Investigation On Heavy Metals Removal In Desulfurization Sludge From Coal-fired Power Plants

A large number of desulfurization sludge with heavy metals exceeding the standard produced by coal-fired power plants cause serious pollution hazards to the ecological environment.At present,there are few reports on the removal of heavy metals from desulfurization sludge in foreign countries,and domestic research is still in the laboratory stage.In this paper,the desulfurization sludge of a coal-fired power plant in Guangzhou was studied,and its physical and chemical properties,heavy metals content and leaching toxicity were determined.The chelating agents of glutamic acid N,N-diacetic acid tetrasodium?GLDA?,polyaspartic acid?PASP?and polyepoxysuccinic acid?PESA?were used to extract heavy metals from sludge to study the effect of different concentration and pH value.to the removal efficiency of heavy metals and the change of species distribution.The stabilizers Na₂S,CaO,FeCl₃ and their combination were used to stabilize heavy metals in sludge to investigate the effects of the pH value and stabilization efficiency by the stabilizer types and stabilization time to stabilization efficiency,and analyze the stabilization ability of stabilizers and the changes of species distribution of heavy metals.The variation of sludge pH value,electrolyte pH value,temperature,current,heavy metal removal rate,species distribution,energy consumption and cathode product characterization were investigated during the process of GLDA,disodium edetate?Na₂EDTA?and tetrasodium iminodisuccinate?IDS?for enhancing the electric repair.Additionally the geological accumulation index method and potential ecological hazard index method were investigated to evaluate the pollution degree of heavy metals in sludge in detail.The main conclusions were as follows:?1?The As,Cd,Cr,Ni,Zn and Hg in the sludge exceeded the third-level standard of soil environmental quality,and the leaching concentrations of As,Cd and Hg exceeded the dangerous solid waste limit.?2?The chemical extraction experiments showed that the extraction rate increased with increasing the concentration.When the molar ratio was 10:1,the extraction rates was up 80.03%;The extraction rate decreased with the increase of pH,specifically,when pH was 2,the extraction rates was up to 86.12%.However,when pH was 6 the extraction rate decreased.to 75.39%.The acid exchangeable,reducible and oxidizable states of heavy metals were extracted easily,while the residual state was relatively stable after extraction.?3?The chemical stability experiments indicated that Na₂S+FeCl₃ had negligible impact sludge pH,and it had the best stability efficiency as well as the strongest stabilization ability.After stabilization,the residual state of heavy metals increased from 6.58%to 94.21%,while the acid exchangeable and reducible states of unstable state decreased to 5.79%.?4?Electrokinetic repair experiments showed that HNO₃ inhibited the anode pH reduction and cathode pH increase in enhanced electric experiments,simultaneously they resulted in higher current and temperature and improved heavy metals removal efficiency in the order of Na₂EDTA>IDS>GLDA in enhanced electric experiments.After the experiment,the acid exchangeable state,oxidizable state and the reducible state were significantly reduced,while the residual state were increased.Ion migration,water electrolysis and heating sludge consumed more energy.The cathode region mainly included the elements such as Cr,Ni,Zn,Cu,and the main substances were CaSO₄·2H₂O,CaCO₃,SiO₂.?5?The risk assessment analysis of heavy metals in sludge showed that Hg caused heavy pollution of the soil environment,Cd and As caused moderate pollution,while the others caused mild pollution.The potential ecological hazard index?RI?of sludge was 829,which indicated a high degree of ecological hazard.After treatment by three methods,the RI index of sludge were decreased to 16.18,5.54 and 3.78,respectively,which indicated a low degree of ecological hazard.Based on the economics and removal rate.This paper recommended that it is suitable to carry out relevant pilot study via chemical stabilization method or electric repair method.