Steel production is a highly technical process which requires careful control of the alloy constituents to achieve the best mechanical properties.
When ores are melted down in a smelter, various minerals (e.g. silica) in the ore combine with the other ingredients of the furnace to produce waste liquid rock called slag. The slag needs to be removed as a by-product from the smelter.
Slag is a mixture of metal oxides and silicon dioxide but can also contain metal sulfides and elemental metals. Although a waste product of smelting, slags can also be used to remove waste materials, assist in the temperature control of the smelt, and also to minimize high-temperature re-oxidation of the final liquid metal product before the molten metal is poured to form solid metal ingots or castings. In many smelting processes, oxides are introduced to control slag chemistry, assisting in the removal of impurities and protecting the furnace refractory lining from excessive wear. In these cases, the slag is termed synthetic as it is used to control the smelting process.
Slag composition and controlling steel chemistry
The requirements for high quality iron and steel products have become increasingly strict and this has led to greater demands for high cleanliness in molten steel. Particularly with regard to high Ni alloys and stainless steels, a molten steel refining process which provides higher cleanliness is important topic.3
In modern steel manufacture, the FeO and chrome oxide content of slag has decreased, and high basicity slags have been embraced in order to achieve higher purity, and lower oxygen levels. However, this has led to MgAl2O4 spinel type inclusions in steel, which has become a problem. Since MgAl2O4 spinels have a high melting point and display a different deformation capacity from that of steel leading to a reduction in the fatigue strength of spring steel and bearing steel. This type of inclusion has also become a cause of surface defects in high Ni alloy steels.
An investigation3 of the influence of the slag composition on MgO–Al2O3 inclusions formation in a small scale furnace (20 kg) showed Mg concentration of the molten steel increase with time after the addition of Al, and the composition of inclusions changes from Al2O3 to Mg Al2O4 spinel. The rate of increase in the MgO concentration increased as the basicity, CaO/SiO2 and CaO/Al2O3, of the top slag increased. By reducing the CaO/SiO2 and CaO/Al2O3 ratio of top slag, the MgO contents in Al2O3 based inclusions decreased.3
The role of XRF spectroscopy in slag analysis
The ability to rapidly analyse the chemical composition of blast furnace slag is central to the operation and control of a blast furnace. XRF spectrometers are the most common analysis tools available to analyses powder samples of slag produced in iron and steel making. The role of the chemical composition of blast furnace slag in the control of the iron production process is well established.3,4 The fast, accurate, and cost-effective elemental analysis of blast furnace slag is a key part of quality and process control in iron production.
XRF is cost-effective, user-friendly fast, non-destructive and environmentally friendly analysis method with a very high accuracy and reproducibility. All of the elements of the periodic table from beryllium up to the rare earths can be measured qualitatively and quantitatively in powders, solids and liquids.5 Concentrations of up to 100% maybe analyzed directly, without any dilution, with reproducibility’s of better than ±0.1% and typical limits of detection (LODs) from 0.1 to 10 ppm.
Most modern X-ray spectrometers in industrial settings have modular sample changers to provide fast, flexible sample handling and adaptation to customer-specific automation processes. XRF spectrometry is the most effective way to perform multi-element analysis of slag, as it offers a number of benefits to steel producers pursuing more efficient productive manufacturing. The number one advantage is that it can be used inline during the manufacturing process, allowing operators to take a sample, analyze it and then quickly adjust the chemistry of the melt. Because of this capability, operators obtain near-instantaneous feedback that they can use to make critical adjustments during production, which greatly enhances efficiency and productivity as well as the quality and consistency of each production batch.
In addition, XRF technology does not use acids during sample preparation, unlike ICP (inductively coupled plasma) and AAS (atomic absorption spectroscopy) methods. XRF technology is clean and generates no by products that require specialized disposal.5
Sample preparation the XRF analysis of slag
For slag analysis three different forms of XRF sample preparation can be used depending upon the speed of analysis and the accuracy required. For in-line systems powder samples can be prepared and run on XRF systems in-line with the blast furnace process. In these cases, analysis is rapid but the powder particles must be fine enough to avoid sample void effects and the powder should be contained in a holder with a flat surface.
An APEX 400 semi-automatic press for XRF sample preparation.
Alternatively, pressed pellets can be produced from powdered samples using a die and a hydraulic press such as Specac’s Autotouch hydraulic press, which is available in 8 Ton, 15 Ton, 25 Ton and 40 Ton load configurations for industrial applications.
The busiest laboratories can opt for the APEX 400 Press, which is capable of rapid, high-throughput sample prep in laboratories preparing up to a few hundred samples per day. The press automatically reconfigures for pellet release and avoids the additional handling associated with traditional pellet dies.
To learn more about what Spectroscopy can do, check out #SpectroscopySolutions for more insights into the applications XRF and FTIR can fit.
References
- Fruehan, Richard (1998). The Making, Shaping, and Treating of Steel, Steelmaking and Refining Volume, 11th Edition. Pittsburgh, PA, USA: The AISE Steel Foundation.
- Huang Yi, Guoping Xu, Huigao Cheng, et al., An overview of utilization of steel slag, Procedia Environmental Sciences 16 ( 2012 ) 791–801
- Goro Okuyama, Koji Yamaguchi, et al., Effect of Slag Composition on the Kinetics of Formation of Al2O3–MgO Inclusions in Aluminium Killed Ferritic Stainless Steel, ISIJ International, Vol. 40 (2000), No. 2, pp. 121–128
- Ko-ichiro Ohno, Masashi Kaimoto, et al., Effect of Slag Melting Behaviour on Metal- Slag Separation Temperature in Powdery Iron, Slag and Carbon Mixture, ISIJ International, Vol. 51 (2011) No. 8, p1279-1284
- Global CCS Institute, IGCC solids disposal and utilisation, 01 May 2012
- http://www.globalccsinstitute.com/publications/igcc-solids-disposal-and-utilisation
- M. Tossavainen , F. Engstrom, et al., Characteristics of steel slag under different cooling conditions, Waste Management 27 (2007) 1335–1344.
Irem Zeynep Yildirim and Monica Prezzi, “Chemical, Mineralogical, and Morphological Properties of Steel Slag,” Advances in Civil Engineering, vol. 2011, Article ID 463638, 13 pages, 2011
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