Reducing volatilization of heavy metals in phosphate-pretreated municipal solid waste incineration fly ash by forming pyromorphite-like minerals

This research investigated the feasibility of reducing volatilization of heavy metals (lead, zinc and cadmium) in municipal solid waste incineration (MSWI) fly ash by forming pyromorphite-like minerals via phosphate pre-treatment. To evaluate the evaporation characteristics of three heavy metals from phosphate-pretreated MSWI fly ash, volatilization tests have been performed by means of a dedicated apparatus in the 100-1000 °C range. The toxicity characteristic leaching procedure (TCLP) test and BCR sequential extraction procedure were applied to assess phosphate stabilization process. The results showed that the volatilization behavior in phosphate-pretreated MSWI fly ash could be reduced effectively. Pyromorphite-like minerals formed in phosphate-pretreated MSWI fly ash were mainly responsible for the volatilization reduction of heavy metals in MSWI fly ash at higher temperature, due to their chemical fixation and thermal stabilization for heavy metals. The stabilization effects were encouraging for the potential reuse of MSWI fly ash.     

Strength, leachability and microstructure characterisation of Na2SiO3-activated ground granulated blast-furnace slag solidified MSWI fly ash.

The chemical composition and the leachability of heavy metals in municipal solid waste incinerator (MSWI) fly ash were measured and analysed. For the leachability of unstabilized MSWI fly ash it was found that the concentrations of Pb and Cr exceeded the leaching toxicity standard. Cementitious solidification of the MSWI fly ash by Na2SiO3-activated ground granulated blast-furnace slag (NS) was investigated. Results show that all solidified MSWI fly ash can meet the landfill standards after 28 days of curing. The heavy metals were immobilized within the hydration products such as C-S-H gel and ettringite through physical encapsulation, substitution, precipitation or adsorption mechanisms.     

Water washing effects on metals emission reduction during municipal solid waste incinerator (MSWI) fly ash melting process

This study investigated that water washing effects on the metals emission reduction in melting of municipal solid waste incinerator (MSWI) fly ash. Experimental conditions were conducted at liquid-to-solid (L/S) ratio 10, 20, and 100 for water-washing process and its subsequent melting treatment at 1450 °C for 2 h. The simple water-washing process as a pre-treatment for MSWI fly ash can remove most of the chlorides, leachable salts, and amphoteric heavy metals from the MSWI fly ash, resulting in the washed ash having lowered chlorine content. MSWI fly ashes washed by L/S ratio 10 and above that were melted at 1450 °C produced slag containing relatively high vitrificaton ratio of Cu and Pb. Besides, the vitrification ratios of Na, K, Ca, and Mg in washed MSWI fly ash were also higher than that of MSWI fly ash. The results indicated that washed MSWI fly ash can reduce the emission of metallic chlorides during its subsequent melting treatment.     

Recycling MSWI bottom and fly ash as raw materials for Portland cement

Municipal solid waste incineration (MSWI) ash is rich in heavy metals and salts. The disposal of MSWI ash without proper treatment may cause serious environmental problems. Recently, the local cement industry in Taiwan has played an important role in the management of solid wastes because it can utilize various kinds of wastes as either fuels or raw materials. The objective of this study is to assess the possibility of MSWI ash reuse as a raw material for cement production. The ash was first washed with water and acid to remove the chlorides, which could cause serious corrosion in the cement kiln. Various amounts of pre-washed ash were added to replace the clay component of the raw materials for cement production. The allowable limits of chloride in the fly ash and bottom ash were found to be 1.75% and 3.50% respectively. The results indicate that cement production can be a feasible alternative for MSWI ash management. It is also evident that the addition of either fly ash or bottom ash did not have any effect on the compressive strength of the clinker. Cement products conformed to the Chinese National Standard (CNS) of Type II Portland cement with one exception, the setting time of the clinker was much longer.     

Distribution of remaining Cd in MSWI fly ash washed with nitric acid

Municipal solid waste incineration (MSWI) fly ash is by-product and hazardous waste produced from MSWI plant. In the MSWI fly ash there are high contents heavy metals, among which cadmium (Cd) is more active and toxic. Although inorganic acid leaching is an effective way to remove heavy metals out from the MSWI fly ash and nitric acid has great efficiency for Cd removal, little literature reported the redistribution of remaining Cd in the MSWI fly ash. This investigation focused on the change of different factions (exchangeable, bound to carbonates, bound to Fe–Mn oxides, bound to organic matter and residual) of Cd in treated (i.e. washed with nitric acid) MSWI fly ash. Sequence extraction procedures (SEP) have been used to derive different fractions of Cd, results indicated that fractions of Cd have changed significantly after nitric acid washing procedures. Due to the changes of main compounds and microstructures stable Cd (bound to organic matter and residual) had opportunity to leach out, which resulted in a higher potential risk (or higher bioavailability index) for living creatures, although the total amount of Cd decreased. X-ray diffraction (XRD) and images of scanning electron microscope (SEM) proved these changes in washed MSWI fly ash.     

Recycling of municipal solid waste incinerator fly ash by using hydrocyclone separation

The municipal solid waste incinerators (MSWIs) in Taiwan generate about 300,000 tons of fly ash annually, which is mainly composed of calcium and silicon compounds, and has the potential for recycling. However, some heavy metals are present in the MSWI fly ash, and before recycling, they need to be removed or reduced to make the fly ash non-hazardous. Accordingly, the purpose of this study was to use a hydrocyclone for the separation of the components of the MSWI fly ash in order to obtain the recyclable portion. The results show that chloride salts can be removed from the fly ash during the hydrocyclone separation process. The presence of a dense medium (quartz sand in this study) is not only helpful for the removal of the salts, but also for the separation of the fly ash particles. After the dense-medium hydrocyclone separation process, heavy metals including Pb and Zn were concentrated in the fine particles so that the rest of the fly ash contained less heavy metal and became both non-hazardous and recyclable.     

Application of MSWI fly ash on acid soil and its effect on the environment

This study evaluated the feasibility of using municipal solid waste incinerator (MSWI) fly ash as acid soil amendment. In particular, changes in soil physicochemical properties and the potential environmental problems caused by the application of MSWI fly ash were investigated. The results showed that application of MSWI fly ash to the acid soil could raise the soil pH. The contents of rapidly available P and K, and slowly available K in the amended soil had a linear relationship with the addition ratio of MSWI fly ash. An addition of less than 20% of MSWI could raise the soil respiratory intensity after incubation for 3-5 days. Application of MSWI fly ash to the soil increased its content of water soluble salts and heavy metals, which could cause phytotoxicity in the plants. Therefore, the addition of MSWI fly ash to the soil should not be excessive, and less than 5-10% is an advisable addition level depending on the acidity of the soil and the plants growing on it.     

Hydrothermal treatment of MSWI bottom ash forming acid-resistant material

To recycle municipal solid waste incinerator (MSWI) bottom ash, synthesis of hydrothermal minerals from bottom ash was performed to stabilize heavy metals. MSWI bottom ash was mixed with SiO(2), Al(OH)(3), and Mg(OH)(2) so its chemical composition was similar to that of hydrothermal clay minerals. These solid specimens were mixed with water at a liquid/solid ratio of 5. The reaction temperature was 200 degrees C, and reactions were performed for 24-240h. Generation of kaolinite/smectite mixed-layer clay mineral was found in the samples after the reaction of the mixture of bottom ash, SiO(2), and Mg(OH)(2). Calcium silicate hydrate minerals such as tobermorite and xonotlite were also generated. X-ray powder diffraction suggested the presence of amorphous materials. Leaching tests at various pHs revealed that the concentration of heavy metals in the leachates from MSWI bottom ash hydrothermally treated with SiO(2) and Mg(OH)(2) was lower than that in leachates from non-treated bottom ash, especially under acid conditions. Hydrothermal treatment with modification of chemical composition may have potential for the recycling of MSWI bottom ash.     

Environmental impact of fly ash utilization in roadway embankments

The leaching potential of heavy metals from a roadway embankment constructed of fly ash and soil mixture was studied. Leaching of eight environmentally concerned metals Ag, As, Ba, Cd, Cr, Hg, Pb, and Se from the fly ash–soil mixtures was examined through batch leaching test and column leaching test. The batch leaching test results showed that the fly ash meets the local regulatory standards for beneficial use of nonhazardous wastes. The column leach test revealed that only Ba, Cr, and Se were detectable in the effluents. The peak concentration of Ba in the effluents was much lower than the US EPA Primary Drinking Water Regulations’ maximum contaminant level (MCL). The peak concentrations of Cr and Se exceeded the MCLs only in the initial flush stage and quickly decreased to below the MCLs. Results of this study suggest a great potential for fly ash to be used in roadway embankments to enhance their mechanical properties, reduce the use of soil, and avoid the disposal of fly ash as waste.     

Calcium phosphate stabilization of fly ash with chloride extraction

Municipal solid waste incinerator by products include fly ash and air pollution control residues. In order to transform these incinerator wastes into reusable mineral species, soluble alkali chlorides must be separated and toxic trace elements must be stabilized in insoluble form. We show that alkali chlorides can be extracted efficiently in an aqueous extraction step combining a calcium phosphate gel precipitation. In such a process, sodium and potassium chlorides are obtained free from calcium salts, and the trace metal ions are immobilized in the calcium phosphate matrix. Moderate calcination of the chemically treated fly ash leads to the formation of cristalline hydroxylapatite. Fly ash spiked with copper ions and treated by this process shows improved stability of metal ions. Leaching tests with water or EDTA reveal a significant drop in metal ion dissolution. Hydroxylapatite may trap toxic metals and also prevent their evaporation during thermal treatments. Incinerator fly ash together with air pollution control residues, treated by the combined chloride extraction and hydroxylapatite formation process may be considered safe to use as a mineral filler in value added products such as road base or cement blocks.     

Thermal and hydrometallurgical recovery methods of heavy metals from municipal solid waste fly ash

Heavy metals in fly ash from municipal solid waste incinerators are present in high concentrations. Therefore fly ash must be treated as a hazardous material. On the other hand, it may be a potential source of heavy metals. Zinc, lead, cadmium, and copper can be relatively easily removed during the thermal treatment of fly ash, e.g. in the form of chlorides. In return, wet extraction methods could provide promising results for these elements including chromium and nickel. The aim of this study was to investigate and compare thermal and hydrometallurgical treatment of municipal solid waste fly ash. Thermal treatment of fly ash was performed in a rotary reactor at temperatures between 950 and 1050 °C and in a muffle oven at temperatures from 500 to 1200 °C. The removal more than 90% was reached by easy volatile heavy metals such as cadmium and lead and also by copper, however at higher temperature in the muffle oven. The alkaline (sodium hydroxide) and acid (sulphuric acid) leaching of the fly ash was carried out while the influence of temperature, time, concentration, and liquid/solid ratio were investigated. The combination of alkaline-acidic leaching enhanced the removal of, namely, zinc, chromium and nickel.     

The effects of the mechanical–chemical stabilization process for municipal solid waste incinerator fly ash on the chemical reactions in cement paste

A water extraction process can remove the soluble salts present in municipal solid waste incinerator (MSWI) fly ash, which will help to increase the stability of the synthetic materials produced from the MSWI fly ash. A milling process can be used to stabilize the heavy metals found in the extracted MSWI fly ash (EA) leading to the formation of a non-hazardous material. This milled extracted MSWI fly ash (MEA) was added to an ordinary Portland cement (OPC) paste to induce pozzolanic reactions. The experimental parameters included the milling time (96 h), water to binder ratios (0.38, 0.45, and 0.55), and curing time (1, 3, 7 and 28 days). The analysis procedures included inductively coupled plasma atomic emission spectroscopy (ICP/AES), BET, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR) imaging. The results of the analyses indicate that the milling process helped to stabilize the heavy metals in the MEA, with an increase in the specific surface area of about 50 times over that of OPC. The addition of the MEA to the OPC paste decreased the amount of Ca(OH)₂ and led to the generation of calcium–silicate–hydrates (C–S–H) which in turned increased the amount of gel pores and middle sized pores in the cement. Furthermore, a comparison shows an increase in the early and later strength over that of OPC paste without the addition of the milled extracted ash. In other words, the milling process could stabilize the heavy metals in the MEA and had an activating effect on the MEA, allowing it to partly substitute OPC in OPC paste.     

Sustainable alkali-activated binders with municipal solid waste incineration ashes as sand or fly ash replacement

The present study investigates the feasibility of using two types of municipality solid wastes incineration ashes, namely, fly ash and bottom ash in the production of sustainable alkali-activated binder. The ashes are collected from the incineration plant and characterized to determine their particle size distribution, specific gravity, chemical composition, and heavy metals content. The ashes are then used as either fly ash or sand replacement with five replacement ratios 0%, 5%, 10%, 15%, and 20% to produce the binder. The produced binder are characterized in terms of strength, workability, density, water absorption, thermal conductivity and stability, chemical composition, and heavy metals content. The results reflect the ability of producing sustainable alkali-activated binder with small dosage of MSWI ashes as either fly ash or sand replacement without negatively affecting its strength, workability, density, and water absorption. The ashes enhance the thermal insulation capability of the binder.

     

Utilization of municipal solid waste incineration (MSWI) fly ash in ceramic brick: product characterization and environmental toxicity

In this study, municipal solid waste incineration (MSWI) fly ash was used as a blending in making ceramic brick based on its characterization and an orthogonal test was performed to determine the optimal mixture ratio of the materials. Besides, the fired bricks made in accordance with the optimal mixture ratio were characterized for performance, phase transformation, microstructure, leaching toxicity of the heavy metals in accordance with GB/T 2542-92 (Detection methods for bricks analysis, China) and by means of XRD, SEM and leaching toxicity analysis. It was found that the optimal mixture ratio of materials (MSWI fly ash:red ceramic clay:feldspar:gang sand) was 20:60:10:10 by mass, and the optimal sintering temperature was 950 °C. Leaching results of heavy metals from sintered bricks were reduced considerably in comparison with those from green bricks prior to sintering process. The results as a whole suggested that utilization of MSWI fly ash in ceramic brick constituted a potential means of adding value.     

Investigation of MSWI fly ash melting characteristic by DSC-DTA

The melting process of MSWI (Municipal Solid Waste Incineration) fly ash has been studied by high-temperature DSC-DTA experiments. The experiments were performed at a temperature range of 20-1450 degrees C, and the considerable variables included atmosphere (O(2) and N(2)), heating rates (5 degrees C/min, 10 degrees C/min, 20 degrees C/min) and CaO addition. Three main transitions were observed during the melting process of fly ash: dehydration, polymorphic transition and fusion, occurring in the temperature range of 100-200 degrees C, 480-670 degrees C and 1101-1244 degrees C, respectively. The apparent heat capacity and heat requirement for melting of MSWI fly ash were obtained by DSC (Differential Scanning Calorimeter). A thermodynamic modeling to predict the heat requirements for melting process has been presented, and it agrees well with the experimental data. Finally, a zero-order kinetic model of fly ash melting transition was established. The apparent activation energy of MSWI fly ash melting transition was obtained.     

The adsorption characteristics of heavy metals by various particle sizes of MSWI bottom ash

The incineration rate of municipal solid waste (MSW) has been increased because of difficulty in securing a proper disposal site for MSW in Korea. The advantage of incineration is reduction of the volume of waste; however, significant amounts of bottom ash and fly ash were generated in the incineration process. Their treatment has attracted growing interest because of the potential toxicity of hazardous heavy metals. Generally, heavy metals are less released from bottom ash than from fly ash. In this study the adsorption characteristics of heavy metals were investigated using various particle sizes of MSWI bottom ash. Since bottom ash has a broad particle size distribution, it was sieved to size classes of +20, -20, -48, -80, -100 mesh. Cation exchange capacity (CEC) was analyzed by the ammonium acetate method to evaluate the potential as an adsorbent. The CEC values and surface areas increase as the range of particle size becomes finer. The adsorption experiment was conducted using synthetic (Cu and Ni) and plating rinse water as a function of reaction time (10-180 min), liquid/solid ratio (2-100) and particle size (+20 to -100 mesh), respectively. The adsorption rate increased with decreasing particle size and with increasing liquid/solid ratio; however, the removal efficiency of Cu was higher than that of Ni. In the case of plating rinse water, the adsorption rate decreased sharply at high liquid/solid ratio, and it showed over 80% of adsorption rates for Cu and Ni at an initial pH of 3.     

Aggregate material formulated with MSWI bottom ash and APC fly ash for use as secondary building material

The main goal of this paper is to obtain a granular material formulated with Municipal Solid Waste Incineration (MSWI) bottom ash (BA) and air pollution control (APC) fly ash to be used as secondary building material. Previously, an optimum concrete mixture using both MSWI residues as aggregates was formulated. A compromise between the environmental behavior whilst maximizing the reuse of APC fly ash was considered and assessed. Unconfined compressive strength and abrasion resistance values were measured in order to evaluate the mechanical properties. From these results, the granular mixture was not suited for certain applications owing to the high BA/APC fly ash content and low cement percentages used to reduce the costs of the final product. Nevertheless, the leaching test performed showed that the concentrations of all heavy metals were below the limits established by the current Catalan legislation for their reutilization. Therefore, the material studied might be mainly used in embankments, where high mechanical properties are not needed and environmental safety is assured.     

Stabilization of heavy metals in MSWI fly ash using silica fume

The objective of this work was to investigate the feasibility and effectiveness of silica fume on stabilizing heavy metals in municipal solid waste incineration (MSWI) fly ash. In addition to compressive strength measurements, hydrated pastes were characterized by X-ray diffraction (XRD), thermal-analyses (DTA/TG), and MAS NMR (²⁷Al and ²⁹Si) techniques. It was found that silica fume additions could effectively reduce the leaching of toxic heavy metals. At the addition of 20% silica fume, leaching concentrations for Cu, Pb and Zn of the hydrated paste cured for 7 days decreased from 0.32 mg/L to 0.05 mg/L, 40.99 mg/L to 4.40 mg/L, and 6.96 mg/L to 0.21 mg/L compared with the MSWI fly ash. After curing for 135 days, Cd and Pb in the leachates were not detected, while Cu and Zn concentrations decreased to 0.02 mg/L and 0.03 mg/L. The speciation of Pb and Cd by the modified version of the European Community Bureau of Reference (BCR) extractions showed that these metals converted into more stable state in hydrated pastes of MSWI fly ash in the presence of silica fume. Although exchangeable and weak-acid soluble fractions of Cu and Zn increased with hydration time, silica fume addition of 10% can satisfy the requirement of detoxification for heavy metals investigated in terms of the identification standard of hazardous waste of China.     

Comparison of leaching characteristics of heavy metals from bottom and fly ashes in Korea and Japan

The objective of this research was to compare the leaching characteristics of heavy metals such as cadmium, chromium, copper, nickel, lead, etc., in Korean and Japanese municipal solid waste incineration (MSWI) ash. The rate of leaching of heavy metal was measured by KSLT and JTL-13, and the amount of heavy metals leached was compared with the metal content in each waste component. Finally, bio-availability testing was performed to assess the risks associated with heavy metals leached from bottom ash and fly ash. From the results, the value of neutralization ability in Japanese fly ash was four times higher than that in Korean fly ash. The reason was the difference in the content of Ca(OH)(2) in fly ash. The amount of lead leached exceeded the regulatory level in both Japanese and Korean fly ash. The rate of leaching was relatively low in ash with a pH in the range of 6-10. The bio-availability test in fly ash demonstrated that the amount of heavy metals leached was Pb>Cd>Cr, but the order was changed to Pb>Cr>Cd in the bottom ash. The leaching concentration of lead exceeded the Japanese risk level in all fly ashes from the two countries, but the leaching concentration of cadmium exceeded the regulatory level in Korean fly ash only.     

Belgian MSWI fly ashes and APC residues: a characterisation study

Municipal Solid Waste Incineration (MSWI) produces different sorts of residues, bottom ash, fly ashes and Air Pollution Control (APC) residues. Generally, fly ashes and APC residues are mixed at the MSWI plant and manage as a sole residue. In this study, fly ashes and APC residues have been sampled separately at different Belgian MSWI plant and analysed by X-ray fluorescence in order to highlight the composition differences that may appear between the solids. Ca and Cl are found to be the major elements in most of the samples. Lithophilic elements, such as Al and Si, are richer in furnace and boiler ashes, as can be expected. Leaching tests also show differences between the residues; leachates from furnace and boiler ashes are alkaline while those from bag filter residues present a pH value of 6, which impacts the leaching of heavy metals (Pb and Zn). The results suggest that it could be advantageous to manage fly ashes and APC residues separately by adjusting the treatment to their specificities.