Experimental investigation and theoretical modeling on scale behaviors of high salinity wastewater in zero liquid discharge process of coal chemical industry

Zero liquid discharge (ZLD) treatment and reuse equipment of high salinity wastewater in coal-chemical industry often occur in various types of blockage problems because of high salt content, affecting the long-term stability of the device. In this study, the effects of solution temperature, steel, reaction time and wall roughness on fouling were investigated. The changes in the contents of fouling and fouling substances were qualitatively and quantitatively analyzed by XRD and EDS respectively, and the formation of scale was observed by SEM. The results show that with temperature increasing, Q235 steel is the most difficult to scale. Scaling rate of all salt scales reaches a maximum after 12 h, and the fouling rate decreases significantly from 12 to 48 h. It gradually stabilizes at 48 to 96 h. With the roughness increasing, the thickness of fouling layer increases, and a linear relationship is presented for 1 to 10 h. By comparing actual and simulated wastewater scaling rates, the relationship between actual and simulated wastewater scaling rates is y=ax^-0.494. The composition of the scale was analyzed, calcium carbonate is the main product and increases with fouling time. Based on the above-mentioned results combining literatures, the hybrid prediction model with calcium carbonate as the main product is put forward. It is discussed microscopically that calcium carbonate is converted from aragonite and vaterite in a thermodynamically metastable state to calcite in a thermodynamically stable state.