11 Aug 2017

XRF analysis for steel production | Spectroscopy Solutions

Steel production is a highly technical process which requires careful control of the alloy constituents to achieve the best mechanical properties.

Part of the steel making process involves the removal of slag. Slag is the remains of the smelted ore generated by high temperature metallurgical activity. Ores (e.g., Iron, copper, lead, and nickel) generally have unwanted minerals associated with them, such as silica, and during the process of smelting this combines with other materials in the furnace and the fuel to produce a liquid rock or slag that is usually discarded.

Steel plant image for an article on the analysis of steel using XRF spectroscopy during steel production.

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 in metal smelting, 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.

Steelmaking slag is a good example of this. In the steelmaking process quicklime and magnesite are added to the smelt for refractory protection to neutralize the alumina and silica separated from the metal, and also to assist in the removal of excessive sulfur and phosphorus from the steel alloy.1 Slag from steel smelting is designed to minimize iron loss and contains oxides of calcium, silicon, magnesium, and aluminum.

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

Steel slag can be analyzed using XRF spectroscopy – Specac's provide excellent XRF sample preparation equipment including XRF pellet presses for steel slag analysis.

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

Separating metal and slag

The properties and chemistry of slag are essential in the production of high-grade steel.4 In a recent Japanese study synthetic slag samples were prepared and different kinds of slag compositions were adopted to change properties, such as the melting temperature and viscosity. Metal-slag separation behavior was dominated largely by agglomeration behavior of the liquid phase of the iron.

Read Environmental testing with XRF.

The separation occurred when both iron and slag changed to liquid phases. At the same time, the effect of slag melting temperature on the separation was larger than the effect of slag viscosity.4 An ability to determine the elemental composition of slag is also critical in terms of meeting the quality and specification for sale of the slag by-product to other industries, because the elemental composition of blast furnace slag is vital to its suitability for certain industry applications such as for cement, concrete, construction filler and as a rich source of trace elements.

Slag foaming is also important in the production of metals. Producing a good ‘foamy’ slag with the correct flow reduces radiant heat loss from the bath and improves the efficiency of the electrical power input to the melt bath. In addition, slag foaming allows for higher rates of electrical energy input to the melt bath without risking damage to the refractory lining, furnace roof, shell and side walls. Slag basicity controls the timing and the extent of foaming. The levels of different oxides present also has an impact - increasing the FeO decreases slag viscosity but too much FeO is equated to iron loss. Increasing the MgO content (refractory component) increases the slag basicity and viscosity. As a result it can be seen that controlling the chemical components of the slag is vital for energy conservation and prolonging the life of the furnace as well as the steel quality.

The role of XRF spectroscopy in slag analysis

The ability to rapidly analyses 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.

Read What is XRF?

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 (Specac’s mill can be used to provide an appropriate powder particle size).

An Autotouch automatic hydraulic press

Alternatively, pressed pellets can be produced from powdered samples and a hydraulic press such as Specac’s AtlasTM Series Autotouch automatic hydraulic press, which is available in 8 Ton, 15 Ton, 25 Ton and 40 Ton load configurations for industrial applications. Again, the pellet needs to be as homogenous as possible and so sample preparation is of the utmost importance.

Read How to Use an Automatic Hydraulic Press.

Homogeneity in samples can be achieved using a hollow pellet die which is filled with the sample then compacted to give a uniform tube of sample. Specac provides stainless steel evacuable dies which possess the mechanical strength to resist strong compressive forces. In the case of samples which do not readily form pellets XRF cups are available to fit around the bottom of the die, keeping the sample intact, whilst leaving the top face of the cylinder exposed for XRF analysis.

Solid samples are another option and can be analyzed with no sample preparation or they can be cut and polished for a more quantitative analysis such as is done for the quality control of the product steel. However, even for largely flat samples, surface finish can affect the outcome and quality of the analysis.6,7

To learn more about what Spectroscopy can do, check out #SpectroscopySolutions for more insights into the applications XRF and FTIR can fit.

References

  1. Fruehan, Richard (1998). The Making, Shaping, and Treating of Steel, Steelmaking and Refining Volume, 11th Edition. Pittsburgh, PA, USA: The AISE Steel Foundation.
  2. Huang Yi, Guoping Xu, Huigao Cheng, et al., An overview of utilization of steel slag, Procedia Environmental Sciences 16 ( 2012 ) 791–801
  3. 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
  4. 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
  5. Global CCS Institute, IGCC solids disposal and utilisation, 01 May 2012
  6. http://www.globalccsinstitute.com/publications/igcc-solids-disposal-and-utilisation
  7. M. Tossavainen , F. Engstrom, et al., Characteristics of steel slag under different cooling conditions, Waste Management 27 (2007) 1335–1344.
  8. 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