Determination of chromium valence state in the CaO–SiO₂–FeO–MgO–CrO_x system by X-ray photoelectron spectroscopy

2.1 Materials

The chemical compositions of chromium-containing steel slag used in the present study are given in Table 1. Analytical reagents CaO (≥98.0%), MgO (≥98.0%), and SiO₂ (≥99.0%) were provided by Chengdu Kelong Chemical Reagent Factory. FeO was synthesized using analytical reagent Fe₂O₃ and reduced iron powder (Fe). Fe₂O₃ (≥70.0% Fe) and Fe were obtained from Tianjin HengXing Chemical Reagent Co., Ltd and Macklin Reagent, respectively. Highly purified Cr₂O₃ (≥99.0%) from Wenzhou Dongsheng Chemical Reagent Factory was used as the starting chromium oxide.

2.2 Methods

A horizontal tube MoSi₂ furnace (SGL-1700; Shanghai Jvjing Precision Instrument Manufacturing Co., Ltd) with a proportional–integral–differential controller was used to prepare the slag samples. Fine powders of the raw materials were carefully weighed and well-mixed together in an agate mortar. A 40 g mixture was charged into a MgO crucible. The MgO crucibles containing the slag samples were positioned inside a square alumina holder. The alumina holder was pushed into the even-temperature zone of the tube MoSi₂ furnace.

According to the equilibrium of C–O reaction and the process conditions of steelmaking end point, the calculated oxygen potential log(PO₂) values are around −5.6 to −3.0. Therefore, the designed log(PO₂) values in the present work are −5.0 to −3.5 and are listed in Table 1. Gas mixtures of CO–CO₂ were employed to obtain the targeted oxygen potential. The gas compositions of CO–CO₂ at equilibrium states were gained by equation (1) [16]. The calculated volume ratios of CO/CO₂ are listed in Table 2.

In view of the sensitivity of chromium to oxygen, the experiments were started by evacuating and filling the furnace chamber with purified argon before the CO–CO₂ gas mixture was introduced. Figure 1 shows the temperature control curves in experiments. When the target temperature of each experiment was reached, the samples were held for 60 min in the furnace chamber. After experiments, the MgO crucibles were quickly pulled out of the reaction tube and quenched in liquid nitrogen. The slag samples were separated from the crucibles and milled to below 0.074 µm, then subjected to XPS analyses.

Figure 1

Temperature control curves in experiments.

The XPS measurements on the slag sample prepared under various conditions were carried out by means of a spectrometer (Thermo Scientific Escalab 250Xi). In the testing, Al K Alpha was used as the source gun type, and the energy type size was 0.1 eV. The measurements were conducted with a spot size of 500 µm. In order to ensure the experimental results, the vacuum degree was controlled below 5 × 10⁻⁹ mbar.

3.1 Chromium spectra

Figure 2 illustrates the typical XPS wide scan spectrum of the CaO–SiO₂–FeO–MgO–CrO_x slag. The auger peaks of the constituent elements in the slag were marked on the spectrum. Two minor peaks around the binding energies of 573–593 eV represent Cr 2p. A major peak at a binding energy of ∼284.6 eV [17] represents C 1s, which appeared due to the contamination of hydrocarbon.

Figure 2

XPS wide scan spectrum of slag sample R3.

For qualitative and quantitative analyses, the valence state of chromium element in the slag samples and the broad peak covering several peaks were analyzed. The Cr 2p spectra were divided into Cr 2p_1/2 and Cr 2p_3/2 by spin–obit interaction. As shown in Figure 3, the peaks of Cr 2p_3/2 and Cr 2p_1/2 locate at the binding energy of ∼577 and ∼586 eV, respectively. The broad peak was deconvoluted into several separate peaks to determine the individual area of each peak. Generally, it can be deconvoluted into three individual valence states, i.e., Cr(ii), Cr(iii), and Cr(vi). The proportions of above three kinds of ions in the slag samples were deduced from the area under the computer-resolved peaks. As can be seen in Figure 3, Cr(iii) is the main existing form of Cr element, Cr(ii) comes second, while only trace amount of Cr(vi) in the slag. The result is consistent with that of Mittal [18]. The reason for the existence of Cr(ii) and Cr(vi) is that the trivalent chromium can be reduced by the FeO in the slag and oxidized by the small amount of oxygen in the atmosphere.

Figure 3

The fitted Cr 2p spectrum of slag example R3.

3.2 Effect of log(PO₂) on the valence state of chromium

In view of the absence of Cr(ii) and Cr(vi) in the starting materials, the redox reactions of Cr(iii) can be described as follows:

According to the stoichiometry of chromium,

Therefore, x can be calculated by the following equation:

The value of x can be obtained by relating Cr(iii)/Cr(ii) and Cr(vi)/Cr(ii) to the oxygen pressure using the law of mass action. As it is very difficult to detect the fractions of Cr(ii)/Cr, Cr(iii)/Cr, and Cr(vi)/Cr, we applied the ion and molecule coexistence theory [19,20,21] to obtain these fractions. The calculation process is detailed in one of our previous study [22].

Figure 4 shows both the calculated x and experimental x in the present study. It can be found that the x in CrO_x increases with increasing oxygen pressure log(PO₂). The result is consistent with Jadid’s finding in the CaO–SiO₂–CrO_x slag and the CaO–SiO₂–Al₂O₃–CrO_x slag [16]. Statistics of the various valence states of chromium in the CaO–SiO₂–FeO–MgO–CrO_x slag are shown in Figure 5. The fraction of Cr(ii)/Cr decreases with increasing oxygen pressure log(PO₂), while Cr(iii)/Cr and Cr(vi)/Cr increase. However, the increase tendency of Cr(vi)/Cr is not obvious, and the fraction of Cr(vi)/Cr is always below 4.5% under the present experimental conditions.

Figure 4

x in CrO_x under different oxygen pressure.

Figure 5

Fractions of Cr(ii)/Cr, Cr(iii)/Cr, and Cr(vi)/Cr as a function of oxygen pressure in the CaO–SiO₂–FeO–MgO–CrO_x slag.

3.3 Effect of basicity on the valence state of chromium

Figure 6 illustrates the fractions of Cr(ii)/Cr, Cr(iii)/Cr, and Cr(vi)/Cr as a function of slag basicity in the CaO–SiO₂–FeO–MgO–CrO_x slag. It is obvious that Cr(iii)/Cr increases with increasing slag basicity, while Cr(ii)/Cr decreases. It is in good agreement with the increase tendency of x in CrO_x in Jadid’s study [8], and the results of comparison are shown in Figure 7. This can be interpreted as more CaCr₂O₄ and MgCr₂O₄ generated with increasing CaO content [23,24]. Meanwhile, it is also found that the fraction of Cr(vi)/Cr is almost constant with the increase in slag basicity.

Figure 6

Fractions of Cr(ii)/Cr, Cr(iii)/Cr, and Cr(vi)/Cr as a function of slag basicity in the CaO–SiO₂–FeO–MgO–CrO_x slag.

Figure 7

x in CrO_x under different basicity.

3.4 Effect of Cr₂O₃ on the valence state of chromium

Compared with ordinary steel slag, the chromium-containing steel slag contains a higher Cr₂O₃ content. Therefore, it is necessary to make clear whether the increase in Cr₂O₃ content has an effect on the valence distribution of chromium. The influence of Cr₂O₃ content on the fractions of Cr(ii)/Cr, Cr(iii)/Cr, and Cr(vi)/Cr in the slag can be seen in Figure 8. With the increase in Cr₂O₃ content, the fraction of Cr(iii)/Cr increases obviously, and Cr(ii)/Cr shows an opposite trend. Furthermore, the fraction of Cr(vi)/Cr shows a slight downward trend. It can be concluded that increasing the amount of Cr₂O₃ in the chromium-containing steel slag does not increase the risk of generating toxic hexavalent chromium substances.

Figure 8

Fractions of Cr(ii)/Cr, Cr(iii)/Cr, and Cr(vi)/Cr as a function of Cr₂O₃ content in the CaO–SiO₂–FeO–MgO–CrO_x slag.

3.5 Effect of temperature on the valence state of chromium

Figure 9 shows the effect of temperature on the fractions of Cr(ii)/Cr, Cr(iii)/Cr, and Cr(vi)/Cr in the CaO–SiO₂–FeO–MgO–CrO_x slag. With the increase in temperature, the fractions of Cr(ii)/Cr and Cr(vi)/Cr decrease slightly, while the fraction of Cr(iii)/Cr increases slightly, but on the whole, all the changes are very small.

Figure 9

Fractions of Cr(ii)/Cr, Cr(iii)/Cr, and Cr(vi)/Cr as a function of temperature in the CaO–SiO₂–FeO–MgO–CrO_x slag.