Hydrothermal treatment of MSWI fly ash for simultaneous dioxins decomposition and heavy metal stabilization

Abstract

Researches on the hydrothermal treatment of municipal solid waste incineration (MSWI) fly ash were conducted to eliminate dioxins and stabilize heavy metals. In order to enhance decomposing polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs) during hydrothermal process, a strong reductant carbohydrazide (CHZ) is introduced. A hydrothermal reactor was set up by mixing raw MSWI fly ash or the pre-treated fly ash with water and then heated to a pre-set temperature; CHZ was spiked into solution according to specially defined dosage. Experimental results showed that under the temperatures of 518 K and 533 K, the decomposition rates of PCDDs/PCDFs were over 80% and 90%, respectively, by total concentration. However, their toxic equivalent (TEQ) decreased only slightly or even increased due to the rising in concentration of congeners 2, 3, 7, 8-TCDD/TCDF, which might be resulted from the highly chlorinated congeners losing their chlorine atoms and being degraded during the hydrothermal process. Better results of TEQ reduction were also obtained under the higher tested temperature of 533 K and reactor with addition of 0.1% wt CHZ was corresponded to the best results. Good stabilization of heavy metals was also obtained in the same hydrothermal process especially when ferrous sulphate was added as auxiliary agent.

Hydrothermal solidification of municipal solid waste incineration fly ash

“…After washing, the peak intensities of these soluble salts decrease substantially. The phases of bottom ash are mainly quartz (SiO 2 ), calcite (CaCO 3 ), anhydrite (CaSO 4 ), and feldspar (R(AlSi 3 O 8 )) (R:Alkali metal) [12,13].…”

Hydrothermal treatment of incineration fly ash for PCDD/Fs decomposition: the effect of iron addition

“…The MSWI fly ash sample was the same as before [13], which was from a full-scale operating incinerator in southeast China, where a spray-dry flue gas scrubbing tower followed by a bag filter was adopted. The sample was well blended and stored in a freezer before experiments.…”

“…The sample was well blended and stored in a freezer before experiments. The total PCDD/Fs concentration of raw fly ash was 11,463.3 ng/kg and its I-TEQ was 628.8 ng-TEQ/kg [13]. It has an original iron content of 1.42%, so it was pre-treated to remove soluble iron by washing with water.…”

“…The hydrothermal process for incineration of fly ash [13] provides a relatively milder condition compared to high-temperature refractory and it is less energy consuming due to its moderate operating temperatures. Elimination of the PCDD/Fs contained in fly ash under hydrothermal condition was formerly explored by Yamaguchi et al [14] and they showed that PCDD/Fs would decompose when the hydrothermal reactions took place under 573 K for 20 minutes in 1 N NaOH solution containing 10% (vol/vol) methanol, and that toxicity of PCDD/Fs in the treated fly ash decreased from the original 18 to 0.03 ng-TEQ/g.…”

Fly Ash Pollutants, Treatment and Recycling

“…In order to enhance decomposing PCDDs/PCDFs during hydrothermal process, Xie et al (2010) studied carbohydrazide as a strong reductant at temperatures of 245 and at 260 °C and achieved by total concentration decomposition efficiencies with PCDDs and PCDFs higher than 80 % and 90 %, respectively.…”