Mechanism on heavy metals vaporization from municipal solid waste fly ash by MgCl₂⋅6H₂O

Towards a low-emission resource circulation of valuable metals from municipal solid waste incineration fly ash

The incineration fly ash (IFA) resulting from municipal solid waste combustion is laden with heavy metals, necessitating proper treatment not only for environmental management but also to reclaim the metal values. The surge in non-traditional metals like cobalt as emerging contaminant within IFA samples further attracts to address this issue. In response, the hydrometallurgical recycling of a cobalt-bearing IFA has been studied. Thereby, approximately 98 % zinc and 96 % cobalt were leached using a 1.0 mol/L H2SO4 solution at 90 °C and 1 h of leaching time. In-depth analysis of the leaching process unveiled metals' dissolution primarily via the ion-exclusion mechanism, as evidenced by lower diffusion coefficients (between 10-9 and 10-11 m2/s) and activation energies (9.6-14.9 kJ/mol). Above 99 % separation of zinc from the cobalt-bearing leach liquor was achieved by extraction with 1.0 mol/L D2EHPA at an equilibrium pH below 3.0, followed by stripping with a 2.0 mol/L H2SO4 solution. Cobalt, remained in the raffinate was efficiently precipitated by adding a 20 % excess dosage of oxalic acid to the stoichiometric ratio of C2O42-:Co2+, resulting in only 5 mg/L cobalt left in the solution when precipitation occurred at a pH of 2.8. Additionally, the conversion of CoC2O4 to high-purity Co3O4 was conducted through heat-treatment at 600 °C. The resulting Co3O4 was mixed with Li2CO3 at a Li/Co molar ratio of 1.1, yielding a LiCoO2 precursor that exhibited good electrochemical properties with a capacity of 128 mAh/g, thus affirming the high quality of the recycled cobalt. A comprehensive life-cycle assessment of the recycling process revealed that cobalt precipitation alone contributes approximately 50 % of the total global warming potential (GWP = 4.2624 kg CO2-eq). Notably, this value is remarkably lower than the GWP reported for primary cobalt production, highlighting the environmentally-friendly approach of this recycling endeavor.

Complementary vitrification of municipal solid waste incineration fly ash from grate furnaces and fluidised bed incinerators via a co-reduction process

The increasing application of municipal solid waste incineration (MSWI) emphasises the need for MSWI fly ash (FA) safe treatment. Based on the compositional complementarity of FA from grate furnaces (G-FA) and fluidised bed incinerators (F-FA), we proposed a co-reduction process to treat G-FA and F-FA together for producing vitrified slag and ferroalloys. The clean vitrified slag and Fe-Cr-Ni-Cu alloy were obtained with the mass ratios of 1:9 ∼ 6:4 (G-FA:F-FA) at 1300℃, which is about 300℃ lower than the conventional G-FA vitrification. The metals Zn, Cd, and Pb were mostly volatilised into the flue gas for potential recovery from the secondary FA. The thermodynamic SiO2-Al2O3-CaO ternary system demonstrated that an optimal mass ratio of the two complementary FA types contributes to the system shifting to the low-temperature melting zone. The co-reduction process of G-FA and F-FA could be a promising option for FA beneficial reutilization with environmental advantages.

Exploring multistep bischofite waste pyrolysis: insights from advanced kinetic analysis and thermogravimetric techniques

Pyrolysis technology is crucial for realizing waste bischofite resource utilization. However, previous studies overlooked the complexity of multistep pyrolysis, resulting in a lack of thorough knowledge of the pyrolysis behavior and kinetics. The pyrolysis products were characterized using XRD and FTIR to indicate the bischofite pyrolysis behavior. Additionally, the multistep kinetics was studied using the segmented single-step reaction (SSSR) and Fraser-Suzuki combined kinetic (FSCK) methods. The results show that the bischofite pyrolysis is divided into dehydration and hydrolysis. The former refers to removing crystalline water from MgCl2·nH2O (n = 4,6). At the same time, the latter is related to the removal of HCl, characterized by the strengthening of the Mg-O bond in the FTIR analysis and the emergence of MgOHCl·1.5H2O in the XRD examination. The two main stages are divided into three dehydration reactions (D-1, D-2, D-3) and three hydrolysis reactions (H-1, H-2, H-3) by DTG-DDTG or Fraser-Suzuki deconvolution. Compared with the SSSR method, the FSCK method has improved model repeatability for multistep kinetic parameters. Following Fraser-Suzuki deconvolution, the FSCK method creates almost the same activation energy results when using the Friedman (FR), Kissinger-Akahira-Sunose (KAS), and Vyazovkin (VZK). This work provides fundamental data to promote the maximizing waste bischofite resource utilization.

Exergy Analysis and Modeling of Pilot-Scale Pyrolysis for Magnesium Oxide Preparation from Salt Lake Bischofite Industrial Waste

Currently, the traditional magnesium oxide production process is facing exceptional challenges arising from carbon emission restrictions and environmental protection. Waste bischofite pyrolysis has attracted much attention as a promising technology to address these challenges. Nonetheless, this process has primarily been demonstrated on a laboratory scale, with limited studies on an industrial scale. A comprehensive exergy analysis was conducted for the entire process and individual subunits within the pyrolysis process to identify potential areas for process enhancement. A FORTRAN subroutine based on empirical correlations of pyrolysis product yields was developed considering the impact of decomposition reactions on the simulation. Furthermore, the optimization of energy and exergy efficiency of the system was discussed in terms of the carbon dioxide emission factor, equivalence ratio, and pyrolysis temperature. The results show that the primary energy bottleneck lies in the combustion phase. In addition, the optimal energy and exergy efficiency conditions are a carbon dioxide emission factor of 5.3, an equivalent ratio of 1.15, and a pyrolysis temperature of 1100 °C. In comparison to the pilot-scale conditions, the energy efficiency and exergy efficiency increase by 2.55 and 3.61%, respectively. At this time, the MgO yield is 100%, and the HCl concentration is above 9.33%.

Onsite treatment of wastes in municipal waste incinerator: Co-sintering of fly ash and leachate sludge into value-added ceramic granule

The leachate sludge (LS) and fly ash (FA) are the foci of hazardous wastes which generated from the municipal solid waste incineration (MSWI). The current work developed a new way to use energy from MSWI process for the on-site sintering of LS and FA at a relatively low temperature. With the assistance of CaF2, granule of LS and MSWI FA were co-sintered. The influence of temperature, the mass of CaF2, and the mass ratio of LS/MSWI FA were investigated. As a result, heavy metals volatilization and leaching in the form of chlorinated salts were controlled. In addition, CaF2 improved the compressive strength of the granule under low-temperature sintering. Moreover, the scale-up co-sintering test was achieved in an MSWI chamber. The results showed that the optimum condition was sintering at 973K for 1 h. The compressive strength of sintered product reached 4.25 MPa, which met the standard of ceramic granule. Moreover, with the addition of CaF2, the volatilization rate of Pb, Zn, and Cd decreased by 6%, 7%, and 6%, respectively. This method can be a promising technique for the utilization of solid wastes.

In situ formation of Ca(OH)2 coating shell to extend the longevity of zero-valent iron biochar composite derived from Fe-rich sludge for aqueous phosphorus removal

Despite being an effective and attractive functional strategy for aqueous phosphorus (P) removal, the use of zero valent iron (ZVI) biochar composites has been severely impeded by rapid self-erosion. We describe a new approach for extending the lifespan of Fe-rich sludge-derived ZVI biochar composites via CaCl2 modification. Preliminary results showed that composites obtained at 900 °C without modification (MBC900) and at 900 °C with 100 g Cl/kg addition (MBC900100) had the highest P removal efficiency. In subsequent batch experiments, MBC900100 exhibited more stable P adsorption capacities than MBC900 over a wide pH range (4-10) and at various dosages, which was enhanced by the presence of HCO3-. The theoretical maximum P adsorption capacities of MBC900 and MBC900100 were 227.14 and 224.15 mg g-1, respectively. Kinetic analysis indicated that chemisorption dominated the removal process. Continuous experimental data using the Yoon-Nelson model indicated that MBC900100 had a considerably longer half-penetration time. The primary mechanism of P removal by MBC900 was Fe/C micro-electrolysis. As the embedded CaO formed a dissolvable Ca(OH)2 shell in situ on the surface of MBC900100, the phosphate formed a precipitate with free Ca2+ before being removed via micro-electrolysis. Overall, CaCl2 modification successfully enhanced the longevity of the ZVI biochar composites.

Synergistic effect of polyvinyl chloride and coal ash on thermal separation of heavy metals from MSWI fly ash through molten salt process

Municipal solid waste incineration fly ash (FA) contains high contents of salts and high concentrations of heavy metals, which makes FA disposal extremely difficult. However, heavy metal elements could potentially be separated from FA during thermal treatment process to make it possible to be recycled. This work aims to study the volatilization of heavy metals in FA treated by molten salt method. The influence of polyvinyl chloride (PVC) and coal ash (CA) on volatilization of heavy metals was investigated. Within the scope of this study, the highest heavy metal removal rate can be under the condition: the calcium chloride/sodium chloride weight ratio 1:1, the FA/molten salt weight ratio 1:10, treatment temperature 1000°C for 2 hours in reducing atmosphere. The volatilization rates of lead, zinc, copper, chromium and manganese were 86.20, 67.53, 65.24, 50.07 and 39.45%, respectively. On the basis of molten salt treatment, adding PVC could promote the volatilization of heavy metals. The volatilization rate of lead was 96.71%, and the volatilization rates of chromium and manganese were higher than 60% when the content of PVC was 5 wt%. When adding 10 wt% CA and 1 wt% polyvinyl chloride, the volatilization rate of lead could reach 100%. The experiments and thermodynamic calculations showed that silicon dioxide and aluminium oxide in CA and hydrochloric acid decomposed from PVC could promote the chlorination and volatilization of heavy metals. The volatilized heavy metal chlorides provided the possibility of recovery and utilization of heavy metals in FA.

Selective Chlorination and Extraction of Valuable Metals from Iron Precipitation Residues

Due to the aggravating situations regarding climate change, resource supply, and land consumption by the landfilling of residual materials, it is necessary to develop recycling processes that allow the recovery of valuable metals from industrial residues with significantly reduced CO2 emissions. In this context, it is conceivable that processes using chlorination reactions will be of importance in the future. The simultaneous selective chlorination and evaporation of nine valuable metals was evaluated theoretically and experimentally in small-scale STA trials; then, it was tested practically on six different iron precipitation residues from the zinc and nickel industries. The metal chlorides FeCl3∙6H2O and MgCl2∙6H2O were identified as the most effective reactants, resulting in high extraction rates for the metals In, Ag, Zn, Pb, Au, and Bi, while lower yields are achievable for Sn, Cu, and Ni. Iron, which is predominant in volume in the residual materials, shows lower chlorination tendencies which allows the effective separation of the valuable elements of interest from the iron containing matrix.

Effects of medical waste incineration fly ash on the promotion of heavy metal chlorination volatilization from incineration residues

High contents of heavy metals and Cl are major challenges for incineration residue disposal. Classification by the Chinese government and the coronavirus disease 2019 pandemic have changed the characteristics of incineration residues, thereby increasing the difficulty of disposal. In this study, medical waste incineration fly ash (MWI FA) was proposed as an additive to promote chlorination volatilization of heavy metals from municipal solid waste incineration fly ash (MSWI FA) and medical waste incineration slag (MWI S). When the mixing ratio of MWI FA to MSWI FA was 1:3, the chlorination volatilization efficiencies of Cu, Zn, Pb, and Cd at 1000 °C for 60 min were 50.2%, 99.4%, 99.7%, and 97.9%, respectively. When MWI FA was mixed with MWI S at a ratio of 1:1, the chlorination volatilization efficiencies of Cu, Zn, Pb, and Cd at 1200 °C for 40 min were 88.9%, 99.7%, 97.3%, and 100%, respectively. Adding MWI FA can replenish Cl in MSWI FA and MWI S while increasing the surface area and forming pore structures by sublimation of NaCl and decomposition of CaSO₄, or can reduce the melting point and viscosity by Na₂O destroying the glass matrix. Therefore, MWI FA can be co-disposed with MSWI FA and MWI S respectively to enhance the chlorination volatilization of heavy metals.

Trace metal elements vaporization and phosphorus recovery during sewage sludge thermochemical treatment - A review

Phosphorus (P) plays essential roles in crops growth. Natural mineral sources of phosphate are non-renewable, overexploited and unevenly distributed worldwide, making P a strategic resource for agricultural systems. The search for sustainable ways to secure P supply for fertilizer production has therefore become a critical issue worldwide. Sewage sludge (SS) is an organic waste material considered as a key alternative source of P. Switzerland and the European Union are about to make it mandatory to recover P from SS or its treatment residues. Among the many technical options to achieve this objective, SS thermochemical treatments spiked with Cl-donors appear as a promising approach to recover P from SS and separate it from mineral pollutants such as trace metal elements (TME). The purpose of Cl-donor additives is to fix P within the mineral residues, possibly in bioavailable P species forms, while promoting TME vaporization by chlorination mechanisms. This review paper compares the various thermochemical treatments investigated worldwide over the past two decades. The influence of process conditions and Cl-donor nature is discussed. The presented results show that, except for nickel and chromium, most TME can be significantly vaporized during a high temperature treatment (over 900 °C) with Cl addition. In addition, the fixation rate and solubility of P is increased when a Cl-donor such as MgCl₂ is added.

Mechanical property and heavy metal leaching behavior enhancement of municipal solid waste incineration fly ash during the pressure-assisted sintering treatment

The conventional sintering process of municipal solid waste incineration (MSWI) fly ash is always energy intensive. The process forms a cracked structure because of the difficulty in forming the liquid phase to enhance the mass transfer process. Therefore, exploring a new disposal method to simultaneously decrease the sintering temperature and improve the mechanical and heavy metal leaching properties of sintered samples is necessary. In this study, a pressure-assisted sintering treatment was introduced to dispose fly ash by varying the chemical composition and mechanical pressure at relatively low temperatures (300-500 °C). The results revealed that the compressive strength of treated samples increased with the CaO/SiO₂ molar ratio increasing from 0.5 to 1.0, and a maximum value of 238.28 ± 8.50 MPa was obtained. The heavy metal leaching concentration results demonstrated a low risk of contamination in the treated samples. Microstructure analyses suggested that the densification process was enhanced with increased mechanical pressure, and the formed calcium silicates and aluminosilicates positively affected the compressive strength. Moreover, smaller crystal lattices were observed during aggregation formation, suggesting the restraint of anomalous crystal growth, which accelerated the densification process and increased the compressive strength. Moreover, the mass transfer process during the pressure-assisted sintering process was enhanced compared with the conventional thermal process, which was reflected by the transformation of elements from homogeneous to heterogeneous distribution. Therefore, the improved mechanical properties and leaching behavior of heavy metals were attributed to the densified microstructure, formation of new minerals, and enhanced driving force during the pressure-assisted sintering process. These findings suggest that pressure-assisted sintering is a promising method for maximizing the reutilization and minimizing the energy consumption simultaneously to dispose fly ash.

Effect of Atmospheres on Transformation of Heavy Metals during Thermal Treatment of MSWI Fly Ash: By Thermodynamic Equilibrium Calculation

The vaporization behaviors of eight heavy metals (Pb, Zn, Cu, Cd, Cr, Co, Mn, and Ni) in municipal solid wastes incineration (MSWI) fly ash during thermal treatment under air atmosphere (21% O₂/79% N₂), an inert atmosphere (100% N₂), and a reducing atmosphere (50% CO/50% N₂) were evaluated based on a thermodynamic equilibrium calculation by FactSage 8.1. The results show that the reducing atmosphere promotes the melting of MSWI fly ash, resulting in a more liquid phase than in air or an inert atmosphere. Except for Cd, the formation of liquids can dissolve heavy metals and reduce their vaporization ratio. In the air and inert atmospheres, Pb, Zn, Cu, Co, Mn, and Ni vaporize mainly in the form of metallic chlorides, while Cd volatilizes in the form of metallic Cd (g) and CdO (g). In the reducing atmosphere, Co, Mn, and Ni still vaporize as chlorides. Zn and Cd mainly vaporize in the form of Zn (g) and Cd (g), respectively. In terms of Pb, in addition to its chlorides, the volatiles of Pb contain some Pb (g) and PbS (g). Cr has a low vaporization ratio, accounting for 2.4% of the air atmosphere. Cr, on the other hand, readily reacts with Ca to form water-soluble CrCaO₄, potentially increasing Cr leaching. Except for Cd, the results of this study suggest that the reducing atmosphere is used for the thermal treatment of MSWI fly ash because it promotes the melting of fly ash and thus prevents heavy metal vaporization.

Effect of additives on melting temperature and energy consumption of municipal solid waste incineration fly ash

This paper presents the melting temperature (MT) and energy consumption (EC) of model municipal solid waste incineration (MSWI) fly ash (FA) under the influence of calcium oxide (CaO), silicon dioxide (SiO₂), aluminium oxide (Al₂O₃) and boron oxide (B₂O₃) based on thermochemistry simulations. Nine different base-to-acid ratios (B/A) of raw FA have been explored. The results show that the effects of CaO, SiO₂ and Al₂O₃ vary for different B/A ranges. SiO₂ and Al₂O₃ play positive roles in decreasing the MT and EC of FA4-FA9 with high B/A (B/A: 2.61, 4.48, 6.43, 6.90, 8.32, 8.82). CaO plays a positive role in decreasing the MT and EC of FA1 and FA2 with low B/A (B/A: 0.22, 0.43). In FA3 (B/A: 1.22), the MT and EC of FA cannot be reduced by adding CaO, SiO₂ and Al₂O₃. The addition of B₂O₃ cannot only further reduce the MT of FA, but also reduce the EC. B₂O₃ and SiO₂ can work together to reduce the MT and EC when B/A is high (2.61-8.82), and SiO₂ and B₂O₃ can be introduced into the FA by adding waste glass and other boron containing waste to realize the coordinated disposal of waste.

Transformation of phosphorus by MgCl2 and CaCl2 during sewage sludge incineration

Phosphorus (P) recovery from sewage sludge (SS) have been regarded as an effective method of P recycling. The effects of incineration temperature, incineration time, and chlorine additives on the distribution of P speciation during sludge incineration were studied. Moreover, the reactions between model compounds AlPO₄ and additives (MgCl₂ and CaCl₂) were investigated by thermogravimetric differential thermal analysis and X-ray diffraction measurements. The results demonstrated that the increase in temperature and time stimulated the volatilization of non-apatite inorganic phosphorus (NAIP) instead of apatite phosphorus (AP). MgCl₂ and CaCl₂ can greatly promote the conversion of NAIP to AP. Additionally, AlPO₄ reacted with MgCl₂ are incinerated at 500-600 °C to form Mg₃(PO₄)₂, which is mainly due to the reaction of the intermediate product MgO and AlPO₄. Reactions between AlPO₄ and CaCl₂ occurred at 700-750 °C and produced Ca₂PO₄Cl, which can be directly used with high bioavailability. These findings suggested that chlorine additives in the SS incineration process can obtain phosphorus-containing minerals with higher bioavailability to realize the resource utilization of P in sludge.

Effect of a Passivator Synthesized by Wastes of Iron Tailings and Biomass on the Leachability of Cd/Pb and Safety of Pak Choi (Brassica chinensis L.) in Contaminated Soil

Cadmium (Cd) and lead (Pb) carry a high heavy-metal-toxic risk for both animals and plants in soil. In this study, iron-based biochar (T-BC) was prepared by co-pyrolysis using wastes of iron tailings and biomass with urea as the functioning agents. Field-emission scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and toxicity-characteristic leaching procedure (TCLP) methods were employed to analyze the physicochemical characteristics of T-BC. Additionally, a pot trial was conducted to examine the effects of T-BC on the physiological characteristics of pak choi (Brassica campestris L.), the availability of heavy metals, and enzyme activities in the soils. The results show that toxic metals have been volatilized by the roasting process and immobilized within T-BC via the formation of stable metal-compounds during the co-pyrolysis process, which satisfies the requirements of a soil passivator. Incubation experiments showed that the DTPA-extractable Cd and Pb in contaminated soils decreased with an increasing amendment rate. Moreover, in the pot experiments, by adding 1% (w/w) T-BC into soils, the soils benefited from its large adsorption, complex precipitation, and immobilization capacity. Approximately 36% Cd and 29% Pb concentrations of edible parts in pak choi were reduced. The amendment proved promising for the stabilization of Cd and Pb in contaminated soils, while providing a strategy for solving the residual waste of tailings and biomass.

A mini-review of heavy metal recycling technologies for municipal solid waste incineration fly ash

This mini-review article summarizes the available technologies for the recycling of heavy metals (HMs) in municipal solid waste incineration (MSWI) fly ash (FA). Recovery technologies included thermal separation (TS), chemical extraction (CE), bioleaching, and electrochemical processes. The reaction conditions of various methods, the efficiency of recovering HMs from MSWI FA and the difficulties and solutions in the process of technical development were studied. Evaluation of each process has also been done to determine the best HM recycling method and future challenges. Results showed that while bioleaching had minimal environmental impact, the process was time-consuming. TS and CE were the most mature technologies, but the former process was not cost-effective. Overall, it has the greatest economic potential to recover metals by CE with scrubber liquid produced by a wet air pollution control system. An electrochemical process or solvent extraction could then be applied to recover HMs from the enriched leachate. Ongoing development of TS and bioleaching technologies could reduce the treatment cost or time.

Simultaneous separation and immobilization of Cr(VI) from layered double hydroxide via reconstruction of the key phases

Layered double hydroxide (LDH) is one of the key host phases of Cr(VI) in the natural environment and chromite ore processing residue (COPR), causing serious pollution by Cr(VI). Therefore, efficient extraction or immobilization of the incorporated Cr(VI) in LDH is urgently needed. In this work, simultaneous separation and immobilization of Cr(VI) in LDH by using MgCl₂·6H₂O under thermal treatment is innovatively proposed. Cr was volatilized as CrCl₃ and was immobilized as MgCr₂O₄ accounted for 62.2% and 37.8%, respectively, under the optimal condition (the mole ratio of Cl/Cr is 9, 700 °C and 120 min). The underlying reaction mechanisms are as follows: (i) HCl produced by MgCl₂·6H₂O accelerates the destruction of Cr(VI)-LDH layer structure, completely exposing the incorporated Cr(VI), (ii) Cr(VI) is reduced to Cr(III) by Cl^-, part of which is directly immobilized as MgCr₂O₄, and the other part generates CrCl₃, which is volatilized or further combined with Mg²⁺ to form MgCr₂O₄. The total Cr leaching concentration of the practical COPR sample treated by this method dramatically decreases from 421 to 0.7 mg/L, well below the landfill standard limit (4.5 mg/L). This work provides an attainable strategy for thorough remediation of COPR and inspires the treatment of heavy metal-containing LDH.

Treatment of municipal solid waste incineration fly ash: State-of-the-art technologies and future perspectives

Municipal solid waste incineration (MSWI) fly ash is considered as a hazardous waste that requires specific treatment before disposal. The principal treatments encompass thermal treatment, stabilization/solidification, and resource recovery. To maximize environmental, social, and economic benefits, the development of low-carbon and sustainable treatment technologies for MSWI fly ash has attracted extensive interests in recent years. This paper critically reviewed the state-of-the-art treatment technologies and novel resource utilization approaches for the MSWI fly ash. Innovative technologies and future perspectives of MSWI fly ash management were highlighted. Moreover, the latest understanding of immobilization mechanisms and the use of advanced characterization technologies were elaborated to foster future design of treatment technologies and the actualization of sustainable management for MSWI fly ash.

Simultaneous recovery of phosphorus and nitrogen from sewage sludge ash and food wastewater as struvite by Mg-biochar

The drawback of biochar as a soil ameliorant is its low-nutrient content while the bottleneck of struvite production is its high chemical cost. This drew the idea of using designed biochar for nutrient recovery from nutrient-rich wastewater as struvite. Mg-biochar was used for simultaneous P and N recovery from sewage sludge ash (SSA) and food wastewater (FW) by using ground coffee bean (GCB) and palm tree trunk (PTT) waste. PTT Mg-biochar could recover 92.2% of PO₄^3--P and 54.8% of NH₄⁺-N while GCB Mg-biochar could recover 79.5% of PO₄^3--P and 38.6% of NH₄⁺-N. Adsorption, precipitation and cation-exchange mechanisms are involved in the Mg-biochar for the simultaneous recovery of PO₄^3--P and NH₄⁺-N as struvite. Mg-biochars also showed higher struvite selectivity than the control samples. This method not only supports waste recycling and pollution mitigation but also highlights economical struvite production and the benefits of CO₂ sequestration.

Integrated thermal behavior and compounds transition mechanism of municipal solid waste incineration fly ash during thermal treatment process

Thermal behavior of municipal solid waste incineration (MSWI) fly ash is extremely complicated due to the simultaneously occurred reactive processes and the products with different chemical compositions, therefore, the investigation of chemical compounds transition behavior and mechanism during the integrated thermal process is of great significance. In this study, the macro-thermal treatment of fly ash and thermo-gravimetric analysis via non-isothermal methods were carried out and Málek method was firstly introduced to explore the mechanism of multi-step reaction for fly ash. The mineral transition results suggested that the halite, sylvite in the raw fly ash transformed to more complex crystals in treated samples, such as chlorellestadite, polyhalite and enstatite during the thermal process. And the heavy metals leaching concentration decreased with the temperature increased from 300 °C to 1200 °C, and the leaching values were lower than the standard limitation after thermal treatment. In addition, three major steps of fly ash reactions (300-380 °C, 650-750 °C and 890-1130 °C) during the thermal process were observed and expressed by first order reaction for the first step, three-dimensional diffusion for the second step and three dimensions of limiting surface reaction between both phases for the third step, respectively. The kinetic study revealed that the mineral transition process was in well accordance with the simulated reaction mechanism during the thermal treatment. The obtained results are expected to provide the research basis for studying detailed thermal characteristics and reaction mechanism during the thermal treatment of MSWI fly ash.

An Experimental Study on the Melting Solidification of Municipal Solid Waste Incineration Fly Ash

Melting solidification experiments of municipal solid waste incineration (MSWI) fly ash were carried out in a high-temperature tube furnace device. An ash fusion temperature (AFT) test, atomic absorption spectroscopy (AAS), scanning electron microscope (SEM), and X-ray diffraction (XRD) were applied in order to gain insight into the ash fusibility, the transformation during the melting process, and the leaching behavior of heavy metals in slag. The results showed that oxide minerals transformed into gehlenite as temperature increased. When the temperature increased to 1300 °C, 89 °C higher than the flow temperature (FT), all of the crystals transformed into molten slag. When the heating temperatures were higher than the FT, the volatilization of the Pb, Cd, Zn, and Cu decreased, which may have been influenced by the formation of liquid slag. In addition, the formation of liquid slag at a high temperature also improved the stability of heavy metals in heated slag.

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.

Addressing environmental sustainability of plasma vitrification technology for stabilization of municipal solid waste incineration fly ash

Fly ash from municipal solid waste incineration is considered as a hazardous waste, which would raise great threats on environmental safety due to the inherent toxic heavy metals and organic pollutants. In this study, we applied the life cycle assessment to evaluate the thermal plasma vitrification process for stabilization of fly ash from municipal solid waste incineration. We established four scenarios: (i) plasma vitrification, including centralized and off-site plasma treatment, (ii) fuel-based vitrification, (iii) water-washing treatment followed by a rotary kiln, and (iv) conventional solidification and landfill. We found that the environmental impacts, especially toxicity to ecosystem quality and human health, could be significantly reduced by deploying plasma vitrification technology. We also found that centralized plasma vitrification facilities possessing larger treatment capabilities with clean electricity could further reduce the environmental impacts. In contrast, the water-washing treatment exhibited the highest environmental impacts due to the emissions of vaporized heavy metals. Based on the LCA and sensitivity analysis, we confirmed that the thermal plasma vitrification should be considered as an environmentally-friendly solution to sustainable treatment of fly ash from municipal solid waste incineration. Lastly, we provided several perspectives and prospects of plasma vitrification for realizing the sustainable materials management.

An overview of removing heavy metals from sewage sludge: Achievements and perspectives

The removal of heavy metals from sewage sludge (SS) is attracting increasing attention because the presence of toxic heavy metals in SS restricts its reuse or disposal, especially on land. This review presents an overview of research on the origin and chemical speciation of heavy metals in SS and describes methods for their removal. SS primarily absorbs heavy metals from wastewater via passive sorption and active uptake of biomass, resulting in the different chemical speciation. The advantages and disadvantages of the current methods for the removal of heavy metals from SS are analysed. The current methods focus on the removal efficiencies of heavy metals, which are high enough to meet the standard of land application, but the treatment cost, the change and retention of nutrients, and the effects on SS properties resulting from heavy metal removal are usually ignored. In this review, the main knowledge gaps are identified and proposals for future research are made. These should comprise determining the underlying mechanisms of current removal methods, optimising and integrating the removal methods, and establishing systematic evaluation standards for these methods. This review will help researchers develop new environmentally and economically friendly methods for the removal of heavy metals from SS.

Fate of heavy metals during molten salts thermal treatment of municipal solid waste incineration fly ashes

Thermal treatment could effectively realize the detoxification of heavy metals in municipal solid waste incineration (MSWI) fly ash through the approach of removal or stabilization process. To lower the operating temperature and suppress the evaporation of heavy metals, a molten salts (NaCl-CaCl₂) thermal treatment method was proposed for the detoxification of heavy metals from MSWI fly ash at a relatively mild condition (600/800 °C). The fate of heavy metals during the heating process and their stabilization properties in the remained ash slag after molten salts thermal treatment were investigated. The results showed that, compared with the traditional thermal treatment, heavy metals were more easily chlorinated by the means of molten salts thermal treatment. The well distributed chloride in molten salts facilitated the direct chlorination of PbO/CdO. Furthermore, Al₂O₃ in ash enhanced the indirect chlorination of CuO/PbO/CdO, except for ZnO. In contrast, SiO₂ showed better performance in promoting the indirect chlorination of heavy metal oxides. Meanwhile, some Zn²⁺ was precipitated from molten salts as Si/Al-Zn composite oxides through the interactions with ash containing Si/Al oxides. On the other hand, the dissolved heavy metals in molten salts showed a good thermal stability during the thermal treatment. The volatilization fractions of all detected heavy metals were less than 5%. After the molten salts thermal treatment, heavy metals in the ash slag were well stabilized and the amount of heavy metals leached was significantly lower than that from the raw fly ash.

Enhanced phosphorus availability and heavy metal removal by chlorination during sewage sludge pyrolysis

Increasing attention has been paid on the application of sewage sludge-derived biochar as soil amendments, but is always limited by heavy metals. This study conducted experiments on heavy metal removal by adding chlorinating agents (PVC, NaCl, MgCl₂, CaCl₂) during sludge pyrolysis. The chlorides addition can largely remove heavy metals by achieving the highest removal efficiency with dosage of 80 g(Cl)/kg(dry sludge) at 700 °C. The most effective removal effect was observed for Zn, Mn, Cu and Pb, with removal efficiency from 37.44% to 99.45%, 5.24% to 93.64%, 9.11% to 86.15% and 16.57% to 90.75%, respectively for the sludge before and after chlorination. Furthermore, the P-solubility in neutral ammonium citrate (P_nac) was enhanced after chlorination and the maximum P-solubility can be obtained at 700 °C for each series. After 700 °C pyrolysis, the P-solubility was significantly increased from 40.08% of the sludge biochar to 72.07%, 74.05%, 74.00% and 76.57% of the biochar obtained after adding PVC, NaCl, CaCl₂, and MgCl₂, respectively. The highest P-solubility was observed in samples with MgCl₂ due to the formation of Mg₃(PO₄)₂. This study proposed a novel method to use the sludge biochar as potential P-fertilizer with effective heavy metal removal, finally achieving a "waste-to-resource" strategy for integrated management of sewage sludge.

The heavy metal leaching property and cementitious material preparation by treating municipal solid waste incineration fly ash through the molten salt process

This research investigated the heavy metal leaching property and cementitious material preparation by treating municipal solid waste incineration fly ash through the molten salt process. The results indicated that the heavy metal thermal evaporation of fly ash in the molten salt was related to molten salt composition, heat treatment temperature and atmosphere. After treatment with sodium chloride molten salts (contains 10-50 wt% calcium chloride) from 900°C to 1000°C for 2 h, the leaching concentrations of lead, cadmium, copper, zinc and other heavy metals in fly ash were decreased more than 90% and they could fully meet with the landfill standard. Moreover, after molten salt treatment, the weight fraction of fly ash was reduced by 50 wt% than the original one, and the fly ash has been changed as a kind of cementitious material, which has excellent cementitious property. The X-ray diffraction result indicated that the main crystal mineral composition of cementitious materials obtained was alite, belite, alinite and calcium sulphate.

A combined kinetic and thermodynamic approach for interpreting the complex interactions during chloride volatilization of heavy metals in municipal solid waste fly ash

This study elucidated complex interactions during the chloride volatilization of heavy metals (Pb, Cu, Zn, Mn, and Cr) from municipal solid waste fly ash by combining thermodynamic and kinetic approaches. Chloride volatilization tests under HCl flow at 900 °C and subsequent rinsing with water achieved almost complete removal of Pb, Zn, and Mn. In contrast, almost 100 % of Cr and ∼40 % of Cu were not removed by either volatilization or rinsing processes. Kinetics indicated that the chlorination of Pb, Zn, and Mn followed a pseudo second order reaction and their apparent activation energies were 96.3, 89.2, and 43.5 kJ/mol, respectively. Further thermodynamic calculation revealed that the components contained in fly ash greatly influenced the chlorination of each heavy metal. Unburned carbon facilitated the chlorination of Pb, Zn, and Mn, while it inhibited Cu chlorination. MgO immobilized Cr and inhibited chlorination. KCl and NaCl promoted Zn and Mn chlorination, respectively. The revealed chloride volatilization behavior and effects of co-existing elements could be useful in the design of high-efficiency recovery process of heavy metals from fly ash and the utilization of residues as raw materials for cement. Furthermore, these findings could guide the realization of a recycling-oriented society in terms of reducing waste disposal.

The effect of hydrolysis on combustion characteristics of sewage sludge and leaching behavior of heavy metals

The aim of this paper is to present the effect of hydrolysis treatment on the thermal reaction characteristics of sludge and the leaching test of heavy metals contained in the sludge. Raw and hydrolysis-treated sludge (at temperature of 200°C and pressure 0.4 MPa) samples were collected from local municipal sewage sludge treatment plant. Thermogravimetric analysis was carried out between 25°C and 900°C at the heating rate of 10°C/min and 20°C/min under incineration and pyrolysis atmospheres. The leaching behavior of toxic heavy metals in the sludge was studied along with ash composition. The heating rate significantly changed the thermogravimetric and differential thermogravimetric curve profiles for studied fuels under different atmospheres. After hydrolysis treatment, the heavy metals of hydrolysis-treated sludge becomes more stable. The leaching concentration of Cr, Mn, Cu and Pb in hydrolysis-treated sludge were greatly reduced, when compared to raw sludge; however, no significant change was observed in Cd concentration. The X-ray diffraction pattern of bottom ash of hydrolysis-treated sludge was significantly reduced than raw sludge. This decrease is due to possible reduction in grain growth, which degrades the crystallinity of hydrolyzed sludge, implying that the hydrolysis treatment of sewage sludge could be a promising and beneficially safe disposal technology.

Assessment of copper and zinc recovery from MSWI fly ash in Guangzhou based on a hydrometallurgical process

Fly ash commonly accumulates a significant amount of heavy metals and most of these heavy metals are toxic and easily leached out to the environment, posing risks to human health. Thus, fly ash has been classified as a type of hazardous waste and requires proper treatment before disposal in specific landfill sites for hazardous waste. In this study, a hydrometallurgical process developed to recover copper and zinc performed in pilot scale close to industrial scale followed by a landfill compliance leaching test of the ash residue is evaluated. LIX860N-I and Cyanex 572 gave high selectively for extractions, a yield efficiency of 95% and 61% was achieved for copper and zinc respectively. Results of pilot experiments reveals that the combining metal recovery/recycling and landfill disposal of the ash residue in a local regular landfill was demonstrated to be a technically and economically effective strategy. Specifically, the economic and environmental aspects of a scenario, in which the fly ash generated in Guangzhou is processed were systematically assessed. the assessment results show that a 7.15 million US$ of total expense reduction, a less energy cost of 19k GJ as well as 2100 tons less CO₂ emissions could be achieved annually comparing to the current alternative, direct disposal of the fly ash as hazardous waste. The results reveal that the hydrometallurgical process has industrial application potential on both economic and environmental aspects and further optimization of the process can give more accurate assessment of the cost and environment effect. In addition, leaching tests and evaluation of solid residue according to the regulations specific to the country should be studied in future.