equilibrium. Ca removal in the complete process reached 95.6 % - 99.5 % while Mg removal was 91 % and 98 % when pH reached 10.7 and 11.2, respectively. CaCO₃ polymorphs were the only detected solid phases using NaOH in step 2, whereas CaMg(CO₃)₂ was also detected using Na₂CO₃. The precipitates recovered in step 3 contained 99 % Mg(OH)_2. Overall base consumption was 1.85 and 2.21 mols per mols of removed Ca and Mg, using NaOH and Na₂CO₃ in step 2, respectively. Hydraulic retention times of 1 – 2 h during step 1 was shown to potentially minimize overall process costs.