Leaching Characteristics Of Heavy Metals In Mswi Fly Ash And Resource Recovery From Wastewater During Pre-treatment Process

Recycling of municipal solid waste incineration(MSWI) fly ash become a focus nowadays. Fly ash co-disposal in cement rotary kiln is one of main recycling methods in China. Water-washing is a method that is usually adopted as a pretreament method before recycling. The wastewater from water-washing pretreatment was characterised by high p H values and high concentrations of alkali chlorides and sulphates and it should be treated before discharged. Furthermore, find a quick test method to judge the toxicity of MSWI fly ash can improve the recycling efficiency. But the reasearch about above is still little in China.This reaserch was conducted in Shanghai solid waste treatment and dispose center. The relations between free Ca O and toxicity leaching behavior of heavy metals in MSWI fly ash were analysed. Different methods of removal heavy metals from fly ash water-washing wastewater were carried out, including using medical waste incineration flue gas, HNO3, Na2CO3 and PAM. And a pilot scale project was set up base on the best technological parameters established by the laboratory research above. The main results of this study are as follows:1.The characterization and evaluation of the MSWI leaching behaviorThe toxicity leaching characteristics of lead and zinc in MSWI was influenced obviously by the concentration of f-Ca O. Both metals showed high toxicity along with the depletion of neutralization capacity, while the concentration of f-Ca O < 6 % and the p H < 8, in the meantime,the toxicity was strengthened severely with the f-Ca O concentration decreased, the qualified rate of lead and zinc were 0-75 % and 0-94 %,respectively.When the f-Ca O concentration among 6-12 %, the leaching toxicity of both metals was unquilified due to the leaching capacity decreasing which was led by the occurred absoption and precipitation. While the f-Ca O concentration was over 12 %, the p H increased over 11, dissolvable hydroxide radical anion complexeswas formed, the quilified rate of lead leaching was among 0-33 %, and the leaching toxicity of zinc was unqualified.The leaching behavoir analysis was determined and approved while the threshold value of lead and zinc were 5ppm and 1ppm, respectively.In general, the characterization by TCLP indicated that the leaching toxicity of lead and zinc were correlated notably by the f-Ca O concentration and p H, while the total amout of f-Ca O and heavy metals were not relevant. Additionaly, the chromium could bonded intimately with the metallic compounds in the MSWI, and existed as Cr O42- or Cr2O72-, the concentration of f-Ca O showed no apparent linear relation with chromium toxicity.The leaching rate of chromium were below 5ppm, which was the limiting value.The f-Ca O concentration could defined efficiently by chemical titration, which could applied in the toxicity evaluation in 30 minutes.2. Key technology of carbonation for detoxify and recycling of heavy metal in wastewater from washing MSWI fly ash(1) Pb and Zn concentrations could be effectively decreased, and met discharging standard DB31/199-2009 by technologies, including acidification, coagulation-flocculation, carbonation with flue-gasof medical waste incineration and Na2CO3, etc. Comparing removal efficiencies of these technologies, optimum technological parameter were obtained for detoxifyof heavy metal in wastewater from washing MSWI fly ash: the wastewater was firstly carbonized by flue-gas when p H reached 8; then it was stirred after adding 4 g/L flocculant; finally 0.25 mol/L Na2CO3 was added. As a result, removals of Pb and Zn could reach 100 % and 91 %, meeting standard DB31/199-2009. Moreover, concentrations of heavy metals obviously fluctuated, since the source and component of fly ash was quite complicated. When Pb and Zn concentrations were relatively high in the wastewater, same removals above could be got by prolonging carbonation time, enlarging the input amounts of flocculant and Na2CO3.(2) Pilot demonstration experiment showed that p Hs and heavy metal concentrations of all wastewaters met the national discharging standard for wastewater, after being treated by carbonation and flocculant. In this technology, the heavy metal wastewater produced by washing fly ash was safely disposed and recycled by processes of acidification, carbonationwith flue-gas of medical waste incineration and multiple-effective evaporation. At the same time, according to the fact that solution after treatment was rich in contents of chlorine and calcium slat, multiple-effective evaporation was utilized to recycle high grade and amount of calcium chloride. The obtained material could be used as industrial salt after further purification.(3) There was 0.00023-0.0039 ng/kg dioxin in the wastewater after washing fly ash. Sicne there was no standard for dioxin in solid waste in China, relative standard of Japan was used. As a result, dioxin concentrations of all the treated waters met the discharging standard for industrial wastewater in Japan after wastewater purification and evaporating recycle. Specifically, the maximum concentration of dixoin was 0.0025 ng/kg,0.0027 ng/kg,0.0021 ng/kg in the discharging water after carbonation with flue-gas, evaporation at high and low temperature, respectively. Dioxin was lower than 0.0020 ng/kg in thewater of condensation. In the technology of carbonation with flue-gas, there was 29 and 22 ng/kg dioxins in the flue-gas precipation and evaporated solid. In the method of carbonation with Na2CO3, there was less than 2.3 ng/kg and 2.1 ng/kg dioxins in the precipation and evaporated solid.