Optimization of the solidification/stabilization process of MSW fly ash in cementitious matrices

The solidification/stabilization (S/S) process of municipal solid waste (MSW) fly ash in cementitious matrices was investigated in order to ascertain the feasibility of a washing pretreatment of fly ash with water as a means of maximizing the ash content of cementitious mixtures. Four types of fly ash resulting from different Italian MSW incineration plants and ASTM Type III Portland cement were used in this study. Ash-cement mixtures with different fly ash/cement (FA/C) ratios were made using untreated and washed fly ash. Washing of fly ash with water was realized by a two-stage treatment (liquid/solid=25; mixing time=15 min for each stage). The cementitious mixtures were characterized for water demand, setting time, mechanical strength, and heavy metals leachability. Comparison between the above properties of mixtures incorporating untreated and washed fly ash (particularly, setting characteristics), coupled with economical evaluation of the S/S process when applied to untreated and washed fly ash, proved the feasibility of washing pretreatment as a means of maximizing the incorporation of MSW fly ash in cementitious matrices (ash content up to 75%-90% by weight of total solid).     

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.     

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.     

Preparation of alinite cement from municipal solid waste incineration fly ash

The feasibility of partially substituting ordinary raw materials with municipal solid waste incineration (MSWI) fly ash in alinite cement production was investigated by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF) and scanning electron microscopy (SEM). The physical properties and leaching behavior of the produced cement were also evaluated. Experimental results show that good quality clinkers can be obtained by firing the raw mixes, in which the replacement of MSWI fly ash reaches to 30%, at 1200 °C for 2 h. Alinite cements have higher early strengths at all gypsum additions, while the best result having acceptable early and 28-day strengths is obtained at 5% of gypsum addition. Results also show that the leaching toxicity of heavy metals is far lower than that of the regulatory limit at all testing ages. Based on this study, MSWI fly ash is viable as an effective, alternative raw material in alinite cement production.     

Polychlorinated biphenyls removal from weathered municipal solid waste incineration fly ash by collector-assisted column flotation

Polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are highly toxic micropollutants emitted from municipal solid waste incineration (MSWI), in particular, concentrated in the unburned carbon (UC) of MSWI fly ash. Because of concerns over their adverse health effects, a number of countries have classified MSWI fly ash as hazardous material and required further treatment before its final disposal in landfills. The technologies for removing the toxic chlorinated micropollutants in the MSWI fly ash have been studied, however, until now no mature technique has been obtained in this purpose. In this research, we used a technique of collector-assisted column flotation to remove PCBs-enriched UC from MSWI fly ash. We found that 36.9% PCBs could be removed from fresh MSWI fly ash with 61.7% UC removal efficiency, whereas only 21.7% PCBs could be removed from weathered MSWI fly ash with a low UC removal efficiency of 33.7%. By adding a mixture of two kinds of surfactants: sorbitan mono-oleate and polyoxyethylene (20) sorbitan mono-oleate to the weathered fly ash slurry as the collector assistant, 39.3% PCBs was removed at the hydrophile–lipophile balance (HLB) value of 13.5, while the UC removal efficiency increased to 49.0%. The results showed that the collector assistant could enhance PCBs and UC removal efficiencies during the column flotation process, and the mechanism has been discussed in detail. Higher PCBs and UC removal efficiencies could be expected by further optimizing the conditions of collector-assisted column flotation.     

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.     

A study on the chemical and mineralogical characterization of MSWI fly ash using a sequential extraction procedure

The presence of heavy metals in municipal solid waste incineration (MSWI) fly ash is of environmental concern due to their leaching potential in landfill environments. Sequential chemical extraction was performed on fly ash samples from a large-scale municipal solid waste incineration plant in East China. The transformation of the mineralogical species of fly ash during the sequential extraction was studied using X-ray fluorescence (XRF) and X-ray powder diffraction (XRD). The leaching behavior of heavy metals such as zinc, lead, cadmium and copper in MSWI fly ash was considered to have a dependency relationship with the components of calcium, such as aphthitalite, calcite, anhydrite and calcium aluminate or calcium aluminosilicate.     

Thermal activation on calcium silicate slag from high-alumina fly ash: a technical report

The fly ash with alumina composition from 45 to 55 % has been found in China in last 10 years, which attracts great attention from Chinese government and related alumina industry. Chinese government and its state-owned enterprises have successfully extracted the Al as alloy product from the high-alumina fly ash. However, to recycle the calcium silicate slag as residue from the Al industry is still undetermined. In this report, an innovative process is introduced to achieve the regional sustainability for the high-alumina fly ash industry, and it is found that the cementitious material composed of calcium silicate slag met with the mechanical requirements of 32.5 cement for road pavement. The chemical and mineral analysis show that the calcium silicate slag has high CaO content, which reaches up to 48.64 %. C₂S and C₃A are the dominant mineral phases by XRD analysis indicating its potential pozzolanic activity during the hydration process. Thermal activation from 200 to 900 °C was applied to enhance its pozzolanic activity for the calcium silicate slag and it proved that 600 °C is the optimal calcination temperature due to the decomposition of calcite and clay minerals. Also the mineral phase amorphization was also observed during the XRD analysis, which might also contribute to the enhanced pozzolanic properties at 600 °C. Although the designed cementitious material contains a large quantity of solid waste, none of the hazardous heavy metals exceed the EPA limits. This short article originally reported a promising direction for managing solid waste for Al industry and enhancing utilization efficiency for the enterprise internal solid wastes.     

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.     

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.     

Utilization of municipal solid waste incineration ash in Portland cement clinker

Municipal solid waste incineration (MSWI) ash is used in part as raw materials for cement clinker production by taking advantage of the high contents of SiO₂, Al₂O₃, and CaO. It is necessary for environmental reasons to establish a material utilization system for the incineration waste ash residue instead of disposing these ashes into landfill. The aim of this paper is to study the feasibility of replacing clinker raw materials by waste ash residue for cement clinker production. MSWI bottom ash and MSWI fly ash are the main types of ashes being evaluated. The ashes were mixed into raw mixture with different portions of ash residue to produce cement clinker in a laboratory furnace at approximately 1400°C. X-ray diffraction and X-ray florescence techniques were used to analyze the phase chemistry and chemical composition of clinkers in order to compare these ash-based clinkers with commercial Portland cement clinker.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.