LICENTIATE T H E S I S
Luleå University of Technology
2007:58
Mineralogical Influence of Different Cooling Conditions on Leaching
Behaviour of Steelmaking Slags
Fredrik Engström
Mineralogical influence of different cooling conditions on leaching behaviour of steelmaking slags
A Laboratory Investigation
by
Fredrik Engström
Licentiate Thesis
Luleå University of Technology
Department of Chemical engineering and Geoscience Division of Process Metallurgy
SE-97187 Luleå Sweden
PREFACE
During the past 35 years, intensive research has been carried out related to the origin of the steel slags generated from the steel industry worldwide.
This research has contributed to the development of new products and the conservation of natural resources. Despite the effort that has been invested, 35 years is a fairly short period of time compared to other industrial sectors. In 2004, approximately 1.7 million tonnes of steel slag was still sent for deposit in Europe. In order to decrease the amount of material to be deposited, further research is needed to determine the influence of different process parameters on the properties of the product.
This thesis presents a study regarding four different types of steelmaking slags: Ladle slag, BOF (Basic Oxygen Furnace) slag and two different EAF (Electric Arc Furnace) slags, modified by different cooling conditions. The aim of this thesis was to determine how different cooling conditions influence the mineralogy and the leaching behaviour of some steelmaking slags. Phases present, crystal size, leaching of metals, volume stability, reactivity and glass content are considered.
The present work was undertaken as a research project within the Minerals and Metals Recycling Research Centre, MiMeR, and in collaboration with the Swedish steel industry.
LIST OF PAPERS
This thesis is based on the following papers:
I. Mia Tossavainen, Fredrik Engström, Qixing Yang, Nourreddine Menad, Margareta Lidström Larsson and Bo Björkman:
Characteristics of steel slag under different cooling conditions, Waste Management (2007), Vol 27, p 1335-1344.
II. Fredrik Engström, Daniel Adolfsson, Qixing Yang, Bo Björkman and Caisa Samuelsson: Crystallisation behaviour of some steelmaking slags, American Ceramic Society, (Submitted November 2007).
F. Engström’s contribution to the papers:
• Practical experiments.
• Responsible for characterization of materials obtained during experimental work using XRD and SEM.
• Interpretation of leaching data and correlation to mineralogical phases.
• Thermodynamic calculations.
• Responsible for writing paper II and the final edition of paper I.
Related publications not included in this thesis:
III. Mia Tossavainen, Fredrik Engström, Nourreddine Menad and Qixing Yang: Stability of modified steel slags. Proceedings of 4th European Slag Conference, June 20-june 21, 2005, Oulu, Finland.
IV. Qixing Yang, Fredrik Engström, Mia Tossavainen and Daniel Adolfsson: Treatments of AOD slag to Enhance Recycling and Resource Conservation. Proceeding of SECURING THE FUTURE, International Conference on Mining and the Environment, Metals and Energy Recovery, June 27-July 1, 2005, Skellefteå, SWEDEN.
V. Qixing Yang, Fredrik Engström, Mia Tossavainen and Mingzhao He:
AOD slag Treatment to recover Metal and to prevent Slag Dusting, Proceedings of The 7th Nordic-Japan Symposium on Science and Technology of Process Metallurgy, Jernkontoret, Stockholm, September 15-16, 2005.
VI. Qixing Yang, Lotta Nedar, Fredrik Engström and Mingzhao He:
Treatments of AOD slag to Produce Aggregates for Road Construction, Proceeding AISTech 2006, Vol. 1, p 573-583.
ABSTRACT
The Swedish steelmaking industry produces large amounts of by-products.
In 2006, the total amount of slag produced reached approximately 1 375 000 metric tons, of which 30% was deposited. Due to its strength,
durability and chemistry, steel slag is of interest in the field of construction due to its similarities with ordinary ballast stone. However, some steel slags face an array of quality concerns that might hinder their use. These concerns generally involve the following physical and chemical properties:
Volume expansion Disintegration Leaching of metals
By controlling and modifying process parameters during slag handling in liquid state, the physical properties of steel slags can be adequately modified to obtain a high-quality product for external application.
The present work was undertaken as a research project within the Minerals and Metals Recycling Research Centre, MiMeR. The major objectives of this work have been to investigate how different cooling methods and cooling rates influence the properties of slag products. Four types of steel slags, Ladle slag, 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.
Experiments were conducted in laboratory scale using an induction furnace. Analysis techniques used in this investigation include:
thermodynamic calculations using FactSageTM, X-ray diffraction analyses
(XRD), scanning electron microscope (SEM) and a standard leaching test (prEN 12457-2/3).
The experimental results show that disintegrating ladle slag is volume stabilized by water granulation resulting in a product consisting of 98%
glass. However EAF slag 1, EAF slag 2 and the BOF slag formed only 17%, 1% and 1% glass, respectively. The leaching tests showed that water granulation 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.
Variations in crystal size as well as phase composition and distribution were observed in the different materials as a result of different cooling methods. The magnesium content of the wustite-type solid solution (Fe,Mg,Mn)O in BOF slag increased when rapid cooling was used.
The reactivity factor, α, was calculated for the BOF and EAF slag 1. A majority of the elements of interests in the slags became more reactive when cooled rapidly. The reactivity for silica in BOF and EAF slag 1 was increased by ~4700% and ~1200%, respectively, and for chromium by
~5300% and ~1500%.
ACKNOWLEDGEMENTS
First and foremost, I wish to thank my thesis advisors, Professor Bo Björkman, Dr Caisa Samuelsson, Dr Margareta Lidström Larsson and Dr Qixing Yang for all their help and guidance during the course of my work.
I would also like to thank my colleagues in the Department of Process Metallurgy for putting up with me all these years. Danny P, Ryro, Ulla, Åke B, Raggsockan, Anaitish, Sempan, Biggish and Secharo; without you, this work would have been considerably more. Thanks also to Dr. Nourreddine Menad, now working at BRGM in France, for all the fruitful and entertaining discussions.
To member companies in MiMeR, Vinnova and Jernkontret via Mistra, I extend my sincere thanks for invaluable financial support and commitment.
Without your help, this study would never have been possible.
Finally, I wish to thank my family. Linda, thanks for all your help and wonderful support. Thank you Moa and Emil, you mean everything to me.
Johan, Mum and Dad, Thank you for helping to make me the person I am.
Fredrik Engström
October 2007, Luleå, Sweden
CONTENTS
1 INTRODUCTION... 1
1.1 BACKGROUND... 1
1.2 STEELMAKINGSLAG... 4
1.3 UTILIZATIONOFSTEELMAKINGSLAG... 7
1.4 CRYSTALLIZATIONTHEORY ... 10
2 EXPERIMENTAL ... 15
2.1 MATERIALS ... 15
2.2 EXPERIMENTALDESIGN ... 15
2.3 EXPERIMENTALPROCEDURE ... 19
3 RESULTS ... 23
3.1 PHYSICALPROPERTIES ... 23
3.2 PHYSICO–CHEMICALANDMINERALOGICALCHARACTERIZATION. ... 23
3.3 X-RAYANALYSES... 25
3.4 THERMODYNAMICCALCULATIONS ... 28
4 DISCUSSION ... 31
4.1 AMINERALOGICALINTERPRETATIONOFTHE SOLIDIFICATION/SOLUBILITY... 31
4.2 GLASSFORMATION ... 42
4.3 REACTIVITY,BOFANDEAFSLAG1 ... 44
4.4 CONCLUDINGDISCUSSION... 46
5 CONCLUSIONS ... 49
6 FUTURE WORK ... 51
1 INTRODUCTION
1.1 BACKGROUND
Large amounts of by-products are produced by the Swedish steelmaking industry each year. In 2006, the total amount of slag produced, reached approximately 1 375 000 tonnes. Only 44%, mostly blast furnace slag, was sold as external products, and approximately 30% was used for landfilling (source: private communication with the steelmaking industry). These figures are very high in comparison to other European countries. In Germany, only 7% of the steel slags produced are dumped, while 93% is used for other applications [3]. In Sweden, a number of goals have been formulated in order to obtain a so-called good building environment [4].
Among these criteria are:
• By 2010, re-used material should represent at least 15% of the aggregates used.
• Landfill waste should be reduced by at least 50% by 2005 compared to 1994.
Due to its strength, durability and chemistry, steel slag could be considered in the field of construction, since the material provides similar properties as granite and flint gravel [3]. Using slag in construction would contribute to a reduction in the amount of landfilled waste. The possibility of using slag is limited due to the lack of rules and guidelines regarding testing, assessment and use of slag in Sweden. The technical and environmental obstacles for not using some slags in construction include:
• Volumetric expansion
• Disintegration
• Leaching of metals.
According to Monaco and Lu [5], the volumetric expansion is considered to be associated with the presence of free lime and free periclase in the solidified slag. Free lime and periclase react with moisture, resulting in an expansion due to the formation of hydroxides [6].
Upon cooling, pure dicalcium silicate undergoes a phase transformation from β-Ca2SiO4 to γ-Ca2SiO4 at approximately 500ºC. The latter results in a volume expansion of approximately 12 vol-% [7]. The polymorphic transformation of β-Ca2SiO4 to γ-Ca2SiO4 is known to occur in AOD slag (Argon Oxygen Decarburization) depending on the cooling rate [8].
However, that has not been reported for EAF slags [9]. Thomas and Stephenson [10], believe that impurities in the EAF slag stabilize the metastable β-Ca2SiO4 from disintegration.
Very little is reported in the literature regarding the influence of cooling on the properties of slag, especially for steelmaking slags (EAF, BOF). Ground granulated blast-furnace slags (GGBS) are known to possess improved hydration reactivity compared to slowly cooled blast furnace slag, due to the formation of glass [11]. The formation of a glassy material depends on both the chemical composition and the cooling conditions. According to Daugherty et al. [12], 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. [13], [27] 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 [14]. Glasses, such as granulated slags, can be regarded as super-cooled liquids having a very high viscosity. Monaco and Lu [9] have reported variations in the composition of the wustite-type solid solution as well as a variation in crystal size when cooling differently.
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 (Cr6+) are generally considered far more toxic than those containing the trivalent form (Cr3+) [15], [16]. According to Lee and Nassarella [17], Cr6+is usually formed at lower temperatures and a rapid cooling reduces the formation by limiting the kinetics of the formation.
1.2 STEELMAKING SLAG
Slag is formed through a chemical reaction of a flux and the gangue of an ore, the ash from a fuel, or with the impurities oxidized during the production and refining of a metal. In most iron- and steelmaking processes, the slag is in intimate contact with the liquid metal and chemical reactions readily occur between the slag and the metal. The main purpose of slag in processes such as basic oxygen furnace (BOF) and electric arc furnace (EAF) is to extract impurity elements from the steel bath [18].
1.2.1 BOF slag
BOF slag is produced as a by-product when iron is converted into steel in the BOF converter, through the injection of oxygen. The slag is produced by the addition of fluxes, such as limestone and dolomite, during the process. Unwanted elements such as carbon, silicon and phosphor are either oxidized to gases or chemically bonded into the slag. The most common minerals found in the BOF slag are listed in Table 1. All minerals may not be present in all slags of a given type.
Table 1: Most abundant minerals usually found in BOF slag.
Mineral name Structural formula
Larnite β-Ca2SiO4
Srebrodolskite Ca2Fe2O5
Tricalcium silicate Ca3SiO5 Spinel (Fe,Mg,Mn,Al) Me2+Me3+2O4
Wustite FeO
Lime CaO
Periclase MgO
1.2.2 EAF slag
EAF slag is produced as a by-product during the melting of scrap, when processed in the electric arc furnace at temperatures above 1600°C.
Phosphorus, sulphur, silicon and sometimes carbon are removed by lancing oxygen into the melt, forming a liquid oxide slag. In Sweden, two types of EAF slag are produced; both from carbon and high-alloyed steel production. EAF slag which originates from the carbon steel manufacturing usually has a high content of iron and basicity, B2 (CaO/SiO2) around 2.5, while the EAF slag generated from the production of high-alloyed steel might differ considerably. The most common minerals found in the two EAF slags are listed in Table 2.
Table 2: Most abundant minerals usually found in EAF slags.
Mineral name Structural formula
Larnite β-Ca2SiO4
Srebrodolskite Ca2Fe2O5
Brownmillerite Ca2(Al,Fe)O5
Spinel (Fe,Mg,Mn,Al,Cr) Me2+Me3+2O4
Wustite FeO
Periclase MgO
Mineral name Structural formula
Bredegite Ca14Mg2(SiO4)8
Merwinite Ca3Mg(SiO4)2
Cuspidine Ca4F2Si2O7
Periclase MgO
Electric arc furnace slag (carbon steel)
Electric arc furnace slag (high alloy)
1.2.3 AOD slag
The AOD (Argon Oxygen Decarburization) converter is often the first refining step after the electric arc furnace, when manufacturing high-alloyed stainless steel. By decreasing the oxygen content in the decarburization gas gradually, using inert gas (nitrogen and argon), a significant removal of carbon is achieved. The AOD slag often shows disintegrating properties due to the formation of γ-Ca2SiO4. The most likely minerals to be found in the AOD slag are listed in Table 3.
Table 3: Most abundant minerals usually found in AOD slag.
Mineral name Structural formula
Ingesonite γ-Ca2SiO4
Larnite β-Ca2SiO4
Merwinite Ca3Mg(SiO4)2
Melilite (Ca,Na)2(Al,Mg,Fe)(Si,Al)2O7
Fluorite CaF2
Spinel (Fe,Mg,Mn,Al,Cr) Me2+Me3+2O4
Lime CaO
Periclase MgO
1.2.4 Secondary metallurgical slag
Secondary metallurgical slag is produced during the treatment of crude steel, and arises from the subsequent treatment of steel produced both in the BOF and EAF processes. Regardless of whether they originate in the ore- or scrap based steelmaking, secondary metallurgical slags often have a high content of CaO, SiO2, Al2O3 and MgO. The most common minerals found in the secondary metallurgical slag are listed in Table 4.
Table 4: Most abundant minerals usually found in secondary metallurgical slag.
Mineral name Structural formula
Ingesonite γ-Ca2SiO4
Larnite β-Ca2SiO4
Bredegite Ca14Mg2(SiO4)8
Mayenite Ca12Al14O33
Cuspidine Ca4F2Si2O7
Spinel (Fe,Mg,Mn,Al,Cr) Me2+Me3+2O4
Lime CaO
Periclase MgO
1.3 UTILIZATION OF STEELMAKING SLAG
The usage of iron and steel slags in external applications such as road building and cement is not a new invention. The earliest reports on the utilization of slags refer to Aristotle, who used slags as a medicament already around 350 BC [19]. During the following centuries, slag was mainly used as construction material. The Romans used slag in road construction about 2000 years ago. In 1813, the first road made of slag in modern times was built in England. However, slag has not only been used in road construction. In 1589, the Germans used slag from ironmaking when manufacturing cannonballs [20].
In Europe, the total steel slag production (BOF, EAF and Secondary metallurgical slags) in 2004 reached approximately 15 million tonnes. 72%
of the slag produced was used in external applications such as road construction and manufacturing of fertilizer, Figure 1.
Figure 1: Use of steel slags in Europe 2004: 15 million tonnes [19].
The extent to which slags are used in Sweden differs significantly compared to other European countries. The total steel slag production reached approximately 810 000 tonnes in 2006, of which only 43% was used in different fields and applications, Figure 2.
Figure 2: Use of steel slags in Sweden 2006: 810 000 tonnes.
In Sweden, 12% (Figure 2) of the produced steel slag was used for road construction purposes in 2006. The physical and chemical properties of the steel slags often make them suitable for asphalt manufacturing. The highly basic steel slags easily react with the acid bitumen, forming a high- performance asphalt product. Tests preformed by the Danish road directorate show an increase of 30% in durability when comparing normal mastic asphalt to asphalt made with EAF slag. The increase in porosity compared to ordinary stone material also helps to keep the road free of water, thereby reducing the risk for aquaplaning while at the same time lowering the noise generated by the rolling tires. Except road construction, steel slags may be suitable for applications such as manufacturing of riverbanks, cement and fertilizer. As a fertilizer, converter lime combines many positive characteristics. Because of its composition of minerals it promotes plant growth. The immune system of the crops is especially strengthened by the high concentration of silica in the slag [28]. Apart from external recycling of steel slags, internal recycling is also applied. Tata Steel [29], Sidera [30] and Ferriere Nord [31] recycle their ladle slag back into the process as slag former. Apart from use as slag formers, ladle slag can also substitute some of the cement needed when manufacturing briquettes, which contributes to the reduction of CO2 emissions.
1.4 CRYSTALLIZATION THEORY
As a new phase is formed in a system, the reaction is always associated with the Gibbs free energy of the system. When comparing Gibbs free energy of two mineral arrangements to determine which is the more stable, it is necessary to have a common basis for comparison. The widely adopted convention in chemistry is to compare Gibbs free energy levels based on the free energy of formation from the elements, ΔGf. The free energy of formation is the energy difference between the elements in their standard state and the elements chemically bonded to form a mineral at a certain temperature and/or pressure. The stability of a mineralogical arrangement may be expressed in terms of Gibbs free energy according to reaction (1).
s) f(reactant )
f(products reaction
cal
mineralogi ΔG ΔG
ΔG = − (1)
If, at a specific temperature – pressure condition, ΔG reaction < 0, the product is more stable and the reaction may occur spontaneously. If ΔG reaction > 0, then the reactants are more stable. The equilibrium condition is obtained if ΔG reaction = 0; thus, there will be no reaction taking place between the product and the reactants. All systems endeavour to minimize the total Gibbs free energy in terms of obtaining a stable state[24-26].
1.4.1 Nucleation
Homogeneous nucleation is by definition the formation of nuclei within one phase. Conversely, where foreign elements are present, the result is heterogeneous nucleation, i.e. formation of crystals through more than one
bumping into other atoms, and forming a variety of chemical combinations.
Some combinations, called embryos, will have the structure and composition of a mineral that could crystallize from the melt through nucleation. Most embryos will be small, consisting of only a few atoms, while some will be larger. Whether these embryos will grow to form minerals or not, depends both on the stability and the size of the embryo. If the new embryo has a higher Gibbs free energy of formation than the melt, the embryo will not have enough power to grow. Instead, the embryos will break apart and constituent atoms will return to the lower energy level represented by the melt. A stable nucleus which can grow further will only exist if the embryo reaches a certain critical radius, rc. If a spherical crystal shape is assumed, then the critical radius of a nucleus that becomes stable can be obtained from equation (2), where γ, ΔGv, ΔGγ and ΔGf correspond to the surface energy per unit area, the volume energy, the surface energy and the free energy of formation [24-26].
( ) ( )
(
ΔG ΔG)
4πr2γ3 r3 γ 4π v ΔG tot ΔG
ΔG f crystal f melt ⎟⎟+
⎠
⎞
⎜⎜
⎝
⎛ −
= +
= (2)
In Figure 3, equation (2) is plotted as a function of the radius. As can be noticed, the Gibbs free energy shows a maximum at a certain radius. The relation between critical radius and ΔGmay therefore be expressed as;
ΔT K
2γ ΔG
2γ rc
f
=
−
= (3)
Figure 3: The Gibbs free energy of formation ΔGtot of crystal nuclei is the sum of the surface energy ΔGγ and the volume energy ΔGv. Only nuclei larger than the critical radius rc are stable with respect to the melt.
For crystals to grow, it is necessary to heavily undercool in terms of reaching sufficient activation energy and thereby overcome the difficulties related to the surface energy. The consequence of equation (3) will therefore be that a strong undercooling (high ΔT) provides a high driving force and a small critical radius, which results in a high probability for nucleation to occur [24-26].
1.4.2 Growth
Once the nuclei are stable, the further growth is characterized by a gradual exchange of atoms at the surface of the nuclei. In order to predict the degree of solidification, both time and rate of growth must be considered.
Figure 4 schematically shows the temperature dependence between growth and nucleation.
rc
Radius (r)
- ← Gibbs free energy → +
Unstable Stable
ΔGγ
0
ΔGV
ΔGtot
Figure 4: The degree of reaction as a function of temperature.
T1 = low degree of undercooling. The nucleation will be the rate-determining step for the crystallization. A small amount of nuclei with high growth will result in few, but large, crystals.
T2 = high degree of undercooling. The growth will be the speed-dependent step in the crystallization. Many small nuclei with low growth rate result in a numerous amounts of small crystals[24-26].
Rate of growth
ΔT Under cooling
Rate of nucleation
T0
(liquid slag)
Nucleation
Growth
T1 T2
2 EXPERIMENTAL
2.1 MATERIALS
20 – 30 kg representative samples of four different steel slags were obtained from steelmaking companies in Sweden:
A. Ladle slag ladle slag
B. Basic oxygen furnace slag BOF slag C. Electric arc furnace slag 1, high alloyed steel EAF slag 1 D. Electric arc furnace slag 2, low alloyed steel EAF slag 2
The materials, except the disintegrated ladle slag, were crushed with a jaw crusher, Retsch BB3, to <30-40 mm before splitting into 1-1.5 kg sub- samples.
2.2 EXPERIMENTAL DESIGN
All slags except the ladle slag were modified in two ways for comparison with the original slags:
1. 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. The result was then compared against the original ladle slag.
2.2.1 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 low content of Fe-oxides and an MgO crucible system for the two different types of EAF and BOF slags with high values of CaO/SiO2 and high contents of Fe- oxides.
The graphite crucible system is shown in Figure 5. 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.
Figure 5: Graphite crucible system (A) and equipment for water granulation (B).
The MgO crucible system consisted of an outer crucible made of castable with 94% Al2O3, enclosing a graphite crucible and an inner MgO crucible. A refractory cover was also placed on the top of the system to minimize air
2.2.2 Modifications
The ladle slag re-melted in the graphite crucible system became liquid within one hour, measuring ≈ 1600°C. For granulation, the liquid slag was poured into the granulation head, as shown in Figure 5. 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 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 at 4-6°C/minute. The time for melting the slag varied from six to eight hours. 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 measured to be five hours. The temperature changes for the experiments are shown in Figure 6.
Figure 6: Temperature profile for semi-rapid and rapid cooling.
2.3 EXPERIMENTAL PROCEDURE
2.3.1 Chemical analyses
The total composition of each material was analyzed by Ovako Steel AB (Sweden) with inductively coupled plasma emission spectroscopy, ICP, and x-ray fluorescence spectroscopy, XRF. The content of Fe and Feoxid was determined through titration. The analyses were done in duplicate and the results are presented as a mean value. The standard deviation for the analyses of FeO and Cr2O3 was 0-0.28 wt% and 0.007-0.02 wt%, respectively.
2.3.2 Specific surface area and density
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.
2.3.3 Glass analyses
The glass content was analyzed by Scancem Research AB using optical microscopy according to ER 9103. A representative slag sample was ground and the fraction 32-40 μm was used. In polarized light, optical isotropic grains, “glass”, has a different colour compared to crystalline anisotropic grains. If the grain contains more than 50% isotropic material it is identified as glass.
2.3.4 X-ray diffraction analyses
For XRD, all samples were prepared in a ringmill for 2x30 seconds.
Between the grinding procedures, magnetic fractions were removed through magnetic separation. The samples were analysed with a Siemens D5000 x-ray diffractometer, using copper Kα radiation. XRD patterns were recorded from 10 to 90° sin2θ, in 0.02°/step. Initially, XRD patterns were recorded by counting 1 s/step, and at a later rerun counting 8 s/step, this time from approximately 30-60° sin2θ. The phase identification was made by reference patterns in an evaluation program supplied by the manufacturer of the equipment.
2.3.5 Scanning electron microscope investigation
The mineralogy of the slag samples was examined in a Philips XL 30 SEM using a beam operation voltage of 20 kV and spot size 6. Semi-quantitative and qualitative elemental analyses were performed with an energy dispersive spectrometer (EDS) fitted with an Everhart and Thornley detector behind a berylium window. Before mapping the texture and mineralogy of the fractured sample surface using the secondary electron (SE) image signal, the samples were sputter coated with a conductive layer of gold.
2.3.6 Leaching test
All the slag samples were crushed to a particle-size of <4 mm and leached according to the one-stage batch test prEN 12457-2 (CEN, 2002a) [1]
except two samples, ladle slag and granulated EAF slag 1, which were
leached according to the two-stage batch test prEN 12457-3 (CEN, 2002b) [2]. The filtrates were analyzed by the laboratory Analytica AB (Sweden). It is generally believed that prEN 12457-2/12457-3 gives more or less the same result. The leaching tests were done in duplicate and the results are presented as a mean value. The standard deviation for Fe and Cr was 0- 0.03 mg/kg and 0.2-410 μg/kg, respectively.
2.3.7 Thermodynamic calculations in Factsage
Thermodynamic calculations were conducted using Factsage [32] version 5.4 using compound database FS53base.cdb, FToxid53base.cdb and solution database FToxid53soln.sda. FToxid-slag and FToxid-MeO were used. During calculation, FS53base.cdb was suppressed contra FToxid53base.cdb to exclude duplications in the data set.
3 RESULTS
3.1 PHYSICAL PROPERTIES
The re-melted slags which were left to cool in the crucibles (semi-rapid cooling) resulted in large pieces that were 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 5 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, except for the ladle slag, mainly due to the reduction of the amount of fines.
Table 5: The compact density (g/cm3), the BET-surface (m2/g) and the glass content (%) in the samples.
Compact density BET-surface Glass content
Sample Original Granulated Original Granulated Original Granulated
g/cm3 g/cm3 m2/g m2/g % %
Ladle slag 3.03 2.76 0.75 0.81 18 98
BOF - slag 3.53 3.65 2.35 0.21 7 1
EAF - slag 1 3.25 3.34 2.23 0.17 2 17
EAF - slag 2 3.59 3.77 1.23 0.59 4 1
3.2 PHYSICO – CHEMICAL AND MINERALOGICAL CHARACTERIZATION
Chemical compositions of the four original slags are shown in Table 6. It shows that the content of iron oxides is high in both the BOF slag and the
chromium content is significantly higher in EAF slag 1 and EAF slag 2 than in the other two slags.
Table 6: Chemical composition of the original samples.
% ppm
Samples Fe2O3 FeO Fe met. Al2O3 CaO MgO MnO SiO2 Cr V Ladle slag 1.1 0.5 0.4 22.9 42.5 12.6 0.2 14.2 2700 280
BOF slag 10.9 10.7 2.3 1.9 45.0 9.6 3.1 11.1 506 14800
EAF-slag 1 1.0 3.3 0.1 3.7 45.5 5.2 2.0 32.2 32700 310 EAF-slag 2 20.3 5.6 0.6 6.7 38.8 3.9 5.0 14.1 26800 1700
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 7. The leaching of silica is increased when cooling rapidly, except for EAF slag 2 compared to semi-rapid cooling, while the aluminium leaching is decreased when cooling rapidly. No systematic changes in the minor elements can be seen when cooling differently. The values reported by the laboratory are in many cases low.
However, there was good agreement between duplicate samples.
Table 7: Results obtained from leaching of investigated slags in (mg/kg).
Slag sample Ca Mg Fe Si Al Cr Mo V
Limit valuea 0.5 0.5
Ladle slag
rapid coolingc 1140 nd 0.37 15.6 298.5 0.08 0.008 0.2
BOF slag
originalb 7095 nd 0.14 4.9 2.63 0.03 0.21 0.3
semi rapid coolingb 4405 nd 0.07 14.9 19.15 0.01 0.07 0.7
rapid coolingb 2070 nd nd 62.5 1.6 0.04 0.07 7.7
EAF slag 1
originalb 1145 nd 0.04 37.4 139 0.73 3.9 0.3
semi rapid coolingb 646.5 2.2 nd 140.5 5.12 0.82 0.11 2.8
rapid coolingc 457 4.34 nd 132.2 2.73 0.93 0.07 0.3
EAF slag 2
originalb 1545 nd 0.171 3.49 636 5.8 0.8 0.3
semi rapid coolingb 2505 nd 0.067 1.08 426 0.008 0.02 0.02
rapid coolingb 684 nd 0.05 50.4 45.6 3.8 0.4 2.5
nd = not detected a = Limit value for inert landfill b = prEN 12457-2 c = prEn 12457-3 Directive 1999/31/EC 16.1.2003
3.3 X-RAY ANALYSES
All investigated slag samples are basic, Mb (CaO+MgO)/(SiO2+Al2O3)>1, also known as B4, which according to Daugherty et al. [12], results in mainly crystalline slags. The values of Mb 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, Figure 7-8, shows that all samples, except the granulated ladle slag, consist largely of crystalline material. The phases present in Figure 7-8 are those that are likely to be present when also results from e.g. the SEM studies are considered.
Figure 7: XRD pattern of the investigated slags, with different cooling conditions.
Figure 8: XRD pattern of the investigated slags, with different cooling conditions.
3.4 THERMODYNAMIC CALCULATIONS
Thermodynamic calculations in Factsage were conducted for EAF 1 and the BOF slag. The result of the thermodynamic calculation is shown in Figure 9. The calculations are based on the following conditions:
Temperature interval 25-1800°C
Pressure 1 atm, constant
Phases formed in less than 3 wt% were omitted from Figure 9.
According to the results given in Figure 9, the first crystalline phase to precipitate from the liquid BOF slag is MgO followed by the crystallization of FeO and CaO entering solid solution. Tricalcium silicate, Ca3SiO5, is the first silicate to crystallize at approximately 1450ºC. Below ~1270ºC, Ca3SiO5 is transformed to α׳-Ca2SiO4 and CaO.
The first phase to be formed in the liquid EAF slag the magnesium chromite, which crystallizes above 1800ºC. In addition, MgCr2O4 is further transformed into chromite (FeCr2O4) at ~1270ºC. Alpha dicalcium silicate, α-Ca2SiO4, is the first silicate to form in liquid EAF slag 1 at approximately 1530ºC.
Liquid slag
Ca2SiO4(s) Ca2SiO4(s)
MgO(s) CaO(s)
Ca3SiO5(s) FeO(s)
Ca2Fe2O5(s)
MnO(s) Ca2Fe2O5(s)
Ca2SiO4(s) Ca2SiO4(s)
Liquid slag Ca3SiO5(s)
MgO
CaO(s) FeO
MnO(s) γ −
γ − α′−
α′−
BOF-slag
T(C)
Phase distribution wt %
25 203 380 558 735 913 1090 1268 1445 1623 1800
0 10 20 30 40 50 60
(MgO)(Cr2O3)(s) FeCr2O4(s)
FeCr2O4(s) ss-spinel
Ca2SiO4(s3) Ca3Si2O7(s)
Ca3Si2O7(s)
Ca3MgSi2O8(s)
Ca3MgSi2O8(s)
Ca2Al2SiO7(s) MnAl2O4(s)
MnAl2O4(s) ss-spinel Liquid slag
Liquid slag (MgO)(Cr2O3)(s) ss-spinel
Ca2SiO4(s)
Ca2SiO4(s) Ca2Al2SiO7(s)
Ca2SiO4(s) α′−
α′−
α−
α−
EAF-slag 1
T(C)
Phase distribution wt %
25 203 380 558 735 913 1090 1268 1445 1623 1800
0 10 20 30 40 50 60
Figure 9: Thermodynamic calculation in Factsage.
4 DISCUSSION
4.1 A MINERALOGICAL INTERPRETATION OF THE SOLIDIFICATION/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. According to both the XRD and the SEM analyses of the modified slag, it could be noted that there was a clear difference in particle size distribution due to the different cooling conditions. A summary of the phases found in the slag samples can be seen in Table 8.
Table 8: A summary of phases identified in the slag samples.
Original Rapid Original Semi-rapid Rapid Original Semi-rapid Rapid Original Semi-rapid Rapid Minerals:
Ca3SiO5 X
α-Ca2SiO4 X
β-Ca2SiO4 X X X X X X
γ-Ca2SiO4 X X X
Ca3Mg(SiO4)2 X X X
Ca2Fe2O5 X X
Ca2(Al,Fe)2O5 X X X
CaAl2SiO6 X
Ca2Al2SiO7 X X
Ca12Al14O33 X
Fe2O3 X X X
(Fe,Mg,Mn)O X X X X X X
MgO X X
CaO (ss) X X
Spinel (ss) X X X
Ladle slag BOF-slag EAF-slag 1 EAF-slag 2
4.1.1 Ladle slag
Ladle slag is difficult to handle and store, due to its disintegrating properties. The XRD graphs, Figure 7, show that the ladle slag is the only
free MgO, β-Ca2SiO4, γ-Ca2SiO4 and Ca2Al2SiO7. The β-form may undergo a phase transformation during cooling at 400-500°C to γ-form and the volume increase (>10%) causes a pulverization of the slag [5]. The formation of γ-Ca2SiO4 is a plausible explanation for the disintegration. It was not possible to filter the leaching solution of the original slag, which might be due to cement-forming properties of the slag.
One crystalline phase, undissolved MgO, was identified in the granulated ladle slag, Figure 7. 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 Figure 10. The MgO particles are well distributed in the matrix (2).
Figure 10: Scanning electron micrograph of rapidly cooled ladle slag.
Dark fragments of (1) MgO in a matrix (2) with high content of calcium, silicon and alumina.
4.1.2 BOF slag
The original BOF slag has high specific surface because of a high content of fines and pores compared to the granulated slag. According to the XRD results, Figure 7, the major phase in the original BOF slag is larnite, β- Ca2SiO4. With SEM and mapping of selected elements, silicon and calcium were detected in the same phase, agreeing with the finding of larnite as the major phase. Parts with high coexistence of iron, manganese and magnesium were also distinguished with SEM; possibly, the (Mg,Fe,Mn)O solid solution also found with XRD.
In the semi-rapidly cooled BOF slag, four crystalline phases were identified, with XRD. A wustite-type solid solution containing magnesium, iron and manganese (Mg,Fe,Mn)O, β-calcium silicate (β-Ca2SiO4), calcium ferrite (Ca2Fe2O5) and a calcium, manganese oxide (Ca,Mn)O phase. All phases except for the (Ca,Mn)O were also detected with SEM and mapping, Figure 11, particle 1-3. The (Mg,Fe,Mn)O is enclosed in the β-Ca2SiO4 structure, indicating an early crystallization, in comparison to β-Ca2SiO4 and Ca2Fe2O5.
Figure 11: Scanning electron micrograph of the semi-rapidly cooled BOF- slag. (1) (Mg,Fe,Mn)O, (2) Calcium silicate, (3) Calcium ferrite.
In the rapidly cooled BOF slag, three crystalline phases were identified through XRD. A wustite-type solid solution containing magnesium, iron and manganese (Mg,Fe,Mn)O, tricalcium silicate and α-Ca2SiO4. These phases were also confirmed by SEM, phase 1-3, Figure 12, where phase 2 is tricalcium silicate (Ca3SiO5) i.e. the euhedral prismatic microphenochrysts, that according to Goldring and Juckes [21] are typical for Ca3SiO5 and phase 3, the matrix phase, crystallizing last, probably containing the α- Ca2SiO4 seen in XRD. All phases identified in the rapidly cooled BOF slag, agree with the thermodynamic calculation, Figure 9, i.e. indicating that the fast cooling enables the presence of metastable phases, such as Ca3SiO5
and the rapidly cooled BOF slag. According to the thermodynamic calculations, Figure 9, MgO is already present as crystals in the liquid slag at 1600ºC. The slightly change in position which occurs in the diffractogram is explained in terms of having a higher concentration of MgO in the wustite-type solid solution. As the slag is cooled rapidly, neither the FeO nor MnO has the same possibility of crystallizing and forming solid solution with MgO, due to its later crystallization in comparison to MgO, Figure 9. The latter was further confirmed by the SEM instrument.
According to semi-quantitative analyses, the solid solution contains 51%
MgO, 42% FeO and 7% MnO, in the semi-rapidly cooled slag, while the solid solutions in the rapidly cooled slag was made up of 78% MgO, 16%
FeO and 6% MnO.
When a phase can form thermodynamically, the crystal size will depend on the temperature to which the crystals are exposed and the duration of the exposure. There was a significant difference in crystal size between the two modified BOF slags, Figure 11-12. The size of the crystals present in the semi-rapidly cooled slag varied between 40-200 μm, indicating that these minerals have had longer time to grow. In the rapidly cooled BOF slag, the variation in crystal size is more pronounced compared to the semi-rapidly cooled BOF slag. The wustite-type solid solution (phase 1) and the tricalcium silicate (phase 2), Figure 12, has a crystal size varying between 20-100 μm. The matrix (phase 3), Figure 12, has a much smaller size than the other two phases discussed. The smaller crystal size of this silicate matrix can thus be explained in terms of not having the same time to develop. Based on the thermodynamic calculations, it can be concluded that both the wustite-type solid solution and the tricalcium silicate were present in the liquid slag at the time the rapid cooling with water was
carried out. However, α-Ca2SiO4 is expected to form during rapid cooling with water.
Figure 12: Scanning electron micrograph of the rapidly cooled BOF-slag.
(1) (Mg,Fe,Mn) oxide, (2) Calcium silicate, (3) Matrix containing (Ca,Si,Ti,V,Mn,Fe) oxides.
The leaching of calcium and iron is reduced in the granulated BOF slag, according to Table 7. 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, Ca3SiO5. 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 7. Vanadium is most
