Шины electric arc furnace

EAF dust is injected through upper tuyeres, melted in a raceway, and reduced during dripping down in packed coke layer.

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Future of Process Metallurgy

4.5.2.4.5.2 PRIMUS [ 162–164 ]

The PRIMUS process treats EAF dust and sludge from rolling mills at steelworks to produce hot metal. Figure 4.5.88 shows the process flow. The process utilizes a multistage RHF to reduce iron oxide powder and granular material with dust mixed together with carbonaceous material.

Figure 4.5.88 . PRIMUS process flow for treatment of EAF dust [ 163 ].

The iron oxide powder and granular material charged into the multistage RHF are pushed down to the lower tiers by rabble arms. The iron oxide is dried and heated by a flame while it falls from the upper to lower hearths. While the iron oxide is reduced, volatile metals, mainly zinc, are vaporized and collected by the cyclone and bag filter.

An EAF melts the reduced iron oxide powder and granular material to produce hot metal. Since there is no need to make carbon composite agglomerates (such as pellets), this method can be used for treating powder and granular material of various grain sizes.

In this process, the plant runs on coal during normal operation because of the use of the multihearth furnace as a reducing furnace and reduction at a temperature of 1373 K or less. Since reduction is performed at a temperature of 1373 K or less, there are problems in that the metallization degree of the product can be as low as 15%. Productivity can be as low as one-half to one-quarter of that of other RHF processes [ 165 ].

Since 2003, a plant (10 tons/h) has been operating on a commercial scale in Luxembourg using EAF dust and rolling mill sludge as the raw material [ 164 ]. In addition, the construction of a second commercial-scale plant (treatment capacity of 100,000 tons/year) was completed at Dragon Steel Corp. (DSC) in Taiwan. Start-up operations began in 2010 [ 162 ].

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Processes for Recycling

4.4.4.2.3.3 MF Process [ 147 , 149 ]

Figure 4.4.24 shows the schematic diagram of this process. EAF dust is mixed with nonferrous smelting residue, especially copper and silver smelting residues, coal, silica sand and binder, and briquetted. Briquette is charged from the top of a special-type shaft furnace and hot blast air is supplied from tuyeres at the bottom of the furnace at 573 K to heat and reduce the briquette. Zinc and lead volatilize and are exhausted as gas from the furnace while the melt is separated into metal and slag. Obtained metal contains from 50 to 60 mass%Cu and 0.2 mass%Ag, which is further treated at smelters. Slag is quenched with water spray and used as a cement resource. In this process, charged dust is completely melted and treated, which is largely different from the electrothermal distillation process or Waelz kiln process. Other feature of this process is a furnace with strong reducing atmosphere, which helps the recovery of copper and noble metals.

Figure 4.4.24 . Schematic view and diagram of the MF process [ 147 ].

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Electric Furnace Steelmaking

1.5.7 Environmental and Safety Issues

The electric arc furnaces bring about some special environmental concerns, namely:

Disposal/recycling of electric arc furnace dust , slag, and refractories

1.5.7.1 EAF Dust

Dust is formed during the melting process typically around 10 kg/t liquid steel. Dust contains mainly iron oxides, CaO, and ZnO. The first two are “common and natural consequences” having their origins in Fe-based charge and lime additions into the furnace and slag. The amount of ZnO in dust is related to the amount of galvanized steel in the scrap. During the melting process, zinc is vaporized as Zn_(g) and leaves the metal bath but is then oxidized to form ZnO when transported with the off-gas and meeting more oxidizing conditions. The EAF dust is typically collected to bag filters in the off-gas treatment installation. Disposal of EAF dust is not allowed in most countries because of the risk of lixiviation of metals like Zn, Cd, and Pb, which are considered dangerous. One way is to treat the dust in order to stabilize metals, as per, for instance, the Super Detox technology. This process involves a series of complex physical and chemical reactions, including oxidation/reduction, insolubilization of metals, polymerization of silicates, puzzolanic bonding, and solidification. Metals change to their less soluble state and are physically immobilized. Stabilized material has low permeability and high strength. The treated residue is not considered dangerous and the disposal as land filling is less expensive [ 37 ].

Another possibility, explored mainly in Europe, is the recycling of the dust in the EAF itself. This procedure has two advantages: total dust generation in a year and per ton steel is less and the Zn content is increasing cycle by cycle, and the dust removed from the circuit has 20% ZnO or more making it more attractive to zinc producers [ 38 ].

The most popular process for zinc recovery is the so-called Waelz kiln, located usually outside the steel plant boundaries. These furnaces have been developed in the early twentieth century by zinc producers to enrich low Zn ore ( Figure 1.5.21 ). They have been adopted in the 1950s for EAF dust treatment. Currently, BEFESA is the leading company in Europe, coming from the waste treatment sector, and Horsehead, a zinc producer, is the leader in North America [ 40 ].

Figure 1.5.21 . Scheme of the Waelz kiln and auxiliary equipment, the most usual tool for the recycling of the EAF dust to enrich it in ZnO for the production of ZnO or Zn metal [ 39 ].

Recently, due to the increase in iron ore cost, some interest arises in the recovery of the Fe units contained in the EAF dust, along with the Zn units. This way, processes based in rotating hearth furnace or channel induction furnace like PRIMUS or PIZO have been developed and installed in a few locations [ 41 , 42 ].

1.5.7.2 EAF Slag

This slag contains typically high CaO, SiO₂, FeO, and MgO, as well as smaller amount of other oxides. Supposing a slag generation of 100 kg/t of crude steel, worldwide EAF slag production would amount to 45 Mtpy. As oxygen steelmaking slag, the recycling of this by-product has been focused in construction use, as inert material in pavement. To this purpose, some kind of stabilization is required. Other less explored possibility is the recycling in the EAF process itself.

1.5.7.3 EAF Refractories

One of the most successful experiences of recycling of spent refractories in EAF steelmaking is that of the Chita plant of Daido Steel, in Japan, which by the year 2000 recycled internally 58% of the spent refractories [ 43 ]. This plant produces 1.7 Mt of special steels. MgO–C and MgO bricks of some zones of the EAF are recycled as refractories in the EAF itself, after eliminating slag and metal adhesions. Others are crushed and used as slag conditioner for Ladle Furnace (LF) and EAF.

1.5.7.4 Noise

Typical noise levels for EAFs given by sound power level are between 125 and 139 dB. Relevant parameters for total noise resulting from electric steelmaking plants are the installed transformer capacity, the size of the furnace, existing enclosures of the EAF and the melting shop, operating conditions, etc. [ 14 ]. From the equipment and process point of view, factors that diminish arc noise are related to good foaming practice, use of shredded scrap, continuous charging, etc. Other measures taken in EAF plants installed in an urban environment include the so-called dog house or elephant house, special isolation measures in the building walls, and tunnels to communicate noisy buildings, high walls.

1.5.7.5 Safety

The furnace area has some inherent risks for operators’ safety (see Figure 1.5.22 ). In recent times, innovations have been introduced to avoid manned operations in the platform and close to the hot furnace. Previously, the special construction of panels to avoid water leakage was mentioned, as well as the spray-cooled upper shell, and hydrogen control in the off-gas to detect leakage. Robots for handling electrodes, taking samples, and measuring temperature and oxygen activity are used in several plants. For refractory demolition, gunning, fettling, and bricklaying robots become utilized, too. Devices for taphole cleansing, for nonspontaneous opening, and for automatic sand filling are offered.

Figure 1.5.22 . EAF hazard main areas [ 44 ].

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Solar thermal processing

18.3.3.3 Extraction of metals from solid waste materials

High-temperature carbothermal conversion of solid waste materials using concentrated solar energy as the source of process heat is an efficient way to extract metals from metal-rich feedstocks [173,174] . Two important sources of waste are electric arc furnace dust (EAFD) and automobile shredder residue (ASR) [173] . Both types of waste occur in millions of tons annually and contain large amounts of metals, mostly in oxidized form. Their typical elemental compositions are listed in Table 18.3 . Solid carbon and methane are considered as reducing agents for EAFD, while the carbon contained within the waste material is used as the reducing agent for ASR [173] .

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Table 18.3 . Main metal elemental compositions of electric arc furnace dust (EAFD) and automobile shredder residue (ASR). Oxygen is complementary to 100%

Thermodynamic equilibrium calculations for the carbothermal reduction of EAFD and ASR indicate that above 1300K the main metallic constituents (Zn(g), Fe, Pb for EAFD; Zn(g), Fe, Pb, Cu for ASR) are present in elemental form, along with CO (and H₂), as well as SiO₂ added as vitrification agent ( Fig. 18.15 ). The theoretical minimum process heat required to convert the feedstocks at 1500K (including heating from ambient temperature) is in the range of 2500–4100 kJ/kg, depending on feedstock and reducing agent used [173] .

Figure 18.15 . Equilibrium compositions for carbothermal reduction of waste materials: (a) EAFD, initial composition 3.46 mol ZnO, 0.91 mol ZnFe₂O₄, 0.12 mol Pb₃O₄, 7.56 mol C, and 5.45 mol SiO₂; (b) ASR, initial composition 29 mol C, 3 mol H₂O, 1.66 mol SiO₂, 1.18 mol Fe₂O₃, 0.39 mol CuO, 0.37 mol Al₂O₃, 0.31 mol ZnO, 0.05 mol Cl₂, and 0.02 mol Pb₃O₄ (Pb not shown in graph due to low concentration) [173] .

Reprinted with permission from [173] . Copyright American Chemical Society.

The conversion of EAFD was conducted in a beam-down solar reactor, shown in Fig. 18.16 . The reactor consists of a windowed inner cavity to absorb concentrated solar radiation and reemit it into the outer rotary reaction chamber. The feedstock, 87% EAFD and 13% activated charcoal, was either placed into the reactor as a batch of 100–200 g, or continuously fed with a screw feeder at a rate of 2–24 g/min. The reactor was purged with a nitrogen flow of 8–25 L_n/min. Gaseous products were continuously pumped out of the reactor via a water-cooled quench tube and a battery of filters to condense and collect solid products. Experiments were conducted with reactor temperatures in the range of 1140–1400K. More than 99% of Zn and Pd initially in the EAFD were extracted in batch mode operation at 1400K; in continuous mode, the corresponding values were 90% and 80% at 1250K. No ZnO was detected when the O₂ concentration in the outlet stream remained below 2% [174] .

Figure 18.16 . 10-kW two-cavity prototype solar reactor used for the carbothermal conversion of EAFD.

Reprinted with permission from [174] . Copyright American Chemical Society.

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Air and Gas Filtration

3.4.1.6 Industrial air filtration

In factories, a dirty atmosphere can produce an unpleasant working environment, reduce operator efficiency, affect health, shorten the life of machines, increase maintenance costs and contaminate products. The problem of air treatment is aggravated by the abnormally high concentration of heavy and/or abrasive contaminants in the industrial atmosphere. Full control may be beyond the scope of conventional HVAC air filtration systems (or be uneconomic to apply). In this case, special dust collecting treatment may need to be applied to specific areas, as discussed later in this section.

Industrial dusts may range in size from 1 mm (1000 μm) down to about 1 μm or even down to 0.1 μm in the case of cupola dust, foundry dust, electric arc furnace dust and paint pigments. Current state-of-the-art surface finishing applications call for superfine air filtration of the air supply side of paint spray plants and downdraught paint booths. An important criterion in this technology is to prevent painted surface-damaging particles 15 μm and larger from migrating downstream after collection in a filter, due to vibration in the system.

When choosing an air filter medium for industrial dusts, it may be preferable to select one that has been tested using a test dust of non-adhesive free-running aluminium oxide particles and proven to have collected this dust without unloading or allowing migration under vibration.

The basic central air treatment plant will have a primary filter at the plant inlet, to protect the air-conditioning units, especially the heat exchanger, a humidifier and the circulating fan. There is then, finally, a second-stage filter to provide finer filtration, sited just before the outlet duct from the plant. The cost of ultra-fine filtration usually prohibits its use for a general factory scheme, it being usually restricted to point-of-use areas, especially clean rooms.

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Recycling of industrial wastes for value-added applications in clay-based ceramic products: a global review (2015–19)

M. Contreras , . J.P. Bolívar , in New Materials in Civil Engineering , 2020

5.3.3.1 Bricks

In the period studied, the greatest effort to reuse mining and metallurgical wastes was aimed at the incorporation into clay brick pastes of different wastes, such as, calamine hydrometallurgical tailings [78] , steel industry waste [79,80,88] , slag from alloy industry [81] , BF slag [82] , aluminum filter dust (AFD) [83] , and Waelz slag [84] .

In this sense, Taha et al. [78] investigated the effect of adding the waste generated by a calamine (Zn₄Si₂O₇(OH)₂·H₂O) hydrometallurgical processing plant to brick production. The waste, mostly composed of gypsum (CaSO₄.2H₂O), quartz (SiO₂), and calcite (CaCO₃), was mixed in different percentages (10–50 wt.%) with natural shale for brick manufacture. The results indicated that the lower proportion of SiO₂ and alumina in calamine waste compared to natural shale has a significant effect on the physical properties of fired bricks. Thus, the incorporation of calamine waste results in higher water absorption and open porosity, which leads to reduced flexural strength and apparent density. However, the authors indicated that the adverse effect of the incorporation of calamine on technological properties could be corrected by the addition of glass waste. In this manner, fired light bricks with appropriate physical and mechanical properties were fabricated from mixtures containing up to 30 wt.% calamine waste and 10 wt.% glass waste.

Quaranta et al. [80] evaluated different steel discards (converter steel slag, white powders, BF sludge, and postmortem aluminosilicate refractories) as feedstock aggregate to clay. Converter steel slag is comprised of quartz, calcite, calcium hydroxide, periclase, wustite, and calcium silicate; the predominant crystalline phases in white powders are calcite, dolomite, and calcium hydroxide; BF sludge contains hematite and quartz as major phases, as well as magnetite, wustite, and iron in minor amounts. Meanwhile, aluminosilicate refractory wastes contains mullite and aluminum oxide as well as cristobalite, quartz, and titanium oxide. The authors studied the effect of waste addition (10–50 wt.%) on the plastic behavior of clay bodies. The results showed that by increasing the proportion of each waste in the body, the plasticity index reduces. They stated that the plastic behavior of steel wastes was not appropriate for processing ceramic bodies by extrusion and that the manufacture of materials using uniaxial pressure should be more suitable. However, recent studies contradict these results. Thus, Karayannis [88] reported on the development by plastic extrusion and firing of clay-based bricks including steel industry electric arc furnace dust (EAFD), and employed pilot-plant simulation procedures of industrial brick manufacturing. EAFD is comprised of enstatite and the results pointed out that efficient extrusion of brick samples incorporating up to 15 wt.% EAFD into the clay mixture was viable, without noteworthy deviations in both their mechanical performance and thermal conductivity. Zong et al. [80] evaluated the influence of steel-making slag (SS) particle size and sintering temperature on the sintering process and end properties of red clay ceramic bricks. The results showed that the water absorption rate of the specimens diminished with decreasing slag particle size and that the compressive strength of the sample was higher at a moderate SS particle size (

Bonet-Martínez et al. [83] carried out an investigation aimed at evaluating the use of AFD, a waste from the aluminum secondary industry, as a alternative for clay in the manufacture of fired bricks. AFD is constituted principally of aluminum oxide (60–70 wt.%), CaO (8 wt.%), sodium chloride (15 wt.%), and potassium chloride (5–10 wt.%). The authors considered the partial substitution of a clay mixture (40% black, 30% red, and 30% yellow clay) by different percentages of AFD (0–25 wt.%). After firing at 950°C, the results indicated that the addition of up to 20 wt.% AFD resulted in bricks with physical features similar to pure clay-based bricks and improved compressive strength and thermal conductivity. These sustainable bricks also met the directives for heavy metals leached to the environment.

However, despite the benefits of manufacturing bricks using waste materials, their commercial production is still irrelevant, which in part is attributed to the lack of information in the industry and the public in general regarding the environmental benefits of these materials. For this reason, Muñoz et al. [85] carried out an investigation for additional information about the environmental consequences of including Waelz slag into fired bricks. The study was carried out using an LCA methodology and a cradle-to-grave approach. The results showed that incorporating Waelz slag into ceramic bricks has a lower impact on climate change and decreases the impact on freshwater ecotoxicity and fossil depletion. These benefits were attributed to impact savings due to avoiding the landfilling of slag and lowered fuel requirements during manufacturing. However, due to the higher SO₂ and HF emissions produced in the firing of slag containing bricks, these advantages are counterbalanced by greater impacts on human toxicity and terrestrial acidification. The results indicated very restricted environmental benefits in this practice even taking into account different end-of-life scenarios.

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The Direct Reduction of Iron

1.2.3.3.3.2 Rotary Hearth Furnaces

Developments in the use of RHFs for coal-based DR of iron go back at least to the development of the HeatFast process at Surface Combustion in the 1960s [ 52 ]. However, Surface Combustion and their parent company Midland-Ross decided to focus on their gas-based technology, now known as the MIDREX process. Inmetco’s technology developed directly from HeatFast, as did FASTMET. As far as is known, the other RHF technologies are similar to Inmetco and FASTMET.

As mentioned earlier, these plants were not included on the master Midrex statistics [ 60 ] because their operations are generally based on processing of waste materials, rather than making highly metallized metallic iron from virgin iron.

The reasons for growth of the RHF market for dealing with waste materials were recently summarized by Southwick [ 65 ]. He makes the point that the economical RHF operations either produce valuable by-products rather than just iron, or basically replace sinter plants as a place for integrated mills to utilize iron-containing dusts and sludges. The product of these mills is too low in iron content and metallization to be considered EAF feed, but can be utilized in the BF. He estimates about 2 million tons of annual capacity in these mills, which are in Japan, Korea, Taiwan, and China.

Southwick goes on to discuss the success of Inmetco, and the failure of several other RHF operations that did not have the high value by-products of Inmetco’s operation. But interest is still high for RHFs in many of these applications because of the following advantages, specifically listed for FASTMET, relative to RKs [ 66 ]:

FASTMET operates at higher temperatures, over 1300 °C.

FASTMET achieves higher metallization and dezincification because of the higher temperatures and higher uniformity of the mixed EAF dust and carbon source before they are fed into the RHF.

FASTMET DRI can be used as a metallic in BFs, BOFs, and EAFs because of the higher metallization and recovery of zinc in the DRI.

Fines generation is lower because the agglomerated raw materials (iron ore and coal) do not roll, but are stationary on the rotary hearth.

The zinc content of the recovered flue dust is higher because the amount of dust generated is lower and the dust can be separated in the flue gas system.

The amount of dioxin in the DRI is lower because the dioxin in the EAF dust is broken down under high temperatures.

The amount of dioxin in the flue gas is lower, because the hot flue gas from the RHF is cooled rapidly in the flue gas system to prevent de novo formation.

The basic principles of an RHF used for iron reduction are described under Section 1.2.3.3.3.2.2 .

1.2.3.3.3.2.1 Inmetco

The INMETCO process is currently operated by the International Metals Reclamation Co. Inc. in Ellwood City, Pennsylvania for the recovery of valuable components of waste materials such as iron and steel dusts and batteries. The solids are fed into an RHF, which is separated by air curtains into oxidizing, reducing, and discharge zones. Burners above the bed are fired with oil or natural gas (coal burners have been tested and can be used as well). The shallow bed depth results in high heat transfer rates. Residence time for the pellets in the bed is approximately 15–30 min [ 38 ].

Inmetco specializes in accepting a wide variety of metallic feeds, particularly batteries and waste metal and oxide streams. In 2008, Inmetco was sold by Vale Inco to Horsehead Resources; at the time, it produced 27,000 tons/year of remelt alloy [ 67 ].

In 1996, Inmetco sold an RHF plant to Nakornthai Steel Mill in Thailand. The furnace was fabricated but never erected.

1.2.3.3.3.2.2 FASTMET

FASTMET®, developed by Kobe Steel and Midrex Technologies, is a coal-based RHF technology that uses a mixture of iron ore fines and/or steel mill wastes, with pulverized coal as carbon reducer, to produce high-quality DRI. The first commercial FASTMET plant started operation in 2000. There are now six operating facilities in Japan, all processing steel mill wastes, with a typical capacity of 200,000 tpy of feed. The end product can be discharged as hot DRI into transfer containers for use in basic oxygen or electric furnaces, cooled, or hot briquetted for later use.

The basic operation of the FASTMET RHF can be seen in Figure 1.2.17 . From the plan view, pellets or briquettes are fed from above onto the hearth of the furnace. A leveler controls the height of pellets in a given location (typically one to two layers). The hearth rotates on a rail-wheel assembly and the pellets pass through a number of zones; each zone is controlled to a different temperature by overhead burners, as shown in the cross section. While the atmosphere at the top of the furnace is oxidizing, the conditions near the pellets are reducing, due to the presence of carbon from coal or similar fuel, intimately mixed with the ore when the pellet or briquette was made. The solids remain on the hearth for about 320° of a circle, where a discharge device carries them away to be cooled or fed directly to a melting furnace. Residence times are typically 10–15 min, and maximum temperatures can easily reach 1350–1450 °C.

Figure 1.2.17 . Schematic diagram of rotary hearth furnace.

1.2.3.3.3.2.3 Others

There are several other RHFs currently operating around the world, handling various waste materials, often with some fraction of virgin iron. The general operation of these facilities is roughly the same, though some elements of the equipment or operation may vary.

Iron Dynamics, in Butler, Indiana, started with green pellets made from coal and iron ore fines, with the resultant hot DRI available for smelting in a submerged arc furnace, or hot briquetted for use in an EAF [ 59 ]. Due to operability problems over the years, the hearth is now used with waste oxides only. Annual production capacity is 260,000 tons [ 68 ].

Little is known about the RHF sold by Shenwu to Rockcheck Steel in 2010. It apparently has (or will have) a capacity of at least 500,000 tpy [ 60 ]. Other RHFs are either operating or under construction by this company in China, but few details are available.

Nippon Steel has built several RHFs, for themselves [ 69 ] and clients in Korea, Taiwan, and China. Recently, Paul Wurth has commercialized operations at the Lucchini-Piombino integrated Iron and Steel works in Italy. This hearth is the front end of the former pilot plant for the RedSmelt process [ 70 ].

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Solidification/Stabilization Process of Fly Ash

8.4.1 Treatment of Fly Ash with NaOH Solution

To 10 g of the fly ash, add 100 mL of 0.1 mol/L, 0.5 mol/L, 1 mol/L, 2 mol/L, and 5 mol/L NaOH solution, stirred on the oscillator for 24 h, and then left to stand for 16 h. It was then filtered, and the aqueous solution analyzed. The leaching residue was also dissolved in a nitric acid solution, and the heavy metal contents determined. Zn, Pb, and Cd appeared in the samples.

Table 8.9 shows the Pb and Cd content levels in the leaching solutions and leaching residues. The quantities of lead extracted increased with the increases of initial NaOH concentrations. This phenomenon can be found also in the alkaline treatment of Electric Arc Furnace dusts and oxidized zinc ores. Inorganic compounds of lead, especially oxides, carbonates, phosphates, arsenates, etc., can dissolve in both strong acidic and strong alkaline solutions. However, as predicted, the extraction of Cd remains unchanged as the initial NaOH concentrations increase.

Table 8.9 . Leaching of Fly Ash with NaOH

The dependence of lead and cadmium leaching on pH and NaOH concentrations clearly shows that extraction of lead increases significantly, while that of cadmium decreases as pH values or NaOH concentrations increase ( Fig. 8.5 ). The chemical reactions can be expressed as follows.

Figure 8.5 . Dependence of leaching of lead and cadmium from fly ash on pH and NaOH concentrations.

At lower pH values, the oxides of lead and cadmium dissolve in acidic leaching solutions:

As pH values increase, insoluble lead and cadmium hydroxides form, so that the leaching rate decreases until reaching pH 9:

Lead hydroxides dissolve when pH (owing to NaOH concentrations) increases so that leaching increases, while the leaching of cadmium remains at a lower value, contributed by chemical equilibrium:

In strong NaOH solutions, PbSO₄ can also dissolve to form soluble Na₂Pb(OH)₂SO₄:

Moreover, the Pb and Cd in the leaching residues are at 628 mg/kg and 20.19 mg/kg respectively, and even the fly ash is treated at 5 M NaOH solutions. According to Table 8.9 , around 20% and 56% respectively of Pb and Cd can be leached at 5 M NaOH solution, and then the possible concentrations in the leaching solutions of the residues would be 10.3 mg/L for Pb and 0.998 mg/L for Cd, which are beyond the scope of the leaching toxicity standard. Therefore, NaOH solutions can be used for the part extraction and recovery of lead and zinc from, but not for ultimate stabilization of, the fly ash.

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Potential hazards from waste based/recycled building materials

L. Tiruta-Barna , R. Barna , in Toxicity of Building Materials , 2012

14.3.3 Recycled aggregates

The potentially hazardous character of recycled aggregates resides in their chemical composition. Generally, these wastes are composed of a solid matrix that is relatively inert (silicates and oxides stable in contact with water), crystalline (often very porous) or vitreous, which also confers the mechanical properties of the aggregates. Various trace elements are also present depending on the origin of these recycled materials: ores, coals, residues from thermal processes, and municipal wastes (in the case of MSWI-BA). Some of these trace elements can have toxic properties depending on their chemical speciation, solubility in contact with water, and means of exposure. Several examples are given below.

The slags issued from different metallurgical processes represent annually huge quantities potentially available for reuse: e.g. Waelz slag from the recycling of electric arc furnace dusts (weight composition: 7–23% CaO, 4–7% Al ₂O₃, 0.1–0.5% Cr₂O₃, 4–40% FeO, 0.01–0.1% As, 0.3–0.5% Cu, 0.4–4.2% Pb, 0.2–4% Zn, 0.8–2% S, etc.) ( Barna et al., 2000a ), Imperial Smelting Furnace slag (average weight composition: 18.1% CaO, 10.4% Al₂O₃, 33.7% FeO, 0.1% As, 0.7% Pb, 7% Zn, 1.6% S, 21.9% SiO₂, etc.) and Lead Blast Furnace slag (average weight composition: 20% CaO, 1.9% Al₂O₃, 33.4% FeO, 0.15% As, 3.5% Pb, 11.2% Zn, 0.7% S, 23% SiO₂, etc.) from the primary non-ferrous metallurgy ( Barna et al., 2004 ), etc.

MSWI-BA composition is variable depending on the municipal solid waste composition and type of incineration process, the major elements being Si, Ca, Al, Fe, Na, Mg, K and C. Elements like Cl, Ti, Zn, Cu, Ba, S and Mn could be found in concentrations of about 1–10 g/kg. Trace elements (less than 1 g/kg) include Pb, N, Sn, Cr, Zr, F, B, Ni, Sb, V, Co, Cd, Ga, Li, La, Mo, Ba, As, Be, Au, Sc, Hg, Se and cyanide ( Jeong et al., 2005 ). The organic part composed of unburned matter varies up to 30% of the MSWI-BA dry mass.

The chemical composition of fly ashes (CCP) depends on the coal origin and combustion process used. Besides the major constituents (Si, Al, Fe, Ca) a large variety of chemical elements are present in trace quantities, some of them being targeted for their potential hazard. These are Cr, Cu, Ni, Pb, V and Zn present in several hundreds of mg per kg of dry fly ash, As and Se as dozens of mg/kg, and Hg, Cd and Sn up to 1 mg/kg ( Rakotoarisoa, 2003 ). Concerning the chemical speciation, the trace elements are mostly captured in stable alumino-silicates and oxides, reducing their solubility and availability in contact with water. Condensation of volatilised elements during combustion can also occur, especially at the surface of ash fine particles.

Significant European research programmes have been dedicated in recent decades to the characterisation of waste materials in parallel with the development of an environmental assessment methodology ( Barna and Blanc, 2011 ). A huge bibliography is available concerning the leaching properties of MSWI-BA, fly ashes and slags (obtained by laboratory leaching tests, Section 14.4 ).

Generally, the amount of pollutants released can be linked to their total content. However, their mineralogical characterisation can explain apparently contradictory behaviours, e.g. the encapsulation of small metallic droplets in the glassy phase of slag (5–47 wt%) explains a reduced release of Pb and Zn between the different Waelz slags ( Barna et al., 2000a ). Leaching conditions such as leachate composition or exposure conditions (carbonation, cycles like wetting/drying, freezing/thawing, etc.) can contribute to particular behaviours. For example, organic compounds having complexation capacity can explain the high level of Cu release in materials containing MSWI-BA ( Bröns-Laot et al., 2004 ), and acidic conditions increase the release from ‘reactive’ materials like phosphates (apatites) or cement-based materials containing pollutants.

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Pyrometallurgical recovery of zinc and valuable metals from electric arc furnace dust – A review

Jie Wang , . Tahani Saad AlGarni , in Journal of Cleaner Production , 2021

Abstract

Electric arc furnace dust (EAFD) has attracted more and more attention as a kind of metallurgical solid waste with huge production. How to reduce its harm to the environment and efficiently recover valuable metals such as zinc, lead and iron from its complex components to realize the comprehensive utilization of EAFD is the aim of various EAFD treatment processes. In this paper, the physical and chemical properties of EAFD are discussed from the micro and macro perspectives, the microstructure, chemical composition and phase composition of EAFD in various countries are summarized in detail. According to the zinc content, EAFD can be divided into three types: high-zinc, medium-zinc and low-zinc. The results show that Fe, Ni, Cr and other elements increase with the decrease of zinc content in EAFD. Not only the zinc oxide but also abundant Fe–Ni–Cr alloy of high value can be recycled from the low-zinc EAFD, which provides guidance for EAFD treatment. At the same time, the general mechanism of direct reduction and smelting reduction pyrometallurgical processes of EAFD is expounded, and the existing pyrometallurgical processes such as rotary kiln process, rotary hearth furnace process, Primus process, ESRF process and Coke-packed bed process are summarized and compared from the aspects of energy consumption, production capacity, process flow to actual application status. In addition, the properties of directly reduced iron and zinc-rich dust obtained by pyrometallurgical processes of EAFD are also discussed. Finally, a new process of treating EAFD by combining direct reduction process and smelting reduction process is proposed to overcome the defects existing in two types of pyrometallurgical processes, and the development of preparing zinc oxide micropowder and high valued Fe–Ni–Cr alloy by pyrometallurgical process of EAFD is prospected.

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