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.     

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.     

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.     

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.     

Effect of SiO2-Al2O3-flux ratio change on the bloating characteristics of lightweight aggregate material produced from recycled sewage sludge

This study investigates the characteristics of lightweight aggregates sintered from sewage sludge ash by modifying the proportion of the main components (SiO(2)-Al(2)O(3)-flux). The ash of incinerated sludge from a municipal sewage treatment plant (STP) was used as the tested material and sintering temperature ranged from 1050 to 1100 degrees C within a time span of 10-30min. The sludge ash appeared to have a high proportion of SiO(2) (44.89%), Al(2)O(3) (11.62%) and Fe(2)O(3) (6.81%) resembling the dilatable shale. When the sintering temperature was raised to above 1060 degrees C, the blowing phenomenon appeared. The aggregates become lighter in weight by prolonging the sintering time and raising the temperature. Cullet powder (amorphous SiO(2)), Al(2)O(3), and fly ash were added to sludge ash to analyse the characteristic changes of the aggregates. The results showed that amorphous SiO(2) lowered the melting point and increased foaming; Al(2)O(3) raised the compression resistance; fly ash lowered the sintering temperature required. However, the composition of fly ash can vary dramatically, resulting in a less predictable characteristic of aggregates.     

Sequential extraction for evaluating the leaching behavior of selected elements in municipal solid waste incineration fly ash

A sequential extraction method has been applied for the determination of binding forms of trace elements in the municipal solid waste incineration (MSWI) fly ash and evaluating their leaching behavior in view of their potential environmental impact. The elemental determinations in the different leachates are performed by ICP-AES and ICP-MS, respectively. The morphology and mineralogical phases after extraction step were performed by scanning electron microscopy (SEM). Total of 20 elements in the samples are investigated. A reference material of city waste incineration fly ash (BCR No. 176) is also tested to examine the applicability as well as accuracy of the proposed method. The sum of most elements present in the individual fractions shows a good agreement with the total elemental concentrations. The extraction efficiencies are generally higher than 80% except for that of Cr and V. The extractable data of most elements give information about the binding forms of various elements in both incineration fly ashes. It was found that the elements such as Ca, K, Na, Pb, Zn, Cd, Cu and Sr have exhibited a remarkable mobility in fly ash. More than half of them would be dissolved or exchanged under a mild leaching condition. The toxic elements such as Pb, Cd, Zn and Cu have a great potential to be released into the environment under normal conditions.     

Sintering of MSW fly ash for reuse as a concrete aggregate

The sintering process of municipal solid waste (MSW) fly ash was investigated in order to manufacture sintered products for reuse as concrete aggregates.Four types of fly ash resulting from different Italian MSW incineration plants were tested in this study. A modification of the chemical composition of MSW fly ash--through a preliminary four-stage washing treatment of this material with water--was attempted to improve the chemical and mechanical characteristics of sintered products.The sintering treatment of untreated or washed fly ash was performed on cylindrical compact specimens (15 mm in diameter and 20mm in height) at different compact pressures, sintering temperatures and times.The sintering process of untreated MSW fly ashes proved to be ineffective for manufacturing sintered products for reuse as a construction material, because of the adverse chemical characteristics of these fly ashes in terms of sulfate, chloride, and vitrifying oxide contents.A preliminary washing treatment of MSW fly ash with water greatly improved the chemical and mechanical characteristics of sintered products and, for all the types of fly ash tested, the sintered products satisfied the Italian requirements for normal weight aggregates for use in concretes having a specified strength not greater than 12 and 15N/mm(2), when measured on cylindrical and cubic specimens, respectively.A compact pressure of 28 N/mm(2), a sintering temperature of 1140 degrees C, and a sintering time of 60 min were the best operating conditions for manufacturing sintered products of washed MSW fly ash.     

Arsenic and lead release from fly ash stabilized/solidified soils under modified semi-dynamic leaching conditions

A fly ash-based stabilization/solidification (S/S) technique was investigated using field soil samples contaminated with arsenic (As) and lead (Pb). A semi-dynamic leaching test was used to evaluate the effectiveness of the S/S treatment. By assessing the cumulative fractions of leached As and Pb, the effective diffusion coefficient (D(e)) and a leachability index (LX) were measured and used for evaluating the effectiveness of the S/S treatment. Overall, As release was reduced by 98.3% and Pb release was reduced by 98.5% upon addition of 25% Class C fly ash. The mean D(e) decreased significantly and the mean LX was always above 9 for all treated samples, indicating that the treated soils were acceptable for "controlled utilization". The mechanism controlling As leaching from all treated samples appeared to be a mixture of wash-off and diffusion. Diffusive As release was proportional to fly ash content. The mechanism controlling Pb leaching when samples were treated with 25% fly ash appeared to be wash-off.     

Effect of sintering temperature and heating mode on consolidation of Al–7Zn–2·5Mg–1Cu aluminum alloy

Densification behaviour, phase transformation, microstructural evolution and hardness values of microwave sintered Al?C7Zn?C2·5Mg?C1Cu (7775) aluminum alloy were investigated and compared with conventionally sintered samples. Microwave sintering was performed in 2·45?GHz multimode microwave furnace at temperatures ranging from 570?C630 °C. Microwave sintering at a heating rate of as high as 22°C/min resulted in ~55% reduction of processing time as compared to conventional sintering. A lower sintered density observed in the case of microwave processed samples was attributed to the inhomogeneity in microstructure and phase distribution. The X-ray diffraction results of conventionally sintered samples showed the presence of MgZn₂, Mg₂Zn₁₁ and CuMgAl₂, while only MgZn₂ and CuMgAl₂ phases were found in the case of microwave sintered samples and in lesser amount. Higher hardness and high standard deviation values were noticed for microwave sintered samples as compared to conventional counterparts.     

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.     

Preparation of aluminium-fly ash particulate composite by powder metallurgy technique

Aluminium-fly ash mixtures containing different weight percentages of fly ash were prepared and compacted at pressures from 138–414 MPa. The compacts prepared at 414 MPa were sintered in nitrogen atmosphere at 600, 625 and 645°C, respectively. The time of sintering ranged from 0.5–6 h. The densification parameter and the green densities of the compacts were determined as a function of compacting pressure and fly ash weight per cent. Density, hardness and strength of the sintered compacts were determined as a function of weight per cent of fly ash particles. Volume changes during sintering of green compacts were also evaluated as a function of increasing fly ash weight per cent. Microscopic studies of green and sintered compacts were done to study the effectiveness of sintering. Green and sintered density of the compacts were found to decrease with increasing weight per cents of fly ash. Sintering results in slight decrease in density and increase in volume of green compacts within the range investigated. Strength of the sintered compacts decreased with increasing weight per cent of fly ash under the present experimental conditions; however, the hardness was found to increase slightly up to 10 wt% fly ash, beyond which it decreased. This revised version was published online in November 2006 with corrections to the Cover Date.     

Solidification/stabilization of electric arc furnace dust using coal fly ash. Analysis of the stabilization process

In this paper, the stabilization of electric arc furnace (EAF) dust containing hazardous metals such as Pb, Cd, Cr or Zn is described. The treatment involves a waste solidification/stabilization (S/S) process, using coal fly ash as the fundamental raw material and main binder. The article also contains a brief review of the most important recent publications related to the use of fly ash as S/S agents.The efficacy of the process has been evaluated mainly through leaching tests on the solidified products and compliance with some imposed leachate limits. The concentration of metals leaching from the S/S products was strongly leachate pH dependent; thus, the final pH of the leachate is the most important variable in reaching the limits and, therefore, in meeting the stabilization goals.In this study, the dependence relationship between the leachate pH and the concentrations of metals in the leachate are analyzed; in some cases, this allows us to estimate the speciation of contaminants in the S/S solids and to understand the mechanism responsible for reduced leachability of heavy metals from solidified wastes.     

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.     

The behaviour of Al in MSW incinerator fly ash during thermal treatment

Fly ash from municipal solid waste (MSW) incinerators contains leachable metals, including potentially hazardous heavy metals. The metal content of the fly ash can be reduced by thermal treatment, which vaporizes the volatile metal compounds. After heat treatment of fly ash at 1000 degrees C for 3 h, less metal was able to be leached from the thermally treated ash than from the ash without thermal treatment. Al and Cr were the exceptions. These metals were more soluble in the ash that had been thermally treated. This paper focuses on the leaching behaviour of Al only. Both simple and sequential extraction leaching tests showed that the leachable Al for the heat-treated fly ash is about twice that of the untreated fly ash. The sequential test further revealed that (i) the majority of the leachable Al is associated with Fe-Mn oxides in the fly ash, and (ii) most of the unleachable Al resides in the silicate matrices of the heat-treated and untreated fly ash. Pure chemicals, Al(2)O(3), CaO and CaCl(2), simulating the relevant ingredients in the fly ash, were used for studying their reactions at 1000 degrees C. The aluminum compounds were identified by X-ray Diffraction (XRD). Two new chemical phases produced by the thermal treatment were identified; Ca(AlO(2))(2) and 12CaO.7Al(2)O(3). Their formation suggests a mechanism whereby thermal treatment of fly ash would produce more soluble Al.     

Chromium behavior during thermal treatment of MSW fly ash.

Energy-from-waste incineration has been promoted as an environmentally responsible method for handling non-recyclable waste from households. Despite the benefits of energy production, elimination of organic residues and reduction of volume of waste to be landfilled, there is concern about fly ash disposal. Fly ash from an incinerator contains toxic species such as Pb, Zn, Cd and Cr which may leach into soil and ground water if landfilled.Thermal treatment of the fly ash from municipal solid waste has been tested and proposed as a treatment option for removal of metal species such as Pb, Cd and Zn, via thermal re-volatilization. However, Cr is an element that remains in the residue of the heat treated fly ash and appears to become more soluble. This Cr solubilization is of concern if it exceeds the regulatory limit for hazardous waste. Hence, this unexpected behavior of Cr was investigated. The initial work involved microscopic characterization of Cr in untreated and thermally-treated MSW fly ash. This was followed by determining leaching characteristics using standard protocol leaching tests and characterization leaching methods (sequential extraction). Finally, a mechanism explaining the increased solubilization was proposed and tested by reactions of synthetic chemicals.     

Trace elements partitioning during co-firing biomass with lignite in a pilot-scale fluidized bed combustor

This study describes the partitioning of 18 trace elements (As, Ba, Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Tl, V, Zn) and 9 major and minor elements (Al, Ca, Fe, K, Mg, Na, S, Si, Ti) during co-firing of olive residue, hazelnut shell and cotton residue with high sulfur and ash content lignite in 0.3 MW(t) Middle East Technical University (METU) Atmospheric Bubbling Fluidized Bed Combustor (ABFBC) test rig with limestone addition. Concentrations of trace elements in coal, biomass, limestone, bottom ash, cyclone ash and filter ash were determined by inductively coupled plasma optical emission and mass spectroscopy (ICP-OES and ICP-MS). Partitioning of major and minor elements are influenced by the ash split between the bottom ash and fly ash and that the major proportion of most of the trace elements (As, Ba, Co, Cr, Cu, Li, Mn, Mo, Ni, Pb, Tl, V and Zn) are recovered in fly ash when firing lignite only. Co-firing lignite with biomass enhances partitioning of these elements to fly ash. Co-firing also shifts the partitioning of Cd, P, Sb and Sn from bottom to fly ash.     

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.     

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.     

Partitioning behavior of trace elements during pilot-scale fluidized bed combustion of high ash content lignite

This study describes the partitioning of 20 trace elements (As, B, Ba, Cd, Co, Cr, Cu, Hg, Li, Mn, Mo, Ni, P, Pb, Sb, Se, Sn, Tl, V, Zn) and eight major and minor elements (Al, Ca, Fe, K, Mg, Na, Si, Ti) during the combustion of high ash content lignite. The experiments were carried out in the 0.3 MW(t) Middle East Technical University (METU) atmospheric bubbling fluidized bed combustor (ABFBC) test rig with and without limestone addition. Inert bed material utilized in the experiments was bed ash obtained previously from the combustion of the same lignite without limestone addition in the same test rig. Concentrations of trace elements in coal, limestone, bottom ash, cyclone ash and filter ash were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). Partitioning of major and minor elements are influenced by the ash split between the bottom ash and fly ash and that the major proportion of most of the trace elements (As, Ba, Cr, Hg, Li, Mo, Ni, Sn, V, Zn) are recovered in fly ash. Limestone addition shifts the partitioning of Ba, Cr, Mo, Ni, Sn, V, Zn from bottom ash to fly ash.     

Pb stabilization in fresh fly ash from municipal solid waste incinerator using accelerated carbonation technology

Carbonation technology with CO(2) absorption was used to enhance the stabilization of heavy metals in fresh fly ash from a municipal solid waste incinerator (MSWI). The influence of fundamental parameters affecting the stabilization of heavy metals, especially Pb and diffusivity and reactivity of CO(2), was evaluated. The results indicated that the addition of 10% or more of water could remarkably accelerate the absorption of CO(2) and could also accelerate the stabilization of MSWI fly ash. The stabilization of MSWI fly ash is not distinct within 1d in the air atmosphere for low content of CO(2) (0.03%). The result of the XRD analysis indicated that CO(2) could combine with Ca(OH)(2) to form CaCO(3) and CO(2) could also combine with heavy metal oxide to form heavy metal carbonate in the adsorption of CO(2). The TGA analysis showed that MSWI fly ash has the sequestration capability of 3% (w/w) CO(2). The sequestration of CO(2) has a large impact on Pb, and the exchangeable Pb can be converted into carbonated form in rich CO(2) condition to be stabilized.