Deep desalination of ash water from coal to methanol gasification: Coupling fluidized-bed crystallization technology and rocking-chair capacitive deionization

This study demonstrates an innovative fluidized bed crystallization (FBC)-rocking-chair capacitive deionization (RCDI) integrated system for efficient desalination of high-hardness coal-to-methanol gasification ash wastewater. Garnet seed crystals were employed in the FBC unit to optimize crystallization conditions (60-80 mesh, pH 10.5, flow rate 70 m/h, 30 cm bed height), achieving 96 % Ca2+ and 81 % Mg2+ removal. The FBC effluent was further treated using RCDI with Fe2O3-activated carbon electrodes, where response surface methodology identified optimal parameters (1.78 V, 185 mL/min, 36 min adsorption) to attain 87 % Cl removal. The cascaded system achieved 85% total desalination efficiency, with the composite electrode exhibiting twice the capacity of commercial activated carbon. This work innovatively couples FBC with RCDI technology through a "chemical crystallization-hardness removal followed by electrochemical chloride elimination" mechanism. This synergistic approach overcomes the mutual interference between hardness ions and chloride encountered in conventional chemical precipitation methods, establishing a cascade purification system of "hardness removal first, chloride elimination later". The integrated system provides an energy-efficient and sustainable solution for large-scale desalination of high-hardness, high-chloride industrial wastewater.