Improving settleability and dewaterability of Friedel's salt for chloride removal from saline wastewater

As an effective method for chloride removal from saline wastewater, Friedel's salt (FS) precipitation generates high concentration of precipitate requiring efficient solid-liquid separation. Effects of temperature, reaction time, polyacrylamides (PAM) type and dosage on settleability and dewaterability of FS precipitate were investigated to explore improving strategies for separation by using actual flue gas desulfurization wastewater. Raw FS precipitate was difficult to separate by gravity settling and mechanical dewatering owing to their small particle size and high solid concentration. Anionic PAM with higher hydrolysis degree and molecular weight greatly improved settleability and enlarged particle size of FS. The composition and crystal structure of FS varied with reaction temperature. Increasing temperature to 40 °C obtained the highest chloride removal, and improved settleability because layers spacing of FS decreased. FS precipitate was gradually broken down to Ca₃Al₂(OH)₁₂ and NaCl at temperature above 60 °C. Prolonging reaction time enhanced chloride removal by adsorption effects, and improved settleability by facilitating crystals growth and aging. The 0.5 m³/h pilot-scale experiment confirmed that the FS precipitation process was operated stably with improved settleability and dewaterability of precipitate by dosing PAM and elevating temperature. Techno-economic analysis showed that 37 °C was optimal for chloride removal and solid-liquid separation.