Evaluation of a mixed chelator as heavy metal stabilizer for municipal solid‐waste incineration fly ash: Behaviors and mechanisms

This paper compares the stabilization performances of three dithiocarbamate and three phosphate chemical stabilizing agents, selects the best agents from each group, combines them as a mixed heavy metal stabilizer for municipal solid‐waste incineration (MSWI) fly ash, and investigates the effectiveness of the mixed chelator in stabilizing heavy metals in MSWI fly ash from southwest China. The mechanism of stabilization is also analyzed. Results indicate that piperazine dithiocarbamate can stabilize Cd, Cu, and Ni effectively, while sodium dihydrogen phosphate can stabilize Pb better. With the addition of the mixed chelator (2% sodium dihydrogen phosphate and 1% piperazine dithiocarbamate), all the targeted heavy metal concentrations in the leachate were below the standards for pollution control on municipal solid waste landfill sites in China. The stabilizing agents combined with the heavy metals in fly ash could not generate insoluble crystals; however, the heavy metals could combine with the agents' effective groups, generating flocs in the pores or on the surfaces of the fly ash and offering significant densification. The mixed chelator could transfer heavy metals from unstable fractions to stable fractions, so the leaching of heavy metals from the fly ash was greatly decreased.     

Thermogravimetry of ternary cement blends

This study reports the microstructure characteristic and compressive strength of multi-blended cement under different curing methods. Fly ash, ground bottom ash, and undensified silica fume were used to replace part of cement at 50 % by mass. Mortar and paste specimens were cured in air at ambient temperature, water at 25, 40, and 60 °C and sealed with plastic sheeting for 28 days. In addition, these specimens were cured in an autoclave for 6, 9, and 12 h. Results indicated that the compressive strength of multi-blended mixes containing silica fume 10 % by mass cured with plastic sealed and cured in water at 25 and 40 °C was similar to or higher than the corresponding Portland cement control at 28 day. Moreover, the mixes containing silica fume 10 % by mass cured in water at 60 °C had higher compressive strength than Portland cement control. X-ray diffraction and thermogravimetry results confirmed that there was increased pozzolanic reaction with increasing silica fume content which relates to the increasing in strength. For autoclaved curing, the compressive strength of multi-blended cement specimens with silica fume (total of 50 % replacement) was noticeably higher than control Portland cement mix and was highest when autoclaving time was 9 h. X-ray diffraction results showed the pattern of 0.9, 1.1, and 1.4 nm tobermorite crystalline phases as the main product of this curing. Thermogravimetry results showed dehydration of 1.4 nm tobermorite and 1.1 nm tobermorite at about 80–90 and 135–150 °C, respectively. Tobermorite (also shown by scanning electron microscope) thereby as a result lead to significant compressive strength improvement in the short time of autoclaved curing.     

垃圾焚烧飞灰理化特性研究

应用能谱分析、灰熔点炉、XRD、压汞仪等仪器手段对国内外8种垃圾焚烧飞灰的成分、熔点、晶相结构、颗粒特性等物理化学性质进行了系统研究。研究表明,飞灰成分因为受原料、炉型、取样位置等因素影响而差异很大。由于飞灰的成分差异导致垃圾焚烧飞灰比煤灰更易于熔融,对熔融处理有利,这主要归因于飞灰中SiO2含量以及SiO2/Al2O3差异,熔点与SiO2含量存在正比关系；添加CaO实现助熔是有条件的,不同飞灰由于焚烧条件和飞灰成分不同导致晶相组成有一定差异；飞灰内部孔径主要分布于0.3 μm～1.5 μm范围内,飞灰的比表面积为20.5 m2/g。     

Dynamic hydrothermal synthesis of Al-substituted 11 Å tobermorite from solid waste fly ash residue-extracted Al2O3

Fly ash residue (hereafter, FAR) is the by-product of pulverized coal combustion fly ash-extracted Al₂O₃. It results in a hazardous industrial solid waste if it does not have appropriate treatment and utilization. Al-substituted 11 Å tobermorite is successfully synthesized from FAR/SiO₂ mixture by dynamic hydrothermal treatment at 220 °C for 6 h. FAR and its hydrothermal reaction products are studied by chemical analysis, XRD, SEM, FTIR, and BET methods. The XRD results show that dicalcium silicate existing in FAR has been converted into tobermorite after the hydrothermal treatment. The main crystalline phases of product are Al-substituted 11 Å tobermorite and minor calcite. SEM results show that the hydrothermal products of the FAR/SiO₂ mixture consist of many tiny needlelike and platy crystals which form micro-porous spherical particles, ranging in size from a few microns to dozens of microns, and have a specific surface area of 49.004 m²/g. The Al-substituted tobermorite-bearing products have a high performance of exclusion of Cr³⁺ from acidified aqueous media, and the adsorption efficiency of Cr³⁺ is 98 %. The exclusion reaction proceeds rapidly, reaching equilibria within 1 h. The results show that this product has a potential to be used in industrial processes for adsorption of heavy metal cations from wastewater.     

垃圾焚烧氯对重金属迁移特性的影响

在管式反应炉上，通过加入有机氯（PVC）和无机氯（NaCl）研究了垃圾焚烧时氯对重金属迁移特性的影响。结果表明，在低温下，氯对重金属的挥发几乎没有影响，只有在较高的反应温度下氯才会对重金属的挥发特性产生影响，且垃圾焚烧过程中氯的存在使重金属更易向飞灰或烟气中迁移，垃圾中有机氯对重金属迁移特性的影响要大于无机氯。氯的存在会显著改变分布在飞灰和烟气或底渣和飞灰之间的重金属的分布特性。     

垃圾焚烧飞灰颗粒的微观形态特征及能谱研究

利用SEM/EDX，观察了垃圾焚烧飞灰颗粒的微观形态特征，探讨了其主要组成元素及其质量分数。实验结果表明，飞灰颗粒形态多样化，其中以不规则形状聚合体居多，球形体、絮状集合体相对较少，少数颗粒为棒状集合体。球形飞灰颗粒很少有重金属分布，不规则形状聚合体或絮状集合体的表面易分布重金属。从飞灰颗粒表面、内部组成元素的质量分数来看，Si、Ca、Al为主要元素。飞灰颗粒表面、内部有重金属Pb、Cu等分布，局部测定点其质量分数可高达16.2%和14.5%。     

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.     

利用旋风炉玻璃化处理垃圾焚烧飞灰实验研究

在自行设计的旋风炉实验台上对焚烧飞灰进行玻璃化熔融实验,系统研究了焚烧飞灰熔融前后微观形貌、灰渣中重金属特性和浸出特性。实验结果表明,焚烧飞灰旋风熔融后成黑色的脆硬玻璃态熔渣,表层呈浅黄色。在熔渣中N i、Cr的固溶率最高,均超过95%以上;低熔点重金属多数挥发至熔融烟气和熔融烟道飞灰颗粒中,Cd的固溶率仅有21%,Pb和Zn部分固溶于玻璃态熔渣中,两者固溶率可达42%以上。Cu和As的固溶率分别为37%和18%。熔融后的玻璃态熔渣重金属浸出率明显低于熔融前的焚烧飞灰,且均低于美国EPA的标准限值。     

Hydrothermal solidification of diatomaceous earth with analcime formation

Diatomaceous earth (DE) solidifies hydrothermally with analcime formation. With analcime formation, strength development was much greater than that with calcium silicate hydrate (CSH) formation. NaOH addition conditioned analcime formation because NaOH solution not only promoted dissolution of quartz and montmorillonite but also provided Na⁺ to form the analcime. Curing temperature and time affected analcime formation, and, in this study, over 6 h (at 200 °C) and 175 °C (for 12 h), analcime seemed to form readily. After hydrothermal treatment, the skeletons of diatoms can still be seen in the solid after analcime formation but can hardly be found after CSH formation.     

Hydrothermal synthesis of porous materials from sepiolite

In order not only to manufacture a tough porous material from sepiolite but also to sustain the inherent properties of the sepiolite, a hydrothermal experiment was carried out under saturated steam pressure at 473 K for up to 24 h by mixing slaked lime. The experimental results show that the addition of slaked lime exerts a positive effect on the strength development, and the strength enhancement is found to be due to tobermorite and calcium silicate hydrate (CSH) gel formation. Curing time seems to condition the strength development. A fine particle size of sepiolite improves its activity during the hydrothermal process, thus offering a higher strength and quicker hardening rate of the synthesized specimen. The acid activation also has a beneficial effect on the strength development. The hardening mechanism of the synthesized specimen is different between the specimens synthesized with or without acid activation. The strength enhancement for sepiolite with acid activating is due mainly to CSH gel formation, instead of tobermorite formation without acid activation.     

城市污水污泥与固体垃圾混烧过程中重金属迁移特性的研究

通过城市污水污泥与固体垃圾合成样品在小型管式焚烧炉内的混烧实验,采集灰渣样品进行XRF化学成分分析和浸出毒性鉴别测试,结合污水污泥焚烧过程中热力学平衡计算,定量表征了污水污泥中Cu、Pb、Zn、 Mn、Ni在焚烧过程中向底灰、飞灰和烟气迁移的特性及对灰渣的毒性影响。研究结果表明,污水污泥混烧显著提高了飞灰和底灰中重金属含量,其中Zn和Pb增加量最大,而Ni、Mn、Cu依次减少;随着有机氯的加入,飞灰中重金属含量呈现递增趋势,其中Zn表现最明显。毒性测试结果显示,污水污泥的混烧也导致了垃圾焚烧飞灰中Zn和Pb的浸出浓度超过中国对危险废物浸出的控制标准值。同时,重金属迁移的热力学平衡模型预测结果与实验结果差距较大。这表明热力学平衡计算方法还需进一步考虑影响重金属氯化物形成的其他因素,如在焚烧过程中反应动力学、焚烧室内氧气含量变化、与其他元素间的化学反应等。     

Trace elements in municipal solid waste incinerator fly ash

Municipal solid waste incineration (MSWI) significantly reduces volume and mass by as much as 80%, prolonging the life span of landfills. The concentration of heavy metals in the ash and their ability to leach into ground water is a serious concern when siting and designing MSWI ash landfills. Improved technology captures most heavy metals in the ash. The distribution of elements among the different ash particle sizes was determined by NAA. The bottom ash residue was separated into fractions ranging from 9.5 mm to 0.3 mm. The fly ash was separated into fractions from 250 m to 20 m. Landfills usually bury a mixture of both. The combined ash was separated into fractions over the entire range from >9.5 mm to <20 m. Thermal and epithermal neutron irradiations of size fractionated MSW bottom, fly and combined ash were performed to determine the distribution of various metals within the ash. Compared to normal soil, the ashes contained elevated amounts of numerous elements. Concentrations of the more enriched elements (Ag, Cd, Cr, Cu, Hg, Sb, Se, Sr and Zn) in fly ash were of particular interest as source markers.     

Lignite fired fly ash modified by chemical treatment for adsorption of zinc from aqueous solution

The purpose of the study described in this paper was to compare removal of Zn(II) from aqueous solutions by use of two adsorbents—alkali-modified fly ash (FAN) and alkali and dye-modified fly ash (FAN-MO). The effects of four conditions (solution pH, contact time, initial metal ion concentration, and dose of adsorbent) on removal of Zn(II) at 27 ± 5 °C were studied in batch mode. Adsorption of Zn(II) was greater at pH 4.0 for FAN (76.49 %) and at pH 5.0 for FAN-MO (24.72 %). Maximum adsorption of Zn(II) by FAN and FAN-MO was achieved after 50 min. The linear forms of the Langmuir, Freundlich, Tempkin, D–R, Harkin–Jura, and Frenkel–Halsey isotherms were used for experiments with different concentrations of the metals. Adsorption of Zn(II) ions satisfied the Langmuir isotherm model only. The adsorption capacity of both adsorbents was also investigated by column studies. Adsorption of Zn(II) ions on FAN in column studies (45.33 %) was lower than in batch mode studies. For FAN-MO, adsorption was 37.88 % in column studies, again lower than in batch mode studies. Fly ash modified by alkali had a higher adsorption capacity for Zn(II) ions than fly ash modified by alkali followed by addition of dye.     

Trace metal analysis of coal fly ash collected plain and on a quartz fibre filter

A simple, uniform procedure has been developed for microwave-based digestion of fly ash samples collected from the hoppers of an electrostatic precipitator (ESP), or collected from flue gases on a heat-resistant quartz fibre filter and for subsequent atomic absorption spectrometric analysis of trace metals (Be, Cd, Co, Cr, Cu, Mn, Ni, Pb, V and Zn). HNO₃, HCl, HF and mixtures have been tested as digestion acids. The combination of HNO₃ and HF has been found to be efficient for the digestion of fly ash samples with and without quartz fibre filter material. In spite of the complicated matrix, results with satisfactory accuracy and precision (relative standard deviation below 10% for most of the elements) have been obtained. In the case of analysis of fly ash together with a quartz fibre filter, the calibration curve has to be determined by standard addition to a blank solution containing a dissolved blank quartz fibre filter.     

Trace metal analysis of coal fly ash collected plain and on a quartz fibre filter

A simple, uniform procedure has been developed for microwave-based digestion of fly ash samples collected from the hoppers of an electrostatic precipitator (ESP), or collected from flue gases on a heat-resistant quartz fibre filter and for subsequent atomic absorption spectrometric analysis of trace metals (Be, Cd, Co, Cr, Cu, Mn, Ni, Pb, V and Zn). HNO(3), HCl, HF and mixtures have been tested as digestion acids. The combination of HNO(3) and HF has been found to be efficient for the digestion of fly ash samples with and without quartz fibre filter material. In spite of the complicated matrix, results with satisfactory accuracy and precision (relative standard deviation below 10% for most of the elements) have been obtained. In the case of analysis of fly ash together with a quartz fibre filter, the calibration curve has to be determined by standard addition to a blank solution containing a dissolved blank quartz fibre filter.     

Dynamic mechanical thermoanalysis of layered calcium silicate hydrates

Dynamic mechanical thermoanalysis (DMTA) was conducted on compacted specimens of calcium silicate hydrates (C-S-H), 1.4 nm tobermorite, jennite, and compacted hydrated Portland cement paste powders, as well as hardened cement paste. The synthetic silicates are key elements for compositional models of the hydrated calcium silicates present in cement paste. The study focuses on the nanostructural effects due to the removal of water from the 11 % RH condition. The DMTA results (E′ and tan? versus temperature curves) in the 25–110 °C range mimicked those of DMA (E′ and tan? versus mass loss curves) conducted at room temperature for C-S-H and cement paste. In addition, the DMTA curves for 1.4 nm tobermorite and jennite in the temperature range 110–300 °C were sensitive to phase changes including the transition of 1.4 nm tobermorite to 1.1 nm tobermorite and other forms, as well as the transition of jennite to metajennite. The DMTA curves of a 50/50 mixture of 1.4 nm tobermorite and jennite exhibit similarities and differences to that of hydrated cement paste that are influenced by porosity and the amorphous nature of C-S-H in the cement paste. The study provides useful data for evaluating Taylor’s concept of a possible tobermorite-jennite model for the C-S-H present in hydrated cement paste.     

垃圾焚烧中氯化物对重金属Pb迁移转化特性的影响

采用管式炉和模拟垃圾对垃圾中氯化物对重金属Pb迁移转化特性的影响进行了研究,使用ICP-AES(美国EPA消解方法)、SEM、EDS和XRD等对重金属浓度、灰渣表面形貌、成分和灰渣X射线衍射物相等进行了分析。结果表明,氯化物含量的增加使得Pb生成更多易挥发性物质,从而向飞灰迁移分布,但有机氯PVC的影响比无机氯NaCl的大。无机氯NaCl和有机氯PVC对焚烧中Pb迁移转化作用机理不同,NaCl易与Pb反应生成PbCl2,而PVC与Pb反应除了生成PbCl2外,还生成了Pb(ClO4)2、PbCl2O4和PbO2。同时焚烧温度和停留时间也是焚烧中Pb迁移分布的两个主要影响因素。     

Behaviour of inorganic constituents of municipal sewage sludge during fluidized-bed combustion

The fate of main heavy metals in municipal sewage sludge (As, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, and Zn) was investigated using a fluidized-bed reactor. The behaviour of metals in the incinerator was explored by comparing metal contents in the two main ash streams: bottom (in-bed) ash and ash separated by a cyclone (cyclone-catch ash). All metals showed enrichment in the cyclone ash. The ash originating from sewage sludge is thermally quite stable consisting of quartz, hematite, spar(s), mica(s), apatite, and anhydrite. It does not soften nor becomes sticky and does not tend to agglomerate at temperatures up to 950°C.     

酸洗污泥与煤共燃烧过程中重金属的迁移分布研究

采用高温管式炉系统进行酸洗污泥与煤共燃烧实验。对共燃烧后As、Cr、Ni、Cd、Cu、Mn、Pb、Zn、Sb、Se重金属元素在烟气、飞灰及炉渣中的分配率进行分析。结果表明,Cd、Se、Zn是易挥发重金属,主要分布在烟气和飞灰中。Cd在烟气中的最大分配率为61%;Se在烟气中的分配率为38.58%~94.612%;Zn在低、高温段分别主要分布在烟气和飞灰中。As、Cu、Pb、Sb是半挥发重金属,Pb在炉渣中的分配率较稳定,分配率为42.67%~64.76%,在烟气和飞灰中的分配率波动较大,其分配率分别是14.176%~45.79%和9.78%~32.55%;Sb在烟气中的最大分配率为37.64%;温度升高反而会抑制As、Cu挥发,这与高温下As、Cu易与矿物质反应生成络合物有关。Ni、Cr、Mn属于难挥发重金属,绝大部分残留在炉渣中,且分配率对温度变化不敏感。赋存于炉渣中的Cr、Ni分别超过95%和97%;随着温度升高,Mn在炉渣中的分配率由71.46%增加到96.89%。     

Innovative stabilization/solidification processes of fly ash from an incinerator plant of urban solid waste

Fly ash samples from an incineration plant of urban solid waste (USW) were submitted to a stabilization/solidification process based on encapsulation with a polyester resin. With this process, a very limited increase of about 25% in weight and a compressive strength as high as 1200 kg/cm² can be obtained. The efficiency of the process to stabilize/solidify the residues and to reduce the pollutant release was evaluated by performing both mechanical and leaching tests following the IRSA–CNR standard method. The leaching test was based on treating the sample with an acetic acid solution adjusted to pH 5.2 for 24 h under magnetic stirring. Toxic metals (Pb, Cd, and Cu) and organic pollutants (PAHs, PCBs, and OCPs) were determined in raw fly ash samples and in the leaching solutions of treated samples. Raw residues showed higher concentrations of trace metals than the regulatory limits, whereas the stabilized/solidified residues showed a concentration of all the pollutants lower than the regulatory limits and a compressive strength much higher than the suggested minimum value. Finally, a critical comparison with conventional stabilization/solidification processes based on the use of Portland cement highlighted that the polyester resin-based process performed much better in terms of the release of both organic and inorganic pollutants, thus substantially lowering the environmental impact of these residues.