Behavior of metals in ash melting and gasification-melting of municipal solid waste (MSW)

In this study, metal behavior in ash-melting and municipal solid waste (MSW) gasification-melting facilities were investigated. Eight ash-melting and three MSW gasification-melting facilities with a variety of melting processes and feedstocks were selected. From each facility, melting furnace fly ash (MFA) and molten slag were sampled, and feedstock of the ash-melting processes was also taken. For the ash melting process, the generation rate of MFA was well correlated with the ratio of incineration fly ash (IFA) in feedstock, and this was because MFA was formed mostly by mass transfer from IFA and a limited amount from bottom ash (BA). Distribution ratios of metal elements to MFA were generally determined by volatility of the metal element, but chlorine content in feedstock had a significant effect on Cu and a marginal effect on Pb. Distribution ratio of Zn to MFA was influenced by the oxidizing atmosphere in the furnace. High MFA generation and distribution ratio of non-volatile metals to MFA in gasification-melting facilities was probably caused by carry-over of fine particles to the air pollution control system due to large gas volume. Finally, dilution effect was shown to have a significant effect on metal concentration in MFA.     

Effect of unburned carbon on lead, zinc, and copper recovery from molten fly ash by chloride-induced volatilization

The present work focuses on investigation of the effective recovery of heavy metals from molten fly ash by applying chloride-induced volatilization. In particular, the effect of unburned carbon on the chloride-induced volatilization of lead, zinc, and copper from model and real molten fly ashes was investigated in the temperature range 873–1173 K under a N₂ atmosphere. As a result, almost 100% of lead and a significant proportion of zinc were volatilized from the real molten fly ash samples at 1173 K. In contrast, for the model fly ash, volatilization ratios of lead and zinc at 1173 K were only 85% and 25%, respectively. Further, the results of X-ray diffraction analysis suggested that PbO in molten fly ash was converted either to Pb₂OCl₂ or Pb by respective chlorination and reduction reactions. Meanwhile ZnO and CuO in the molten fly ash were reduced to Zn and Cu by reaction with unburned carbon. Subsequently, Pb, Zn, and Pb₂OCl₂ were volatilized, but Cu remained in the solid residue. Finally, the volatilization ratio of zinc increased with the addition of carbon, and more than 98% of zinc was volatilized at 1173 K from a fly ash with a carbon content of 20%.     

Metal distribution in incineration residues of municipal solid waste (MSW) in Japan

This study aimed to identify distribution of metals and the influential factors on metal concentrations in incineration residues. Bottom ash and fly ash were sampled from 19 stoker and seven fluidized bed incinerators, which were selected to have a variety of furnace capacity, furnace temperature, and input waste. In the results, shredded bulky waste in input waste increased the concentration of some metals, such as Cd and Pb, and the effect was confirmed by analysis of shredded bulky waste. During MSW incineration, lithophilic metals such as Fe, Cu, Cr, and Al remained mainly in the bottom ash while Cd volatilized from the furnace and condensed to the fly ash. About two thirds of Pb and Zn was found in the bottom ash despite their high volatility. Finally, based on the results obtained in this study, the amount of metal in incineration residues of MSW was calculated and the loss of metal was estimated in terms of mass and money. A considerable amount of metal was found to be lost as waste material by landfilling of incineration residues.     

Effect of chlorine on volatilization of Na, K, Pb, and Zn compounds from municipal solid waste during gasification and melting in a shaft-type furnace

In the present work, a shaft-type furnace model in which the furnace column is divided into multiple cells was proposed and equilibrium reaction calculation software was used to describe the model. The model was used to study the effects of gasification and melting conditions such as temperature, oxygen partial pressure, and chlorine content on the volatilization behaviors of the low-boiling-point metals Na, K, Pb, and Zn during the gasification and melting process of municipal solid waste in a shaft-type furnace. Consequently, the volatilization ratios of Na, K, Pb, and Zn compounds in the exhaust gas from a pilot plant shaft-type furnace were found to be in good agreement with the calculation results, and the Na, K, Pb, and Zn compounds were volatilized mainly as metal chlorides in the temperature range up to approximately 1173 K. With a further rise in temperature, these low-boiling point metals were volatilized as metallic forms. It was found that almost 100% of Pb and Zn compounds were volatilized regardless of the chlorine content in municipal solid waste; in contrast, the volatilization rates of Na and K increased when the chlorine content increased. Finally, Na, K, Pb, and Zn compounds were converted from reduced metals to metal chlorides such as NaCl, KCl, PbCl₂, and ZnCl₂ with an increase in the ratio of chlorine to each metal.     

Leachability of municipal solid waste ashes in simulated landfill conditions

In Japan the volume of municipal solid waste is reduced by incineration, with fly ash and bottom ash disposed in controlled landfills. The leachability of anions and heavy metal cations, Zn, Cu and Pb, from MSW fly ash and bottom ash at different pHs was examined using batch- and column-leaching tests. The MSW ashes had a high capacity for neutralizing acids. Behaviour during leaching depended on the pH of the solution. For the volumes applied, the leachabilities of MSW fly ash were very similar at pHs from 3 to 6. Due to its amphoteric nature, Pb is leachable at pHs of approximately 10 or more, with leachate concentrations of about 3 and 3-10mg/L for the fly ash and bottom ash, respectively, much higher than for Zn and Cu. Pb concentrations for most leaching solutions were 1 and 3mg/L for the fly ash and bottom ash, respectively. Zn, and Cu leached at low concentrations for solutions of pH 3-6. Na and K ions leached at high concentrations of approximately 5000 mg/L in the first batch leaching test, decreasing to 10mg/L by the fourth leach. Ca and Mg ions leached more gradually than Na and K. Cl(-) and SO(4)(2+) ions were the major anions in the MSW ash. The high pH and cation leaching are expected to have negative impacts on the performance of clay liners.     

Characteristics of element distributions in an MSW ash melting treatment system

Thermal treatment of municipal solid waste (MSW) has become a common practice in waste volume reduction and resource recovery. For the utilization of molten slag for construction materials and metal recovery, it is important to understand the behavior of heavy metals in the melting process. In this study, the correlation between the contents of elements in feed materials and MSW molten slag and their distributions in the ash melting process, including metal residues, are investigated. The hazardous metal contents in the molten slag were significantly related to the contents of metals in the feed materials. Therefore, the separation of products containing these metals in waste materials could be an effective means of producing environmentally safe molten slag with a low hazardous metals content. The distribution ratios of elements in the ash melting process were also determined. The elements Zn and Pb were found to have a distribution ratio of over 60% in fly ash from the melting furnace and the contents of these metals were also high; therefore, Zn and Pb could be potential target metals for recycling from fly ash from the melting furnace. Meanwhile, Cu, Ni, Mo, Sn, and Sb were found to have distribution ratios of over 60% in the metal residue. Therefore, metal residue could be a good resource for these metals, as the contents of Cu, Ni, Mo, Sn, and Sb in metal residue are higher than those in other output materials.     

An investigation on the potential of metal recovery from the municipal waste incinerator in Taiwan

This study aimed to identify distribution of metals and to estimate the amount of these metals that can be potentially recovered from incineration residues. First, the partitioning behavior of Cr, Cu, Fe, Cd, Al, Zn, and Pb in bottom ash and fly ash was investigated in one large municipal waste incinerator in Taiwan. In addition, the material flow analysis (MFA) method was used to estimate the material flux of metals within incinerator plant, and to calculate the amount of metal recovery. According to the findings of this study, six metals (Fe, Al, Cu, Zn, Cr, and Pb) concentrated in bottom ash mostly, while Cd existed primarily in fly ash. The weight percentages of Fe (4.49%), Al (5.24%), Cu (1.29%), Zn (2.21%), and Pb (0.58%) in incinerator ash are high, and even higher than the compositions of natural minerals. Finally, the amount of Cr, Cu, Fe, Cd, Al, Zn and Pb that can be potentially recovered from incineration residues will reach 2.69 x 10(2), 1.46 x 10(4), 4.91 x 10(4), 6.92 x 10(1), 5.10 x 10(4), 1.85 x 10(4) and 4.66 x 10(3) ton/yr, respectively.     

Chemical properties of heavy metals in typical hospital waste incinerator ashes in China

Incineration has become the main mechanism for hospital waste (HW) disposal in China after the outbreak of Severe Acute Respiratory Syndrome (SARS) in 2003. However, little information is available on the chemical properties of the resulting ashes. In the present study, 22HW ash samples, including 14 samples of bottom ash and eight samples of fly ash, were collected from four typical HW incineration plants located across China. Chemical analysis indicated that the HW ashes contained large amounts of metal salts of Al, Ca, Fe, K, Mg, Na with a concentration range of 1.8-315gkg(-1). Furthermore, the ashes contained high concentrations of heavy metals such as Ag, As, Ba, Bi, Cd, Cr, Cu, Mn, Ni, Pb, Ti, Sb, Sn, Sr, Zn with a vast range of 1.1-121,411mgkg(-1), with higher concentrations found in the fly ash samples. Sequential extraction results showed that Ba, Cr, Ni and Sn are present in the residual fraction, while Cd existed in the exchangeable and carbonate fractions. As, Mn, Zn existed in the Fe-Mn oxide fraction, Pb was present in the Fe-Mn oxide and residual fractions, and Cu was present in the organic matter fraction. Furthermore, toxicity characteristic leaching procedure (TCLP) results indicated that leached amounts of Cd, Cu and Pb from almost all fly ash samples exceeded the USEPA regulated levels. A comparison between the HW ashes and municipal solid waste (MSW) ash showed that both HW bottom ash and fly ash contained higher concentrations of Ag, As, Bi, Cd, Cr, Cu, Pb, Ti, and Zn. This research provides critical information for appropriate HW incineration ash management plans.     

Demobilisation of critical contaminants in four typical waste-to-energy ashes by carbonation

Two bottom ashes, one air pollution control (APC) residue and one fly ash from three different Swedish municipal solid waste incineration (MSWI) plants were characterised regarding the leaching of environmentally relevant components. Characterisation was performed using a diffusion tank leaching test. The impact of carbonation on the release of eight critical components, i.e., Cl(-), Cr, Cu, Mo, Pb, Sb, Se, SO(4)(2-) and Zn, was assessed at a lab-scale and showed carbonation to have a more pronounced demobilising effect on critical components in bottom ashes than in APC residue and fly ash. From grate type incinerator bottom ash, the release of Cr decreased by 97%, by 63% for Cu and by 45% for Sb. In the investigated APC residue, the releases of Cr, Se and Pb were defined as critical, although they either remained unaffected or increased after carbonation. Cl(-) and SO(4)(2-) remained mobile after carbonation in all investigated residues.     

Thermodynamic and experimental studies on condensation behavior of low-boiling-point elements volatilized in the melting process

To study the volatilization and condensation behaviors of low-boiling-point elements in the waste melting process, experiments were conducted to collect the dust from a coke-type incineration residue melting furnace. Then, a comparison was made between the experimental results and the calculated values obtained from a thermodynamic equilibrium model in terms of the chemical composition of the dust. The composition of the dust collected from a cylindrical filter in the exhaust gas duct was determined by chemical methods, scanning electron microscopy/energy-dispersive X-ray spectroscopy, and X-ray analysis. As a result, the sampled dust was classified into two different types: fine particles mainly containing Na, K, Pb, and Zn collected from the side face of the cylindrical filter, and large particles containing Ca, Si, and Al collected from the bottom face of the cylindrical filter. From X-ray analysis of the sampled dust, NaCl, KCl, PbS, and ZnS were identified in the fine-particle dust, while CaO, SiO₂, Al₂O₃ were detected in the large-particle dust. From the results of the thermodynamic equilibrium calculation in the gas cooling process from 2000 to 600 K, it was found that Na, K, Pb, and Zn volatilized as metals in the melting furnace were condensed as alkali chlorides such as NaCl and KCl and heavy metal sulfides such as PbS and ZnS. These computational results were in good agreement with the X-ray diffraction results of the sampled dust at a gas temperature of 823 K for the formation of NaCl, KCl, PbS, and ZnS.     

Effect of multi-factors interaction on trace lead equilibrium during municipal solid waste incineration

The thermodynamic equilibrium of trace lead during the waste incineration was calculated on the basis of the minimization of the total Gibbs energy. The effect of incineration condition and MSW components on Pb distribution was investigated mainly in the view of the interaction of related elements. In the oxygen-rich condition, incineration temperature affects Pb distribution by the interaction of Cl, Ca and Na. In the fuel-rich condition, incineration temperature affects Pb distribution directly by the thermal transition of PbS(s) to PbCl(g) and the thermal transition of PbCl(g) to Pb(g). Air ratio has significant effect on Pb distribution by the interaction of H, O and Cl. The liberated Cl in oxidizing condition is far less than that in reducing condition. Na has the top priority to bond with Cl, than Ca only at low temperature and H only at high temperature, so the effect of Cl on Pb distribution depends on the content of Na and Ca. S promotes Pb volatilization by the interaction with Na in oxygen-rich and chlorine-poor condition and depresses Pb volatilization by the formation of PbS(s) directly without interaction with other elements in fuel-rich condition.     

Water extraction with CO2 bubbling as pretreatment of melting-furnace fly ash for metal recovery

In Japan, melting-furnace fly ash (MFA) generated from ash melting and gasification/melting plants is considered an “urban mine” due to its high metal content. This study aimed to develop a novel approach to pretreating MFA for metal recovery. Water extraction with CO₂ bubbling was investigated because MFA mainly consists of water-soluble salts containing elements such as Cl, Ca, Na, and K. Instead of acid addition, CO₂ bubbling was applied to maintain the optimal pH for minimizing the release of target metal elements and maximizing the removal of undesirable elements during water extraction. The results revealed that CO₂ bubbling effectively decreased the release of Pb, Zn, and Cd into the treatment water. This was mainly due to coprecipitation with CaCO₃, which was primarily formed by the reaction of Ca²⁺ from the MFA with CO₃ ²⁻ from the CO₂ gas. The bubbling process also helped accelerate the removal of Cl from MFA. Furthermore, the study showed that it is possible to lower the water-to-solid ratio to 5 with only a slight reduction in water extraction effect. Finally, approximately four times the concentration of target metals (rare metals and Cu, Pb, and Zn) was achieved by removing 90% of Cl, 70%–90% of Na and K, and 30%–40% of Ca through water extraction with CO₂ bubbling, resulting in a concentration of target metals that was nearly equal to that of ore.     

Leaching characteristics of rare metal elements and chlorine in fly ash from ash melting plants for metal recovery

In terms of resource recovery and environmental impact, melting furnace fly ash (MFA) is attracting much attention in Japan due to its high metal content. The study aims to obtain fundamental information on using a water extraction method not only to concentrate valuable rare metals but also to remove undesirable substances such as chlorine for their recovery from MFA. The composition and leaching characteristics of MFA was investigated. The results revealed that the metal content in MFA is nearly equal to raw ore quality. The content of Ag, In, Pd, Pb, and Zn is, in fact, higher than the content of raw ore. As for leaching behavior, Ag, Bi, In, Ga, Ge, Sb, Sn, and Te showed the lowest release at a neutral pH range. Pd was leached constantly regardless of pH, but its concentration was quite low. On the other hand, most of the Tl was easily leached, revealing that water extraction is not appropriate for Tl recovery from MFA. Major elements Cl, Ca, Na, and K, occupying about 70% of MFA, were mostly leached regardless of pH. Base metal elements Cu, Pb, and Zn showed minimum solubility at a neutral pH. The leaching ratio of target rare metal elements and base metal elements suggests that the optimal pH for water extraction is 8-10, at which the leaching concentration is minimized. The water extraction process removed most of the Cl, Ca, Na, and K, and the concentration of rare metals and base metals increased by four or five times.     

Trace element partitioning in ashes from boilers firing pure wood or mixtures of solid waste with respect to fuel composition,chlorine content and temperature

Trace element partitioning in solid waste (household waste, industrial waste, waste wood chips and waste mixtures) incineration residues was investigated. Samples of fly ash and bottom ash were collected from six incineration facilities across Sweden including two grate fired and four fluidized bed incinerators, to have a variation in the input fuel composition (from pure biofuel to mixture of waste) and different temperature boiler conditions. As trace element concentrations in the input waste at the same facilities have already been analyzed, the present study focuses on the concentration of trace elements in the waste fuel, their distribution in the incineration residues with respect to chlorine content of waste and combustion temperature.Results indicate that Zn, Cu and Pb are dominating trace elements in the waste fuel. Highly volatile elements mercury and cadmium are mainly found in fly ash in all cases; 2/3 of lead also end up in fly ash while Zn, As and Sb show a large variation in distribution with most of them residing in the fly ash. Lithophilic elements such as copper and chromium are mainly found in bottom ash from grate fired facilities while partition mostly into fly ash from fluidized bed incinerators, especially for plants fuelled by waste wood or ordinary wood chips. There is no specific correlation between input concentration of an element in the waste fuel and fraction partitioned to fly ash. Temperature and chlorine content have significant effects on partitioning characteristics by increasing the formation and vaporization of highly volatile metal chlorides. Zinc and cadmium concentrations in fly ash increase with the incineration temperature.     

Production of cement clinkers from municipal solid waste incineration (MSWI) fly ash

This communication reports the laboratory scale study on the production of cement clinkers from two types of municipal solid waste incineration fly ash (MSW ash) samples. XRD technique was used to monitor the phase formation during the burning of the raw mixes. The amount of trace elements volatilized during clinkerization and hydration, as well as leaching behaviours of the clinkers obtained from optimum compositions, were also evaluated. From the results it is observed that all of the major components of ordinary Portland cement (OPC) clinkers are present in the produced clinkers. Results also show the volatilization of considerable amounts of Na, K, Pb, Zn and Cd during the production of clinkers. However, major parts of the toxic elements remaining in the clinkers appear to be immobilized in the clinkers phases. Hydration studies of the clinkers obtained from optimum compositions show that the clinkers prepared from raw MSW ash are more reactive than the washed MSW ash based clinkers. TG/DTA analyses of the hydrated pastes show the formation of hydration products, which are generally found in OPC and OPC derived cements. The initial study, therefore, shows that more than 44% of MSW ash with the addition of very small amounts of silica and iron oxide can be used to produce cement clinkers. The amount of CaCO3 necessary to produce clinkers (approximately 50%) is also smaller than the same required for the conventional process (more than 70%).     

Influence of calcium chloride on the thermal behavior of heavy and alkali metals in sewage sludge incineration

In order to separate and reuse heavy and alkali metals from flue gas during sewage sludge incineration, experiments were carried out in a pilot incinerator. The experimental results show that most of the heavy and alkali metals form condensed phase at temperature above 600 degrees C. With the addition of 5% calcium chloride into sewage sludge, the gas/solid transformation temperature of part of the metals (As, Cu, Mg and Na) is evidently decreased due to the formation of chloride, while calcium chloride seems to have no significant influence on Zn and P. Moreover, the mass fractions of some heavy and alkali metals in the collected fly ash are relatively high. For example, the mass fractions for Pb and Cu in the fly ash collected by the filter are 1.19% and 19.7%, respectively, which are well above those in lead and copper ores. In the case of adding 5% calcium chloride, the heavy and alkali metals can be divided into three groups based on their conversion temperature: Group A that includes Na, Zn, K, Mg and P, which are converted into condensed phase above 600 degrees C; Group B that includes Pb and Cu which solidify when the temperature is above 400 degrees C; and Group C that includes As, whose condensation temperature is as low as 300 degrees C.     

煤燃烧过程中金属元素迁移特性

利用ICP-AES光谱仪对原煤、飞灰和底灰中铝、钾、钙、镁、铁、锰、锌、铜、硒、铬、砷等11种元素的含量进行了分析；对上述元素在煤燃烧过程中的迁移挥发特性进行了讨论，结果是钙、锰、铁挥发性较小，主要沉积在底灰中，而锌、铜挥发性较大，其他元素属于中等挥发性。     

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

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

Leaching from solid waste incineration ashes used in cement-treated base layers for pavements

Waste incineration bottom ash and treated flue gas cleaning products mixed with 2.5% of cement (50 kg/m3) were tested in the laboratory in terms of compressive strength and tank leaching tests over a 64-day period. Although the material displayed lower mechanical strength than a reference concrete, the strength still was sufficient for use as a base layer for roads. The metal content in the incineration-residue-based specimens was up to 100 times higher than in the reference concrete, suggesting that the mixed waste incineration residue should be used only for dedicated purposes. The leaching of Cl and Na was increased by a factor of 20-100 from the incineration-residue-based specimens as compared to the reference, while the leaching of K, Ca and SO4 was increased by a factor of 2-10. The leaching of heavy metals was also higher from the incineration-residue-based specimens than from the reference with respect to Cu (50 times), Cd, Pb and Zn (5 times), but not with respect to Cr and Ni. The leaching curves did only allow for a closer evaluation of the leaching process in a few cases. The physical retention of the constituents seemed to be the same in the reference as in the incineration-residue-based specimens. Heavy metal leaching was limited by enhanced chemical retention in the incineration-residue-specimens as compared to the reference. Since no quality criteria in terms of leaching from a monolithic material are currently available, the leaching issue must be evaluated case by case.     

Effect of carbon particles on heavy metal emissions under ash-melting conditions

In Japan, incineration ash is subjected to a melting process to reduce waste volume and to stabilize hazardous heavy metals. In previous articles, we reported that large quantities of volatile metals are emitted under ash-melting conditions at temperatures higher than 1200°C and that such emissions are considerably increased under reducing conditions. However, the emission behavior in the presence of large amounts of char particles was unclear, and we suspected that emissions under these conditions might differ from emissions under the previous conditions. Therefore, we investigated heavy metal emissions and the melting characteristics of ash in the presence of carbon particles. In this experiment, a small crucible with ash and carbon was rapidly heated using a high-frequency induction-heating furnace to simulate the melting ash gasification with carbon. As a result, it was found that additive carbon can promote emissions of heavy metals such as zinc and lead and control the melt of the ash.