Preparation and characterization of fully waste-based glass-ceramics from incineration fly ash,waste glass and coal fly ash

Municipal solid waste incineration (MSWI) fly ash (FA) is a typical hazardous waste due to its high contents of toxic heavy metals, and hence its disposal has attracted global concern. In this work, it was recycled into environmental-friendly CaO–Al₂O₃–SiO₂ system glass-ceramics via adding coal fly ash (CFA) and waste glass (WG). The effects of CaO/SiO₂ ratios and sintering temperatures on the crystalline phases, morphologies, mechanical and chemical properties, heavy metals leaching and potential ecological risks of glass-ceramics were investigated. The results showed that wollastonite (CaSiO₃), anorthite (CaAl₂Si₂O₈) and gehlenite (Ca₂Al₂SiO₇) were the dominant crystals in the glass-ceramics, which were not affected by CaO/SiO₂ ratio and sintering temperature. The compressive strength increased, while the Vickers hardness and microhardness decreased as increasing the sintering temperatures from 850 to 1050 °C, which reached their maximum values of 660.69 MPa, 6.14 GPa, and 7.43 GPa, respectively. However, the increase of CaO/SiO₂ ratio resulted into the reduction of the three mechanical parameters. As varying CaO/SiO₂ ratio from 0.48 to 0.86, the maximum compressive strength, Vickers hardness and microhardness were 611.80 MPa, 5.43 GPa, and 6.56 GPa, respectively. Besides, all the glass-ceramics exhibited high alkali resistance of >97%. The extremely low heavy metals leaching concentrations and low potential ecological risk of glass-ceramics further revealed its environmentally friendly property and potential application feasibility.     

固体废物焚烧灰渣毒性评估及其控制方法研究进展

固体废物焚烧产生的灰渣时环境是一种二次污染物,其中,焚烧飞灰因其含有重金属及有毒有机污染物而被列为危险固体废物,因此在其最终处置前,必须进行稳定化处理.作者介绍了灰渣的污染特性,尤其是飞灰中所含重金属、有毒有机污染物的毒性及其各种评估方法,包括水平震荡法、浸出柱实验法、毒性浸出程序(TCLP),并综述了当前灰渣的控制技术方法和资源化利用手段,如水泥固化法、熔融固化法、化学药剂固化法.     

The leachability of heavy metals in hardened fly ash cement and cement-solidified fly ash

The effect of mix proportion, leachant pH, curing age, carbonation and specimen making method etc. on the leaching of heavy metals and Cr(VI) in fly ash cement mortars and cement-solidified fly ashes has been investigated. In addition, a method for reducing the leaching of Cr(VI) from cement-solidified fly ashes is proposed. The results mainly indicate that: (1) either Portland cement or fly ash contains a certain amount of heavy and toxic metals, and the leaching of them from hardened fly ash incorporated specimens exists and is increased with fly ash addition and water to cement ratio; (2) the leachability of some heavy metals is greatly dependent on leachant pH; (3) when carbonation of cement mortars occurs the leaching of chromium ions is increased; (4) the amount of heavy metals leached from cement-solidified fly ashes depends more on the kind of fly ash than their contents in fly ash; and (5) with ground granulated blast furnace slag addition, the leaching of Cr(VI) from solidified fly ashes is decreased.     

The influence of municipal solid waste incinerator fly ash slag blended in cement pastes

This study investigates the effect of mixing Type I, Type II, and Belite cements with municipal solid waste incinerator (MSWI) fly ash slag-blended cement (FASBC). The experimental results showed that a 10-40% slag replacement of by caused an increase in the initial and the final setting time. The toxicity characteristic leaching procedure (TCLP) results show that the heavy metal content met the Environmental Protection Administration (EPA) regulatory limits. From the results, it can be seen that the effect of the replacement of 10-40% of the cement by slag caused an increase in the initial and final setting time. Compressive strength results indicate that the slag-blended cement (SBC) pastes had slower compressive strength development in the early stages, but this strength obviously increased at later ages. Variations in the Portland cements can affect early strength development but have no significant effect on the degree of hydration at later ages. MSWI slag gives a relatively slower increase in early strength but may show a greater degree of reaction at later ages.     

Preparation of zeolitic adsorbents from waste coal fly ash

Power plants burning coal generate a large amount of fly ash as waste matter. The objective of this study is to produce zeolitic adsorbents that possesses high adsorptive capacity for toxic cations. The sample was first pretreated with a High Intensity Magnetic Separator for the removal of iron and magnetic materials (mainly Fe₂O₃ and TiO₂). The zeolitic adsorbents were prepared under the various conditions of NaOH concentration (1–5 N), reaction time from 3 to 96 hours and at the various temperatures of 60, 80 and 100°C. The results of the experiment showed that the coal fly ash should be synthesized with 4 N NaOH for 48 hours at 100°C in order to have good adsorptive capacity. The zeolitic adsorbents showed higher cation exchange capacity values than the natural zeolite in removing NH ₄ ⁺ , Pb²⁺, Ca²⁺and Cd²⁺ions.     

Effect of CO2 on leaching from a cement-stabilized MSWI fly ash

A cement-stabilized MSWI fly ash was leached according to the diffusion test NEN 7345. The influence of carbonation during leaching was tested by bubbling N₂, air or CO₂ through the leachant. The original test, where no gas is bubbled, was used as a reference. The periodically renewed leachant was analysed for its ionic concentrations. Porosity measurements, XRD and scanning electron microscopy (SEM) analysis were performed.For some metals, leaching is seriously underestimated when precipitation on the tank walls is not included. Thus, the sum of the dissolved portion and the precipitated portion has to be taken into account. Diffusion tests with N₂ and air bubbling gave results that were not significantly different from those obtained in the reference test. Bubbling of pure CO₂, however, influenced leaching profoundly. This was mainly due to the lowering of pH. The reaction of hydroxides to carbonates—another effect of CO₂ bubbling—had a negligible effect on leaching.     

Conversion of coal fly ash into zeolite and heavy metal removal characteristics of the products

Fly ash obtained from a power generation plant was used for synthesizing zeolite. Zeolites could be readily synthesized from the glassy combustion residues and showed potential for the removal of heavy metal ions. By the use of different temperatures and NaOH concentration, five different zeolites were obtained: Na-P1, faujasite, hydroxy sodalite, analcime, and cancrinite. The synthesized zeolites had greater adsorption capabilities for heavy metals than the original fly ash and natural zeolites. Na-P1 exhibited the highest adsorption capacity with a maximum value of about 1.29 mmole Pb g^-1 and had a strong affinity for Pb²⁺ ion. The metal ion selectivity of Na-P1 was determined as: Pb²⁺> Cu²⁺> Cd²⁺> Zn²⁺, consistent with the decreasing order of the radius of hydrated metal ion. The adsorption isotherm for lead by Na-P1 fitted the Freundlich rather than the Langmuir isotherm.     

Release to the gas phase of metals, S and Cl during combustion of dedicated waste fractions

The release to the gas phase of inorganic elements such as alkali metals, Cl, S, and heavy metals in Waste-to-Energy (WtE) boilers is a challenge. Besides the risk of harmful emissions to the environment, inorganic elements released from the grate may cause severe ash deposition and corrosion problems in the boiler, subsequently leading to decreased overall efficiency and costly, unscheduled shut-downs. The objective of this study was to obtain quantitative data on the release of inorganic elements from dedicated, well-characterized waste fractions; in order to understand the release pattern and the link to the formation of fly ash and aerosols in full-scale waste incinerators. The release of metals, S and Cl from four dedicated waste fractions was quantified as a function of temperature in a lab-scale fixed-bed reactor. The waste fractions comprised chromated copper arsenate (CCA) impregnated wood, shoes, automotive shredder waste and PVC (poly-vinyl-chloride). The waste fractions were characterized by use of wet chemical analysis, and, based on the chemical composition of the initial fuel sample and the ash residue after the experiments; the release of inorganic elements was quantified. The lab-scale release results were then compared with results from a related, full-scale partitioning study, in which test runs with the addition of similar, dedicated waste fractions to a base-load waste had been performed in a grate-fired WtE boiler. In general, the elements Al, Ca, Cr, Cu, Fe, Mg, Si and Ti were not released, in consistency with the non-volatile nature of these elements. The elements Pb, Zn, Cl, S, Na, K and As, on the other hand, were released to a significant extent, and the release pattern was found to be both temperature dependent and fuel specific. Possible release mechanisms were discussed in relation to the chemical characteristics of each fuel, and some of the same release mechanisms as previously suggested for e.g. biomass-based fuels in the literature also seemed to apply for the present waste fractions.     

Neutralizing capacity of bottom ash from municipal solid waste incineration of different particle size

This study measures the neutralizing capacity of bottom ash from municipal solid waste incineration of different particle sizes. We examine the effect of particle size on the weathering process, a method popularly used for stabilization of heavy metals in incineration of bottom ash. The distribution of particle sizes in municipal solid waste incineration bottom ash is rather broad, ranging from fine powder to as large as 40 mm in diameter. Although considered a by-product highly suitable as a road construction material, the high level of heavy metal leaching is an obstacle to its reuse. Weathering, a method used to reduce heavy metal leaching, is a lengthy process taking over thee months to complete. The chief reaction involved in weathering is carbonation occurring between Ca(OH)2 in bottom ash and CO₂(g) in the atmosphere. During this process, CaCO₃ is produced, causing the pH level to drop from over 12 to about 8.2 and reducing heavy metal leaching. In this paper, we attempt to determine the particle size best suited for reducing the period required for weathering bottom ash by identifying characteristics of different particle sizes that affect heavy metal leaching and neutralization.     

Studies of Cd, Pb and Cr distribution characteristics in bottom ash following agglomeration/defluidization in a fluidized bed boiler incinerating artificial waste

This study focused on the effects of agglomeration/defluidization on the Cd, Pb and Cr distribution in bottom ash particles of different sizes. In this study we have incinerated artificial waste, which was a mixture of sawdust, polypropylene, selected metal solutions, and polyethylene. The experimental parameters included Na concentration, addition of Ca and Mg and operating temperature. The results indicated that particle size decreased by attrition and thermal impact in the absence of added Na. When Na was added to the system, this metal reacted with silica sand to form eutectics, which increased particle size. Further addition of Ca and Mg was found to prolong the operating time, with greater amounts of liquid eutectic accumulating, leading to increase particle size.The heavy metal concentrations in coarse and fine particles were greater than those present in particles of intermediate sizes over a range of experimental conditions. As the particle size decreased below 0.59 mm or increased above 0.84 mm, the heavy metal concentrations increased. As Ca and Mg were added, the heavy metal concentrations in particles of all sizes increased relative to those present in identical particles when no metals or only Na were added. Additionally, the ratio of Cd sorption to silica sand decreased with increasing Na concentration, but Cr sorption had the opposite tendency. Therefore, while the addition of Na tended to increase agglomeration, it also increased the tendency for heavy metals to remain in the sand bed of fluidized bed incinerators. Addition of Ca and Mg not only inhibited the agglomeration/defluidization process, resulting in increased operating time, but also enhanced the removal of heavy metals by silica sand, decreasing the concentration of heavy metals in reactor exhaust.     

Manipulation of the ash flow temperature and viscosity of a carbonaceous Sasol waste stream

In 2001 Sasol investigated the possibility to convert a Sasol-Lurgi MK III fixed bed dry bottom coal gasifier at the former Sasolburg coal-to-liquids plant to a slagging gasification process and selected the Lurgi Multi Purpose Gasification (MPG) process for this purpose. At the time the MPG process was considered as a possible technology option suitable for the gasification of feedstocks which are difficult to manage, for example solid-liquid mixtures which are unsuitable for conventional solids or liquids processes and furthermore tars with a large variety of properties and high solids content have been gasified successfully.The most obvious differences between the feedstocks previously gasified, compared to the Sasol dusty tar, were found to be the viscosity and melting point of the dusty tar. The viscosity of the Sasol dusty tar mixture (as received) was higher than a factor of 10 of the previously used feedstocks. Another important feedstock property is the ash melting point of the feed within the gasifier. Ash particles fed with the tar melt in the high temperature zone of the flame. Molten ash particles, which hit the gasifier wall, will solidify and stick to the wall, if the wall temperature is below the melting point of the ash. The melting point of the dusty tar ash is 1380 °C and a fluxing agent has to be added to the dusty in order to reduce the melting temperature below 1250 °C to limit excessive wear of the refractory lining.Two approaches were evaluated in order to lower the viscosity, i.e. milling of the dusty tar and addition of low viscosity feedstocks to the dusty tar. It was concluded from this study that the viscosity of dusty tar can be decreased with the addition of specific waste solvent streams. The ash fusion temperatures of dusty tar can be lowered with the addition of a fluxing agent. The addition of spent Fe-catalyst as fluxing agent was also investigated, but was found to be not as effective as limestone. In some cases the addition of Fe can cause the acid/base ratio to change in such a way that the ash fusion temperature increases.In conclusion it can be highlighted that the ash flow temperature and viscosity of the Sasol dusty tar stream, and possibly other carbonaceous sources as well, can be decreased to acceptable values for slagging gasification applications. The results clearly show in both oxidizing and reducing atmospheres the Fe-catalyst was transformed into the slag melt as either Fe₂O₃ under oxidizing conditions and FeO under reducing conditions. The slag showed no sign of metallic Fe and was very homogeneous under oxidizing and reducing conditions. Spent Fe-catalyst can be discarded in a safe and environmentally acceptable way during high temperature gasification.     

Influence of ash composition on heavy metal emissions in ash melting process

Two kinds of fly ash, discharged in the combustion of either refused derived fuel (RDF) or car shredder dust (SD), were examined for the emission of heavy metals in melting process under oxidizing and reducing conditions. The residual fractions of heavy metal in slag were experimentally estimated. As a result, it was confirmed that several volatile heavy metals were readily emitted during melting process. The type of atmosphere provided for the melting process was found to affect the emission of some volatile metals in RDF ash, but not in SD ash. The emission of volatile heavy metals in RDF ash under oxidizing conditions was lower than under any other conditions in this study. The emission behavior of iron and heavy metals in RDF ash under reducing conditions was similar to that in SD ash. These facts indicated that phosphorous in RDF ash had the property of fixing the volatile metals in the slag only under oxidizing conditions. Then the mixture of SD ash with phosphorous oxide powder was also tested in a melting process, and the result was consistent with the above inference of the effect of phosphorous.     

Agglomeration problems during fluidized bed gasification of olive-oil residue: evaluation of fractionation and leaching as pre-treatments

The effect of two pre-treatment techniques leaching and fractionation on the agglomeration problems observed during the gasification of the olive-oil residue material in a fluidized bed reactor was studied. The obtained results proved to be very positive in the case of leaching showing a substantial decrease of the agglomeration phenomena in the bed during the gasification tests compared with the results from the tests with the un-treated material. On the other hand, fractionation appeared to increase the ash reactivity leading to a dramatic increase of the agglomeration problems during the gasification process.     

Heavy metal characterization of CFB-derived coal fly ash

The present study investigates the heavy metal content of coal fly ash (FA) samples coming from three different sampling points (secondary cyclone, cooler and filter) of a pilot plant combustion facility. The combustion experiments were carried out in a 0.1 MW Circulated Fluidized Bed (CFB) boiler using South African coal, with the addition of limestone for sulfur capture. FA was tested for the presence of selected heavy metals using inductively coupled plasma optical emission spectrometry (ICP-OES); batch leaching tests were conducted as well. The samples were also characterized in terms of their microstructure, chemical and mineralogical composition, total surface area and particle size distribution. Most of the studied metals (Cd, Cr, Cu, Ni, Mn, Zn) showed enrichment in the fine, filter FA particles, while Pb was mostly concentrated in the cooler sample. Regarding leaching characteristics of the examined samples, Cr was found to occur in considerable amounts. Although the use of CFB technology for the combustion of solid fuels steadily gets bigger worldwide, only a very limited number of studies have environmentally assessed CFB-coal FAs to date. Thus, the current study aims to contribute toward building a more integrated knowledge on the environmental impact of this abundant power production by-product.     

垃圾焚烧底渣中重金属的研究

采用Tessier四步分级提取法对垃圾焚烧飞灰中的4种重金属(锌、镍、铜、镉)的化学形态进行了研究。结果表明,金属Cd主要以底渣态的形式存在(占总镉质量的61.22%~62.41%),Ni,Zn以底渣态和铁锰氧化态为主,Cu主要以有机结合态为主(占总铜质量的60.97%~62.29%)。同时4种金属的生物有效性由高到低顺序依次为Ni,Cd,Zn,Cu。     

Coleaching behavior of lead of incineration ash waste mixtures

In this paper, the authors reported the coleaching behavior of lead of incineration ash waste mixtures, i.e., fabric filter residues [called fly ash (FA)] and semidry scrubber residues [called reaction product (RP)]. The leaching potential of lead metal of those mixtures was estimated by toxicity characteristic leaching procedure (TCLP) with acidic extractant. And it was found to be under the controls of dissolution rate and interparticulate interactions. The author has developed a linear regression equation to fit the coleaching behavior well, the equation is basing on the chemical compositions of (RP/FA) mixtures. The impact of this study is that the leaching toxicity of mixture could be reduced to be nonhazardous, just only to adjust the mixing ratio of these two ash wastes. And the authors are able to use these ash wastes for wider recycling applications without any intermediate stabilization treatment.     

Use of municipal solid waste incineration fly ash in concrete

With a view of reducing the quantities to be landfilled, the Solvay Company has been working on the development of a new physicochemical treatment for municipal solid waste incineration (MSWI) fly ashes: the Revasol process. This process allows reducing the soluble fraction, fixing heavy metals and eliminating dioxins. This article reports on the characteristics of a treated ash and on its use in concrete. For the latter point, three characteristics were chosen: the compressive strength and the durability of the hardened concrete and its behavior to leaching. From mechanical and durable points of view, the ash incorporated in the concrete behaves like ordinary sand. The leaching tests carried out on the concrete confirm that the process makes it possible to obtain materials without major risks for the environment. Also, these results as a whole suggest that the use of waste in concrete constitutes a potential means of adding value.     

电镀污泥中Cr~(3+)的堆浸工艺研究

利用湿法冶金的核心技术-酸浸法,对电镀污泥中的重金属铬进行堆浸。实验证明,利用H2SO4作堆浸酸液,当固液比为1∶50,堆浸液pH=1,静态堆浸4 h或动态堆浸3 h,处理后堆浸液可用于回收有用金属,堆浸残渣中的铬的含量低于国家电镀污泥农用排放标准,可进行填埋等处理。     

Electrokinetic remediation of bottom ash from municipal solid waste incinerator

The electrokinetic remediation was studied to verify the possibility to reclaim the bottom ash from municipal solid waste incineration (MSWI). In Italy, a production of 1 million tons per year of this kind of residue has been estimated, 90% of which is still landfilled. This work shows the results of four electrokinetic remediation tests for the removal of Pb, Cu, Zn, Cd and chlorides, using an open cell with graphite electrodes and without enhancing agents. The four tests have, respectively, been performed at a constant current density of 0.89, 1.67, 2.04 and 2.48 mA cm⁻², with duration of 42, 68, 47 and 40 h. Heavy metals occur in ashes in various forms, such as exchangeable, adsorbed, precipitated, organically complexed and residual phases. In order to determine the nature of any given system, in terms of specific chemical species and pertaining mobilities, sequential extraction analyses have been performed. The release of pollutants was investigated for treated and untreated ash. After treatment, the concentration of pollutants in the leachate was reduced by 31-83%, better results being obtained for chlorides. Both the low amount of heavy metal extracted and the increase of ash pH during the electrokinetic tests, suggest to use enhancing agents or a cation exchange membrane at the cathode, to prevent the precipitation of metals as hydroxides.