Study On Solid-liquid Separation And Heavy Metal Removal For A Dual-alkali FGD System

During the process of dual-alkali Hue gas desulphuri/ation (FGD). it is necessary for the recycle of the absorption solution to separate suspended solids and remove heavy metal ions from the regenerated slurry; it is indispensable for practical utilization of FGD gypsum to separate suspended solids (SS) from the oxidated slurry; it is required for pollution abatement to remove heavy metal ions from FGD wastewater. The study above can be in favor of the water circulation in the FGD with well promotion and the FGD project with well development.Firstly, it was studied that hydrocyclone and flocculation were used to raise the removal efficiency of SS. The results showed that the removal efficiency of hydrocyclone performance was not acceptable due to the self-assemble of SS from the regenerated slurry. The SS concentration of the overflow from hydrocyclone could not meet the demand of the absorption solution. However, flocculation-sedimentation could improve its settling characteristic.Secondly, the top loading method was capable of producing a settling velocity curve. which made the settling characteristic of SS in the regenerated slurry quantified. For the case studied, while a basic pH enhanced the separation. a high ionic strength had an adverse effect, especially more than 0.035mol/L. Based on the method, fractal geometry was introduced in order to characterize highly irregular geometric shapes. It showed that the settling velocity was in proportion to the largest size of SS. On the other hand, the relation between the surface loading rate of the sedimentation tank and the flocculant's dosage could be predicted by computional fluid dynamics (CFD). The CFD results were in accordance with the experimental data and could be employed for designing the most cost effective sedimentation tank.Thirdly, hydrocyclone experiment studies on the oxidated slurry were made on relationships among adjustable parameters (input pressure, input concentration and some construction parameters of hydrocyclone). The results were that center piece in hydrcyclone could raise the input flowrate and the underflow percentage and promote the concentration effect. Dealing with the data gained from experiments with similarity theory, a series of equations were established for the relationships between adjustable parameters and separation efficiency. The equations could provide a foundation for the designing of separation operation.Fourthly, it took neutralization, sulfide precipation and chitosan flocculation to treat FGD wastewater. All of the treatments could remove heavy metal ions effectively and the removal efficiencies could achieve more than 95%. The occurrence of other cations in the solution had little influence on chitosan flocculation for heavy metal ions. The associations between chitosan and heavy metal ions, such as Mn²⁺ and Zn²⁺, were also verified. The pH investigation revealed that there were three stages for different actions: adsorption of chitosan for heavy metal ions, precipitation of heavy metal hydroxide and coprecipitation of heavy metal hydroxide and chitosan- heavy metal complex.Finally, the feasibility of coprecipitation of heavy metal and SS from the regenerated slurry by water-soluble chitosan was studied in a lab scale experiment. Based on this research finding, a composite flocculant of chitosan and polysilicic asid was developed to improve performance and achieve stability. The lab experiment showed that application of the composite flocculant could remove heavy metal ions efficiently and meanwhile improve the settling characteristics of SS from the regenerated slurry.The research work resolved some problem. It was possible to separate suspended solids easily and remove heavy metal ions efficiently from the regenerated slurry. The SS of the oxidated slurry could be separated effectively at the hydrocyclone performance. The treatment of heavy metal ions from FGD wastewater was improved. Based on above, a modified water circulation was proposed and had a potential utilization prospect for its flexibility with the dual-alkali FGD process.