Use of carbon dioxide to enhance the brine purification and flocculation performance of PAM flocculants

The “CO2–NaOH” brine purification method and flocculation performance of polyacrylamide (PAM) flocculants in vacuum salt making brine purification process are reported in this paper. The scale-forming trends of the brine were assessed by the Langlier index (LSI), the Ryznar stability index (RSI) and the Larson-Skold index (LR). The thermodynamic calculation results and PAM flocculant coagulation mechanism show that removal of Ca2+ and Mg2+ ions and suspended solids (SS) from brine by the “CO2–NaOH” method and PAM flocculants is feasible. The total concentration of Ca2+ and Mg2+ ions was decreased from 486.1 mg/L to 3.41 mg/L, and SS was decreased from 20 mg/L to 5 mg/L at the optimal experimental condition of CO2 flow velocity of 0.04 m3/h, injection time of 30 min, equilibrium pH of 11.5, and the dosage of the FL2949 and AN945SH PAM flocculants of 1 mg/L. LSI, RSI and LR show that the scale-forming was slowed down by brine purification. PAM flocculants were not retained in the brine as confirmed by the analysis of total organic carbon and chemical oxygen demand, and the X-ray diffraction, Fourier transform infrared, and scanning electron microscopy electron dispersive spectroscopy measurements for the flocs sludge during the brine purification process, results shown that the flocs sludge is calcite CaCO3. PAM flocculants purification was applied in industrial testing and achieved deep brine purification to remove scale-forming ions and SS. Additionally, it was found that the “CO2–NaOH” brine purification method promotes the utilization of CO2.