Influence of Organic Impurities on Fractional Crystallization of NaCl and Na2SO4 from High-Salinity Coal Chemical Wastewater: Thermodynamics and Nucleation Kinetics Analysis.

It is a valid path to realize the zero discharge of coal chemical wastewater by using the fractional crystallization method to recycle the miscellaneous salt in high-salinity wastewater. In this study, the thermodynamics and nucleation kinetics of sodium chloride (NaCl) and sodium sulfate (Na2SO4) crystallization in coal chemical wastewater were systematically studied. Through analyses of solubility, metastable zone width, and induction period, it was found that the impurity dimethoxymethane would increase the solid-liquid interface energy and critical crystal size during the nucleation of Na2SO4. Ternary phase diagrams of the pseudo-ternary Na2SO4-NaCl-H2O systems in simulated wastewater were plotted in the temperature range of 303.15 to 333.15 K, indicating that a co-ionization effect existed between NaCl and Na2SO4, and NaCl had a strong salting out effect on Na2SO4. Finally, the nucleation rate and growth rate of Na2SO4 crystals under simulated wastewater conditions were determined by the intermittent dynamic method, and the crystallization kinetic models of Na2SO4 were established. The crystallization nucleation of Na2SO4 crystals was found to be secondary nucleation controlled by surface reactions. The basic theoretical research of crystallization in this study is expected to fundamentally promote the application of fractional crystallization to realize the resource utilization of high-salinity wastewater in the coal chemical industry.

Biochemical Fulvic Acid Modification for Phosphate Crystal Inhibition in Water and Fertilizer Integration.

Biochemical fulvic acid (BFA), produced by organic wastes composting, is the complex organic matter with various functional groups. A novel modified biochemical fulvic acid (MBFA) which possessed stronger chelating ability had been synthesized by the grafting copolymerization of BFA and acrylic acid (AA). Results showed that MBFA effectively inhibited the crystallization of calcium phosphate and increased the concentration of phosphate in water solution. The optimum reaction conditions optimized by Box-Behnken design and response surface methodology were reaction temperature 69.24 °C, the mass of monomer to fulvic acid ratio 0.713, the initiator dosage 19.78%, and phosphate crystal-inhibition extent was 96.89%. IR spectra demonstrated AA was grafted onto BFA. XRD data and SEM images appeared the formation and growth of calcium phosphate crystals was effectively inhibited by MBFA.