Using an EDXRF Spectrometer to Analyze Metallurgical Slags

Slags originate from different stages in the iron and steel process, for example, electric arc furnace, blast furnace, basic oxygen furnace, converter, or ladle. In the blast furnace, slag is generated from impurities in the iron ores (called the gangue), coke ash, and the flux; it is a complex mixture of silica, sulfides, alumina, and oxides of magnesium and calcium, as well as smaller amounts of iron oxides and manganese.

In an electric arc furnace, the slag formation process can be regulated by incorporation of oxygen, carbon, and slag formers such as magnesia (MgO) and lime (CaO) to the melt. Oxygen and carbon encourage the formation of CO instead of FeO and MnO and preserve these elements in their (valuable) elemental metallic form, while the standard slag formers help neutralize the acidity of the slag and extend the life of refractory (heat-resistant) bricks in the furnace. Proper and consistent monitoring of slag composition is a vital factor in regulating the contemporary smelting process.

X-Ray Fluorescence spectrometry (XRF) has been established as an exclusively rapid and reliable technique for measuring slags in solid-state with low sample preparation, outstanding repeatability, and minimal operator skill. Based on the equipment type and configuration, measurement times vary from minutes down to seconds. This article shows the procedure and standard results obtained with the Thermo Scientific™ ARL™ QUANT’X XRF Spectrometer, which has been used to measure magnesium (Mg) and heavier elements in slags at numerous sites across the world.

Instrumentation

The ARL QUANT’X Spectrometer is based on the principle of Energy-Dispersive (EDXRF) that enables a single extremely sensitive detector to measure the emission lines of all elements from Sodium (Na, Z = 11) to Uranium (U, Z = 92). While EDXRF has progressively limited sensitivity to lighter elements and is seldom used to measure Magnesium (Mg) or Sodium (Na) at concentrations lower than 1%, advances in technology now spread its range into applications traditionally worked by the more sensitive and larger Wavelength-Dispersive (WDXRF) spectrometers.

The large active area, full vacuum chamber, and the low operating temperature and noise features of the Silicon Drift Detector (SDD) enable the ARL QUANT’X Spectrometer to realize extraordinary repeatability and sensitivity. In contrast to a majority of laboratory-grade instruments, compact footprint, durable mechanical design, and minimal site requirements of the ARL QUANT’X Spectrometer allow placement in harsh environments such as next to the furnace. Regular analysis can be carried out by untrained personnel, and the hardware is regulated exclusively by software to assure data integrity and cut operational complications because of mechanical buttons or controls.

Excitation Conditions

In EDXRF, precision and sensitivity are attained by targeted excitation of the sample to fluoresce only the elements of interest. An instrument with better flexibility and control over the excitation efficiency and background generally demonstrates better performance. The ARL QUANT’X Spectrometer provides a nearly limitless mixture of excitation voltages of 4–50 kV and numerous primary beam filters for ideal background control. Table 1 shows two spectra that had been obtained from each slag sample, for a full counting time of 180 seconds in a low-vacuum atmosphere.

Table 1. Analytical settings

An example of three very different slag samples under the two excitation conditions can be seen in Figure 1. The concentrations of these slags are given in Table 3.

Figure 1. Spectra of three slags under two excitation conditions.

Sample Preparation

To perform qualitative analysis, XRF can be used without the need for any sample preparation, irrespective of sample size or shape. However, for relevant quantitative analysis, slag samples are normally crushed and ground in a mill to create a particle size of less than 50 µm, which helps reduce particle-size effects. Although the powder can be examined directly, accuracy and sample-to-sample repeatability are enhanced if the powder is pressed into what is known as a “pellet” using an automatic or manual press at 15–20 tons.

Calibration

A majority of applications of XRF in industrial process control are calibrated using empirical calibrations on the foundation of primary or secondary standards. The outcomes stated below were acquired with the help of a multi-variable regression curve based on 20 secondary slag standards. All calibration tasks, curve display, and fit evaluations are included in the basic software package delivered with the ARL QUANT’X Spectrometer.

The Standard Error of Estimate (SEE) for all the oxides of interest in arc-furnace slags can be seen in Table 2. The SEE is the average difference between the calculated and certified concentrations of a particular analyte.

Table 2. Summary of calibration accuracy

Repeatability

Repeatability tests were carried out to reveal the typical precision attained by the ARL QUANT’X Spectrometer for the 3 minute total counting time. The statistical summary of 10 measurements of three slag samples is shown in Table 3.

Table 3. Measurement precision

In certain cases, the difference between the nominal (given) composition and the average measured result was much greater than three-sigma of variation because of the instrument itself. In this case, the real variation is the result of the sample (pellet) preparation and the large range of slags used in this test. Therefore, the ideal accuracy for slag measurement would be accomplished by fusing the samples with lithium tetraborate, which eliminates grain size and mineralogical effects.

Conclusion

The ARL QUANT’X EDXRF Spectrometer has been used to effectively examine many components in slags. While the EDXRF’s performance in relation to repeatability, speed, and sensitivity to light elements is not similar to that of WDXRF, the small size, mechanical simplicity, robust architecture, and outstanding analytical value of the ARL QUANT’X Spectrometer make it a practical choice for numerous industrial settings.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

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