Characteristics of steel slag under different cooling conditions

Transcription

1 Waste Management 27 (2007) Technical paper Characteristics of steel slag under different cooling conditions M. Tossavainen a, F. Engstrom b, *, Q. Yang b, N. Menad b, M. Lidstrom Larsson b, B. Bjorkman b a Division of Mineral Processing, Luleå University of Technology, SE Luleå, Sweden b Division of Process Metallurgy, Luleå University of Technology, SE Luleå, Sweden Accepted 8 August 2006 Available online 26 September Abstract 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 /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. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction Large quantities of materials are used in the construction and maintenance of roads each year. In Sweden, the production of rock material (aggregates) in 2003 was 70 million tonnes, 50% of which was used for road making and 10% in the manufacture of concrete (SGU, 2004). In Sweden, two interim targets regarding the environmental quality objective A Good Built Environment have been set, according to which, by 2010, the reused materials will represent at least 15% of the aggregate used and by 2005 the landfilled waste will be reduced by at least 50% compared to Gravel is used in concrete and according to the environmental quality objectives (A Good Built Environment, 2004), by 2010, the extraction of natural gravel in Sweden shall not exceed 12 million tonnes per * Corresponding author. Tel.: ; fax: address: Fredrik.i.engstrom@ltu.se (F. Engstrom). year, as compared to the 20.3 million tonnes produced in 2003 (SGU, 2004). Due to its high strength, durability and chemistry, steel slag is a suitable material in the field of construction, and its use also contributes to a reduction in the amount of landfilled waste. Unfortunately, in spite of its potential in 2002, only 25% of the Swedish steel-slag production (896 kt) was sold as external products (source: private communication with steel industry representatives). This is due to the fact that in addition to the lack of rules and guidelines regarding testing, assessing and using slag in Sweden, the technical and environmental obstacles for some slags in construction include low volume stability and leaching of elements. Other impediments are a long tradition and knowledge of using rock material and the fact that in Sweden there are still quite good resources of highquality rock material. The fear that some slags are environmentally hazardous is also something that has to be considered X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.wasman

2 1336 M. Tossavainen et al. / Waste Management 27 (2007) Rapid cooling by water granulation can result in an amorphous slag, encapsulating metals and oxides, and thereby lowering the solubility of the heavy metals compared to rock material used for road making (Tossavainen and Forssberg, 2000). The formation of a glassy material depends on both the chemical composition and the cooling conditions. According to Daugherty et al. (1983), glass was easier to produce, as the acidity of the slag increased for a series of synthetic slag compositions that was quenched and annealed. Ionescu et al. (1998, 2001) have shown how water quenching of steel slag results in products with a high content of glassy material. Silicate melts have high viscosity due to long molecule chains, and rearrangement into crystals only takes place slowly. If the cooling is rapid, the slag passes from a liquid state to a solid without development of a crystalline structure (Lea, 1983). Glasses, such as granulated slags, can be regarded as super-cooled liquids having a very high viscosity. By enhancing the amount of amorphous material in a slag, the potential hydrating properties are increased and the material can also be used in cement and concrete products of higher quality compared to conventional road-making materials (Murphy et al., 1997; Ionescu et al., 1998, 2001; Shij, 2004). For a disintegrated slag, use in concrete is particularly interesting, as grinding costs can be reduced. Besides glass formation, controlling cooling conditions can be a means of affecting mineral transformation and consequently the solubility of elements like chromium. Chemical compounds containing hexavalent chromium (Cr 6+ ) are generally considered far more toxic than those containing the trivalent form (Cr 3+ )(Plunkett, 1976; Windholz, 1976). According to Lee and Nassarella (1998), Cr 6+ is usually formed at lower temperatures and a rapid cooling reduces the formation by limiting the kinetics of the formation. This paper presents a study regarding four different types of steelmaking slags; a ladle slag, a basic oxygen furnace (BOF) slag and two types of electric arc furnace (EAF) slags, modified by different cooling conditions. The aim was to determine if rapid cooling by water granulation would result in a glassy slag with improved properties regarding leaching and volume stability. For EAF slag, the leaching of metals such as chromium and molybdenum is a concern. The qualities of the matrix of the modified slag were studied and the possibility of encapsulating metals in a glassy matrix, and thereby reducing the leaching, is discussed. 2. Materials 2.1. Investigated steel making slags Representative samples (20 30 kg) of four different steel slags were obtained from steelmaking companies in Sweden: C. Electric arc furnace slag 1, high alloyed steel, slag 1 D. Electric arc furnace slag 2, low alloyed steel, slag 2 EAF EAF The materials, except the disintegrated ladle slag, were crushed with a jaw crusher, Retsch BB3, to <30 40 mm before splitting into kg sub-samples. 3. Methods for characterization 3.1. Physico-chemical and mineralogical composition The total composition of each material was analyzed by Ovako Steel AB with inductively coupled plasma emission spectroscopy, (ICP), and X-ray fluorescence spectroscopy, (XRF). Titration was used for analysis of Fe and FeO and infrared adsorption spectroscopy, IR, for carbon and sulphur. The specific surface area was determined according to the BET-method with a Micromeretics Flowsorb 2300 and density was measured with a Micromeretics Multivolume Pycnometer 1305 on material prepared for leaching, <4 mm. All of the slag samples were crushed to a particle-size of <4 mm and leached according to the one-stage batch test pren (CEN, 2002a) except two samples, ladle slag and granulated EAF slag 1, which were leached according to the two-stage batch test pren (CEN, 2002b). The leaching tests were done in duplicate and the results are presented as a mean value. The filtrates were analyzed by the laboratory Analytica AB (Sweden). The mineralogy of the slag phases was studied on polished thin samples using scanning electron microscopy (SEM), Philips XL 30, with energy dispersive analysis (EDX). X-ray diffraction analysis (XRD) was performed on pulverized material using a Siemens D5000 automatic diffractometer with a step and continuous scanning device. Diffraction patterns were measured in a 2h range of using (Cu Ka) radiation of 50 kv and 30 ma. The glass content was analyzed according to the ER method (Scancem Research AB, Sweden) using optical microscope. To be able to better understand the crystallization procedure and the formation of glass, thermodynamic calculations were conducted using Factsage (Bale et al., 2002) version 5.4 using compound database FS53base.cdb, FToxid53base.cdb and solution database FToxid53- soln.sda. FToxid-slag and FToxid-MeO were selected as standard stable. During calculating, FS53base.cdb was suppressed contra FToxid53base.cdb to exclude duplications in the data set. 4. Modification trials A. Ladle slag, ladle slag B. Basic oxygen furnace slag, BOF slag All slags except the ladle slag were modified in two ways for comparison with the original slags:

3 M. Tossavainen et al. / Waste Management 27 (2007) Re-melting and water-granulation (rapid cooling). 2. Re-melting and cooling in the crucible (semi-rapid cooling). The ladle slag was only modified by re-melting and water-granulation Crucible systems Two different crucible systems for re-melting the materials were developed in order to minimize reactions between the refractory material and the slag. A graphite crucible system was used for the ladle slag with a low content of Fe-oxides and MgO crucible system for the two different types of EAF and BOF slags with high values of CaO/ SiO 2 and high contents of Fe oxides. The graphite crucible system is shown in Fig. 1. The outer crucible was made of refractory castable (MgO 80%). With a refractory cover, the system could be closed to minimize air intrusion and oxidation of the inner, graphite crucible. The MgO crucible system consisted of an outer crucible made of castable with 94% Al 2 O 3, enclosing a graphite crucible and an inner MgO crucible. A refractory cover was also placed on the top of the system to minimize air intrusion during slag melting and cooling Modifications The ladle slag re-melted in the graphite crucible system became liquid within 1 h. For granulation, the liquid slag (1600 C) was poured into the granulation head, as shown in Fig. 1. Water jets formed in the granulation head hit the pouring slag, and the generated slag granules were collected at the bottom of the water tank at the end of Furnace temperature, C the granulation. The duration for the tapping and granulation was approximately a few seconds. For re-melting the EAF- and BOF slags in the MgO crucible system, a thermocouple was placed above the slag and the heating rate was controlled to 4 6 C/min. The time for melting the slag varied from 6 to 8 h. The water granulation of the slags re-melted in the MgO crucible system was carried out in the same way as for the ladle slag. For the semi-rapid cooling, the re-melted slag was left to cool in the MgO crucible. The cooling time from a temperature of 1600 C to room temperature was estimated to be 5 h. The temperature changes for the experiments are shown in Fig Results Physical properties Semi rapid Rapid Time in hours Fig. 2. Temperature profile for semi-rapid and rapid cooling. The re-melted slags, which were left to cool in the crucibles (semi-rapid cooling), resulted in large pieces that were a b Fig. 1. Graphite crucible system (a) and equipment for water granulation (b).

4 1338 M. Tossavainen et al. / Waste Management 27 (2007) crushed to <4 mm for leaching tests. The water-granulated material of the BOF slag, the EAF slag 1 and the EAF slag 2 consisted of granular particles, 2 4 mm. During the rapid cooling process the ladle slag reacted with water to produce a volumetric stable, brittle and porous product. Table 1 summarizes the compact density, the BET surface and the results from the glass measuring test of the original and water granulated slag samples. From this table, it can be seen that the BET surface was reduced substantially in the granular particles, mainly due to the reduction of the amount of fines Physico-chemical and mineralogical characterization Chemical compositions of the four original slags are shown in Table 2. It shows that the content of iron oxides is high in both the BOF slag and the EAF slag 2, and the amount of Al 2 O 3 and MgO is high in the ladle slag. The chromium content is higher in EAF slag 1 and significantly higher in EAF slag 2 than in the other two slags. The solubility of five major elements (Ca, Mg, Fe, Si, Al) in the matrix and of three minor elements (Cr, Mo, V), expressed as mg/kg of the element dissolved, is shown in Table 3. The leaching of silicon is increased in all samples except for EAF slag 2 compared to semi rapid cooling, while the aluminium leaching is decreased when cooling rapidly. No significant changes in the minor elements can be seen when cooling differently. The values reported by the laboratory are in many cases low and there was good agreement between duplicate samples. Table 1 The compact density (g/cm 3 ), the BET-surface (m 2 /g) and the glass content (%) in the slag samples Sample Compact density BET-surface Glass content Original (g/cm 3 ) Granulated (g/cm 3 ) Original (m 2 /g) Granulated (m 2 /g) Original (%) Granulated (%) Ladle slag BOF slag EAF slag EAF slag Table 2 Chemical composition of the original slag samples Samples % ppm Fe 2 O 3 FeO Fe met. Al 2 O 3 CaO MgO MnO SiO 2 Cr Mo Zn Ni Cu K Na P Ti V Ladle slag <20 < BOF slag < EAF slag < EAF slag <20 < Table 3 Results obtained from standard test leaching of investigated in mg/kg Slag sample Ca Mg Fe Si Al Cr Mo V Limit value a Ladle slag Rapid cooling c 1140 nd BOF slag Original b 7095 nd Semi rapid cooling b 4405 nd Rapid cooling b 2070 nd nd EAF slag 1 Original b 1145 nd Semi rapid cooling a nd Rapid cooling c nd EAF slag 2 Original b 1545 nd Semi rapid cooling b 2505 nd Rapid cooling b 684 nd nd = not detected. a Limit value for inert landfill. b pren c pren

5 M. Tossavainen et al. / Waste Management 27 (2007) Fig. 3. XRD patterns of the investigated slags, with different cooling conditions XRD All investigated slag samples are basic M b (CaO + MgO)/ (SiO 2 +Al 2 O 3 ) > 1, which according to Daugherty et al. (1983), result in mainly crystalline slags. The values of M b are 1.5, 3.9, 1.4 and 2.1 for ladle slag, BOF slag, EAF slag 1 and EAF slag 2, respectively. The comparison of the XRD pattern of the original and the modified slags, Fig. 3, shows that all samples, except the granulated ladle slag, consist largely of crystalline material. It is important to note that the very complex composition makes the identification of phases difficult. The examination program for XRD analysis gives a high probability of several phases. The phases present in Fig. 3 are those that are likely to be present according to other knowledge, e.g., from the SEM studies. 6. Discussion 6.1. A mineralogical interpretation of the solubility The investigations with XRD were complemented with SEM studies in order to evaluate the impact of different cooling methods on the matrix of the slags and the effect on solubility (leaching) of minor elements Ladle slag The ladle slag is difficult to handle and store due to the disintegrating properties. The XRD graphs, Fig. 3, show that the ladle slag is the only one that becomes almost completely amorphous by granulation. The major mineral in the original slag is mayenite, Ca 12 Al 14 O 33, followed in order by free MgO. b-ca 2 SiO 4, c-ca 2 SiO 4 and Ca 2 Al 2 SiO 7 were also identified. The b-form may undergo a phase transformation during cooling at C to c-form and the volume increase (>10%) causes a pulverization of the slag (Monaco and Lu, 1996). The expansive c-ca 2 SiO 4 is a plausible explanation for the disintegration. The leaching solution of the original slag was not possible to filter, which might be due to cement-forming properties of the slag. One crystalline phase was identified in the granulated ladle slag: unassimilated MgO. With SEM and mapping of selected elements, two phases were identified, a matrix consisting mainly of calcium, silicon and aluminium (glass matrix) enclosing small fragments of MgO (1), see Fig. 4. The MgO particles are well distributed in the matrix (2) BOF slag The original BOF slag has high BET surface because of a high content of fines and pores compared to the granulated material. According to the XRD results, Fig. 3, the major phase in the original BOF slag is larnite, b-ca 2 SiO 4. With SEM and mapping of selected elements, Fig. 5, silicon and calcium coexist (particle 1) in the same phases, which agree with the findings of larnite as the major phase. Parts with high co-existence of iron, manganese and magnesium (particle 3) were distinguished, possibly the (Fe, Mn, Mg)O solid solution also found in XRD, as well as pure MgO grains (particle 2).

6 1340 M. Tossavainen et al. / Waste Management 27 (2007) Fig. 4. SEM picture of the water granulated ladle slag. Dark fragments of (1) MgO in a matrix and (2) with high content of calcium, silicon and aluminium (glass). Fig. 5. SEM investigation of the original BOF slag: (1) calcium silicate, (2) MgO, and (3) fragment rich in iron, manganese and magnesium. According to the XRD analysis, Fig. 3, the main mineral in the granulated BOF slag is Ca 3 SiO 5. This phase exists at high temperatures, and is liable to transform on cooling to Ca 2 SiO 4 and lime (Goldring and Juckes, 1997). Quick cooling (Luxán et al., 2000; Monaco and Lu, 1996), as well as presence of impurity ions (Ionescu et al., 1998), prevent

7 M. Tossavainen et al. / Waste Management 27 (2007) formation of Ca 2 SiO 4. In our tests, the transformation has probably taken place in the original and the semi-rapidly cooled slags. With SEM and mapping, a calcium silicate phase was observed as big crystals (particle 1) as well as fibre-shaped particles in the granulated BOF slag, Fig. 6. The euhedral prismatic microphenochrysts, that according to Goldring and Juckes (1997) are typical for Ca 3 SiO 5, were clearly distinguished. MgO is present as small spherical particles (2) distributed in the matrix that contains high content of calcium and iron (area 3). The leaching of calcium and iron is reduced in the granulated BOF slag, see Table 3. Iron is present in the matrix, as discussed above, and the leaching is very low in all three slag samples. Calcium, on the other hand, is also present in the major silicate phase, Ca 3 SiO 5. The solubility of silicon is increased in the granulated slag compared to the original. The leaching result shows that the dissolution of the minor elements is not prevented by the rapid cooling procedure, see Table 3. Vanadium is most soluble in the granulated BOF slag, correlating to the silica leaching EAF slag 1 The XRD graphs of the EAF slag 1, Fig. 3, show that the original and the two modifications contain a large proportion of crystalline phases. Merwinite, Ca 3 Mg(SiO 4 ) 2 was identified as the main mineral in both the original slag and the two modifications. c-ca 2 SiO 4 was only found in the original and the semi-rapid slags, which might explain the high BET surface in these samples. With SEM and mapping of selected elements, two matrix-forming phases and a spinel phase were differentiated in the semi-rapidly cooled EAF slag 1, see Fig. 7. Two of the phases correlate with the XRD identification of merwinite (particle 3) and solid solution spinel phase (particle 1), (Mg, Mn)(Cr, Al) 2 O 4. The other matrix-forming phase contains aluminium (particle 2) and SEM results show co-existence with primarily silicon, calcium and oxygen. The content of calcium and silicon is high in the EAF slag 1, Table 2. The solubility of these two major elements, as well as aluminium, iron and magnesium, is shown in Table 3. The leachability is very low and varies in the three modifications. The solubility of aluminium is reduced substantially in the semi-rapidly cooled and the granulated slag, which indicates that one of the matrix-forming phases is stable. On the other hand, the mobility of silica seems to increase when granulating. There does not seem to be any obvious correlation between the solubility of the major and the minor elements. The varying dissolution of the metals chromium, molybdenum and vanadium is more likely a result of the presence in different minerals. The solubility Fig. 6. SEM picture of the granulated BOF slag: (1) silicate, (2) MgO, and (3) matrix with high content of iron.

8 1342 M. Tossavainen et al. / Waste Management 27 (2007) Fig. 7. SEM picture of semi rapidly cooled EAF slag 1: spinel phase (1), Al Ca Si O phase (2), and silicate phase (3). of chromium is very low, 20 ppm of the total chromium content, in all three samples. Vanadium, on the other hand, is most leachable in the semi-rapidly cooled slag EAF slag 2 The XRD analysis, Fig. 3, shows that the slag is very complex and some phases have varying contents of substituted ions. The identified main mineral in the original slag and the two modifications is b-ca 2 SiO 4. A wustite-type solid solution ((Fe, Mg, Mn)O), Ca 2 (Al, Fe) 2 O 5 and Fe 2 O 3 were also identified. A broadening in the diffraction peaks, indicating smaller crystallite size, could be seen when cooling rapidly. Calcium, iron and silicon are the major elements in the matrix of the EAF slag 2. As can be seen in Table 3, calcium, aluminium and iron have the lowest leachability in the granulated slag, while silicon, as well as the minor elements chromium, molybdenum and vanadium are most insoluble in the semi-rapidly cooled slag. A similar behaviour of the major elements takes place for the BOF slag and the EAF slag 1 as well, but the effect on the leaching of the minor elements is different Glass formation As mentioned earlier, Daugherty et al. (1983) claimed that an acid slag M 3 b < 1 produces a glassy material more readily compared to a more basic slag when cooling rapidly. However, the investigated slags are considered to be basic ( ) and should therefore mainly contain crystalline material. The measured glass content for the original and granulated slag is listed in Table 1. Both the EAF slag 1 and the ladle slag show significant changes in glass content. In order to determine the glass formation possibility in the slag, it is not enough to look at the chemical analyses. It is also of importance to consider the chemical analyses of the rest melt due to high temperature crystallization. To better understand the glass formation, the crystallization path and corresponding melt composition at equilibrium conditions were calculated using Factsage 5.4, see Fig. 8. According to the thermodynamic calculations, the MgO crystallization from the liquid ladle slag already starts at approximately 1800 C. Only 38% of the total MgO content is present in the liquid slag at 1400 C. The remaining 60% has already been crystallized as pure MgO. This phenomenon can be seen in both Figs. 3 and 4. When MgO crystallization takes place, the M b in the liquid material is changed from original 1.5 to 1.25 at 1400 C, influencing the glass forming properties in the material. Due to early crystallization of solid solution, spinel phases, the liquid slag composition of the EAF slag 1 is changed during cooling. Thermodynamically, the formation of spinel already starts at 1950 C. When the formation takes place, MgO is reacting with chromium, forming magnesiochromite (MgCr 2 O 4 ). This formation lowers the M b factor, lowering it as the temperature decreases and the formation of spinel increases. The M b ratio is decreased form original 1.41 to As seen in Fig. 8, both the EAF slag 1 and the ladle slag tend to become more acidic as the liquid slag temperature decreases. Neither the BOF slag nor the EAF slag 2 slags show any tendency of forming glass when cooling rapidly, according

9 M. Tossavainen et al. / Waste Management 27 (2007) Fig. 8. Glass forming tendency M b, as a function of liquid slag temperature. to Table 1. The high M b value indicates a basic slag, even at low temperatures, Fig. 8. According to the previous results discussed, it can be concluded that different behaviour was observed in relation to the cooling conditions. In view of recycling, i.e., having a lot of by-products generated each year within the steel industry, the knowledge gained from the trials will hopefully facilitate the possible use of residues in new fields and applications. 7. Conclusions From this investigation, the following conclusions are drawn: 1. The mineralogical composition is complex for the evaluated slag samples. XRD reveals the presence of different kinds of calcium silicate in all samples. 2. The results obtained from the test leaching show that the solubility of elements such as chromium, molybdenum and vanadium for the different investigated slags is in most cases very low in percentage. On the other hand, the differences between the original and modified samples are low. The leaching of chromium is not prevented by cooling rapidly according to the Official Journal of the European Communities (2003). 3. Slags with a M b factor higher than 1 may form glass when cooling rapidly, depending on chemical analyses of the smelt. Still, the formation of glass in the investigated granulated slag samples has not been sufficient to enclose the heavy metals and prevent them from leaching. 4. The disintegration ladle slag became stable after performing rapid cooling, due to the formation of glass. 5. To choose one appropriate modification method for all slags is difficult, depending both on slag composition and the product to be manufactured. Acknowledgements This work was financed by MiMeR (Minerals and Metals Recycling Research Centre) and Vinnova. The authors thank the members of MiMeR for the opportunity to present the data and for fruitful discussions about the tests and results. References A Good Built Environment, Available from: < Bale, C.W., Chartrand, P., Decterov, S.A., Eriksson, G., Hack, K., Ben Mahfoud, R., Melançon, J., Pelton, A.D., Petersen, S., Fact sage thermochemical software and databases. Calphad Journal 62, CEN, 2002a. Final draft pren Characterization of wasteleaching-compliance test of leaching of granular waste material and sludges Part 2: one-stage batch test at a liquid to solid ration of 10 l/ kg for materials with particle size below 4 mm (with or without particle reduction). CEN, 2002b. Final draft pren Characterization of wasteleaching-compliance test of leaching of granular waste material and sludges Part 3: two-stage batch test at a liquid to solid ration of 2 l/kg and 8 l/kg for materials with high solid content and with particle size below 4 mm (with or without particle reduction). Daugherty, K.E., Saad, B., Weirich, C., Eberendu, A., The glass content of slag and hydraulic activity. Silicates Industriels 4 (5), Goldring, D.C., Juckes, L.M., Petrology and stability of steel slags. Ironmaking and Steelmaking 24 (6), Ionescu, D., Meadowcroft, T.R., Barr, P.V., Hydration potential of high iron level glasses: criteria for the recycling of steel slag as a portland cement additive. In: 1998 ICSTI/Ironmaking Conference Proceedings, pp

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