Sulfate removal by Mg–Al layered double hydroxide precipitates: Mechanism, settleability, techno-economic analysis and recycling as demulsifier

This study investigated sulfate (SO₄ ²⁻) removal by Mg–Al hydrotalcite-type (HT) compound (Mg₆Al₂SO₄(OH)₁₆•mH₂O), and analyzed applicable condition of the method from techno-economic perspective. Under optimized condition of pH 8.80, molar ratio of Mg²⁺ and SO₄ ²⁻ (Mg/S ratio) of 6.63 and molar ratio of Al³⁺ and SO₄ ²⁻ (Al/S ratio) of 2.53, HT compound achieved a greatly lower concentration of SO₄ ²⁻ (3.76 mmol/L) than conventional gypsum precipitation. The composition and morphological analysis of precipitates showed that HT compound was transformed to Mg(OH)₂ at pH > 9.0, and Mg(OH)₂, Al(OH)₃ and MgAl₂(OH)₈ at Al/S ratio > 2.5. HT compound had better settleability than Mg(OH)₂ and ettringite, and was easier to be flocculated by anionic polyacrylamide to obtain a lower SV₃₀ of 16.4%. Turbiscan showed that HT compound had the highest instability and aggregation property among the three systems. Techno-economic analysis showed that HT compound coupled ettringite route was more economic than Mg(OH)₂ coupled ettringite route at Mg/S ratio > 6.08, but was not applicable when concentrations of Mg²⁺ and SO₄ ²⁻ were very low. The precipitated HT compound was recovered as demulsifier and showed more efficient COD removal than conventional demulsifier for chemical cleaning wastewater treatment.