Technical paperCharacteristics of steel slag under different cooling conditions
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 1994. 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 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 high-quality rock material. The fear that some slags are environmentally hazardous is also something that has to be considered.
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, Ionescu et al., 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, Ionescu et al., 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 (Cr6+) are generally considered far more toxic than those containing the trivalent form (Cr3+) (Plunkett, 1976, Windholz, 1976). According to Lee and Nassarella (1998), Cr6+ 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.
Section snippets
Investigated steel making slags
Representative samples (20–30 kg) 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.
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,
Modification trials
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.
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 1 summarizes the compact density, the BET surface and the results from the glass measuring
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.
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.
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.
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