Removal of chloride from MSWI fly ash

Recovery of Zn and Cu from municipal solid waste incineration fly ash by integrating ammonium leaching and ammonia removal

This study introduces an environmentally friendly process for recovering zinc (Zn) and copper (Cu) from municipal solid waste incineration (MSWI) fly ash using ammonium chloride leaching and ammonia removal. The leaching rates for Zn and Cu were 54.39% and 86.23%, respectively, with total recovery rates reaching 52.21% and 85.28%, respectively. The recovered precipitate demonstrated significant Zn (33.62%) and Cu (14.19%) contents, making it ideal for metal smelting. The ammonium leaching process also showcased effective reduction and dechlorination effects on the fly ash. The treated fly ash had a reduced mass of only 30.63% of the original, and chlorine content decreased from 26.23% to 0.84%. The results of this study support the sustainable utilization of MSWI fly ash by facilitating valuable resource recovery and promoting its conversion into construction materials.

Removal of heavy metals in water-extracted solution through adsorption by palygorskite and stabilization by comilling

Removing water-soluble chlorides (WSCs) through water extraction is a common pretreatment technology for recycling municipal solid waste incineration (MSWI) fly ash (FA). However, the extracted solution often contains heavy metals, the concentrations of which exceed standards for effluent. This study aims to investigate the adsorption of heavy metals by palygorskite in water-extracted solution and explore the feasibility of stabilizing heavy metals through comilling palygorskite-adsorbed heavy metals (PAHMs) with water-extracted fly ash (WFA). The experimental parameters include: two-stage water extraction with a liquid-to-solid ratio of 5, adding 0, 0.125, 0.25, 0.5, 1, 2 or 3 g of palygorskite to 100 mL of water-extracted solution, and comilling the mixture of PAHMs and WFA for 0, 0.5, 1, 2, 4, 8, 12, 24 or 96 hours. The experimental results revealed that 3 g of palygorskite in 100 mL of extracted solution could absorb Pb, Cd, Cr, Cu and Zn, meeting the effluent standards. The total amount of Pb, Cd, Cr, Cu and Zn removal rate reached 99.7%. Moreover, 98.44% of the WSCs were not adsorbed, the water extraction process for removing WSCs was not compromised. After the comilling of PAHMs and WFA, the distribution of the heavy metals in the milled blended powder was greater than 99.44%; moreover, toxicity characteristic leaching procedure concentrations were determined to conform to regulatory standards, and the sequential extraction procedure revealed that the heavy metals tended to be in stable fractions. This achieves the goal of preventing secondary pollution from heavy metals during the MSWI FA recycling process.

Synthesis of layered double hydroxides from municipal solid waste incineration fly ash for heavy metal adsorption

The process of urbanization has resulted in a continuous growth of the production of municipal solid waste, consequently leading to the increase of municipal solid waste incineration fly ash (MSWI FA) over time. This has prompted the need for effective disposal and value-added utilization strategies for MSWI FA. In this study, a hydrothermal method was employed to synthesize CaAl layered double hydroxides (LDHs) using MSWI FA as the raw material. The main objective was to investigate how different synthesis parameters affect the crystallinity of the layered bimetallic hydroxides. Subsequently, the synthesized LDHs were characterized using various techniques such as BET, SEM, XRD, FT-IR, and XPS. The results revealed the presence of calcium and aluminum cations in the interlayer region of the synthesized material, with chloride ions, sulfate ions, and acetate ions being the predominant anions. Moreover, the formation of LDHs presents an effective approach for the self-purification of leachates derived from MSWI FA. The LDHs exhibited excellent adsorption capacity for Cd2+ and Cu2+ in wastewater, with maximum values of 730 mg·g-1 and 446 mg·g-1, respectively. The adsorption mechanisms involved isomorphous substitution, complexation, as well as the precipitation of hydroxides or interlayer anions. This method presents a novel approach for effectively utilizing MSWI FA to produce environmentally friendly value-added adsorbents.

Research on oxygen enrichment for municipal solid waste fly ash melting: A pilot-scale study on natural gas and coal as the melting fuel

High energy consumption is the main obstacle of melting/vitrification technology for the disposal of municipal solid waste incineration fly ash (MSWIFA) for industrial applications. To reduce energy consumption and lower operating costs, oxygen enrichment melting was proposed and studied in this work. This research was conducted in a pilot-scale melting furnace, and three melting conditions were compared and discussed. The results showed that 66% of natural gas was saved and the operating cost was reduced by 55% when oxygen enrichment technology was applied to MSWIFA melting. When coal was used as the fuel with the oxygen enrichment melting technology, the operating cost was even lower at 66.39 dollar/ton of fly ash. Because MSWIFA was a Ca-rich material, the relatively high content of Si and Al in the coal fly ash promoted the formation of vitrificated slag, leading to a reduction in the overall pollution toxicity index (OPTI) of MSWIFA by 99.98%. Meanwhile, SO2, HCl, and secondary fly ash were the main pollutants during MSWIFA melting, and when coal was used as the fuel, the emissions of SO2 and HCl could be reduced and the OPTI of secondary fly ash was suppressed. These results suggested that to obtain the lowest operating cost and reduce secondary pollution during MSWIFA melting, the best option consisted of oxygen enrichment technology with coal as the fuel.

Study on dechlorination salt characteristics of pickling sludge by a water-washing process

Steel hydrochloric acid pickling sludge (SHPS), containing the heavy metals Fe, Zn, and Ni and a high chloride salt content, is considered a hazardous solid waste. With the gradual reduction of high-grade metal mineral resources such as Fe, Zn and Ni, it is particularly urgent to recycle valuable metals such as Fe, Zn and Ni in solid waste SHPS in order to realize the resource utilization of SHPS and reduce the environmental harm caused by SHPS. In addition, SHPS usually contains different amounts of alkali chloride, which will have a serious adverse impact on the subsequent extraction and smelting process of Fe, Zn and other metals. Therefore, the removal of chloride plays an important role in the resource utilization of valuable metals in SHPS. Thus, in this study, the effects of water washing dechlorination process parameters such as liquid-solid (L/S) ratio, SHPS particle size, washing time and washing frequency on the chloride removal rate were investigated. The best experimental parameters of SHPS washing were obtained. At the same time, the microscopic morphology and crystal phase composition of SHPS before and after washing were explored. The results showed that the optimized conditions were as follows: room temperature, a L/S ratio of 3 : 1, an SHPS particle size of 100 mesh, and 10 min of water washing, repeated two or three times; under these conditions, the removal rate of Cl, Na, Ca, K, Mg, and S reached 96.64-99.68%, 97.38-99.89%, 36.40-60.37%, 49.11-54.82%, 39.18-40.22%, and 36.98-42.13% respectively. The contents of Cl, K, and Na in filter residue (FR) meets the requirements in GB/T 36144-2018 and GB/T 32545-2016. Conversely, the contents of Fe, Zn, Mn and Ni in the FR are enriched, which is more conducive to the subsequent resource utilization of SHPS. The scanning electron microscope (SEM) image shows the particle size of the FR particles is reduced after washing. The X-ray diffractometer (XRD) results proved that the chlorine salt content in the FR after washing was significantly reduced, the diffraction peaks of Al2O3 appeared in the FR, and the diffraction peak intensity of CaCO3, Fe2O3 and SiO2 increased.

Fate of Cl and chlorination mechanism during municipal solid waste incineration fly ash reutilization using thermal treatment: a review

Safe and sustainable treatment of municipal solid waste incineration fly ash (MSWI FA) is urgently needed worldwide because of its high heavy metals, dioxin, and chlorine (Cl) contents. Thermal treatment is widely considered as a promising method for treating MSWI FA owing to its high toxic content removal efficiency and resource recovery; however, residual Cl is a concurrent critical problem faced during reutilisation of thermal treatment products. This review summarises the innovative thermal treatment methods of MSWI FA, such as those employed in production of cement, lightweight aggregates, glass slag, and metal alloys. The characteristics of Cl in MSWI FA, removal rate, transformation of water-soluble Cl into water-insoluble Cl, and the effect of different influencing factors such as temperature, composition, superheated steam, and mechanical pressure were analysed. The volatilization and decomposition of NaCl, KCl and CaClOH dominates Cl removal; however, the degradation of organic Cl and heavy metal chlorination volatilization process that generate HCl and heavy metal chlorides, respectively, also contributed to Cl removal. To promote the reutilisation of MSWI FA-based products, the leaching behaviour of residual Cl in products obtained by different thermal treatments was investigated.

Chlorine and heavy metals removal from municipal solid waste incineration fly ash by electric field enhanced oxalic acid washing

Electric field enhanced oxalic acid (H₂C₂O₄) washing was conducted to examine the simultaneously removal efficiency of heavy metals (HMs) and chlorine, especially insoluble chlorine from municipal solid waste incineration fly ash (MSW FA). Results show that chlorine and HMs can be effectively removed with a total chlorine, As, Ni and Zn removal rate of 99.10%, 79.08%, 75.42% and 71.43%, when the electrode exchange frequencies is 40 Hz, current density is 50 mA/cm², H₂C₂O₄ adding amount is 0.5 mol/L and the reaction time is 4 h. Insoluble chlorine removal efficiency is up to 95.32%, much higher than reported studies. And the chlorine content in the residue is lower than 0.14%. Meanwhile, HMs removal efficiency is remarkable, 41.62%-67.51% higher than that of water washing. The high-efficient removal effect is caused by the constantly changing direction of electrons hitting the fly ash surface, which provides more escape channels for internal chlorine and HMs. These results proved that electric field enhanced oxalic acid washing could be a promising method for removing contaminants from MSWI fly ash.

Municipal Solid Waste Incineration Fly Ash: From Waste to Cement Manufacturing Resource

This study investigates the possibility of using municipal solid waste incineration fly ash as a supplementary cementitious material to replace part of the clinker in cement. Life cycle assessment has shown that the partial replacement of clinker with blast furnace slag (CEM III) reduces cement's global warming potential by ~30%, while replacing clinker with fly ash reduces it by up to 55%. When using CEM III as the control binder in cement in which 55 wt% of the clinker was replaced with hydrothermally treated fly ash, the flexural strength decreased by ~60% and the compressive strength by ~65%. When the fly ash was mixed with calcined and vitrified demolition materials, flexural strength decreased by ~30% and compressive strength by ~50%. The hardening of the hydraulic binders fixed the heavy metals in the municipal solid waste incineration fly ash.

Research on the Optimization and Application of the Washing Dechlorination Process for Municipal Solid Waste Incineration Fly Ash

In this paper, the fly ash cyclic gradient washing dechlorination process is systematically studied through experiments, and the effects of process parameters such as liquid-solid ratio, the number of leaching, and process pulping on the dechlorination effect of fly ash are investigated and analyzed with the currently operating three-stage counter-current washing dechlorination process. The experimental results indicate that with the liquid-solid ratio of 3:1, the number of leaching of 4, and the primary process pulping, the chlorine content of washing fly ash is reduced to 0.5-0.6%. The Baume degree in the washing filtrate is increased to 11-12 °Bé, the total amount is reduced by about 15%, and the average turbidity value is ≤5NTU. Meanwhile, the moisture content of the washing fly ash is reduced to 28-30%. By comparing with the actual construction project, it is found that under a disposal capacity of 100 t/d, the cyclic gradient washing dechlorination process can reduce the installed power by 30.3%, the floor space by 32.9%, the treatment volume of washing filtrate by 11.1%, and the drying load by 27.9% compared to the traditional three-stage counter-current washing and dechlorination process.

Carbonation of municipal solid waste gasification fly ash: Effects of pre-washing and treatment period on carbon capture and heavy metal immobilization

Carbon capture has become an important technology to mitigate ever-increasing CO₂ emissions worldwide, and alkali waste is a potential source of CO₂ capture material. Slagging-gasification is a novel technology for treating municipal solid waste (MSW), and the gasification fly ash (GFA) is the only solid residue that is not reused at present due to its high heavy metal content. GFA contains high amounts of Ca(OH)₂ and Ca(OH)Cl, making it protentional for CO₂ capture. In this study, GFA and washed gasification fly ash (WGFA) were treated with CO₂ for different treatment periods. Weight changes of samples were recorded to evaluate the efficiency of CO₂ capture. To assess the properties of treated GFA, pH value, leached heavy metal concentration, mineral composition, and microscopic morphology were studied. The results revealed that GFA and WGFA could adsorb 18.8% and 23.7% CO₂ of their weights, respectively. Carbonation could immobilize heavy metals including Pb, Zn, and Cu when a proper treatment period was applied. An excessive treatment period decreased the efficiency of heavy metal immobilization. Pre-washing is recommended as a pre-treatment method for GFA carbonation, which increased the efficiency to adsorb CO₂, improved the pH of carbonated GFA, and enhanced the effect to immobilize heavy metals.

A new co-processing mode of organic anaerobic fermentation liquid and municipal solid waste incineration fly ash

A new co-processing mode of waste liquid from anaerobic fermentation of organic wastes and municipal solid waste incineration fly ash (MSWI-FA) dechlorination is reported in this paper. Taking acetic acid, the most common organic acid in anaerobic fermentation systems, as the representative of anaerobic fermentation organic acids, the improvement of the dechlorination effect and the mechanism of washing MSWI-FA with low concentrations of organic acid lotion were explored. The chlorine content of MSWI-FA was reduced to 0.82% after the optimal process washing pretreatment. Three anaerobic fermentation waste liquids (AFWLs) were used to verify that the chlorine content of MSWI-FA could be reduced to less than 1%, and the dechlorination effect of brewery wastewater, which reduced the chlorine content of MSWI-FA to 0.91%, was the best at this. The influence of the washing process on MSWI-FA pyrolysis was reflected in the whole process. The release of chloride decreased and the weight loss was mainly due to the release of CO₂. The melting point of MSWI-FA, washed by the optimal process, was reduced by nearly 30 ℃, and only 0.06% chlorine remained after calcination at 1100 ℃, which was extremely beneficial in reducing the release of trace elements in MSWI-FA during heat treatment, and for the preparation of cement raw meal.

Removal of harmful components from MSWI fly ash as a pretreatment approach to enhance waste recycling

Municipal solid waste incineration (MSWI) fly ash contains many harmful components that may limit its potential for recycling. An effective pretreatment is therefore required before any recycling can be implemented. In this study, the effects of four pretreatment methods (water washing, CO₂-aided washing, CO₃^2--aided washing, and CO₂ and CO₃^2--aided washing) on the extraction behavior of chloride, sulfate, and heavy metals were evaluated. Water washing was found to be effective for the extraction of all easily and moderately soluble Cl-bearing salts, achieving Cl extraction ratios of 88%, 90%, and 96% for ash from Chongqing (CQ), Qingdao (QD), and Tianjin (TJ), respectively. Injection of CO₂ during washing facilitated decomposition of the hardly soluble Cl-bearing salts, increasing the Cl extraction efficiency by 6% for CQ ash and 9% for QD ash. However, for the TJ ash that contained few insoluble Cl-bearing minerals, CO₂ injection decreased the Cl extraction rate. The addition of CO₃^2- had a negative influence on Cl extraction for all ashes, but it slightly promoted sulfate extraction. Despite the high Cl removal rate, only 23-37% of the sulfate and 0.1-12% of heavy metals were removed. Overall, water-based pretreatment, especially CO₂-aided washing, significantly altered the physical, chemical, and mineralogical characteristics of the fly ash, making it more suitable for recycling. Consequently, the blending ratio of the fly ash for cement clinker manufacture increased from 0.2 to 0.3% in the raw ash to 3.5-5.5% in the treated ash, enabling the extensive use of ash materials.

Chemical stabilization of heavy metals in municipal solid waste incineration fly ash: a review

Sufficient attention should be attached to the large amount of fly ash containing high levels of toxic heavy metals generated after municipal solid waste incineration. Because heavy metals could be leached out of the fly ash under specific conditions, it is necessary to stabilize the heavy metals in fly ash before landfill disposal. Processing technologies of incineration fly ash include solidification/stabilization technology, thermal treatments, and separation processes. This study reviewed the current treatment technologies of municipal solid waste incineration (MSWI) fly ash, with the main focus on the treatment of heavy metals in fly ash with chemical stabilization. Chemical stabilization processes involve chemical precipitation of heavy metal and chelation of heavy metals. In multiple studies, chemical stabilization technology has shown practical feasibility in terms of technology, economy, and effect. In addition, the combination of two or more stabilization agents broadens the general applicability of the agents to heavy metals and reduces the cost. The application of joint processing technology realizes the remove of soluble salt from fly ash. To minimize pollutants while increase their usable value, effective use of waste and co-disposal of several kinds of wastes have gradually become the research hotspots. New developments in chemical stabilization are progressively moving towards the sustainable direction of harmlessness and resource utilization of MSWI fly ash.

Sustainable Recovery of Valuable Nanoporous Materials from High-Chlorine MSWI Fly Ash by Ultrasound with Organic Acids

The new technology development for municipal solid waste incineration fly ash treatment and reuse is urgent due to landfill shortage and environmental effect of leached hazardous substances. Chlorine (Cl) is worth considering due to its high levels in fly ash. In this study, a treatment process of ultrasound combined with organic acid was used to eliminate Cl from fly ash to enhance its properties for reuse. Taguchi methodology was implemented to design the experiments by controlling four impact factors and the contribution of each factor was evaluated by the ANOVA analysis of variance. Following two treatment steps within 5 min with a solid/liquid ratio of 1:10 at 165 kHz, 98.8% of Cl was eliminated. Solid/liquid ratio was the most prominent factor that contributed to the Cl removal with more than 90%, according to the ANOVA analysis of variance. Tert-butyl alcohol (tBuOH), an •OH radical scavenger, was utilized to examine different effects of ultrasonic cavitation on Cl removal efficiency. A 20 kHz ultrasound was used to explore the influence of multi-frequency ultrasound with different mechanical and sonochemical effects on the fly ash dechlorination. This ultrasonic-assisted organic acid treatment was found to be a time and cost-effective pathway for fly ash Cl removal.

CO2 pretreatment of municipal solid waste incineration fly ash and its feasible use as supplementary cementitious material

In this study, municipal solid waste incineration fly ash (MSWIFA) was pretreated with CO₂ via slurry carbonation (SC) and dry carbonation coupled with subsequent water washing (DCW). Both the treated MSWIFAs were then used as cement replacement in cement pastes by weight of 10%, 20% and 30% to investigate the influence on hydration mechanisms, physico-mechanical characteristics and leaching properties. The results showed that carbonates formed on the surface of SC-MSWIFA particles were finer (primarily 20-50 nm calcite) than those from the corresponding DCW-MSWIFA (mostly 130-200 nm vaterite). Hence, SC-MSWIFA blended cement pastes led to shorter setting time and higher early compressive strength than the DCW-MSWIFA pastes. In contrast, the presence of vaterite-rich DCW-MSWIFA in the blended cement pastes could accelerate the cement hydration after 24 h. Both the CO₂-pretreated MSWIFA can replace cement up to 30% without sacrificing the long-term strength and mechanical properties of cement pastes, demonstrating excellent performance as a supplementary cementitious material. Moreover, volume stability in terms of expansion and lead leaching of CO₂-pretreated MSWIFA cement pastes were far below the regulatory limits.

Effect of interactions during co-combustion of organic hazardous wastes on thermal characteristics, kinetics, and pollutant emissions

Thermal disposal of organic hazardous wastes (OHWs) in a rotary kiln is an effective method to destroy organic pollutants and reduce the volume, but the complex interactions between various OHWs may result in sharp degradation on combustion performance and the increase in gaseous pollutants emission. Herein, three typical types of OHWs (i.e., pesticide waste, dyeing waste, and organic resin waste, labeled as HW1, HW2, and HW3, respectively) were chosen and thermally co-treated, and the co-combustion characteristics, kinetics, and gas evolutions were systematically studied. A strong positive interaction between HW1 and HW2 was found between 440 and 680 °C possibly due to the catalytic effect of Fe (III) and alkali metals in HW1. The experimental DTG peaks of mixtures at 480 °C were advanced by 60 °C compared with the calculated ones, resulting from the volatiles combustion of HW2 and the catalytic effect from Fe₂O₃ formed during the combustion. The decrease of ignition temperature of mixtures was found helpful for stable combustion, while the decrease of burnout temperature during co-combustion of HW2 and HW3 exhibited the potential to reduce the clinker ignition loss.

Investigation of properties change in the reacted molten salts after molten chlorides cyclic thermal treatment of toxic MSWI fly ash

To realize the thermal detoxification of municipal solid waste incineration (MSWI) fly ash in a relatively mild environment, molten salts thermal treatment technology was proposed in our previous research, which showed good effects. To investigate the properties of molten salts (NaCl-CaCl₂) during cycling reusing, the change of the main components and the physical properties of the used molten salts were estimated. Results showed that the salts in fly ash would dissolve into molten salts. During this process, the concentration of K⁺, SO₄^2- kept increasing while Cl^- was decreased. The changing trend of Na⁺ and Ca²⁺ was dependent on the ratio of Ca/Na in raw fly ash. Ca(OH)₂ in fly ash would react with CaCl₂ to form CaClOH. Moreover, the introduction of the salt components on the thermal properties of molten salts were also studied. The melting point hardly changed by NaCl, CaSO₄, and SiO₂. Nevertheless, it was lowered to 431 °C with 15% CaCO₃ addition, while increased to 523 °C with 20% KCl. Besides, there were no significant influences on the viscosity, stability, and thermal diffusivity of molten salts. KCl had the greatest influence on the specific heat capacity of molten salt, with an increase of about 20%.

Implementation effect of municipal solid waste mandatory sorting policy in Shanghai

In Shanghai, the biggest metropolis in China, the source classification of domestic waste began in the 1990s but achieved little success over the past thirty years. Surprisingly, the compulsory classification provision of Shanghai residents' domestic waste since 2019 has been successful. In this work, the overall status, and challenges of municipal solid waste (MSW) management in Shanghai under compulsory waste sorting rules are investigated. We tracked the classification efficiency of municipal solid waste, physicochemical characteristics of separated waste, and the carbon emissions arising from waste disposal system. It was found that almost 83.62 % of household food waste has been effectively separated with high purity of 99.50 %, while the lower heating value (LHV) of residual waste was increased by 96.4 % compared to previous years. With effective waste classification, the net carbon emission of separated processing was reduced to 0.11 ton CE/ton waste. However, this system generated some negative outcomes as well such as the lower recovery value of metal in bottom ash and the higher chloride content in fly ash. Moreover, we have documented and discussed several challenges of sustainable waste management system in Shanghai in the text. In order to overcome those challenges, we recommend to: (i) establish a finer source separation and recycling system; (ii) develop highly-efficient resource recovery technologies of byproducts from waste disposal system; and (iii) propose optimization adjustment strategy for overcapacity in incineration facilities.

Municipal Solid Waste Incineration Ash-Incorporated Concrete: One Step towards Environmental Justice

Municipal solid waste and cement manufacture are two sources of environmental justice issues in urban and suburban areas. Waste utilization is an attractive alternative to disposal for eliminating environmental injustice, reducing potential hazards, and improving urban sustainability. The re-use and recycling of municipal solid waste incineration (MSWI) ash in the construction industry has drawn significant attention. Incorporating MSWI ash in cement and concrete production is a potential path that mitigates the environmental justice issues in waste management and the construction industry. This paper presents a critical overview of the pretreatment methods that optimize MSWI ash utilization in cement/concrete and the influences of MSWI ash on the performance of cement/concrete. This review aims to elucidate the potential advantages and limitations associated with the use of MSWI ash for producing cement clinker, alternative binder (e.g., alkali-activated material), cement substitutes, and aggregates. A brief overview of the generation and characteristics of MSWI ash is reported, accompanied by identifying opportunities for the use of MSWI ash-incorporated products in industrial-scale applications and recognizing associated environmental justice implications.

Phase changes during various treatment processes for incineration bottom ash from municipal solid wastes: A review in the application-environment nexus

Incineration technology has been widely employed, as an effective method to decrease the volume of waste disposal. In this review, relationships between municipal solid waste (MSW) inputs and residues after combustion-specifically, the incineration bottom ashes (IBA) of MSW, were discussed, with an emphasis on the geoenvironmental impacts of IBA associated with the complex crystal and amorphous phase reactions and changes during combustion and from their downstream treatments, whereas, their influences on IBA leaching behaviors are considered as another focus. This review summarizes the IBA leaching behaviors based on literature, showing the leaching variabilities induced by natural weathering and artificial intervention conditions, such as accelerated carbonation, washing treatment, stabilization/solidification, and thermal treatments, all of which can be attributed to changes of mineral phases and microstructure. It helps to understand IBA characteristics and transitions in application-environment nexus, and better reuse it for multiple applications.

Roles of chlorine and sulphate in MSWIFA in GGBFS binder: Hydration, mechanical properties and stabilization considerations

In this study, municipal solid waste incineration fly ash (MSWIFA) was first washed (pretreatment) with pure water with liquid to solid (L/S) ratio of 2, 3, 6, 10, to understand the removal efficiency of chlorine and sulphate, as well as its consequent ability as alkaline activator for granulated blast furnace slag (GGBFS). Washed MSWIFA was blended with GGBFS at a fixed ratio of 3:7 to examine their impact on mechanical properties, reaction mechanism, microstructure and leaching behavior. The results showed that chlorine in MSWIFA (>70%) can be washed out easily, while the removal of sulphate was largely depended on the L/S. GGBFS can be better activated by a low L/S (e.g. 2) washed-MSWIFA with attaining the compressive strength of 45.2MPa at 28 days. The higher chlorine and sulphate contents retained in the washed-MSWIFA, the higher the total heat release in the activated GGBFS system. Calcium silicate hydrate (C-S-H), ettringite (AFt) and Friedel's salt were the main hydration products of the activated binders. The rapid formation of AFt was mainly responsible for the 1-day strength development. Large amounts of Friedel's salts were formed from 1 day to 3 days associated to the inhibition of sulphate, and the presence of C-S-H played the key role in long-term strength development. The leaching test of heavy metals and soluble ions also demonstrated that washed MSWIFA activated GGBFS binders were harmless to the environment.

Combustion wastes from thermal power stations and household stoves: A comparison of properties, mineralogical and chemical composition, and element mobilization by water and fertilizers

Wood and coal combustion generate wastes, which may negatively influence the environment. However, studies concerning coal combustion products serving as soil additives are currently in progress. Hence, this study was conducted to compare properties (mineralogy, metallic element content, and rare earth element content) of combustion wastes of different genesis (ash and soot after wood and coal combustion in households, ash from thermal power stations) and to assess possible risk posed to the soil environment when used as soil additive. This study demonstrated the diversity of chemical and mineralogical features of ashes of household genesis originating from thermal power stations. Ash from household stoves showed a higher concentration of metallic elements (i.e., Zn) compared to those originating from thermal power stations. Antimony (Sb) content in household ash can serve as an indicator of plastic (polyethylene) combustion, which is legally prohibited. Leaching tests using water and common mineral fertilizers showed that ammonium sulfate mobilizes metallic elements (Cu, Zn, Pb) more significantly than potassium nitrate or deionized water. The leaching of metallic elements from household stove's ash certainly excludes the possibility of applying the ash as a soil additive even when the ash contains a source of beneficial elements for plants (i.e., Ca).

Chloride removal from municipal solid waste incineration fly ash using lactic acid fermentation broth

As far as improvement of chlorine removal from fly ash by lactic acid fermentation broth (LAFB) was concerned, it is particularly important to explore the instinct mechanism and understand how leaching protocols (i.e. lactic acid addition amount and timing) affect the dechlorination efficiency. Results revealed that the WLL leaching protocol yielded the highest dechlorination efficiency (i.e. removed 98.7% of the total chlorine content of fly ash). The undissolved chlorine in fly ash residue might wrap inside the crystal structure of CaAlSiO₄(OH). Given that the chlorine removal from fly ash might prohibit by the newly formed calcium salt precipitation, exclusively increase the addition amount of LAFB (i.e. LLL protocol) did not necessarily stimulate the dechlorination efficiency. Conversely, it might accelerate the fly ash mass reduction (compared with WLL protocol), resulting in a high chlorine content in fly ash residue. Therefore, instead of increasing lactic acid strength, reducing the thickness of the calcium salt precipitation layer or breaking the crystal structure of CaAlSiO₄(OH) during the leaching process was suggested for efficient fly ash dechlorination.

Co-disposal of municipal solid waste incineration fly ash and bottom slag: A novel method of low temperature melting treatment

Conventional melting for disposing municipal solid waste incineration (MSWI) fly ash or bottom slag needed high temperature and consumed high energy. High calcium content in fly ash and high silicon content in bottom slag brought them high melting point, respectively. Based on the analysis of chemical composition and phase diagram, suitable contents, namely 30%-40% CaO, 45%-60% SiO₂ and 10%-15% Al₂O₃, were proposed to obtain a lower-melting-point mixture system. When the mass ratio of fly ash to bottom slag was 1:5, lowest melting point can be obtained. It was 1,190 ℃, lower than that of fly ash (1,448 ℃) and bottom slag (1,310 ℃). The toxicity characteristic leaching procedure of slags obtained from low melting treatment met the leaching toxicity of Chinese standard GB 5085.3-2007, and the slags containing about 25 wt% CaO, 10 wt% Al₂O₃ and 45 wt% SiO₂ can be used for preparing CaO-Al₂O₃-SiO₂ glass ceramics. The co-process of fly ash and bottom slag realized the low temperature melting treatment with low energy consumption.

Municipal solid waste incineration fly ash to produce eco-friendly binders for sustainable building construction

Municipal solid waste incinerator (MSWI) fly ash is a residue of municipal solid waste incineration whose recycling is currently a worldwide problem. Therefore, considerable efforts are being made to establish effective recovery techniques so MSWI can be used as a substitute for natural resources in construction, as in masonry blocks, roads and so on, or in the manufacture of new materials. MSWI fly ashes contain elements such as Ca, Si and Al, which make it possible for them to be used as raw material to manufacture cements. This paper presents the results obtained from the physicochemical characterization of two MSWI fly ashes from two Spanish cities. The research aims to explore the feasibility of using MSWI fly ash as raw material for sintering belite cements. The results show that MSWI fly ashes have a suitable composition. However, appropriate pre-treatment will be required to eliminate chloride and possible traces of heavy metals and to improve pozzolanic activity. Furthermore, the addition of vitreous silica in the proper proportions is required. The phases generated after calcination of the blend at 800 °C are not those corresponding to pure belite cements. Nevertheless, the possibility of using these ashes as supplementary cementitious material in the manufacture of eco-cements should be contemplated.

Utilisation of municipal solid waste incinerator (MSWI) fly ash with metakaolin for preparation of alkali-activated cementitious material

The proper treatment on hazardous municipal solid waste incineration fly ash (MSWIFA) is important. The application of alkali-activation technology to prepare alkali-activated MSWIFA (AAFA) material provides a potential not only to immobilise the heavy metals, but also to trigger its pozzolanic property in manufacturing building material. In this study, in addition to investigate the feasibility of alkaline activation technology in preparing AAFA with sodium silicate activator, the effect of metakaolin in AAFA (AAFM) was also explored to enhance its performance. The results showed that, compared to the AAFA, blending 10 % metakaolin in AAFA significantly improved both 28-day and 90-days compressive strengths, which was almost 200 % higher than that of AAFA. The compressive strength was increased with increasing the dosage of sodium silicate. The C-S-H gel was observed as the main hydration product of AAFA and AAFM. Moreover, the ettringite was observed in AAFM due to the reaction between the CaSO₄ in MSWIFA and aluminate phase from metakaolin. Finally, the 28 and 210-day leaching behaviours of AAFM on Zn, Cu, Pb, Cd, Cr and Ni were successfully suppressed to less than 1 % of that originally from MSWIFA, which can meet the requirement from Chinese standards.

Incineration Fly Ash and Its Treatment to Possible Utilization: A Review

Incineration has gained popularity over landfill as a key solution for the reduction of massively increasing volumes of municipal solid waste (MSW) generation worldwide and in particular China. However, it is not the end solution. The disposal of the incineration residues, which are enriched with a wide range of heavy metals and soluble salts, has become a challenge for the environmental managers. The aim of this study was to review the increasing urbanization and its repercussion on waste generation in China; waste management options were compared for possible environmentally friendly considerations. Treatment techniques of incineration fly ash were discussed to determine the effectiveness of obtaining environmentally stable material, and, finally, possible applications of incineration fly ash for utilization were discussed based on identifying the processing suitability, performance and environmental impact of incineration fly ash for its applications.

High-Performance Flocculants for Purification: Solving the Problem of Waste Incineration Bottom Ash and Unpurified Water

The silicon-aluminum-iron flocculant (PAFSi) combines the most abundant resources of waste incineration bottom ash and unpurified water, being regarded as one of the most promising approaches toward water purification. Herein, in this research, waste incineration bottom ash was employed to produce a cost-effective and highly efficient flocculant. PAFSi with a particle size of 214 nm and a zeta potential of 8.63 mV reached the optimum performance using a dosage of 2 mL/50 mL at pH from 8 to 11. The results with the copolymer exhibited the following: (1) a good flocculation efficiency over a wide pH range, (2) superior flocculation performance compared to those of polyaluminum chloride and polyferric sulfate, (3) three-dimensional branching structure of PAFSi micelles with a high aggregation degree, (4) charge neutralization and bridging as the main flocculation mechanism, and (5) recycling the floc. Thus, this work provides an attractive solution to the pressing global clean water shortage problem.

Chemical speciation, distribution and leaching behavior of chlorides from municipal solid waste incineration bottom ash

Municipal solid waste incineration (MSWI) bottom ash is an environmentally harmful solid waste that cannot be recycled without pre-treatment. The chloride content in bottom ash (BA) is a major obstacle that restricts its application as secondary building materials. Here, the chemical speciation of the chlorides in BA is systematically studied with multiple analytical techniques, i.e., quantitative XRD, microanalysis and XPS. In addition to halite (NaCl), several chloride-rich minerals are present in BA. These phases are hydrous metal oxides, ettringite, decomposed hydration products (C₄A₃) and incineration slag with a chloride content of 3.2%, 1.4%, 2.1% and 1.3%, respectively. For the first time, the real-time leaching profiles of chloride (up to 80 h) from BA were obtained with a chloride-ion specific electrode to explain the leaching mechanism. In the initial stage of leaching, highly soluble alkali salts (NaCl) and physisorbed chlorides (especially those adsorbed on hydrous metal oxides) are released, which is controlled by diffusion. Later, the leaching is controlled by the solubility/reactivity of the chloride-containing phases, such as ettringite and incineration slag. The results show that the release of chloride is not only a diffusion-controlled process, as reported in the literature, but also a reaction-controlled phenomenon, during which the chloride-rich phases decompose and release chlorides that are associated with them via sorption/incorporation.

Chlorine removal from MSWI fly ash by thermal treatment: Effects of iron/aluminum additives

The high content of alkali chlorides in municipal solid waste incineration (MSWI) fly ash limit its resource reuse due to the potential environmental risks. In this paper, with superheated steam as the gasifying agent and inducer, chlorides in fly ash were removed by thermal treatment within a moderate temperature range. Thermal treatment experiments were performed under different conditions: temperature (500-800°C), steam addition (mass ratio of steam to fly ash = 0.25-1) and residence time (0.5-3 hr). Iron and aluminum powders were added to fly ash to improve the chlorine removal efficiency. Water-soluble chlorides included NaCl and KCl, and insoluble chlorides mainly included Ca(OH)Cl. The heating process with the addition of water steam was more efficient than that without steam in terms of the removal performance of water-soluble chlorides. The removal efficiency of soluble chlorides reached 75.25% for a mass ratio of 1:1 after 1-hr thermal treatment at 700°C. When the residence time was increased above 1 hr, the total dechlorination efficiency was not increased dramatically. Moreover, adding iron and aluminum powder into the fly ash improved the removal of water-insoluble chlorides, and the total dechlorination efficiency was increased by 11.41%-16.64%.

Mechanisms of chloride and sulfate removal from municipal-solid-waste-incineration fly ash (MSWI FA): Effect of acid-base solutions

A general approach to managing municipal solid waste is by incineration. Unfortunately, large amounts of municipal-solid-waste-incineration fly ash (MSWI FA) is produced in the process, with their heavy metals content posing further problems to the environment. One fundamental treatment of MSWI FA heavy metals is called solidification-stabilization, where MSWI FA is solidified in cement-based materials to cap hazardous elements from being released into the environment. Mortar formed from this cement mixed with MSWI FA suffer from decreased compressive strength due to their chloride and sulfate contents. Thus, pre-treatment of MSWI FA to remove these salts before producing mortar is desirable. This study investigated treating MSWI FA with deionized water, 0.01 M and 0.1 M nitric acid, and 0.1 M and 0.25 M sodium carbonate to remove chloride and sulfate. Physical and chemical structures of treated and untreated MSWI FA was studied to understand the chloride and sulfate removal mechanisms. Treated MSWI FA was used as cement replacement in mortar, and the compressive strength was tested. Results suggest that all of the treatment solutions tested in this study can equally remove chloride (around 250,000 mg/kg), but sodium carbonate can remove sulfate at the highest extent (15,821 mg/kg). In addition, mortar with deionized-water-treated MSWI FA gave the highest compressive strength. Heavy metals leaching was tested by the Toxicity Characterization Leaching Procedure (TCLP) method, with results passing the standard.

Research on synergistically hydrothermal treatment of municipal solid waste incineration fly ash and sewage sludge

To explore a feasible method of utilizing municipal solid waste incineration fly ash (IFA) rather than releasing it into solidified landfill, in this work, IFA was pretreated by mixing it with municipal sewage sludge (MSS) and applying hydrothermal treatment (HTT). The influences of the IFA dosage, HTT temperature, HTT time, and liquid to solid ratio (L/S) on the dewatering, chlorine migration, solidification, and leaching of heavy metals (HMs) in MSS were investigated. The results show that the synergistic effect was obtained, IFA enhanced the dewatering of MSS and in return, MSS improved the release of chlorine in IFA. The optimal pretreatment conditions were an IFA dosage of 5%, HTT temperature of 180 °C and HTT time of 60 min. The moisture of the solid residue after HTT could be controlled below 40%. Under a fixed IFA dosage, the chlorine content of the liquid could be reached almost 50% with increasing HTT temperature, and the chlorine distribution exhibited a strong positive correlation with the L/S ratio (R² > 0.90). The migrating chlorine was mainly derived from its soluble state, which was controlled by the HTT liquid volume. After the soluble chlorine was dissolved, bound chlorine compounds, such as CaCl(OH), gradually neutralized and released chlorine into the liquid during HTT, and finally reached an equilibrium as the L/S ratio continued to increase. In addition, during HTT, satisfactory HM immobilization performance was achieved and the fraction of HMs, such as Cr, Ni, Cu and Zn, stabilized.

Investigation of controlling factors on toxic metal leaching behavior in municipal solid wastes incineration fly ash

Municipal solid wastes incineration (MSWI) fly ash has drawn worldwide attention for its substantial annual generation capacity and high toxic metals leachability. Although many factors have been shown to affect the leachability of metals in fly ash, the controlling factors, which guide the selection of appropriate risk reduction method, remain unclear. The purpose of this study was to evaluate the effects of the two most important factors, total metal content, and remaining alkaline substances of MSWI fly ash, on the leaching behavior of toxic metals. In this work, a series of leaching tests and sequential extraction procedures were performed for seven fly ash samples collected from one MSWI plant. Results show that particulate size distribution, morphology, and mineralogy of all samples are similar, indicating the effects of these properties on metal leaching behavior can be ignored. In leaching tests, although the leaching behavior in terms of metal species and concentration levels vary as expected, only the leachate Pb concentration in four samples (up to 17.32 mg/L) exceeds the threshold in Chinese regulation (0.25 mg/L). The variation of the leachate Pb concentration is not consistent with the change of the total Pb concentration in fly ash. Further correlation analysis evidences that the acid-soluble Pb, which is highly correlated to the calcium content of fly ash, dominates the concentration of leachate Pb. Notably, when the addition of lime is about 1.5 times over the theoretical value, the concentration of leachate Pb would exceed the threshold regardless of the total Pb concentration in fly ash. Overall, this study demonstrates that the remaining alkaline substances (mainly calcium-bearing compounds), rather than the total content of metals, are the controlling factor of metal leaching behavior in fly ash. Thus, strategies to delicately optimize the quantity of lime addition in acid gas purification process should be considered to minimize MSWI fly ash environmental risks in the future.

A mixed separation-immobilization method for soluble salts removal and stabilization of heavy metals in municipal solid waste incineration fly ash

This work presents the results of a treatment process of municipal solid waste incineration (MSWI) fly ash using a solution of sodium carbonate as a stabilizing agent. The effectiveness of the treatment was evaluated by means of leaching test for waste characterization according to European Standard, with special focus on soluble chlorides and heavy metals (Zn, Cd, Pb and Cu). Chemical, XRD and DTA/DTG analysis were used to gain insight into the chemical changes induced in the fly ash by the treatment. In the fresh fly ash, the total dissolved solids and chloride concentration exceed the acceptance limits for hazardous waste whereas fresh fly ash was classified as hazardous waste concerning Pb. The carbonated fly ash was considered as non-hazardous waste according to all studied parameters. XRD and DTA/DTG analysis of treated fly ash showed that chlorine compounds have been transferred into the liquid phase during the stabilization process. The chloride removal from the ash was complete and fast irrespective of the sodium carbonate concentration and solid/liquid ratio in the stabilization process within the range studied. The treated fly ash was mainly composed by calcite and portlandite and the chemical analysis after the leaching test demonstrated that more than 98% of heavy metals remained in the treated fly ash. Therefore, the stabilization procedure of MSWI fly ash with a solution of carbonate ions achieved the separation of soluble salts and the leaching stabilization of heavy metals simultaneously in one step.

Mechanical activation of fly ash from MSWI for utilization in cementitious materials

In the present study, the physicochemical characteristics of municipal solid waste incineration fly ashes (FA) from circulating fluidized bed (CFB) or grate furnaces are studied in detail. It is identified that the CFB FA, containing high amount of Si and Al, has better potential and properties for utilization than the grate FA, which is much richer in chlorides. Mechanical activation (MA) allows amending the properties of CFB FA, thus preparing for its subsequent utilization in cementitious materials. Compared to simple water washing, MA treatment of CFB FA further reduces the residual amount of chlorine in fly ash from 0.72 to 0.33 wt%, giving the possibility of doubling the capacity of cement kiln for fly ash disposal. The improvement in chlorine removal relates to the conversion of FA compounds from a crystalline to an amorphous state, increasing the solubility of sparingly soluble chlorides. During the curing of mortars, traces of aluminum or other nonferrous metals in CFB FA are oxidized, liberating lots of hydrogen gas; this would cause expansion problems and significantly reduce the flexural and compressive strength. MA treatment of CFB FA solves such expansion problems by exhausting these reactions in advance, respectively enhancing flexural and compressive strength from 5.7 and 35.3 MPa to 9.1 and 56.9 MPa, which is comparable to the performance of Ordinary Portland Cement. Finally, an innovative pre-treatment technique for CFB FA, combining wet ball milling and counter-current two-stage water washing, is proposed for facilitating its recycling.

Changes in composition and lead speciation due to water washing of air pollution control residue from municipal waste incineration

Changes in elemental and mineralogical composition, and lead speciation, of air pollution control residue (APCR) from municipal solid waste incineration, due to treatment by water washing, were investigated in this work and are reported in the context of a review of the literature. Water washing was shown to substantially modify the nature of APCR by: 1) removing 23% dry mass soluble salts to disagglomerate particles and significantly reduce concentrations of the associated major elements, and increase concentrations of insoluble matrix elements and potential pollutants; and 2) respeciating elements to form new phases. X-ray absorption near edge spectroscopy (XANES) showed that the 500 mg/kg of Pb in raw and washed APCR were comprised mainly of Pb-glass, with some PbSO₄, and small amounts of PbO and PbCl₂. Semi-quantitative linear combination fitting suggests that the glass in the APCR may be unstable and release Pb under the alkaline pH of water washing, to reprecipitate as PbO. Chemical analysis suggests that some Pb may be removed by washing. Scientific understanding of the composition of raw and washed APCR, and particularly the speciation of potentially toxic metals, such as Zn and Pb, can help in developing effective element recovery and residue treatment, utilization or disposal strategies.

Characteristics and Treatment Methods of Medical Waste Incinerator Fly Ash: A Review

Medical waste incinerator fly ash (MWIFA) is quite different from municipal solid waste incinerator fly ash (MSWIFA) due to its special characteristics of high levels of chlorines, dioxins, carbon constituents, and heavy metals, which may cause irreversible harm to environment and human beings if managed improperly. However, treatment of MWIFA has rarely been specifically mentioned. In this review, various treatment techniques for MSWIFA, and their merits, demerits, applicability, and limitations for MWIFA are reviewed. Natural properties of MWIFA including the high contents of chlorine and carbonaceous matter that might affect the treatment effects of MWIFA are also depicted. Finally, several commendatory and feasible technologies such as roasting, residual carbon melting, the mechanochemical technique, flotation, and microwave treatment are recommended after an overall consideration of the special characteristics of MWIFA, balancing environmental, technological, economical information.

Technologies for the management of MSW incineration ashes from gas cleaning: New perspectives on recovery of secondary raw materials and circular economy

Environmental policies in the European Union focus on the prevention of hazardous waste and aim to mitigate its impact on human health and ecosystems. However, progress is promoting a shift in perspective from environmental impacts to resource recovery. Municipal solid waste incineration (MSWI) has been increasing in developed countries, thus the amount of air pollution control residues (APCr) and fly ashes (FA) have followed the same upward trend. APCr from MSWI is classified as hazardous waste in the List of Waste (LoW) and as an absolute entry (19 01 07*), but FA may be classified as a mirror entry (19 0 13*/19 01 14). These properties arise mainly from their content in soluble salts, potentially toxic metals, trace organic pollutants and high pH in contact with water. Since these residues have been mostly disposed of in underground and landfills, other possibilities must be investigated to recover secondary raw materials and products. According to the literature, four additional routes of recovery have been found: detoxification (e.g. washing), product manufacturing (e.g. ceramic products and cement), practical applications (e.g. CO₂ sequestration) and recovery of materials (e.g. Zn and salts). This work aims to identify the best available technologies for material recovery in order to avoid landfill solutions. Within this scope, six case studies are presented and discussed: recycling in lightweight aggregates, glass-ceramics, cement, recovery of zinc, rare metals and salts. Finally, future perspectives are provided to advance understanding of this anthropogenic waste as a source of resources, yet tied to safeguards for the environment.

Potential application of pre-treated municipal solid waste incineration fly ash as cement supplement

Pre-treatment process carried out on raw municipal solid waste incineration (MSWI) fly ash (FA) sample that did not comply initially with the USEPA method 1311 toxicity characteristic leaching procedure (TCLP) regulatory limits of 2011 reduced its chloride and heavy metal content appreciably, thereby making it compliant with the aforementioned regulatory requirement. More than 98% of each heavy metal was successfully removed. The process was excellent in removing cadmium (Cd) (99.99%) and very good at removing the rest. About 99.96% of Cu was eliminated, 99.96% of cobalt (Co), 99.95% of Zn, 98.61% of Cr, and 98.12% of nickel (Ni). Also, the leachate resulting from these pre-treatment steps met the China (GB 16889-2008) and USEPA method 1311 regulatory standards for safe discharge. The density and compressive strength of all the samples prepared increased as the FA content decreased. The compressive strength of all the samples passed the 1989 USEPA-recommended value of 0.34 MPa (50 psi), thereby making them suitable for industrial application. The sample that attained the highest compressive strength (2.08 MPa) after 28 days of curing incorporated 55% fly ash (FACS55), while the one with the least compressive strength (0.65 MPa) after the same period was made from only fly ash (FACS100). Moreover, the concentration of heavy metals in all the samples generally decreased with the FA content. Chromium was not detected in all the samples, thereby making this an excellent method for its immobilization. From the leachability test results, all the samples prepared met the USEPA method 1311 TCLP regulatory limits.

Effect of water-washing on the co-removal of chlorine and heavy metals in air pollution control residue from MSW incineration

The present study systemically investigated the effect of a water-washing process on the removal of harmful chlorides, sulfates, and heavy metals in the air pollution control (APC) residue from municipal solid wastes incineration (MSWI), for sake of a better reuse and disposal of this kind of waste. In addition, the kinetic study was conducted to reveal the releasing mechanism of relevant element in the residue. The results show that, over 70wt.% of chlorides and nearly 25wt.% of sulfates in the residue could be removed by water washing. Based on an economical consideration, the optimal operation conditions for water washing of APC residue was at liquid/solid (L/S) ratio of 3mL:1g and extracting time of 5min. As expected, the concentrations of Co, Cr, Fe, Ni, V and Cu in the washing effluent increased with time during the washing process. However, the extracting regime differs among different heavy metals. The concentrations of Ba and Mn increased firstly but declined afterwards, and concentrations of Pb and Zn gradually declined while Cd and As kept constant with the increase of extracting time. It is worth mentioning that the bubbling of CO₂ into the washing effluent is promisingly effective for a further removal of Pb, Cu and Zn. Furthermore, kinetic study of the water washing process reveals that the extracting of heavy metals during water washing follows a second-order model.

Reductive solidification/stabilization of chromate in municipal solid waste incineration fly ash by ascorbic acid and blast furnace slag

Fly ash is a hazardous byproduct of municipal solid waste incineration (MSWI). Cementitious material that is based on ground-granulated blast furnace slag (GGBFS) has been tested and proposed as a binder to stabilize Pb, Cd, and Zn in MSWI fly ash (FA). Cr, however, still easily leaches from MSWI FA. Different reagents, such as ascorbic acid (VC), NaAlO₂, and trisodium salt nonahydrate, were investigated as potential Cr stabilizers. The results of the toxicity characteristic leaching procedure (TCLP) showed that VC significantly improved the stabilization of Cr via the reduction of Cr(VI) to Cr(III). VC, however, could interfere with the hydration process. Most available Cr was transformed into stable Cr forms at the optimum VC content of 2 wt%. Cr leaching was strongly pH dependent and could be represented by a quintic polynomial model. The results of X-ray diffraction and scanning electron microscopy-energy dispersive analysis revealed that hollow spheres in raw FA were partially filled with hydration products, resulting in the dense and homogeneous microstructure of the solidified samples. The crystal structures of C-S-H and ettringite retained Zn and Cr ions. In summary, GGBFS-based cementitious material with the low addition of 2 wt% VC effectively immobilizes Cr-bearing MSWI FA.

Influence of ignition process on mineral phase transformation in municipal solid waste incineration (MSWI) fly ash: Implications for estimating loss-on-ignition (LOI)

This research focused on the mineral phase transformation under varied ignition conditions with the objective of estimating loss-on-ignition (LOI) parameter in municipal solid waste incineration (MSWI) fly ash residues. LOI is commonly used to measure the volatile species, unburned carbon and moisture in the solid materials. There are criteria for LOI measurement in some research fields, while there is no standard protocol for LOI measurement in MSWI fly ash. Using thermogravimetry technique, the ignition condition candidates were proposed at 440/700/900°C for 1 and 2h. Based on X-ray diffractometry results, obvious mineral phase transformation occurred as a function of ignition temperature variation rather than ignition time. Until 440°C, only some minor phases disappeared comparing with the original state. Significant mineral phase transformations of major phases (Ca- and Cl-based minerals) occurred between 440 and 700°C. The mineral phase transformation and the occurrence of newly-formed phases were determined not only by the ignition condition but also by the content of the co-existing components. Mineral phase components rarely changed when ignition temperature rose from 700 to 900°C. Consequently, in order to prevent critical damages to the original mineralogical composition of fly ash, the lowest ignition temperature (440°C) for 2h was suggested as an ideal measurement condition of LOI in MSWI fly ash.

The impact of thermal treatment and cooling methods on municipal solid waste incineration bottom ash with an emphasis on Cl

Municipal solid waste incineration (MSWI) bottom-ash products possess qualifications to be utilized in cement production. However, the instant use of bottom ash is inhibited by a number of factors, among which the chlorine (Cl) content is always strictly restricted. In this paper, the unquenched MSWI bottom ash was used as the experimental substance, and the influences of thermal treatment and cooling methods on the content and existence of Cl in the ash residues were investigated. The characterization of the MSWI bottom-ash samples examined by utilizing X-ray diffraction, optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy. The experimental results show that as a function of thermal treatment, the reduction rate of Cl is slight below 15.0%, which is relatively low compared with water washing process. Different cooling methods had impacts on the existing forms of Cl. It was understood that most of Cl existed in the glass phase if the bottom ash was air cooled. Contrarily in case of water-quenched bottom ash, Cl could also be accumulated in the newly-formed quench products as chloride salts or hydrate substances such as Friedel's salt.

Stabilization/solidification of municipal solid waste incineration fly ash via co-sintering with waste-derived vitrified amorphous slag

Municipal solid waste incineration (MSWI) fly ash (FA) is classified as hazardous waste and requires special treatment prior to landfilling due to its high levels of alkali chlorides and heavy metals. In this paper we presented and discussed a novel method of converting FA into an inert and non-hazardous material, by using the metastable state of vitrified amorphous slag (VAS). XRD results showed that VAS remained in the amorphous state when sintered at 700 and 800°C and were in the crystalline state at 900 and 1000°C. Heavy metals- and Cl-containing phases appeared during phase transformation process. The residual rates of heavy metals and Cl increased with the decrease of FA:VAS ratios. The prolonged leaching test and potential ecological risk assessment of heavy metals showed that the heavy metals were well immobilized into the sintered samples and presented no immediate threat to the environment. The results indicated that the immobilization of heavy metals was due to the reaction with silicate or aluminosilicate matrices within VAS and/or the incorporation into the new generated crystals. The proposed method can be considered as a potential promising technique for the stabilization/solidification of MSWI fly ash with high Cl content.

Heavy metal removal from MSS fly ash by thermal and chlorination treatments

The thermal behavior of heavy metals in the co-incineration of municipal solid waste-sludge incinerator fly ash (MSS fly ash) was studied using a laboratory-scale tube furnace. The results indicate that without the addition of chlorinating agents, temperature was an important parameter and had significantly influenced on heavy metal removal, whereas the residence time had a weak effect. Between 900 and 1000 °C for 60 to 300 min, heavy metals reacted with chloride-inherent in the fly ash, and approximately 80 to 89% of Pb, 48% to 56% of Cd, 27% to 36% of Zn and 6% to 24% of Cu were removed. After the adding chlorinating agents, the evaporation rate of the heavy metals improved dramatically, where the evaporation rates of Cu and Zn were larger than that of Pb and Cd. As the amount of added chlorinating agents increased, the removal rate of heavy metals increased. However, the effect of the type of chlorinating agent on the chlorination of heavy metals differed considerably, where NaCl had the weakest effect on the removal rate of Cu, Cd and Zn. In terms of resource recovery and decontamination, MgCl₂ and CaCl₂ are the best choices due to their efficient removal of Zn.

Possibilities of municipal solid waste incinerator fly ash utilisation

Properties of the waste treatment residual fly ash generated from municipal solid waste incinerator fly ash were investigated in this study. Six different mortar blends with the addition of the municipal solid waste incinerator fly ash were evaluated. The Portland cement replacement levels of the municipal solid waste incinerator fly ash used were 25%, 30% and 50%. Both, raw and washed municipal solid waste incinerator fly ash samples were examined. According to the mineralogical composition measurements, a 22.6% increase in the pozzolanic/hydraulic properties was observed for the washed municipal solid waste incinerator fly ash sample. The maximum replacement level of 25% for the washed municipal solid waste incinerator fly ash in mortar blends was established in order to preserve the compressive strength properties. Moreover, the leaching characteristics of the crushed mortar blend was analysed in order to examine the immobilisation of its hazardous contents.

Selective leaching process for the recovery of copper and zinc oxide from copper-containing dust

The purpose of this study was to develop a resource recovery procedure for recovering copper and zinc from dust produced by copper smelting furnaces during the manufacturing of copper-alloy wires. The concentrations of copper in copper-containing dust do not meet the regulation standards defined by the Taiwan Environmental Protection Administration; therefore, such waste is classified as hazardous. In this study, the percentages of zinc and copper in the dust samples were approximately 38.4% and 2.6%, respectively. To reduce environmental damage and recover metal resources for industrial reuse, acid leaching was used to recover metals from these inorganic wastes. In the first stage, 2 N of sulphuric acid was used to leach the dust, with pH values controlled at 2.0-3.0, and a solid-to-liquid ratio of 1:10. The results indicated that zinc extraction efficiency was higher than 95%. A selective acid leaching process was then used to recover the copper content of the residue after filtration. In the second stage, an additional 1 N of sulphuric acid was added to the suspension in the selective leaching process, and the pH value was controlled at 1.5-2.0. The reagent sodium hydroxide (2 N) was used as leachate at a pH greater than 7. A zinc hydroxide compound formed during the process and was recovered after drying. The yields for zinc and copper were 86.9-93.5% and 97.0-98.9%, respectively.