Environ. Sci. Technol. 2010, 44, 5383–5388
Physico-Chemical Characterization of Steel Slag. Study of its Behavior under Simulated Environmental Conditions C A R L A N A V A R R O , † M A R I O D ´I A Z , ‡ A N D M A R ´I A A . V I L L A - G A R C ´I A * , † Department of Organic and Inorganic Chemistry, and Departament of Chemical Engineering and Environmental Technology, University of Oviedo, Julia´n Claverı´a 8, 33006 Oviedo, Spain
Received March 9, 2010. Revised manuscript received June 10, 2010. Accepted June 10, 2010.
The chemical and mineralogical composition of steel slag produced in two ArcelorMittal steel plants located in the North of Spain, as well as the study of the influence of simulated environmental conditions on the properties of the slag stored in disposal areas, was carried out by elemental chemical analysis, XRF, X-ray diffraction, thermal analysis, and scanning electron microscopy with EDS analyzer. Spectroscopic characterization of the slag was also performed by using FTIR spectroscopy. Due to the potential uses of the slag as low cost adsorbent for water treatment and pollutants removal, its detailed textural characterization was carried out by nitrogen adsorption-desorption at 77 K and mercury intrusion porosimetry. The results show that the slag is a crystalline heterogeneous material whose main components are iron oxides, calcium (magnesium) compounds (hydroxide, oxide, silicates, and carbonate), elemental iron, and quartz. The slags are porous materials with specific surface area of 11 m2g-1, containing both mesopores and macropores. Slag exposure to simulated environmental conditions lead to the formation of carbonate phases. Carbonation reduces the leaching of alkaline earth elements as well as the release of the harmful trace elements Cr (VI) and V. Steel slags with high contents of portlandite and calcium silicates are potential raw materials for CO2 long-term storage.
Introduction Steel industry plants produce large amounts of byproduct. The solid coproducts generated during steel manufacturing are in the form of slag and sludge. The annual world production of slag from iron and steel industries reaches almost 50 million tons (1, 2). There are different types of steel industry slag, each one named for the process from which they are generated: blast furnace slag (BF) also called iron slag, basic oxygen furnace slag (BOF), electric arc furnace acid slag (EAF) and ladle furnace basic slag (LF) also called refining slag. In this work we studied a mixture of BOF and LF slag that we named as steel slag. * Corresponding author phone: 34 985102976; fax: 34 985103446; e-mail:
[email protected]. † Department of Organic and Inorganic Chemistry. ‡ Departament of Chemical Engineering and Environmental Technology. 10.1021/es100690b
2010 American Chemical Society
Published on Web 06/22/2010
Due to their high CaO content, the steel slag containing low amounts of phosphorus and sulfur, are reused as fluxing material in steel making process to replace limestone (3). Steel slag is also being used in other areas such as materials for road construction, in ballast for railway tracks, hydraulic engineering, as fertilizers and soil conditioners, for metal recovery (1-10). During last decades, a few research groups approached a new way of recycling steel slag by investigating their use as low cost materials capable to substitute high cost adsorbents, such as activated carbons, for water treatment and the removal of different types of pollutants. Special attention deserves the work of V.K. Gupta et al. (11-20) that have studied the use of a number of inexpensive natural materials and also low cost adsorbents from different industrial wastes, for this purpose. Nevertheless, very large amounts of this material are being dumped in slag disposal areas without any profit and new alternative uses have to be developed. It is then interesting to obtain new data on the structure and chemical nature of slags from different steel plants that will allow a better knowledge of this material to facilitate its recycling, which will favor the reduction of the environmental impact associated with its disposal. The objective of this work was to study the chemical composition and the structural and textural properties of steel slags produced in the ArcelorMittal Aviles and Verin ˜a (Spain) steel plants. The accurate knowledge of their characteristics is necessary in order to recycle these industrial byproduct. The study has been accomplished by means of chemical analyses, X-ray diffraction, FT-IR spectroscopy which is a technique not often used to characterize this type of materials, scanning electron microscopy, electron probe microanalysis, TG-DTA analysis, N2 adsorption-desorption analysis, and mercury intrusion porosimetry. The use of the last two techniques to characterize the slag texture is novative, because only a few articles report partial data on the porous structure of steel slag. However, a detailed systematic study of the surface area and porosity are needed to recycle this material as potential low cost adsorbent. The data obtained provide insights that may be useful for finding alternative uses of this material. Even if steel slag are not considered as hazardous material, from an environmental point of view when millions of tons of slag are stored in dumpsites during long time periods, it is necessary to study the environmental impact associated to its disposal for security reasons, which was evaluated by analyzing any possible change in the chemical composition and structural properties of slags treated under simulated environmental conditions. To ensure inoffensive dumpsites for the environment, the release of harmful elements such as Cr (VI) and V, was also studied.
Experimental Procedures The steel slags used in the present work were provided by the steel plants ArcelorMittal Avile´s (sample SA) and ArcelorMittal Verin ˜ a (sample SV), both located in the North of Spain. BOF and LF slag are being mixed in these plants, so the used samples contained both types of slag. The slags, size gradation 0-10 mm, were produced during 2009 and were sent to the laboratory without any storage time. Once received, to facilitate the characterization analysis, the samples were milled and sieved to obtain a powder with particle sizes e100 µm. After milling we did not detect metallic iron. Solubility measurements were performed to asses the mobility of metals, or any soluble compound present in the slag, under neutral conditions. Samples of both types of slag VOL. 44, NO. 14, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Simulated Environmental Conditions sample
conditions
flow (mL/s)
temp. (°C)
exposure time
SA1 SA2 SA3
air current water vapor saturated air current water vapor saturated air current air current passed through slag suspended in distilled water under stirring
50 50 50
20 20 20
1 week 1 week 2 months
50
20
1 week
SA4
were thoroughly washed with distilled water, then were dried in an oven during 24 h at 120 °C, the resulting materials were named SAW and SVW, respectively. Due to the similarity between both types of slags, the influence of the environmental conditions was evaluated using slag from Avile´s Steel Plant. Treatment conditions appear in Table 1 After the treatment the samples were filtered and dried at 120 °C for 24 h. The chemical composition of the samples was determined by X-ray fluorescence spectroscopy (XRF) using a Philips PW 2404 instrument. Elemental analysis of carbon, nitrogen, sulfur and hydrogen was performed with a Perkin-Elmer 2400 elemental analyzer. Mineralogical composition was studied at room temperature (25 °C) using a X-ray diffractometer Philips X’PERT PRO, operating at 45 kV and 40 mA, using Cu KR radiation, adquisition interval 5-80° (2θ) and step size 0.02°. FTIR spectra were obtained with a Perkin-Elmer PARAGON 1000 spectrometer, using the KBr pellet technique. Thermal analysis was carried out using a Mettler SDTA851e with a Mettler TA-4000 TG-50 type thermobalance. Samples (40-50 mg) were heated in alumina crucibles under nitrogen atmosphere at 10 °C/min from 25° to 1000 °C. Nitrogen adsorption-desorption isotherms at 77 K were obtained with a Micromeritics ASAP 2020 instrument, using static adsorption procedures. Before the analysis the samples were degassed at 120 °C during 4 h (sample mass 490-510 mg). Macroporosity and total pore volume were measured by mercury intrusion porosimetry with a Micromeritics Autopore IV instrument. The analysis was performed at low and high pressure (0.1-60000 psi) using a bulb penetrometer for powder with stem (sample mass 490-510 mg). The microscopic morphology of the slag was studied at a working distance of 15 mm using a Jeol 6100 scanning electron microscope operating at 20 kV in SE micrograph mode. Concerning EPMA a Cameca SX 100 coupled with wavelength dispersive spectrometers was used (acceleration tension 15 kV, intensity 40 nA and counting time 15 s/element).
Results and Discussion Slag Characterization. Chemical Composition. Elemental analysis was performed in both types of steel slag to determine the possible presence of any carbon, nitrogen or sulfur compound. The presence of carbon, nitrogen or sulfur compounds in the slags is negligible (see Table S1 in the Supporting Information (SI)). Elemental analysis of samples SVW and SAW showed same values for C, N, H, and S content than those obtained for samples SV and SA, respectively. The chemical composition of the major constituents of the slag, determined by X-ray fluorescence spectroscopy (XRF) for sample SV was 39.19% CaO; 31.47% Fe2O3; 14.40% SiO2; 5.45% MgO; 3.55% MnO; 3.11% Al2O3; 1.11% P2O5; 0.50% TiO2; 0.10% K2O; 0.06% Na2O; and 1.06% the loss of ignition. For sample SA the results obtained were 44.31% CaO; 26.30% Fe2O3; 14.03% SiO2; 3.89% MgO; 4.04% MnO; 3.50% Al2O3; 1.25% P2O5; 0.57% TiO2; 0.10% K2O; 0.03% Na2O; and 1.98% the loss of ignition. The main constituents of the slag samples are calcium, iron, silicon, magnesium, manganese, aluminum, and phosphorus compounds. Most of the minor metal constituents 5384
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are present as traces, with a concentration less than 200 ppm (see SI Table S2). Chromium and vanadium are present in higher concentrations than the rest of the trace elements, due to its toxicity, special attention has to be dedicated to its leaching behavior. Different researchers already studied the leachability of these toxic trace elements present in the slag (1, 21-25). The chemical composition of samples SVW and SAW, determined by XRF analysis, is analogous to that of unwashed samples, indicating that there is not significant dissolution of major constituents nor harmful metals present as traces, the last ones probably are bound within stable crystalline phases, as it occurs in natural ore deposits. Structural Characterization. X-ray Diffraction. XRD was performed in as received samples from Avile´s and Verin ˜a steel plants and after thoroughly washing with distilled water. The diffraction patterns of samples SA and SV are similar (see diffractogram in SI Figure S1). Both type of slags are heterogeneous materials consisting of a mixture of crystalline phases. The analysis of the diffraction lines revealed the presence of the following major constituent phases: portlarnite (Ca2SiO4), merwinite landite (Ca(OH)2), (Ca3Mg(SiO4)2), dicalcium magnesium ferrite (Ca2MgFe2O6), calcite (CaCO3), dolomite (CaMg(CO3)2), magnetite (Fe3O4), hematite (Fe2O3) and quartz (SiO2). The slags also contain free CaO and elemental iron (see SI Figure S2). Any other minor constituent phases are very difficult to assign due to the complexity of the diffractograms and the low intensity of their diffraction lines. The XRD diffractogram of samples SAW and SVW show the same major constituent phases than SA and SV but the CaO minor constituent phase was not detected, which indicates that after washing CaO is transformed to Ca(OH)2. The great content of calcium hydroxide and basic oxides gives to steel slag a high alkalinity (pH ∼ 12.5), having its suspensions a high capacity to neutralize strong acid media. FTIR Spectroscopy. In many cases, infrared absorption spectra of crystalline substances give valuable information on their structure and, in particular, supply conclusive evidence on the nature of the functional groups present in the crystal lattice. Spectroscopic studies have been carried out to understand the structure of the slags. The FTIR spectrum of sample SA is shown in Figure 1 and is similar to that of sample SV (see SI Figure S3). The very sharp bands that appear between 3695 and 3530 cm-1 can be assigned to hydroxyl stretching modes, the band at 945 cm-1 corresponds to hydroxyl deformation modes. These bands confirm the presence of hydroxides and oxyhydroxides in the slags. Assignment of the hydroxyl vibration modes found in the slag to the different crystalline phases is not an easy task. The difficulty with using this region of the spectrum is due to the overlapping of the hydroxyl stretching region of the different slag constituents. The band at 3692 cm-1 and the shoulder at 3624 cm-1 are characteristic of hydroxyl stretching vibrations of mineral layered aluminum silicates containing structural hydroxyl groups (26, 27). The very strong and sharp band that appears at 3644 cm-1 corresponds to hydroxyl stretching vibrations of calcium hydroxide. The system Al2O3-H2O also shows IR absorption bands in the hydroxyl stretching region, at 3622, 3530, and
FIGURE 1. FTIR Spectrum of Sample SA. 3450 cm-1 (28), the band at 3538 cm-1 could be assigned to hydroxyl stretching modes of hydrated aluminum compounds.Itwasalsoreported(29)thathydratedcalcium-magnesium carbonates show a broad band centered around 3435 cm-1 and a sharp band at 3629 cm-1, therefore the broad shoulder centered at 3442 cm-1 could be assigned to both hydroxyl stretching modes of hydrated carbonates and also to adsorbed or coordinated water molecules. The band at 1638 cm-1 corresponds to hydroxyl bending modes of the hydroxides and hydrated phases present in the slag. The bands that appear at 1797 cm-1 and 1425 cm-1 (very strong and very broad) correspond to C-O stretching modes of the CO32ion, the bands at 873 and 715 cm-1 can be assigned to deformation modes of the C-O vibrations (30). The absorptions with maxima between 1100 and 950 cm-1 are characteristics of Si-O stretching modes of silica and silicates, moreover, absorption bands corresponding to hydroxyl vibrations of aluminum oxyhidroxydes (31) and iron oxyhidroxydes (32) also appear in this region of the spectrum. The band at 753 cm-1 corresponds to Al-O stretching vibrations (33-35) and indicates the presence in the slag of aluminum compounds such as silicates, Al2O3 and aluminates (spinel, mayenite, or tricalcium aluminate) not detected by X-ray diffraction as major constituents. Absorptions between 600 and 450 cm-1 are a common feature of different oxides. Bands in this region have been observed in Al2O3, Fe2O3, MgO, ferrites and most metallic oxides. Due to the complex composition of the slag, it is very difficult to obtain additional structural information from this region of the spectrum because there is an overlap of the bands characteristic of the different constituents. FTIR spectroscopy also identifies as the most abundant components of the slag the following phases: calcium hydroxide, calcium and magnesium carbonates, silicates, iron oxides, and calcium aluminates, which corroborates the XRD results. Thermal Analysis. The TG and DTA curves of sample SA are similar to those obtained for sample SV (see SI Figure S4). The TG plot shows that the weight loss takes place in several steps. Between 80 and 220 °C the slag undergo a weight reduction of 1% due to the removal of adsorbed moisture. A very small endothermic effect between 220 and 340 °C (1.5% weight loss) can be attributed to the loss of structural H2O from hydrated carbonates (29) and partial dehydration of silicates (36, 37). A second endothermic peak from 340 to 420 °C, which corresponds to a weight loss of 1.7%, can be attributed to dehydration of iron and magnesium hydroxides (38). A large endothermic reaction occurring between 420 and 560 °C (6.6% weight loss) is caused by dehydroxylation of Ca(OH)2 to lime and partial dehydration of silicates that continues up to 800 °C. Finally, in the range
FIGURE 2. Pore size distribution of sample SA measured by mercury intrusion porosimetry. from 600 to 900 °C takes place the endothermic decomposition of carbonates with the liberation of CO2, and total dehydration of silicates, the weight loss associated to this process is 3.4%. Textural Properties. The use of industrial wastes which are not currently being recycled or given new uses, as potential low cost adsorbents for water treatment and pollutants removal has a great potential interest. Very large amounts of slag produced by steel plants are being kept in disposal areas without any profit, their use as low cost adsorbents with little or no pretreatment should be object of more detailed systematic studies, because their low cost and availability is of great advantage, even if they perform half as well as high cost adsorbents such as activated carbon. It is a known fact that the adsorption behavior depends on various factors, which include adsorbate-adsorbent interactions controlled by the nature of the functional groups present on the surface of both adsorbent and adsorbate, pH, temperature, batch or column conditions, surface area, and pore size distributions of adsorbent. The chemical nature of functional groups on the adsorbent surface was studied by FTIR spectroscopy. Furthermore, a detailed study of the surface area and the type of porosity, micro/meso/macro, as well as the pore size distribution are needed previously to studying the behavior of steel slag as inexpensive adsorbents. Specific surface areas and mesopore size distributions of theslagswerecalculatedfromthenitrogenadsorption-desorption isotherms at 77 K. The profile of the isotherms is typical of type IV of the BDDT classification (39), with very narrow hysteresis loops type H3, which is usually associated to porous solids consisting of particle aggregates. The low nitrogen uptake at relative pressures
