Influence of flue gas SO2 on the toxicity of heavy metals in municipal solid waste incinerator fly ash after accelerated carbonation stabilization

The influence of CO₂ content and SO₂ presence on the leaching toxicity of heavy metals in municipal solid waste incinerator (MSWI) fly ash was studied by examining the carbonation reaction of MSWI fly ash with different combinations of simulated incineration flue gases. Compared with raw ash, the leaching solution pH of carbonated ash decreased by almost 1 unit and the leaching concentrations of heavy metals were generally lower, with that of Pb decreasing from 19.45 mg/L (raw ash) to 4.08 mg/L (1# carbonated ash). The presence of SO₂ in the incineration flue gas increased the leaching concentrations of heavy metals from the fly ash to different extents after the carbonation stabilization reaction. The pH of the leaching solution was the main factor influencing the leaching concentrations of heavy metals. The increase in buffer capacity with the pH of carbonated ash caused an increase in heavy metal stability after the carbonation reaction. Accelerated carbonation stabilization of MSWI fly ash could reduce its long-term leaching concentrations (toxicity) of Cu, Pb, Se, and Zn. The leaching concentrations of heavy metals from carbonated ash also likely had better long-term stability than those from raw ash. The presence of SO₂ in the incineration flue gas increased the proportion of exchangeable state species of heavy metals; slightly increased the long-term leaching toxicity of Cu, Pb, Se, and Zn; and reduced the long-term stability of these metals in the fly ash after the carbonation reaction.     

Effect of water-extraction on characteristics of melting and solidification of fly ash from municipal solid waste incinerator

This study was conducted to investigate the effect of water-extraction process on the removal of major elements and heavy metals in the fly ash from Municipal Solid Waste Incinerator (MSWI), and their thermal stability in the following melting process. The water-extraction was first applied to extract soluble elements and heavy metals from the fly ash from MSWI at different liquid-to-solid rates (L/S) of 2, 5, and 10, respectively. The extracted fly ash and the raw fly ash were then melted at the temperatures of 1000-1350 degrees C in an electrically heated furnace. The results showed that the compounds of Ca, Na, K, and Cl achieved high removal rates of 30.7-72.8% at L/S=10, respectively, Cr was the most extractable heavy metal with removal rate of 12.3% among the several heavy metals tested. The water-extracted fly ash had better stability as compared to raw one, which was indicated by lower weight loss and better immobilization ability of heavy metals such as Zn, Cu, and Pb in the melting process. The results showed that combing water-extraction and melting process could provide one of the alternatives for treating MSWI fly ash in China for reutilization.     

Utilization of MSWI fly ash for stabilization/solidification of industrial waste sludge

This work investigated the potential for utilization of MSWI incineration fly ash as solidification binder to treat heavy metals-bearing industrial waste sludge. In the study, Municipal Solid Waste Incineration (MSWI) fly ash was used along with ordinary Portland cement to immobilize three different types of industrial sludge while MSWI incineration fly ash was stabilized at the same time. The results showed that the matrixes with heavy metals-bearing sludge and MSWI fly ash have a strong fixing capacity for heavy metals: Zn, Pb, Cu, Ni and Mn. Specimens with only 5-15% cement content was observed to be sufficient to achieve the target compressive strength of 0.3 MPa required for landfill disposal. An optimum mix comprising 45% fly ash, 5% cement and 50% of the industrial sludge could provide the required solidification and stabilization. Addition of MSWI can improve the strength of matrix. Meanwhile, the main hydration products of new S/S matrix are ettringite AFt, Friedel's salt and C-S-H. These hydration products play an important role in the fixing of heavy metals. The co-disposal of MSWI fly ash with heavy metals-bearing sludge can minimize the enlargement of the landfill volume and stabilize the heavy metals effectively.     

Sintering of MSWI fly ash by microwave energy

This study presents the sintering of municipal solid waste incineration (MSWI) fly ash assisted by microwave energy. The composition of fly ash was investigated by chemical sequential extraction and modified microwave digestion method. Effects of process time, container materials, aging time and salt contents were also discussed. The major elements of fly ash are Ca, Cl, Na, Si, K, Al, Mg, and Zn, and the metal species, Zn, Cr, Pb, Ca, and Cu, are mainly in the oxide phase. Under microwave processing, the fly ash was sintered into a glass-ceramics and the leaching concentrations of heavy metals were restrained. The stabilization efficiency increased with an increase in processing time in most of the cases. Better stabilization efficiency of fly ash was discovered by using the SiO(2) or Al(2)O(3) container than by using the graphite plate/SiC plate. The presence of salt in the fly ash could enhance the sintering and stabilization of fly ash. During the aging time of 0-30 days, negligible Pb in the sintered fly ash was leached out, and the leaching concentration was lower than the criterion.     

Electrodialytic removal of heavy metals from different fly ashes. Influence of heavy metal speciation in the ashes

Electrodialytic remediation, an electrochemically assisted extraction method, has recently been suggested as a potential method for removal of heavy metals from fly ashes. In this work, electrodialytic remediation of three different fly ashes, i.e. two municipal solid waste incinerator (MSWI) fly ashes and one wood combustion fly ash was studied in lab scale, and the results were discussed in relation to the expected heavy metal speciation in the ashes. The pH-dependent desorption characteristics for Cr differed between the two MSWI ashes but were similar for Cd, Pb, Zn and Cu. Thus, it was expected that the speciation of Cd, Pb, Zn and Cu was similar in the two ashes. However, in succeeding electrodialytic remediation experiments significant differences in removal efficiencies were observed, especially for Pb and Zn. In analogous electrodialytic remediation experiments, 8% Pb and 73% Zn was removed from one of the MSWI ashes, but only 2.5% Pb and 24% Zn from the other. These differences are probably due to variations in pH and heavy metal speciation between the different ashes. Cd, the sole heavy metal of environmental concern in the wood ash, was found more tightly bonded in this ash than in the two MSWI ashes. Approximately 70% Cd was removed from both types of ashes during 3 weeks of electrodialytic remediation, although the total concentration was a factor of 10 lower in the wood ash. It was suggested that complex Cd-silicates are likely phases in the wood ash whereas more soluble, condensed phases are dominating in the MSWI ashes.     

Electrodialytic removal of heavy metals from municipal solid waste incineration fly ash using ammonium citrate as assisting agent

Electrodialytic remediation, an electrochemically assisted separation method, has previously shown potential for removal of heavy metals from municipal solid waste incineration (MSWI) fly ashes. In this work electrodialytic remediation of MSWI fly ash using ammonium citrate as assisting agent was studied, and the results were compared with traditional batch extraction experiments. The application of electric current was found to increase the heavy metal release significantly compared to batch extraction experiments at comparable conditions (same liquid-to-solid ratio, same assisting agent, and same extraction time). Up to 86% Cd, 20% Pb, 62% Zn, 81% Cu and 44% Cr was removed from 75 g of MSWI fly ash in electrodialytic remediation experiments using ammonium citrate as assisting agent. The time range for the experiments varied between 5 and 70 days.     

Thermal treatment of the fly ash from municipal solid waste incinerator with rotary kiln

Reuse of the fly ash from the municipal solid waste incinerator (MSWI) is a policy of Taiwan EPA. However, the fly ash is often classified as a hazardous waste and cannot be reused directly because the concentrations of heavy metals exceed the TCLP regulations. The main objective of this study is to investigate the continuous sintering behavior of fly ash with a rotary kiln and seek a solution to reduce the concentrations of heavy metal to an acceptable value. The partitions of the heavy metals in the process are also considered. The results of TCLP showed that among the metals of Cr, Cd, Cu and Pb, only the concentrations of Pb in raw fly ash exceeded the regulation. At sintering temperatures of 700, 800 and 900 degrees C, the concentration of Pb decreased in sintering products, however, the concentration of Pb still exceeded the limitation at 700 and 800 degrees C. Additionally, the water-washing was used to pre-treat the fly ash before sintering process. The washing treatment effectively reduced the leaching concentrations of Pb to agree the regulations. Therefore, water-washing followed by a sintering treatment is an available process for detoxifying the fly ash of MSWI.     

Characteristics and heavy metal leaching of ash generated from incineration of automobile shredder residue

Bottom and fly ash collected from automobile shredder residue (ASR) incinerator have been characterized in terms of particle size, compositions, and heavy metal leaching by the standard TCLP method. Two alternative methods were also examined for the treatment of heavy metals in ASR incinerator ash from the aspect of recycling into construction or lightweight aggregate material. It was remarkable that the concentration of Cu was very high compared to common MSWI bottom and fly ash, which was probably originated from copper wires contained in ASR. As a whole, the results of characterization of ASR fly ash were in good agreement with common MSWI fly ash in terms of particle size, pH, and water-soluble compounds. It was clearly found that heavy metals could be removed thoroughly or partly from ASR fly ash through acid washing with dilute HCl solution so that the remaining fly ash could be landfilled or used as construction material. It was also found that the amount of heavy metal leachability of lightweight aggregate pellet prepared with ASR incineration ash could be significantly decreased so that the application of it to lightweight aggregate would be possible without pre-treatment for the removal of heavy metals.     

Evaluation of the distribution patterns of Pb, Cu and Cd from MSWI fly ash during thermal treatment by sequential extraction procedure

Municipal solid waste incinerator (MSWI) fly ash was frequently classified as hazardous materials as the metals' concentration of toxicity characteristic leaching procedure (TCLP) exceeded regulations. Many studies have focused on reducing the concentration of TCLP using thermal treatment and increasing the application of thermally treated slag. However, the metal patterns in MSWI fly ash with or without thermal treatment have seldom been addressed. The main objective of this study was evaluation of the distribution patterns of Pb, Cu and Cd from MSWI fly ash during thermal treatment by sequential extraction procedure. The experimental parameters included the form of pretreatment, the proportion of bottom ash (bottom ash/fly ash, B/F=0, 0.1 and 1) and the retention time. The results indicated that (1) In comparison to raw fly ash, the distribution patterns of Pb, Cu and Cd become stable in thermally treated slag. (2) Washing pretreatment caused the Pb pattern to become stable, while the influence on Cu and Cd were not significant. (3) The distribution patterns of Pb, Cu, and Cd became more stable as the retention time increased. (4) Adding bottom ash could make the distribution patterns of Pb and Cd more stable.     

Study on use of MSWI fly ash in ceramic tile

In this work, MSWI (municipal solid waste incineration) fly ash is used as a blending in production of ceramic tile by taking advantage of its high contents of SiO(2), Al(2)O(3) and CaO. Besides, macro-performance and microstructure of the product as well as its leaching toxicity in practical application were studied by means of XRD, IR and SEM analysis, and leaching toxicity and sequential chemical extraction analysis of the product. It is found that when 20% fly ash is added, the product registers a high compressive strength of 18.6MPa/cm(2) and a low water absorption of 7.4% after being sintered at 960 degrees C. It is found that the glazed tile shows excellent resistance against leaching, in accordance with HVEP stand, of heavy metals with Cd<0.0002ppm, Pb<0.0113ppm and Zn<0.0749ppm, and Hg below the low detection limit. These results show that heavy metals are cemented among the solid lattice in the product and can hardly be extracted. Leaching toxicity of heavy metals in the product, especially Hg, Pb, Zn and Cd, is substantially reduced to less than one-tenth of that in fly ash. In addition, specifications of Hg, Pb, Zn and Cd are largely changed and only a small portion of these heavy metals exists in soluble phases. These results as a whole suggest that the use of MSWI fly ash in ceramic tile constitutes a potential means of adding value.     

Stabilization of a chlorine-rich fly ash by colloidal silica solution

Fly ash from municipal solid waste incinerators (MSWI) consists of various substances, including a lot of heavy metals. In cases where fly ash contains a lot of chlorides, it is very difficult to apply general treatment methods because chlorides could hinder hydration in cementation and cause great loss in vitrification. In this study, we report a promising method for the treatment of fly ash containing a high concentration of chlorides. A colloidal silica solution was induced to stabilize the fly ash. The fly ash used in this research has a chlorine level over 35 wt.% as well as containing heavy metals, such as Pb (1120 ppm) and Zn (5430 ppm). The samples were prepared at a temperature of 600-800 degrees C for 2h after mixing with the fly ash and a 4 wt.% colloidal silica solution. The effect of the colloidal silica was evaluated by the leaching test, Toxicity Characteristic Leaching Procedure (TCLP). The solidified products with fly ash and colloidal silica at temperatures of more than 700 degrees C had an excellent resistance, with Pb(2+) <0.02 ppm, Zn(2+) <0.52 ppm, and Cd(2+), Cr(2+), Cu(2+), Mn(2+) <0.01 ppm, and it could be influenced by the phases containing Ca and Si formed above 600 degrees C and reduction of a chemically weak phase.     

Disposal of MSWI fly ash through a combined washing-immobilisation process

The objective of this work was to investigate the feasibility of a combined washing-immobilisation process as a means of optimising the disposal of fly ash resulting from municipal solid waste incineration (MSWI) in cementitious matrices. Two different types of Italian MSWI fly ash and an ordinary Portland cement (ASTM Type I) were used. Washing pre-treatment of fly ash with water always produces a wastewater that can be successfully treated by reducing the pH to values of 6.5-7.5. This treatment is capable of removing the detected contaminants (Al, Cd, Pb, Zn) through two different mechanisms: precipitation of aluminium hydroxide and adsorption of cadmium, lead and zinc ions onto floc particles of Al(OH)(3). Setting and leaching tests on cementitious mixes prove that the hazardous sludge produced from wastewater treatment can be completely mixed with washed fly ash and this mixture can be incorporated into cementitious matrices to a great extent (75 wt.% of total solid) without the risks of an unacceptable delay of cement setting and an excessive heavy metals leachability from solidified products. The better performance of the combined washing-immobilisation process as compared to the immobilisation process of unwashed fly ash may be ascribed primarily to the ability of the washing step in promoting the formation of hydrate phases that incorporate and/or convert heavy metal compounds into less reactive forms and, secondarily, to its ability of removing significant amounts of alkali chlorides and sulphates from fly ash. As a result, MSWI fly ash is transformed into a material that adversely affects cement hydration to a much lower extent than unwashed fly ash.     

Evaluation of assisting agents for electrodialytic removal of Cd,Pb, Zn,Cu and Cr from MSWI fly ash

Different assisting agents (0.25 M ammonium citrate/1.25% NH(3), 0.25 M Na-citrate, 2.5% NH(3), DI water) have been used for aiding the removal of heavy metals during electrodialytic treatment of municipal solid waste incineration (MSWI) fly ash. In this study, the effectiveness of the different agents was evaluated. The heavy metal speciation in solution was discussed and simulated at the different conditions using the geochemical equilibrium model Visual MINTEQ. The heavy metals examined were Cd, Pb, Zn, Cu and Cr.The 2.5% NH(3) solution was the best assisting agent for removal of Cd, probably due to formation of stable tetraammine complexes; whereas the best Pb removal was obtained with 0.25 M Na-citrate (Pb forms very stable chelates with citrate). The best compromise for removal of all five metals was obtained with the 0.25 M ammonium citrate/1.25% NH(3) solution.     

Leaching characteristics of slag from the melting treatment of municipal solid waste incinerator ash

This study investigated the composition and leaching characteristics of municipal solid waste incinerator (MSWI) ash and slag. The modified slags were characterized after the melting of MSWI ash mixtures at 1400 degrees C for 30 min. The ash mixtures were composed of different types of MSWI ash, including cyclone ash, scrubber ash and bottom ash, in various proportions. The results indicate that the Cd leaching concentration of the cyclone ash and the Pb leaching concentration of the scrubber ash reached 1.82 and 8.7 mg/L, respectively, which exceeds the ROC EPA's current regulatory thresholds, and can thus be classified as hazardous. The results of the analysis of the metal content and the leaching behavior of heavy metals, showed high concentrations of Cu and Zn, but a low leaching ratio of these metals. Concerning the characteristics of the modified slags, the X-ray diffraction patterns of the MSWI fly ash slag showed that it contained large amounts of glass. The toxicity characteristic leaching procedure (TCLP) leaching concentrations of the target metals of all the slags, met the ROC EPA's regulatory thresholds. The leaching concentrations of heavy metals in the F- and B1-slag were lower than those in the cyclone and the bottom ash, because there was a high amount of SiO(2), which formed a net-like structure in the bottom ash.     

Properties of MSW fly ash-calcium sulfoaluminate cement matrix and stabilization/solidification on heavy metals

In this paper, investigations were undertaken to formulate the properties of fly ash-calcium sulfoaluminate (CSA) cement matrix by blending MSW fly ash with CSA cement. The compressive strength, pore structure, hydration phases, and leaching behavior of Zn and Pb doped MSW fly ash-CSA cement matrices were determined by XRD, MIP, DSC, FTIR, EDX, TCLP leaching test and other experiments. The results showed that the addition of MSW fly ash to form fly ash-CSA cement matrix reduced the compressive strengths of matrices and made the pore distribution of matrices coarser, compared to that of pure CSA cement matrix. However, fly ash-CSA cement matrix could effectively immobilize high concentration of heavy metal such as lead and zinc with much lesser leaching of TCLP. Besides ettringite AFt, Friedel phase was a new hydration phase formed in the matrix. The formation of these hydration phases was responsible for huge reservoir of heavy metal stabilization by chemical fixing. Therefore, it could be postulated that MSW fly ash-CSA cement matrix was a potential new constituent of S/S matrix for high concentration of heavy metals such as Zn and Pb ions.     

Leaching behavior of heavy metals from municipal solid wastes incineration (MSWI) fly ash used in concrete

The characteristics of municipal solid waste incineration (MSWI) fly ash, surface leaching toxicity and successive leaching concentration of heavy metals from MSWI fly ash-cement hardened pastes were studied. And, the relationships between leaching concentrations of heavy metals and leaching time were also discussed. Experimental results showed that immobilization effect of cement on MSWI fly ash is good. Even if MSWI fly ash-cement hardened pastes were damaged, the leaching toxicity is still in a safety range. In early leaching stage, the surface leaching rate is relatively a little high, up to 10(-5)-10(-4)cmd(-1) order of magnitude, in the later time of leaching, its rate rapidly declined, down to 10(-7). Most of leached heavy metals are produced at early ages. The leaching concentration of heavy metals and leaching time has strong positive relationships. In factual utilizing circumstances, heavy metals' leaching from MSWI fly ash-cement hardened pastes is a very slow and gradually diluting process. The leaching toxicity of heavy metals is far lower than that of the National Standard of China, and minimum harmful matters can be contained and released in the environment. Reusing of MSWI fly ash as partial replacement for cement in concrete mixes is potentially feasible.     

Melting characteristics during the vitrification of MSWI fly ash with a pilot-scale diesel oil furnace

Treatment of municipal solid waste incineration (MSWI) fly ash is becoming an important issue in China. A pilot-scale experiment was carried out to treat MSWI fly ash by using a diesel oil furnace (DOF) for more than 6 months. The effects of melting temperature on volume reduction, weight loss, compositional changes, and toxicity of leach water for molten slag have been investigated and reported. Results indicated that the volume reduction fraction of raw fly ash (RFA) and washed-fly ash (WFA) was 75–80% and the weight loss fraction was 23.8–30% at 1260–1350 °C. During the vitrification, CaO, A1₂O₃, and SiO₂ percentages in fly ash increased as the temperature increased, especially for SiO₂, which was caused by both the decomposition of carbonates or sulfates and the volatilization of metal chlorides because the main components in secondary fly ash collected from fabric filter bags were NaCl and KCl. The leaching concentrations of heavy metals in molten slag were lower than the standard values of TCLP. The releasing levels of dioxin and other pollutants (such as SO₂, HCl, CO, NO_x, etc.) in flue gas were all lower than the Chinese standard.     

SEM/EDS and XRD characterization of raw and washed MSWI fly ash sintered at different temperatures

The disposal of fly ash generated during municipal solid waste incineration (MSWI) may pose a significant risk to the environment due to the possible leaching of hazardous pollutants, such as toxic metals. Sintering technology attracted more attention than the vitrification process because of its low energy needed. Generally, a preliminary washing treatment of raw fly ash with water was necessary for this sintering technology. This study investigated the composition and morphology of raw fly ash (RFA) and washed fly ash (WFA) at different sintering temperatures, and examined the newly formed minerals during sintering. Toxicity characteristic leaching procedure (TCLP) tests were carried out to investigate the effect of the washing treatment and sintering process on the leaching performance of heavy metals in fly ash. Results showed that, with an increase of sintering temperature more complex aluminosilicates were formed; the incorporation of Mg, Fe and Pb into the aluminosilicates occurred during the sintering process at higher temperatures (800 and 900 degrees C). The washing treatment reduced the leachable concentration of Cd, Pb and Ni, but increased that of Cr. A CaCrO(4) compound was considered as a potential soluble species.     

Emission of Pb and PAHs from thermally co-treated MSWI fly ash and bottom ash process

Municipal solid waste incinerator (MSWI) fly ash was regarded as a hazardous material because concentrations of TCLP leaching solution exceeded regulations. Previous studies have investigated the characteristics of thermally treated slag. However, the emissions of pollutant during the thermal treatment of MSWI fly ash have seldom been addressed. The main objective of this study was to evaluate the emission of Pb and PAHs from thermally co-treated MSWI fly and bottom ash process. The experimental parameters included the form of pretreatment, the proportion of bottom ash (bottom ash/fly ash, B/F=0, 0.1 and 1) and the retention time. The toxicity of thermally treated slag was also analyzed. The results indicated that (1) Pb emission occurred only in the solid phase and that PAHs were emitted from both solid and gas phases during thermal treatment process. (2) Washing pretreatment reduced not only the TCLP leaching concentration of Pb (from 15.75 to 1.67 mg/L), but also the emission of PAHs from the solid phase during thermal treatment process. (3) Adding bottom ash reduced the TCLP leaching concentration of thermally treated slag. (4) The concentration of Pb emission increased with retention time. (5) The thermal treatment reduced the toxicity of raw fly ash effectively, the inhibition ratio of raw fly ash and thermal treated slag were 98.71 and 18.35%, respectively.     

生物质与生活垃圾共气化过程重金属的迁移转化规律

为了研究流化床生物质气化协同处置生活垃圾衍生燃料过程中重金属的迁移转化规律,在湖北某循环流化床气化炉耦合燃煤发电厂进行掺烧试验。研究表明重金属主要赋存于飞灰和炉渣中。空白工况下86.2%的Cr赋存于飞灰中,13.3%于炉渣中;75.5%的Pb在飞灰中,23.8%在炉渣中;79.5%的As迁移至飞灰中,11.7%在炉渣中。RDF工况下75.8%的Cr迁移至飞灰,20.7%迁移至炉渣;44.6%的Pb存在于飞灰中,52%存在于炉渣。协同处置后,重金属在飞灰和炉渣中的分配比例明显发生了变化,飞灰中重金属含量减少12.5%~31.3%,炉渣中重金属增加7.33%~20.1%,气化气的引入改变了炉内重金属的分配情况。协同处置可以有效处理生活垃圾,对固废中的热量进行资源化利用,且出炉物料中重金属含量均低于我国现行标准限值。