Metallic-phase lead in slag of municipal solid waste incineration ash and leaching characteristics

Metallic phases in slags and their influence on the leaching characteristics were investigated. The proportions of metallic phase in four slags were 0.028%, 0.24%, 1.87%, and 3.05% by weight. The lead content was 10–248 mg/kg in bulk slag after metal removal, while in the metallic phase it was 579–7390 mg/kg. Lead concentrations in the metallic phase were more than ten times higher than in slags after metal removal. Lead was distributed in the metallic phase at 2.0%, 8.3%, 10.3%, and 47.4%. The concentrations of all metallic elements in metallic phases were much higher than in bulk slag. Iron, copper, and nickel had accumulated in magnetic metals, while aluminum and zinc were found in nonmagnetic metals. As regards chromium, manganese, lead, and tin, the proportion of metallic phases depended on the slag samples. By removing metallic phases, both water and pH 4 leachable lead decreased. The basic principles of melting residues containing lead are the separation of lead as a metal in reductive melting, and the containment of lead ions into uniform glassy particles in oxidization melting. Melting slag can be seen to contribute to environmental preservation by facilitating the recycling of materials through the separation of metals from melting slag. Received: February 21, 2000 / Accepted: July 27, 2000     

Reduction–melting combined with a Na2CO3 flux recycling process for lead recovery from cathode ray tube funnel glass

With large quantity of flux (Na₂CO₃), lead can be recovered from the funnel glass of waste cathode-ray tubes via reduction–melting at 1000 °C. To reduce flux cost, a technique to recover added flux from the generated oxide phase is also important in order to recycle the flux recovered from the reduction–melting process. In this study, the phase separation of sodium and the crystallization of water-soluble sodium silicates were induced after the reduction–melting process to enhance the leachability of sodium in the oxide phase and to extract the sodium from the phase for the recovery of Na₂CO₃ as flux. A reductive atmosphere promoted the phase separation and crystallization, and the leachability of sodium from the oxide phase was enhanced. The optimum temperature and treatment time for increasing the leachability were 700 °C and 2 h, respectively. After treatment, more than 90% of the sodium in the oxide phase was extracted in water. NaHCO₃ can be recovered by carbonization of the solution containing sodium ions using carbon dioxide gas, decomposed to Na₂CO₃ at 50 °C and recycled for use in the reduction–melting process.     

An overview of recovery of metals from slags

Various slags are produced as by-products in metallurgical processes or as residues in incineration processes. According to the origins and the characteristics, the main slags can be classified into three categories, namely ferrous slag, non-ferrous slag and incineration slag. This paper analysed and summarised the generation, characteristics and application of various slags, and discussed the potential effects of the slags on the environment. On this basis, a review of a number of methods for recovery of metals from the slags was made. It can be seen that a large amount of slags is produced each year. They usually contain a quantity of valuable metals except for blast furnace slag and they are actually a secondary resource of metals. By applying mineral processing technologies, such as crushing, grinding, magnetic separation, eddy current separation, flotation and so on, leaching or roasting, it is possible to recover metals such as Fe, Cr, Cu, Al, Pb, Zn, Co, Ni, Nb, Ta, Au, and Ag etc. from the slags. Recovery of metals from the slags and utilisation of the slags are important not only for saving metal resources, but also for protecting the environment.     

In situ remediation of arsenic in contaminated soils

In situ chemical fixation represents a promising and potentially cost‐effective treatment alternative for metal‐contaminated soils. This article presents the findings of the use of iron‐bearing soil amendments to reduce the leachability and bioaccessibility of arsenic in soils impacted by stack fallout from a zinc smelter. The focus of this investigation was to reduce the lead bioaccessibility of the soils through addition with phosphorus‐bearing amendments. However, as phosphorus addition was expected to increase arsenic mobility, the fixation strategy also incorporated use of iron‐bearing amendments to offset or reverse these effects. The findings of this investigation demonstrated that inclusion of iron‐bearing chemicals in the amendment formulation reduced arsenic leachability and bioaccessibility without compromising amendment effectiveness for reducing lead bioaccessibility. These results suggest that in situ chemical fixation has the potential to be an effective strategy for treatment of the impacted soils. © 2003 Wiley Periodicals, Inc.     

Application of magnetic separation to steelmaking slags for reclamation

Integrated iron and steel plants generate large amounts of metallurgical slag, which usually contains some quantity of metals or mixtures of oxides that could be treated to be recycled in various applications. The conventional method for disposal of slags is dumping. However, it is possible to process the slags to be used in the production of metallic iron, or as an additive in cement making. In this study, a basic oxygen furnace (BOF) steelwork slag obtained from the Kardemir integrated iron and steel works, Karabuk, Turkey is used. A drum magnetic separator system with pre-engineered crucial processing parameters of drum revolution speed, drum radius, drum flesh thickness, and magnitude of the magnetic field applied is utilized, as these parameters have a competing influence on the results. Subsequently, the effects of slag grain size and the drum-blade gap are investigated in the separation efficiency of magnetic grains. It is found that collection of magnetic grains is improved by decreasing the grain size of slags and moreover, the collection of magnetic grains fraction is increased with an increase in the gap between the blades and drum.     

Volatilization behavior of lead from molten slag under conditions simulating municipal solid waste melting

To reutilize molten slag derived from an ash melting process, the lead volatilization mechanism under reducing conditions was investigated. Reducing conditions were established by introducing a CO-CO₂-N₂ gas mixture to the reactor or by adding graphite to the molten slag prior to the experiments. As samples, two types of simulated molten slag composed of CaO-SiO₂-Al₂O₃ mixed with PbO were used and the lead volatilization behavior was studied at 1773 K. It was found that the lead volatilization rate increased on increasing the amount of reducing agent for both graphite and the CO-CO₂ gas mixture. For the CO-CO₂ reducing gas mixture, this increase was mainly attributed to PbO conversion to Pb. For the addition of graphite, the increase in lead volatilization ratio was considered to partially result from PbO conversion to Pb and partially from a reaction of graphite with SiO₂ yielding volatile SiO. The volatile SiO gas was then emitted from the furnace, which brought about a reduction in the SiO₂ content of the slag. As a result, the slag viscosity decreased, which led to an enhancement of the lead volatilization ratio.     

The hydration characteristics and utilization of slag obtained by the vitrification of MSWI fly ash

This study investigated the effects of slag composition on the hydration characteristics of slag blended cement (SBC) pastes. Synthetic slag samples were prepared by melting CaO-modified and Al(2)O(3)-modified municipal solid waste incinerator (MSWI) fly ash. MSWI fly ash was mixed with 5% CaO and 5% Al(2)O(3) (by weight), respectively, resulting in two fly ash mixtures. These mixtures were then melted at 1400 degrees C for 30 min to produce two types of slag with different contents, designated at C-slag and A-slag. Both the C-slag and A-slag samples exhibited a pozzolanic activity index higher than the unmodified slag sample. The results show that the synthetic slags all met the Taiwan EPA's current regulatory thresholds. These synthetic slags were then blended with ordinary Portland cement (OPC) at various weight ratios ranging from 10 to 40%. The 28-day strength of the C1 paste was higher than that developed by the OPC paste, suggesting that the C-slag contributed to the earlier strength of the SBC pastes. At curing times beyond 28 days, the strength of the A1 paste samples approached that of the OPC paste samples. It can be seen from this that increasing the amount of calcium and aluminum oxide increases the early strength of SBC. The C-slag blended cement paste samples showed an increase in the number of fine pores with the curing time, showing that the C-slag enhanced the pozzolanic reactions, filling the pores. Also, the incorporation of a 10% addition of C-slag also tended to enhance the degree of hydration of the SBC pastes during the early ages (3-28 days). However, at later ages, no significant difference in degree of hydration between the OPC pastes and the SBC pastes was observed with the 10% C-slag addition. However, the incorporation of A-slag did decreased the degree of hydration. A slag blend ratio of 40% significantly decreased the hydration degree.     

Chemical and mineralogical evaluation of slag products derived from the pyrolysis/melting treatment of MSW

This paper provides the results of studies on the characteristics of novel material derived from pyrolysis/melting treatment of municipal solid waste in Japan. Slag products from pyrolysis/melting plants were sampled for the purpose of detailed phase analysis and characterization of heavy metal-containing phases using optical microscopy, electron probe microanalysis (EPMA), XRF and XRD. The study revealed that the slag material contains glass (over 95%), oxide and silicate minerals (spinel, melilite, pseudowollastonite), as well as individual metallic inclusions as the major constituents. A distinct chemical diversity was discovered in the interstitial glass in terms of silica content defined as low and high silica glass end members. Elevated concentrations of Zn, Cr, Cu, Pb and Ba were recorded in the bulk composition. Cu, Pb and Ba behave as incompatible elements since they have been markedly characterized as part of polymetallic alloys and insignificantly sulfides in the form of spherical metallic inclusions associated with tracer amounts of other elements such as Sb, Sn, Ni, Zn, Al, P and Si. In contrast, an appreciable amount of Zn is retained by zinc-rich end members of spinel and partially by melilite and silica glass. Chromium exhibits similar behavior, and is considerably held by Cr-rich spinel. The intense incorporation of Zn and Cr into spinel indicates the very effective enrichment of these two elements into phases more environmentally resistant than glass. There was no evidence, however, that Cu and Pb enter into the structure of the crystalline silicates or oxides that may lead to their easier leachability upon exposure to the environment.     

Diffusion test of 20 kinds of waste molten slags and competitive materials

“Waste molten slag” is a glass-like material produced by the vitrification of solid waste or solid waste incineration residue. When using slags of this kind in a natural environment, their impact is anticipated to be at the same level as competitive or substituted materials. In this study, we made comparative evaluations between waste molten slags and competitive materials, using 20 samples in total. It was proved that release fluxes of metals from molten slags of municipal solid waste were almost at the same level as competitive or substituted materials. However, a larger impact will be caused from some types of slag that contain harmful metals in high concentrations, such as the slag from shredded automobile residues. The results of release flux showed that nearly 80% of the slope of the flux did not fit with the diffusion range. However, the linearity of every flux was extremely high, regardless of the slope.     

Evaluation of waste slags produced by zinc industry in bituminous hot mixtures

Owing to the large amount of waste slags produced by zinc industry, it has become necessary to recycle it in some areas. Road construction has significant potential for the use of waste materials because more material is always needed. In this study, the engineering behaviour of asphalt concrete was investigated using mineral aggregates with waste slag, which is a by-product of the zinc–lead production industry. The asphalt concrete tested in this study was fabricated using 25, 50, 75 and 100 % mixing ratios instead of the conventional fine mineral aggregate (11, 22, 33 and 44 % rate of total aggregate mixture) to determine the possibility of using slags in the binder course of bituminous hot mixtures. The asphalt concretes, made of waste slags and conventional asphalt concrete, were evaluated in terms of their fundamental engineering properties such as Marshall stability, flow, Marshall quotient (MQ), bulk specific gravity, air voids and voids filled with bitumen in the total mix characteristics. The results indicate that the addition of waste slag as mineral aggregate improves the engineering characteristic performance and that it can be used in bituminous hot mixtures. In addition, principal component analyses were applied to examine the significance of each Marshall parameter, and a regression model was developed to estimate the MQ value using effective parameters.     

Resurrection of the iron and phosphorus resource in steel-making slag

 This research focused on the treatment of steel-making slags to recycle and recover iron and phosphorus. The carbothermal reduction behavior of both synthesized and factory steel-making slag in microwave irradiation was investigated. The slags were mixed with graphite powder and heated to a temperature higher than 1873 K to precipitate a lump of Fe–C alloy with a diameter of 2–8 mm. The larger the carbon equivalent (C_eq, defined in the text), the higher the fractional reduction of iron and phosphorus. An increase in the SiO₂ content of slag led to a considerable improvement in the reduction for both iron and phosphorus because of the improvement in the fluidity of the slags and an increase in the activity coefficient of P₂O₅ in the slags. The extraction behavior of phosphorus from Fe–P–C_satd alloy was also investigated at 1473 K by carbonate flux treatment. For all the experiments with a processing time longer than 10 min, the phosphorus in the fluxes could be concentrated to more than 9% (w/w) showing that it could be used as a phosphorus resource. Compared with K₂CO₃ flux treatment, that using Na₂CO₃ was more effective for the extraction of phosphorus, and this was attributed to the lower evaporation of Na₂CO₃. Finally, a recycling scheme for steel-making slag is proposed. Received: March 16, 2001 / Accepted: November 12, 2001     

Characteristics of steel slag under different cooling conditions

Four types of steel slags, a ladle slag, a BOF (basic oxygen furnace) slag and two different EAF (electric arc furnace) slags, were characterized and modified by semi-rapid cooling in crucibles and rapid cooling by water granulation. The aim of this work was to investigate the effect of different cooling conditions on the properties of glassy slags with respect to their leaching and volume stability. Optical microscopy, X-ray diffraction, scanning electron microscope and a standard test leaching (prEN 12457-2/3) have been used for the investigation. The results show that the disintegrated ladle slag was made volume stable by water granulation, which consisted of 98% glass. However EAF slag 1, EAF slag 2 and the BOF slag formed 17%, 1% and 1% glass, respectively. The leaching test showed that the glass-containing matrix did not prevent leaching of minor elements from the modified slags. The solubility of chromium, molybdenum and vanadium varied in the different modifications, probably due to their presence in different minerals and their different distributions.     

Treatment Of Electric Arc Furnace Dust With A Sustained Shockwave Plasma Reactor

In the reprocessing of steel from automotive scrap, lead, cadmium and zinc, which are volatile constituents, vaporize under the extremely high localized temperatures in an electric arc furnace at the points of contact between the graphite electrodes and the charge. These vapors condense to produce a hazardous dust termed electric arc furnace dust (EFD). This study was undertaken to satisfy the need for a mobile system which could be moved from site to site of generators of EFD to avoid transportation of hazardous waste, and to recycle slags and metallic iron products within the plant. A new type of plasma device, the Sustained Shockwave Plasma (SSP) has been developed and tested for treatment of EFD on a laboratory scale (500 g h^-1). Batch tests show that EFD can be treated to produce a non-hazardous slag and metallic zinc and lead to recycle.     

Stabilization and utilization of hospital waste as road and asphalt aggregate

This article reports the operational results of the effective utilization of hospital waste molten slag produced using a high-temperature melting system, and being operated at a hospital in Selangor, Malaysia. The hospital waste is incinerated and subsequently melted at 1200°C. Scanning election microscope (SEM)/EDX results showed that the slag produced after melting contained amounts of SiO₂, CaO, and Al₂O₃ in excess of 53%, 9%, and 16%, respectively. The results from a leaching analysis on the slag produced proved that the melting process had successfully stabilized the heavy metals. The use of this slag as an alternative material to replace conventional aggregates for road construction was studied. The results from aggregate and asphalt mix tests showed that the slag produced fulfills all the requirements of an alternative aggregate. The average asphalt content, or the optimum asphalt content to be mixed with hospital waste molten slag, was found to be about 5.53%.     

Enhancing performance and durability of slag made from incinerator bottom ash and fly ash

This work presents a method capable of melting the incinerator bottom ash and fly ash in a plasma furnace. The performance of slag and the strategies for recycling of bottom ash and fly ash are improved by adjusting chemical components of bottom ash and fly ash. Ashes are separated by a magnetic process to improve the performance of slag. Analytical results indicate that the air-cooled slag (ACS) and magnetic-separated slag (MSS) have hardness levels below 590 MPa, indicating fragility. Additionally, the hardness of crystallized slag (RTS) is between 655 and 686 MPa, indicating toughness. The leached concentrations of heavy metals for these three slags are all below the regulatory limits. ACS appears to have better chemical stability than MSS, and is not significantly different from RTS. In the potential alkali-silica reactivity of slag, MSS falls on the border between the harmless zone and the potentially harmful zone. ACS and RTS fall in the harmless zone. Hence, the magnetic separation procedure of ashes does not significantly improve the quality of slag. However, RTS appears to improve its quality.     

Effects of cement on redistribution of trace metals and dissolution of organics in sewage sludge and its inorganic waste-amended products

The suitability of using cement-stabilized sludge products as artificial soils in earth works was evaluated. The sludge products investigated were cemented sludge, cement-treated clay-amended sludge (SS+MC), and cement-treated copper slag-amended sludge (SS+CS). The leachability of lead (Pb), zinc (Zn), copper (Cu), and chromium (Cr) were assessed using the sequential extraction technique, toxicity characteristic leaching procedure (TCLP), NEN 7341 availability test, and column leaching test. The results indicated that Zn leachability was reduced in all the cement-stabilized sludge products. In contrast, Cu was transferred from the organic fraction to the readily leachable phases in the cement-stabilized sludge products and therefore exhibited increased leachability. The increased Cu leachability could be attributed to dissolution of humic substances in the sludge as a result of elevated pH. Good correlation between dissolved organic carbon (DOC) and heavy metal leaching from the cement-stabilized sludge products was observed in the column leaching experiment. Even with a cement percentage as small as 12.5%, calcium silicate hydrate (C-S-H) was formed in the SS+MC and SS+CS products. Inclusion of the marine clay in the SS+MC products could reduce the leaching potentials of Zn, and this was the great advantage of the marine clay over the copper slag for sludge amendment.     

Leaching assessment of road materials containing primary lead and zinc slags

Characterisation of the leaching behaviour of waste-containing materials is a crucial step in the environmental assessment for reuse scenarios. In our research we applied the multi-step European methodology ENV 12-920 to the leaching assessment of road materials containing metallurgical slag. A Zn slag from an imperial smelting furnace (ISF) and a Pb slag from a lead blast furnace (LBF) are investigated. The two slags contain up to 11.2 wt% of lead and 3.5 wt% of zinc and were introduced as a partial substitute for sand in two road materials, namely sand-cement and sand-bitumen. At the laboratory scale, a leaching assessment was performed first through batch equilibrium leaching tests. Second, the release rate of the contaminants was evaluated using saturated leaching tests on monolithic material. Third, laboratory tests were conducted on monolithic samples under intermittent wetting conditions. Pilot-scale tests were conducted for field testing of intermittent wetting conditions. The results show that the release of Pb and Zn from the materials in a saturated scenario was controlled by the pH of the leachates. For the intermittent wetting conditions, an additional factor, blocking of the pores by precipitation during the drying phase is proposed. Pilot-scale leaching behaviour only partially matched with the laboratory-scale test results: new mass transfer mechanisms and adapted laboratory leaching tests are discussed.     

Utilizing a database to interpret leaching characteristics of lead from bottom ashes of municipal solid waste incinerators

This study focuses on evaluation and quantification of factors affecting leachability of lead from bottom ashes of municipal solid waste incinerators (MSWI) by utilizing a database. The database, which was constructed with data collected from sources such as research papers, questionnaires and reports, consists of 1149 data sets on 508 MSWI plants. Factors chosen as affecting lead leachability included: pH in the leachate, loss on ignition of bottom ash, total content of lead, and content of main elements such as Fe, Mn, Si and Al. The lead leachability was reduced to a minimum at neutral pH, increased with increasing pH and, especially, showed an abrupt increase at pH levels above 12. The main factor controlling the pH of the leachate appeared to be leachable Ca(2+) originating from portlandite (Ca(OH)(2)). Leaching concentration increased with increasing total contents of lead, and the relationship between leaching of lead and loss on ignition showed no distinct tendencies. The lead leaching ratio increased with decreasing total contents of Si, Mn and Fe. It is evident, therefore, that these numerous factors determine the leachability of lead in a simultaneous and complex manner.     

Metallurgical use of glass fractions from waste electric and electronic equipment (WEEE)

Within the European Union, it is estimated that between 8 and 9 million tonnes of waste electric and electronic equipment (WEEE) arises annually, of which television sets and computers account for an important part. Traditionally, Cathode Ray Tubes (CRT) have been used for TVs and computer monitors, but are rapidly being replaced by flat-screen technology. Only part of the discarded CRT glass is being recycled. Primary smelters use large amounts of silica flux to form iron-silicate slag, and can, in most cases, tolerate lead input. Use of discarded CRT glass in copper smelting is a potential alternative for utilization of the glass.The mineralogical composition of a slag sampled during ordinary slag praxis has been compared with that of a mixture of slag and CRT glass when re-melted and slowly cooled. Slag (iron-silicate slag) from Boliden Mineral AB, Sweden, was used for the experiments. Slag and glass have been mixed in various proportions: pure slag, pure glass, 90% slag-10% glass and 65% slag-35% glass, and heated in an inert atmosphere up to 1400 °C in a Netzsch Thermal Analysis (TA) instrument. The re-melted material has been analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) to determine changes in mineralogical composition after mixing with glass.The results show that the main mineralogical component of the slag is fayalite; the CRT glass is amorphous. The main crystalline phases of the slag do not change with addition of glass. An amorphous phase appears when the addition of glass is increased, which gives the sample a different structure.     

Assessment of plasma gasification of high caloric waste streams

Plasma gasification is an innovative technology for transforming high calorific waste streams into a valuable synthesis gas and a vitrified slag by means of a thermal plasma. A test program has been set up to evaluate the feasibility of plasma gasification and the impact of this process on the environment. RDF (refuse derived fuel) from carpet and textile waste was selected as feed material for semi-pilot gasification tests. The aim of the tests was: (1) to evaluate the technical feasibility of making a stable synthesis gas; (2) to characterize the composition of this synthesis gas; (3) to define a suitable after-treatment configuration for purification of the syngas and (4) to characterize the stability of the slag, i.e., its resistance to leaching for use as a secondary building material. The tests illustrate that plasma gasification can result in a suitable syngas quality and a slag, characterized by an acceptable leachability. Based on the test results, a further scale-up of this technology will be prepared and validation tests run.