The weathering of municipal solid waste incineration bottom ash evaluated by some weathering indices for natural rock

The weathering of municipal solid waste incineration (MSWI) residues consists of complicated phenomena. This makes it difficult to describe leaching behaviors of major and trace elements in fresh/weathered MSWI bottom ash, which was relevant interactively to pH neutralization and formation of secondary minerals. In this study, mineralogical weathering indices for natural rock profiles were applied to fresh/landfilled MSWI bottom ash to investigate the relation of these weathering indices to landfill time and leaching concentrations of component elements. Tested mineralogical weathering indices were Weathering Potential Index (WPI), Ruxton ratio (R), Weathering Index of Parker (WIP), Vogt’s Residual Index (V), Chemical Index of Alternation (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alternation (PIA), Silica–Titania Index (STI), Weathering Index of Miura (Wm), and Weatherability index of Hodder (Ks). Welch’s t-test accepted at 0.2% of significance level that all weathering indices could distinguish fresh and landfilled MSWI bottom ash. However, R and STI showed contrasted results for landfilled bottom ash to theoretical expectation. WPI, WIP, Wm, and Ks had good linearity with reclamation time of landfilled MSWI bottom ash. Therefore, these four indices might be applicable as an indicator to indentify fresh/weathered MSWI bottom ash and to estimate weathering time. Although WPI had weak correlation with leachate pH, other weathering indices had no significant correlation. In addition, all weathering indices could not explain leaching concentration of Al, Ca, Cu, and Zn quantitatively. Large difficulty to modify weathering indices correctly suggests that geochemical simulation including surface sorption, complexation with DOM, and other mechanisms seems to be the only way to describe leaching behaviors of major and trace elements in fresh/weathered MSWI bottom ash.     

Recovery of MSWI and soil washing residues as concrete aggregates

The aim of the present work was to study if municipal solid waste incinerator (MSWI) residues and aggregates derived from contaminated soil washing could be used as alternative aggregates for concrete production.Initially, chemical, physical and geometric characteristics (according to UNI EN 12620) of municipal solid waste incineration bottom ashes and some contaminated soils were evaluated; moreover, the pollutants release was evaluated by means of leaching tests. The results showed that the reuse of pre-treated MSWI bottom ash and washed soil is possible, either from technical or environmental point of view, while it is not possible for the raw wastes.Then, the natural aggregate was partially and totally replaced with these recycled aggregates for the production of concrete mixtures that were characterized by conventional mechanical and leaching tests. Good results were obtained using the same dosage of a high resistance cement (42.5R calcareous Portland cement instead of 32.5R); the concrete mixture containing 400 kg/m³ of washed bottom ash and high resistance cement was classified as structural concrete (C25/30 class). Regarding the pollutants leaching, all concrete mixtures respected the limit values according to the Italian regulation.     

Solidification and recycling of incinerator bottom ash through the addition of colloidal silica (SiO2) solution

The possibility of using incinerator bottom ash as a substitute for natural aggregates was investigated. Rough, porous surface of bottom ash, which diminishes the strength of solidified products, was improved by colloidal silica solution. As a result, a significant increase of mechanical strength was accomplished by a slight amount of silica (<1 wt% to total). Moreover, pozzolanic reaction was induced in initial cement hydration due to the nano-particle size of about 20 nm in colloidal silica solution. Cylindrical specimens and bricks were prepared from bottom ash added to a colloidal silica (SiO2) solution and cement, and then their compressive strengths were evaluated. Cylindrical specimens showed an increase of approximately 60% in compressive strength when colloidal solution containing 4 wt% silica particles was sprayed onto the bottom ash. The strength of bricks containing colloidal silica was in excess of 20 MPa, which meets the requirement of construction materials. Results of leaching tests based on Toxicity Characteristic Leaching Procedure (TCLP) proved that the solidified bottom ash possessed good chemical stability.     

Glass–ceramic from mixtures of bottom ash and fly ash

Along with the gradually increasing yield of the residues, appropriate management and treatment of the residues have become an urgent environmental protection problem. This work investigated the preparation of a glass–ceramic from a mixture of bottom ash and fly ash by petrurgic method. The nucleation and crystallization kinetics of the new glass–ceramic can be obtained by melting the mixture of 80% bottom ash and 20% fly ash at 950 °C, which was then cooled in the furnace for 1 h. Major minerals forming in the glass–ceramics mainly are gehlenite (Ca₂Al₂SiO₇) & akermanite (Ca₂MgSiO₇) and wollastonite (CaSiO₃). In addition, regarding chemical/mechanical properties, the chemical resistance showing durability, and the leaching concentration of heavy metals confirmed the possibility of engineering and construction applications of the most superior glass–ceramic product. Finally, petrurgic method of a mixture of bottom ash and fly ash at 950 °C represents a simple, inexpensive, and energy saving method compared with the conventional heat treatment.     

Accelerated carbonation of different size fractions of bottom ash from RDF incineration

This paper investigates the effects of accelerated carbonation on the characteristics of bottom ash from refuse derived fuel (RDF) incineration, in terms of CO₂ uptake, heavy metal leaching and mineralogy of different particle size fractions. Accelerated aqueous carbonation batch experiments were performed to assess the influence of operating parameters (temperature, CO₂ pressure and L/S ratio) on reaction kinetics. Pressure was found to be the most relevant parameter affecting the carbonation yield. This was also found to be largely dependent on the specific BA fraction treated, with CO₂ uptakes ranging from ～4% for the coarse fractions to ～14% for the finest one. Carbonation affected both the mineralogical characteristics of bottom ash, with the appearance of neo-formation minerals, and the leaching behaviour of the material, which was found to be mainly related to the change upon carbonation in the natural pH of the ash.     

High fire resistance in blocks containing coal combustion fly ashes and bottom ash

Fire resistance recycled blocks, containing fly ash and bottom ash from coal combustion power plants with a high fire resistance, are studied in this paper by testing different compositions using Portland cement type II, sand, coarse aggregate and fly ash (up to 50% of total weight) and bottom ash (up to 30% of total weight). The fire resistance, physical-chemical (density, pH, humidity, and water absorption capacity), mechanical (compressive and flexural strength), and leaching properties are measured on blocks made with different proportions of fly ash and bottom ash. The standard fire resistance test is reproduced on 28 cm-high, 18 cm-wide and 3 cm-thick units, and is measured as the time needed to reach a temperature of 180 °C on the non-exposed surface of the blocks for the different compositions.The results show that the replacement of fine aggregate with fly ash and of coarse aggregate with bottom ash have a remarkable influence on fire resistance and cause no detriment to the mechanical properties of the product. Additionally, according to the leaching tests, no environmental problems have been detected in the product. These results lead to an analysis of the recycling possibilities of these by-products in useful construction applications for the passive protection against fire.     

Mercury leaching characteristics of waste treatment residues generated from various sources in Korea

In this study, mercury (Hg) leaching characteristics of the waste treatment residues (fly ash, bottom ash, sludge, and phosphor powder) generated from various sources (municipal, industrial, medical waste incinerators, sewage sludge incinerator, oil refinery, coal-fired power plant, steel manufacturing plant, fluorescent lamp recycler, and cement kiln) in Korea were investigated. First, both Hg content analysis and toxicity characteristic leaching procedure (TCLP) testing was conducted for 31 collected residue samples. The Hg content analysis showed that fly ash from waste incinerators contained more Hg than the other residue samples. However, the TCLP values of fly ash samples with similar Hg content varied widely based on the residue type. Fly ash samples with low and high Hg leaching ratios (R_L) were further analyzed to identify the major factors that influence the Hg leaching potential. Buffering capacity of the low-R_L fly ash was higher than that of the high-R_L fly ash. The Hg speciation results suggest that the low-R_L fly ashes consisted primarily of low-solubility Hg compounds (Hg₂Cl₂, Hg⁰ or HgS), whereas the high-R_L fly ashes contain more than 20% high-solubility Hg compounds (HgCl₂ or HgSO₄).     

The effects of accelerated carbonation on CO2 uptake and metal release from incineration APC residues

This work presents the results of a study on accelerated carbonation of incinerator air pollution control residues, with a particular focus on the modifications in the leaching behaviour of the ash. Aqueous carbonation experiments were carried out using 100% CO₂ at different temperatures, pressures and liquid-to-solid ratios, in order to assess their influence on process kinetics, CO₂ uptake and the leaching behaviour of major and trace elements. The ash showed a particularly high reactivity towards CO₂, owing to the abundance of calcium hydroxides phases, with a maximum CO₂ uptake of ～250 g/kg. The main effects of carbonation on trace metal leaching involved a significant decrease in mobility for Pb, Zn and Cu at high pH values, a slight change or mobilization for Cr and Sb, and no major effects on the release of As and soluble salts. Geochemical modelling of leachates indicated solubility control by different minerals after carbonation. In particular, in the stability pH range of carbonates, solubility control by a number of metal carbonates was clearly suggested by modelling results. These findings indicate that accelerated carbonation of incinerator ashes has the potential to convert trace contaminants into sparingly soluble carbonate forms, with an overall positive effect on their leaching behaviour.     

Mechanical properties of incineration bottom ash: The influence of composite species

The mechanical properties, including strength, deformational behavior, and wetting softening phenomena of municipal solid waste incinerator (MSWI) bottom ash are one of the major concerns for reuse applications. However, owing to the complex constituents of municipal solid waste, the properties of MSWI bottom ash are often highly variable. A series of artificial specimens with controlled chemical components were tested in this study. The test results show that the artificial bottom ash possesses the following mechanical characteristics: (1) for the strength, the frictional angles of the bottom ash under dry and saturated conditions vary from 34.8° to 51.1° and 26.0° to 37.2°, respectively; (2) for the deformation, the shear stiffness increases with the normal stress arises and degrades upon increased shearing; (3) significant wetting degradation of the strength and stiffness were observed.The multi-variable regression analysis was conducted to evaluate the associated influence of the chemical components on the strength. Among the evaluated components, Fe₂O₃ and Al₂O₃ are key factors; an increase in either results in higher strength at both dry and saturated conditions. The results were used to propose empirical relationships for ?_dry and ?_sat, expressed in terms of Fe₂O₃ and Al₂O₃. Accordingly, a strength classification chart is proposed for engineering purposes.     

Utilization of blended fluidized bed combustion (FBC) ash and pulverized coal combustion (PCC) fly ash in geopolymer

In this paper, synthesis of geopolymer from fluidized bed combustion (FBC) ash and pulverized coal combustion (PCC) fly ash was studied in order to effectively utilize both ashes. FBC-fly ash and bottom ash were inter-ground to three different finenesses. The ashes were mixed with as-received PCC-fly ash in various proportions and used as source material for synthesis of geopolymer. Sodium silicate (Na₂SiO₃) and 10 M sodium hydroxide (NaOH) solutions at mass ratio of Na₂SiO₃/NaOH of 1.5 and curing temperature of 65 °C for 48 h were used for making geopolymer. X-ray diffraction (XRD), scanning electron microscopy (SEM), degree of reaction, and thermal gravimetric analysis (TGA) were performed on the geopolymer pastes. Compressive strength was also tested on geopolymer mortars. The results show that high strength geopolymer mortars of 35.0–44.0 MPa can be produced using mixture of ground FBC ash and as-received PCC-fly ash. Fine FBC ash is more reactive and results in higher degree of reaction and higher strength geopolymer as compared to the use of coarser FBC ash. Grinding increases reactivity of ash by means of increasing surface area and the amount of reactive phase of the ash. In addition, the packing effect due to fine particles also contributed to increase in strength of geopolymers.     

Understanding the chemical and mineralogical properties of the inorganic portion of MSWI bottom ash

This paper investigates the changes of mineralogical composition of bottom ash in the environment. The chemical and mineralogical bulk composition was determined by X-ray fluorescence (XRF) and X-ray powder diffraction (XRPD) Rietveld method. Single bottom ash particles were investigated by optical microscopy, scanning electron microscopy with quantitative energy-dispersive X-ray microanalysis (SEM/EDX) and electron probe micro analysis (EPMA). SEM/EDX and EPMA are valuable complement to bulk analysis and provide means for rapid and sensitive multi-elemental analysis of ash particles. The fresh bottom ash consists of amorphous (>30 wt.%) and major crystalline phases (>1 wt.%) such as silicates, oxides and carbonates. The mineral assemblage of the fresh bottom ash is clearly unstable and an aging process occurs by reaction towards an equilibrium mineral phase composition in the environmental conditions. The significant decrease of anhydrite and amorphous contents was observed in the aged bottom ash, leading to the formation of ettringite, hydrocalumite and rosenhahnite under atmospheric conditions. In the water-treated sample, the calcite contents increased significantly, but ettringite was altered by the dissolution and precipitation processes in part, to produce gypsum, while the remaining part reacted with chloride to form hydrocalumite. Gypsum and other Ca based minerals may take up substantial amounts of heavy metals and subsequently control leaching behaviour of bottom ash.     

Municipal incineration bottom ash treatment using hydrothermal solidification

Solidification of municipal incineration bottom ash (MIBA) has been carried out using a hydrothermal processing method, in which the MIBA was first compacted in a mold at 5-20 MPa, and then hydrothermally cured in an autoclave under saturated steam pressure at 150-250 degrees C for 10-72 h. Experimental results showed that the tensile strength of the solidified body was greatly influenced by the addition of NaOH solution and fresh cement in the MIBA. The hydrothermal curing temperature and time exerted a significant influence on the development of tensile strength of solidified body. The strength development is speculated to be due primarily to the formation of 1.1 nm tobermorite. Laboratory leaching tests were conducted to determine the amount of heavy metals dissolved from the solidified bodies and the results showed that under the hydrothermal conditions of this study the leaching of heavy metals was very low. As such, the hydrothermal processing method may have a high potential for recycling MIBA.     

Preparation of zeolitic material with simultaneous removal of NH4 + and PO4 3− from paper sludge ash via acid leaching

This study investigates the preparation of zeolitic material with removal of both NH₄ ⁺ and PO₄ ^3? from paper sludge ash (PSA) via acid leaching. PSA typically has a low Si and high Ca content, owing to the presence of calcite fillers. Acid leaching with 3 M HCl was used firstly to reduce the Ca content of the PSA, whereafter a zeolite-P (Na-P) product with high cation exchange capacity (CEC) was synthesized through reaction with 2.5 M NaOH solution at 80 °C. Ca-P zeolitic products were prepared by Ca-treatment with the leachant that had been in contact with the PSA. The product with high CEC capacity including Na-P could be synthesized from the acid-leached ash, and the high Ca content in the ash could be reduced by extraction of the Ca into the leachant via the acid leaching. The Ca-P zeolitic product could be prepared by Ca-treatment with the solution obtained from neutralization of the leachant with NaOH. This product was capable of removing NH₄ ⁺ and PO₄ ^3? from aqueous solution, simultaneously.     

Mechanochemical dechlorination of polyvinyl chloride with calcium sulfates

Polyvinyl chloride (PVC) was milled with hydrated or unhydrated calcium sulfates (CaSO₄·2H₂O or CaSO₄) in air by using a planetary mill to investigate mechanochemical dechlorination behavior. The milling process resulted in size reduction and in the breaking of bonds leading to mechanically induced solid state reaction, forming CaCl₂ and dechlorinated hydrocarbon with C=C double bonds in the product. Washing the milled mixtures with water at room temperature allowed removal of the chloride formed during milling, and more than 95% of the chlorine in PVC was removed from a mixture milled for 4 h. This process could offer a potential route for the handling and disposal of both PVC and gypsum wastes. H₂S gas was generated during milling; more H₂S was released from the unhydrated sample than from the hydrated sample.     

Manufacture of artificial aggregate using MSWI bottom ash

This paper reports the results of an investigation on material recovery by stabilization/solidification of bottom ash coming from a municipal solid waste incineration plant. Stabilization/solidification was carried out to produce artificial aggregate in a rotary plate granulator by adding hydraulic binders based on cement, lime and coal fly ash. Different mixes were tested in which the bottom ash content ranged between 60% and 90%. To avoid undesirable swelling in hardened products, the ash was previously milled and then granulated at room temperature. The granules were tested to assess their suitability to be used as artificial aggregate through the measurement of the following properties: density, water absorption capacity, compressive strength and heavy metals release upon leaching. It was demonstrated that the granules can be classified as lightweight aggregate with mechanical strength strongly dependent on the type of binder. Concrete mixes were prepared with the granulated artificial aggregate and tested for in-service performance, proving to be suitable for the manufacture of standard concrete blocks in all the cases investigated.     

Artificial carbonation for controlling the mobility of critical elements in bottom ash

In municipal solid waste incineration (MSWI), bottom ash, generated at a stoker grate type incinerator, the critical elements were identified in terms of EU regulation. The stabilizing effect of moderate carbonation (pH 8.28 ± 0.03) on critical contaminants was studied through availability and diffusion leaching protocols. Data from the performed tests were evaluated with the goal of reusing MSWI bottom ash as secondary construction material. To investigate the mobilizing effect of CO₂, suspended MSWI bottom ash was severely carbonated (pH 6.40 ± 0.07). The effect of CO₂ and its interaction with other leaching factors, such as liquid/solid (L/S) ratio, leaching time, pH, ultrasound treatment, and leaching temperature, were examined using a reduced 2^6-1 experimental design. Contaminants identified as critical were Cr, Cu, Mo, Sb, Cl⁻, and SO₄ ²⁻. Although moderate carbonation decreased the release of Cr, Cu, Mo, and Sb from compacted bottom ash, the main disadvantage remains its inability to demobilize Cl⁻ and SO₄ ²⁻. The hypothesized mobilizing effect of severe carbonation was proven. The treatment enhanced the separation of critical components (α = 0.05) (except for Cl⁻), i.e., about fivefold for Sb and about twofold for Cr, Cu, and S. Nevertheless, the prospect is good that severe carbonation could constitute the deciding key parameter to facilitate the technical feasibility of a future washing process for MSWI bottom ash.     

Recycling of combined coal-biomass ash from electric power plant waste as a cementitious material: characteristics and improvement

Combined coal-biomass ash has an enormous impact on environmental quality near electric power plants. This paper describes an alternative to disposal in which the ash is used to produce cementitious materials. Ash was obtained from combustion of coal and biomass containing four mass ratios of anthracite, bitumen, rice husks, and eucalyptus bark. The cement-forming properties were systematically characterized including compressive strength development, durability, and expansion in water. The ash samples were ground to increase the specific surface area, and then used to partially replace ASTM Type I Portland cement in mixtures containing 15, 30, or 45 % ash by mass. The water-binder material's (Portland cement with or without combined coal-biomass ash) ratios (w/c) were held constant at 45, 55, or 65 % by mass. Types A, B, and D ash behaved similarly, while the properties of type C ash were slightly different. Increasing the ash fraction in Portland cement mixtures increased the water requirement and resulted in lower compressive strength. Thorough mechanical grinding reduced the porosity and significantly enhanced the material properties.     

Metal leaching from MSWI bottom ash as affected by salt or dissolved organic matter

In order to manage municipal solid waste incineration (MSWI) bottom ash safely, risk assessments, including the prediction of leaching under different field conditions, are necessary. In this study, the influence of salt or dissolved organic matter (DOM) in the influent on metal leaching from MSWI bottom ash was investigated in a column experiment. The presence of salt (0.1M NaCl) resulted in a small increase of As leaching, whereas no impact on leachate concentration was found when lakewater DOM (35.1mg/l dissolved organic carbon) was added. Most of the added DOM was retained within the material. Further, X-ray spectroscopy revealed that Cu(II) was the dominating form of Cu and that it probably occurred as a CuO-type mineral. The Cu(2+) activity in the MSWI bottom ash leachate was most likely determined by the dissolution of CuO together with the formation of Cu-DOM complexes and possibly also by adsorption to (hydr)oxide minerals. The addition of lake DOM in the influent resulted in lower saturation indices for CuO in the leachates, which may be due to slow CuO dissolution kinetics in combination with strong Cu-DOM complexation.     

Comparative study on the characteristics of fly ash and bottom ash geopolymers

This research was conducted to compare geopolymers made from fly ash and ground bottom ash. Sodium hydroxide (NaOH) and sodium silicate (Na(2)SiO(3)) solutions were used as activators. A mass ratio of 1.5 Na(2)SiO(3)/NaOH and three concentrations of NaOH (5, 10, and 15M) were used; the geopolymers were cured at 65 degrees C for 48 h. A Fourier transform infrared spectrometer (FT-IR), differential scanning calorimeter (DSC), and scanning electron microscope (SEM) were used on the geopolymer pastes. Geopolymer mortars were also prepared in order to investigate compressive strength. The results show that both fly ash and bottom ash can be utilized as source materials for the production of geopolymers. The properties of the geopolymers are dependent on source materials and the NaOH concentration. Fly ash is more reactive and produces a higher degree of geopolymerization in comparison with bottom ash. The moderate NaOH concentration of 10 M is found to be suitable and gives fly ash and bottom ash geopolymer mortars with compressive strengths of 35 and 18 MPa.