Redox Equilibrium of Niobium in Calcium Silicate Base Melts

Abstract

The oxidation state of niobium has been determined at 1873 K (1600 °C) in CaO-SiO₂-NbO _x melts with CaO/SiO₂ ratios (mass pct) of 0.66, 0.93 and 1.10, and 5.72 to 11.44 pct Nb₂O₅ (initial). The slag samples were equilibrated with gas phases of controlled oxygen pressure, then quenched to room temperature and analyzed chemically. The niobium is mainly pentavalent with small amounts in the tetravalent state. It was found that the Nb⁵⁺/Nb⁴⁺ ratio increases with oxygen pressure at a constant CaO/SiO₂ ratio and constant content of total niobium, closely according to the ideal law of mass action, which is proportional to ${\text{p}}_{{{\text{O}}_{2} }}^{1/4} .$ The ratio also increases with total niobium content, and it seems to have a maximum at a basicity of about 0.93. The color of the solidified slag samples is described and is explained with the help of transmission spectra.

Introduction

It is well known that transition metals are present in steelmaking slags with different valence states. The quantitative treatment of the slag/metal reactions requires a good knowledge of the redox equilibria of these metals. Previously, we have investigated the oxidation state of vanadium[1] and chromium[2] in silicate slags as a function of oxygen pressure. In this article, we present such data on niobium.

Niobium can occur in solid oxide phases in the divalent, quatrovalent, and pentavalent forms (NbO, NbO₂, and Nb₂O₅).[3] At 1873 K (1600 °C), the Nb/NbO equilibrium[4] is at an oxygen pressure of about 10⁻¹⁴ atm (1.0133 × 10⁻¹⁴ bar), which is much lower than the range investigated in the current study. In aqueous solutions, the trivalent (Nb³⁺), tetravalent (Nb⁴⁺), and pentavalent (Nb⁵⁺) valence states seem to exist.[5–7] With respect to oxygen containing liquid steel, niobium is a (moderate) deoxidizer.[8] The deoxidation phases are FeO·Nb₂O₅ at low niobium contents (≤0.165 mass pct Nb at 1873 K (1600 °C)) and NbO₂ at high niobium contents.[8] In liquid slags, the oxidation state is usually taken to be Nb⁵⁺. Several studies of the partition behavior of niobium between slag and metal have been carried out at temperatures between 1473 K and 1873 K (1200 °C and 1600 °C) using graphite[9–12] or magnesia crucibles,[13,14] assuming that the niobium oxide in the slag is Nb₂O₅ (Nb⁵⁺), except in a study by Tsukishashi et al.,[11] in which also the valence state Nb⁴⁺ was considered. The Nb⁵⁺/Nb⁴⁺ ratio of approximately 3 can be deduced from the capacity data given in this investigation[11] for a CaO/3CaO·SiO₂ saturated CaO-SiO₂-CaF₂ slag at 1573 K (1300 °C) at graphite saturation and under carbon monoxide gas $\left( {\log {\text{p}}_{{{\text{O}}_{2} }} \left( {\text{atm}} \right) = - 16.6} \right).$ It seems that no other equilibrium data of the redox reaction between niobium species are available for the basic steelmaking slag systems and at higher temperature.

We have determined the oxidation state of niobium in calcium silicate base melts with selected CaO/SiO₂ ratios as a function of oxygen pressure at 1873 K (1600 °C). The investigation was confined to the range of relatively low niobium contents in the slags from about 3.7 to 8.5 mass pct. The results are reported in the following text.

Experimental Procedure

The experimental procedure was similar to that described in the investigation on chromium, and the reader is referred to Reference 2 for details. The oxide samples of 1.5 to 6 g were melted in platinum-rhodium crucibles and were equilibrated with gas phases of controlled oxygen pressure, and then were quenched and analyzed chemically for total niobium and tetravalent niobium. The content of pentavalent niobium was obtained as the difference between the contents of total and tetravalent niobium.

The gas mixtures were composed with capillary flow meters. Starting gases were high purity carbon dioxide and carbon monoxide. Subsequent purification was carried out as usual.[2] The oxide samples were prepared from reagent grade CaCO₃, SiO₂, and Nb₂O₅.

In most experiments, the applied reaction times were 4 hours. At the beginning of the investigation, time series were carried out to check the rate of the oxidation/reduction reaction and the extent of evaporation. It was confirmed that the equilibration of the sample with the gas phase occurs fast so that the oxidation state pertaining to the oxygen pressure of the gas phase is established in 1 hour or less.

At the end of the reaction time, the samples were pulled into the water-cooled brass head of the used Tammann furnace and quenched to room temperature under the reaction gas. The samples were powdered under carbon dioxide in a glove box to prevent oxidation and pick up of moisture. As the first step, about five times an amount of Fe³⁺, compared with that of total niobium, was added to the slag powder. Then, the material was dissolved in a mixture of sulfuric and hydrofluoric acids (1/1), which was performed, under purified nitrogen, using a closed vessel made of polytetrafluorethylene. The Fe³⁺ oxidizes the Nb⁴⁺ to Nb⁵⁺ and forms an equivalent quantity of Fe²⁺. The residual hydrofluoric acid was complexed with oxygen-free boric acid solution. Then, the Fe²⁺ was determined by potentiometric titration with Ce⁴⁺. The total niobium was obtained by atomic absorption spectroscopy. The accuracy of the Nb⁴⁺ analyses is estimated to be better than ±5 pct and that of the Nb_total analyses better than ±3 pct, yielding an uncertainty of the derived Nb⁴⁺/Nb⁵⁺ ratios of less than ± 8 pct. This error can be considered to be small in view of the change of Nb⁴⁺/Nb⁵⁺ ratio by a factor of 15 in the covered range of oxygen pressure. The experimental data are listed in Table I.

Table I Oxidation State of Niobium in Calcium Silicate Slags as Determined by Chemical Analysis of Quenched Samples*

Redox Equilibrium of Niobium

The reaction to be considered is

$$ {\text{Nb}}^{4 + } + \frac{1}{4}{\text{O}}_{2} = {\text{Nb}}^{5 + } + \frac{1}{2}{\text{O}}^{2 - } $$

(1)

with

$$ {\frac{{{\text{Nb}}^{5 + } }}{{{\text{Nb}}^{4 + } }}} = {\text{Kp}}_{{{\text{O}}_{2} }}^{{\frac{1}{4}}} $$

(2a)

or

$$ \log\, \left( {{\frac{{{\text{Nb}}^{5 + } }}{{{\text{Nb}}^{4 + } }}}} \right){\text{ = log}}\;{\text{K + }}\frac{1}{4}{ \log }\,{\text{p}}_{{{\text{O}}_{2} }}. $$

(2b)

The factor K in Eq. [2] contains the activity coefficients of the metal ions and the activity of the oxygen anion. If the ratio of the activity coefficients of the niobium cations and the oxygen ion activity approach to constant values, then K will be constant at given contents of majority components of the slag, and the log–log plot of the Nb⁵⁺/Nb⁴⁺ ratio against ${\text{p}}_{{{\text{O}}_{2} }}$ results in a straight line with slope 1/4. This slope is so to a close approximation as shown in Figure 1. But there is an influence of total niobium content on K, as seen in Figure 2. The effect of CaO/SiO₂ (basicity) is not clear, Figure 3. There is an increase of K from CaO/SiO₂ = 0.66 to 0.93, which corresponds to the behavior of other transition metals, e.g., an increase of Cr³⁺/Cr²⁺ and Cr⁶⁺/Cr³⁺ with basicity as observed in the preceding investigation.[2] But from CaO/SiO₂ = 0.93 to 1.10, there seems to be a decrease again, which indicates a maximum of K in the K-basicity function.

Fig. 1

Ratio Nb⁵⁺/Nb⁴⁺ as a function of oxygen pressure in niobium containing calcium silicate melts at 1873 K (1600 °C)

Fig. 2

Effect of total niobium content on ratio Nb⁵⁺/Nb⁴⁺ in niobium containing calcium silicate melts at 1873 K (1600 °C). Basicity = 0.93. The symbols are explained in Fig. 1

Fig. 3

Plot of Nb⁵⁺/Nb⁴⁺ ratio vs mass pct CaO/mass pct SiO₂. Calcium silicate melts with about 8 mass pct total niobium. 1873 K (1600 °C). The symbols are explained in Fig. 1

Mention should be made on the possibility of other valence states of niobium in the slag. Particularly, it should be considered whether divalent niobium Nb²⁺ might exist because NbO is a stable phase in the Nb-O system.[3,8] In this case, the equilibrium

$$ {\text{Nb}}^{2 + } + \frac{3}{4}{\text{O}}_{2} = {\text{Nb}}^{5 + } + \frac{3}{2}{\text{O}}^{2 - } $$

(3)

$$ {\frac{{{\text{Nb}}^{5 + } }}{{{\text{Nb}}^{2 + } }}} = {\text{K}^{\prime}}\text{p}_{{{\text{O}}_{ 2} }}^{{{3/4}}} $$

(3a)

would be established. In the limiting case that only two species are present rather than three, there would be Nb²⁺, instead of Nb⁴⁺, and Nb⁵⁺. But this alternative can be excluded. The potentiometric titration method that we used for determination of Nb⁴⁺ (via Fe²⁺) cannot distinguish whether the slag contained Nb⁴⁺ or Nb²⁺. If Nb²⁺ existed instead of Nb⁴⁺, the Nb²⁺ concentration obtained from the titrated Fe²⁺ would be one third of the Nb⁴⁺ concentration. The Nb⁵⁺/Nb²⁺ ratio would just be about three times the value of the Nb⁵⁺/Nb⁴⁺ ratio. But in a log–log plot against oxygen pressure the slope 3/4 should arise, which is not the case. The slope is ¼, which proves that the main species are Nb⁵⁺ and Nb⁴⁺ and not Nb⁵⁺ and Nb²⁺ in the range of oxygen pressure investigated. But possibly Nb²⁺can exist at lower oxygen pressure.

Color of The Quenched Slag Samples And Transmission Spectra

The solid slag samples containing transition metals have characteristic colors that depend on the state of oxidation of the transition metal. This is well known and has been elaborated for chromium containing slags in the preceding article.[2] Figure 4 shows the colors in samples with CaO/SiO₂ = 0.93. It is evident that the slags are violet with the color intensity increasing with decreasing oxygen pressure or increasing Nb⁴⁺ content of the sample. In the transmission spectra, there is a minimum in the range of wavelength from 530 to 575 nm[15] caused by Nb⁴⁺, which has been found previously for niobium Nb⁴⁺ containing fluoride salt melts,[16] for Nb⁴⁺ containing nonaqueous solutions,[17] and for Nb⁴⁺ containing Nb₂O₅ films.[18] The minimum becomes deeper with decreasing oxygen pressure (increasing Nb⁴⁺ fraction) and increasing total niobium content.[15] The color of a transparent slag represents the complementary color of the absorbed light. The absorbed color is “green” for the range 495 to 570 nm, see Table VI in Reference 2, making an observed color of "violet-red violet", which agrees with that found in our slag samples (Figure 4). The transmission spectra are given elsewhere.[15]

Fig. 4

Colors of the niobium containing calcium silicate slags. Basicity of samples: mass pct CaO/mass% SiO₂ = 0.93

Conclusions

This article presents the experimental results on the dependence of the redox state of niobium on oxygen pressure, in calcium silicate base melts at steelmaking temperature. Such an investigation has not been carried out hitherto. It was confirmed that niobium is mainly pentavalent in liquid slags, as is usually assumed in the metallurgical literature. A small fraction is tetravalent. The apparent equilibrium constants of the redox reaction were determined. The experiments were confined to oxygen pressure above 10⁻⁹ atm (1873 K (1600 °C)). It might be interesting to perform similar measurements at lower oxygen pressures to check whether divalent niobium can occur. The phase NbO is a stable solid in the binary niobium-oxygen system. Possibly, niobium can exist in liquid slags in three valence states (Nb²⁺, Nb⁴⁺, and Nb⁵⁺) as is the case for vanadium (V³⁺, V⁴⁺, and V⁵⁺) and chromium (Cr²⁺, Cr³⁺, and Cr⁶⁺).

Niobium colors the slags violet; the intensity increases as oxygen pressure decreases or Nb⁴⁺ content increases.