Electric furnace dust and filter cake collected from a stainless steel plant as well as EF dust collected from a ferrochrome plant were characterised. Electric furnace dusts consist of very fine particles, from which Cr(VI) can be leached by shallow groundwater. When heated in air H2O, CO2, SO2, SO3, fluorine, calcium and silicon are expelled from these materials, while metallic particles oxidise. The main phases present in the stainless steel plant dust are the (Mg,Fe,Mn,Cr)3O4 spinel phase, quartz, Ca(OH)2 and nickel. The coarse fraction of ferrochrome dust mainly contains chromite and partly altered chromite, quartz and carbon, while the main components of the fine fractions of ferrochrome dust are chromite, SiO2, ZnO, NaCl and Mg2SiO4. CaF2 is the major phase in the filter cake. Cr(VI) containing phases are possibly generated at the top of the submerged arc furnace or in the off gas duct, as Cr(VI) is found on the surface of the dust.
Extracts
Introduction
Stainless steel is typically smelted in an electric arc furnace (EAF) from scrap, molten or lump ferrochrome and slag formers (lime, fluorspar and dolomite), after which it is refined in an argon oxygen decarburisation (AOD) vessel or Creusot–Loire Uddeholm (CLU) converter. Ferrochrome on the other hand, is produced by the carbothermic reduction of chromite ore in a submerged arc furnace (SAF) or direct current (DC) furnace. Emissions from these processes are cooled and the particulate matter collected by dust treatment systems. Figure 1 schematically shows the dust treatment systems of the ferrochromium and stainless steel plants. In the ferrochromium plant (Fig. 1a), the coarse dust is collected by a cyclone separator, while the bag house filters gather the fine dusts. The particulate matter from the EAF and refining converter are also collected by the bag house filter in the stainless steel plant (Fig. 1b). These electric furnace (EF) dusts contain valuable components (e.g. chromium, zinc and iron) as well as toxic substances (e.g. chromium (VI) and lead), which can leach into the groundwater when stockpiled or land filled. Of these toxic substances, chromium(VI) is also carcinogenic, and is present in the dusts at levels which exceed the regulation thresholds for the disposal of hazardous waste in many countries.1 The EF dust is therefore considered to be a hazardous material and it needs to be treated before being stockpiled or land filled.
Stainless steel plants also produce filter cake that contains significant levels of Cr(VI) and fluoride, which is also potentially harmful to human health and the environment. This filter cake is produced in the waste pickling acid treatment plant from a neutral solution of sodium sulphate as well as a mixture of nitric acid and hydrofluoric acid, which are used to dissolve the oxide scale on the surface of the stainless steel by electrochemical and chemical ways in order to improve the surface quality of the stainless steel (Fig. 2). The waste pickling acid is highly acidic (typically at pH of y1) and contains high concentrations of fluorine, iron, nickel, Cr(III) and Cr(VI). These waste acids are treated through neutralisation with lime, followed by iron sulphate addition to reduce Cr(VI) species to Cr(III), and then the precipitation of metals by lime (typically at pH of y9.5). Finally, the precipitate is pressed into a filter cake.2
The existence and treatment of wastes from stainless steel and ferrochrome production remain a challenge and an issue of concern. The increase of environmental legislation globally and the trend towards sustainable development are drives for alternatives to landfill.
In the past decades, a number of alternative technologies have been developed to handle EF dusts and other metallurgical wastes. These technologies can be divided into three categories, i.e. direct recycling processes,3,4 recovery processes5–10 and solidification/stabilisation processes.11–14 Direct recycling of dust back to the EF or blast furnace is the easiest way to treat the EF dusts. However, the alkaline metals and zinc in the dust could increase the energy consumption in the EF.5 The recovery processes include pyrometallurgical methods,7–9 which mostly recover Cr, Ni, Fe, Zn, Pb and Cd, and hydrometallurgical methods that focus on the recovery of zinc.10 The pyrometallurgical recovery processes require high investment costs, while in the hydrometallurgical recovery processes it is often difficult to both economically recover the valuable elements and let the treated residue meet the toxicity limits.10 Solidification/ stabilisation processes are therefore widely considered to be an effective method that can encapsulate, glassify or combine the toxic elements, and simultaneously add value to the wastes.
In order to develop a technique whereby the Cr(VI) containing EF dust and filter cake can successfully be stabilised, it is important to first comprehensively characterise these waste materials. The present paper subsequently describes the characteristics and microstructures of Cr(VI) containing EF dust and filter cake from a stainless steel and ferrochromium plant.
The mechanisms of formation of the dusts and a more detailed study of their leaching behaviour are presented in a later paper. ...
Results
Particle size distribution, bulk density, moisture content and Ph
The EF dusts are very fine particles, with d50 values of 3.2, 2.9, 5.5 and 79.8 µm for samples SPD, FCD1, FCD2 and FCD3 respectively. The bulk density, moisture content and pH of these wastes are presented in Table 1. It can be seen from Table 1 that sample FCD3 has the highest density (2.42 g cm23) while sample FCD1 (0.49 g cm23) has the lowest density of the examined waste materials. The moisture contents of the dusts range between 0.4 and 1.06%. The leaching behaviour of Cr species by ground water is related to the pH of the environment and the redox potential (EMF) of the aqueous solution.18 In natural groundwater chromium has two stable oxidation states, i.e. Cr (III) and Cr(VI). Cr(VI) is the stable species under oxidising conditions that are found in shallow ground waters, whereas Cr(III) is thermodynamically stable under reducing conditions in deeper ground waters.18 The dominant species of Cr(VI) are highly soluble HCrO4 2 and CrO4 22. Under oxidising conditions, in natural groundwater with a pH value between 6 and 8, the predominant species is CrO4 22.18 CrO4 22 prevails at higher pH.18 It was found that the EF dusts and filter cake all generate basic solutions (pH.8) when leached in water. The CrO4 22 species can therefore potentially be leached from these materials in shallow groundwater. A more detailed study on the leachability of Cr(VI) from the electric furnace dusts and filter cake is presented in a later paper.
Chemical composition and phase composition of EF dusts and filter cake
The chemical compositions of the EF dusts and filter cake are given in Table 2. The SPD is iron oxide, chromium oxide and CaO rich, but also contains some MgO, MnO, SiO2, ZnO and nickel. The fine fractions of the ferrochrome plant dust (FCD1 and FCD2) contain high concentrations of SiO2, ZnO, MgO and alkali metal oxides, but also some sulphur and chlorine, while the coarse fraction (FCD3) is SiO22Cr oxide–iron oxide2Al2O32MgO2C based. The concentrations of calcium, fluorine, iron and sulphur are high in the filter cake.
X-ray diffraction (Fig. 3) and EDS analyses indicated that the SPD mainly contains a (Mg,Fe,Mn,Cr)3O4 spinel phase, quartz, CaF2, pure Ni particles, stainless steel particles, Ca(OH)2 and a glassy slag phase. The major phases that are present in dust samples FCD1 and FCD2 include NaCl, ZnO, MgO, Mg2SiO4, chromite particles, cristobalite, ferrochrome particles and a glassy slag phase. Small amounts of zinc hydroxychlorosulphate hydrate [NaZn4(SO4)Cl(OH)6.6H2O] and zinc sulphate hydroxide hydrate [Zn4SO4(OH)6.5H2O] could also be detected by XRD. The coarse ferrochrome dust sample (FCD3) contains chromite and partially altered chromite (PAC) particles, carbon, quartz, (Ca,Mg)(CO3)2 and a glassy slag phase from which anorthite ((Ca,Na)(Si,Al)4O8) precipitated. Fluorite (CaF2) is the major phase in the filter cake, while CaSO4, an amorphous metal oxide rich phase, a few lime particles and stainless steel scale are also present. ...
Microstructure of EF dusts and filter cake
Steel plant dust
More than 85% of the SPD particles are smaller than 40 mm in diameter. These dust particles are of varying microstructure, and can be divided into three categories:
(i) particles that are irregular in shape
(ii) spherical or near spherical particles
(iii) particles coated with slag or oxides.
The particles that are spherical or near spherical, as well as the spherical particles that are coated with slag or oxides, are the most abundant in the SPD sample.
The particles that are irregular in shape include pure nickel (Fig. 5), quartz, lime, fluorspar and ferrochrome particles. It is clear that these particles were captured by the off gas during charging.
The spherical or near spherical particles include metal particles (Fig. 6) and slag particles. These particles range from submicron to ≥ 200 μm in diameter. The slag particles are either hollow (Fig. 7) or consist of a glassy silicate based matrix that contain oxide crystals (Figs. 8– 10) and metal droplets (Fig. 11). The oxide crystals are either cubic (Fig. 8), dendritic (Fig. 9) or needle like (Fig. 10). EDS analysis indicated that the cubic and dendritic crystals are (Mg,Fe,Mn)(Cr,Al)2O4 spinel crystals, while the needles are CaCr2O4. Spherical stainless steel particles (Fe–3.0Cr–7.2Ni–3.9Mo) that are coated with slag were also found (Fig. 11).
Slag particles that are covered with an oxide layer, which is of different chemical composition to the centre of the particle, could also be distinguished. Such slag particles are shown in Figs. 12 and 13. X-ray mapping of one of these particles (Fig. 13) indicated that the centre of the particle is enriched in Cr, Ca and Al, while the rim is rich in Zn and Fe.
Ferrochrome plant dust
The fine fractions of ferrochrome dust (FCD1 and FCD2) mostly consist of agglomerated particles that formed clusters (Fig. 14). Approximately 90, 75 and 20% of the particles are ( 40 μm diameter in samples FCD1, FCD2 and FCD3, respectively. Such clusters typically contain chromite and partly altered chromite (PAC) particles, reductant (C based particles), hollow metallic ferrochrome droplets, flux (quartz) and SiO2 based slag droplets that are embedded in a matrix of very fine particles. This matrix is mainly SiO2–MgO– ZnO–(Na, K)2O based.
The coarse fraction (FCD3) mostly contains particles that are irregular in shape, but also spherical or near spherical particles and particles coated with a slag layer (Fig. 15). The particles that are irregular in shape include chromite ore particles, quartz particles and carbon based particles. The spherical particles include metal particles, Si–Ca–Mg–Fe based slag particles, particles that contain spinel crystals and ferrochrome droplets that are embedded in a porous sodium rich silicate slag layer (Fig. 16), as well as chromite particles that are surrounded with a porous layer that contains some very small Cr–Fe based metal droplets, followed by a more solid layer of Mg2SiO4 (Fig. 17).
Filter cake
The filter cake consists of very finely intergrown areas that are Ca–F–S–O based, and areas that contain high concentrations of metal oxides of iron, chromium and nickel (Fig. 18). XRD analysis indicated that the Ca–F– S–O based areas consist of a mixture of CaF2 and CaSO4 while the metal rich oxide phase is amorphous. The Ca–F–S–O based precipitate presumably formed owing to supersaturation of the waste acid with respect to CaF2 and CaSO4 (solubility limits of 0.016 and 2.09 g L21 respectively24) in the neutralisation and reduction steps. It is assumed that the metal oxide rich precipitate is the reaction product between the metal ions and the lime particles during the final precipitation step. Free lime particles could also be distinguished in the filter cake (Fig. 19). ...
Conclusions
The size distribution, bulk density, moisture content, pH, thermal properties, chemical composition, phase composition and microstructure of chromium containing EF dusts and filter cake were characterised. The following conclusions can be drawn.
1. The EF dusts are very fine particles, have bulk densities that vary between 0.49 and 2.42 g cm23, and have low moisture contents.
2. On leaching in water the examined EF dusts and filter cake produce alkaline solutions (pH.8). Since water soluble Cr(VI) species are associated with shallow ground water where conditions are oxidising and alkaline (pH.6), it can be expected that Cr(VI) species will leach from these materials.
3. The main phases that are present in the SPD are the (Mg,Fe,Mn,Cr)3O4 spinel phase, quartz, Ca(OH)2 and nickel. The dominant phases of the coarse fraction of ferrochrome plant dust are chromite, partly altered chromite, quartz and carbon, while the main components of the fine fractions include chromite, SiO2, ZnO, NaCl and Mg2SiO4. The major phase present in the filter cake is CaF2.
4. TG/DTA analysis in air indicated that mass losses and gains occur during heating of these waste materials owing to reactions in which H2O, CO2, SO2, SO3, fluorine, calcium and silicon are driven off, and metallic particles oxidise.
5. It is assumed that Cr(VI) containing species in ferrochrome dust are generated at the top of the SAF or in the off gas duct, as Cr(VI) is found on the surface of the dust.
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