Reaction Mechanism of Magnesium in Roasting of Vanadium Slag

Abstract

The influence of magnesium on roasting vanadium slag was investigated by simulating the roasting process of vanadium by mixing V₂O₅ and MgO. The calcination products of V₂O₅ reacted with MgO at different temperatures which were characterized by X-ray diffraction (XRD), Thermogravimetry-differential scanning calorimetry (TG-DSC) and scanning electron microscope (SEM). There were two mass loss intervals, three endothermic peaks and one exothermic peak appearing during the reaction by the integrated thermal analysis as the temperature increased from room temperature to 1273.15 K. The samples of mixed V₂O₅ and MgO began to melt at 573.15 K and reacted at 773 K, and the shape of the particles changed from block to ovoid or irregular sphere at 773.15 K. With increasing the reaction temperature from 973.15 to 1073.15 K, the intermediate of VO₂ was produced, and MgV₂O₆ was partially decomposed at 1073.15 K. During this process, the particle shape gradually returned to block shape. The conversion rate of vanadium is 99.4% with MgO of 1.65%.

Key words
vanadium slag; roasting; simulation; magnesium

INTRODUCTION

Up to now, about 88% reserves of vanadium in the world are extracted from vanadium-titanium magnetite (Qiu et al. 2011), and the rest of that is collected from various minerals such as stone coal, and spent catalyst (Moskalyk & Alfantazi 2003, Li et al. 2013).

Vanadium slag is a crucial raw material for preparing vanadium and steel material containing vanadium in the present research. For the traditional vanadium production process, the vanadium slag is used to be roasted with sodium salt or calcium salt (Voglauer et al. 2004, Gabra & Malinsky 1981, Bradbury 2002, Song et al. 2014, Li et al. 2015, Cao 2012, Chen et al. 2013), such as NaCl, Na₂CO₃, or Na₂SO₄ in rotary kiln or multiple hearth furnace filled with oxidizing atmosphere at the temperature around 1023.15 to 1123.15 K.

It is worth noting that various elements in the vanadium slag will affect the roasting process. Magnesium exists in vanadium slag in the form of spinel and complex oxides containing various metals (Wang 2011). Sodium magnesium slag has advantages over sodium slag for the next process to extract vanadium pentoxide (Sun 1995). However, there are few researches about effects of magnesium except chromium (Liu et al. 2016) and calcium (every 1% increase in CaO in slag will lose 4.7%–9.0% of V₂O₅ (Peng et al. 2007)). Therefore, the effects of magnesium on salt roasting have been investigated and a theoretical basis for vanadium extraction was provided. The changes in the mineralogy of vanadium slag are studied by XRD, SEM, and TG-DSC. The effect of the roasting temperature on vanadium extraction and the characterization of leach residues are also discussed.

MATERIALS AND METHODS

Vanadium slag and the residue are from a steel group in Chengde, China. The vanadium slag is tail slag after the vanadium extraction from the converter, and the vanadium residue refers to the vanadium tailing obtained after the vanadium slag by sodium calcining-leaching-filtration.

The main mineral phases in vanadium slag are shown in Figure 1. The chemical compositions of vanadium slag and vanadium residue is given in Table I and Table II, respectively.

Figure 1
The XRD patterns of vanadium slag and vanadium residue. a: vanadium slag, b: vanadium residue.

Table I
Chemical compositions of vanadium slag (wt. %).

Component	TFe	V	SiO2	CaO	Mn	MgO	TiO2	Cr	Na
wt. %	30.55	6.07	22.25	1.34	7.40	1.87	8.6	1.86	0.01

Table II
Chemical compositions of vanadium residue (wt. %).

Component	TFe	V	SiO2	CaO	Mn	MgO	TiO2	Cr	Na
wt. %	25.30	1.03	17.38	1.57	7.56	1.75	8.23	1.80	3.87

Vanadium extraction has been researched a lot by China and foreign scholars (Gabra & Malinsky 1981, Peng et al. 2007, Huang 2000, Wang et al. 1998, Wu 2008, Qiu et al. 2010, Li et al. 1994, Kozlov & Demidov 2000, Wen & Ding 1999, Fu et al. 2009, Shi et al. 2008). Vanadium pentoxide was extracted from vanadium slag using magnesium roasting technology in the study, and the reactions involving vanadium pentoxide and the reactions of spinel decomposition are as follows:

$$\text{FeO}·{\text{V}}_2{\text{O}}_3+\text{FeO}+1/2{\text{O}}_2={\text{Fe}}_2{\text{O}}_3·{\text{V}}_2{\text{O}}_3$$ (1)

$${\text{Fe}}_2{\text{O}}_3·{\text{V}}_2{\text{O}}_3+1/2{\text{O}}_2={\text{Fe}}_2{\text{O}}_3·{\text{V}}_2{\text{O}}_4$$ (2)

$${\text{Fe}}_2{\text{O}}_3·{\text{V}}_2{\text{O}}_4+1/2{\text{O}}_2={\text{Fe}}_2{\text{O}}_3·{\text{V}}_2{\text{O}}_5$$ (3)

$${\text{Fe}}_2{\text{O}}_3·{\text{V}}_2{\text{O}}_5={\text{Fe}}_2{\text{O}}_3+{\text{V}}_2{\text{O}}_5$$ (4)

And the reactions involving magnesium and vanadium in the slag are as follows:

$$\text{MgO}+{\text{V}}_2{\text{O}}_5=\text{MgO}·{\text{V}}_2{\text{O}}_5$$ (5)

$$\text{2MgO}+{\text{V}}_2{\text{O}}_5=\text{2MgO}{\text{V}}_2{\text{O}}_5$$ (6)

$$\text{2MgO}+{\text{3V}}_2{\text{O}}_5=\text{2MgO}·{\text{3V}}_2{\text{O}}_5$$ (7)

Experimentation

As shown in figure 2, the vanadium slag, vanadium residue, Na₂CO₃, NaCl and magnesium oxide were pretreated by mixing ingredients for 3 hours, subsequently were roasted with stirring in a vertical muffle furnace. The predetermined furnace temperature was maintained by a temperature controller. The furnace’s door was open to ensure the oxidizing atmosphere in the procedure above.

Figure 2
Flow chat of experiment.

Cooled the roasted sample with water and heat the solution up to 353.15 K with stirring at 200 r/min, subsequently separate it via vacuum filtration. The concentration of vanadium in the solution was determined with ammonium ferrous sulfate titration.

The vanadium conversion rate was calculated by:

$$\text{Conversion rate}=1-\frac{[V]s}{[V{]}_0}\times 100\mathrm{\%}$$

Where [V]₀ is the mass of vanadium in the mixed sample, and [V]_s is the mass of vanadium in the vanadium residue II. The conversion rate was calculated using the concentration of leaching solution instead of the concentration of leaching residue, this is because it is difficult to determine the mass of all residues accurately. The parallel experiments were conducted 3 times and the obtained extraction data was averaged to ensure the validity.

MgO and V₂O₅ were pretreated by mixing ingredients for 3 hours, subsequently were roasted with stirring in a vertical muffle furnace.

Effect of MgO on vanadium conversion rate

Table III shows that the mass ratios of Na₂CO₃/V₂O₅ and Na₂CO₃/NaCl were 1.2 and 3.5, respectively. The vanadium content in the sample was kept at 4.7% in the experiment, and the sample was kept at 1053.15 K for 100 minutes.

Table III
Experimental schemes for the effects of MgO on vanadium conversion.

No.	vanadium slag/%	vanadium residue/%	Na2CO3/%	NaCl/%	MgO/%
1	76.38	6.19	10.91	3.12	4.94
2	76.18	7.34	10.91	3.12	4.00
3	76.02	8.28	10.91	3.12	3.23
4	75.95	8.69	10.91	3.12	2.89
5	75.90	9.03	10.91	3.12	2.62
6	75.81	9.53	10.91	3.12	2.21
7	75.77	9.79	10.91	3.12	2.00
8	75.68	10.29	10.91	3.12	1.65

The silicate reacted with vanadium and additive to form glassy silicon vanadate, and there were 4 or 5 equivalents of vanadium moved into vitreous and insoluble vanadium compounds. Figure 3 shows that the conversion rate of vanadium decreased in the whole, with the increasing of the MgO content in the sample. The MgO content increased by 0.1% (with MgO in the range 1.65-3.23%), the rate of vanadium conversion decreased by 1.54%.

Figure 3
Effect of MgO content on vanadium conversion rate.

The conversion rate of vanadium sharply decreased from 99.4% to 75.14%, with the increase of MgO content from 1.65% to 3.23%. When the MgO content was less than 3.23%, the orthosilicate content was higher than metasilicate (olivine). However, the opposite situation occured when the MgO content was more than 3.23%, and the destruction temperature of metasilicate (olivine) was higher than orthosilicate during the roasting process (the breaking temperature of (FeMgCa)SiO₃ needs to reach 1273.15 K (Chen 1993)), that was the reason why vanadium was difficult to diffuse in to diffuse in metasilicate (olivine). In addition, as the contact area between MgO and vanadium increased, the ability of MgO to react with vanadium to form magnesium vanadate increased, resulting in a slight increase in vanadium conversion rate.

Magnesium content at 1.65% was the control group without adding magnesium oxide. In general, the addition of magnesium was detrimental to the conversion rate of vanadium in sodium roasting, since the increase of magnesium content in mixed slag would reduce the conversion rate of vanadium.

Mechanism of magnesium on roasting vanadium slag

According to the research of vanadium slag calcined by sodium(or calcium) salt, when the ratio of sodium salt(or calcium salt) to vanadium pentoxide was 1, the primary product is metavanadate. When the molar ratio was 2, the main product is pyrovanadate. The main product was orthovanadate with increasing the molar ratio to 3 (Li et al. 2011, Gao et al. 2020, Zhang et al. 2015). Metavanadate was the most stable among them, followed by pyrovanadate, then orthovanadate. Orthovanadate was unstable and generally hydrolyzes after formation and was converted into pyrovanadate (Yang et al. 2014). In this experiment, magnesium metavanadate was used as the product. Magnesium oxide and the equal molar amount of vanadium pentoxide were roasted to study the reaction of magnesium on vanadium.

XRD Analysis

As shown in Figure 4a, V₂O₅ did not react with MgO at the temperature below 673.15 K, while the appearance of VO₂ indicated that V₂O₅ decomposes at low temperature. When the temperature rose to 773.15 K, the phase peaks of MgV₂O₆ and Mg₂V₂O₇ appeared nevertheless, the phase peak of V₂O₅ was still the strongest. Furthermore, the phase peak of VO₂ disappeared, indicating that VO₂ was mesophase.

Figure 4
(a)The XDR pattern and (b) TG-DTG-DTA curve of sample roasted at different temperatures for 1 hour with the molar ratio of V₂O₅ - MgO is 1:1.Instrument Name: SDT 2960 Simultaneous DSC-TGA (USTA Instruments).

When the temperature increased to 873.15 K, the intensity of the diffraction peak of MgV₂O₆ increaseed sharply and became the main peak with the disappearance of V₂O₅ and MgO, indicating that MgO and V₂O₅ reacted vigorously and consumed several samples at 773.15 ~ 873.15 K. The phase peak of MgO disappeared and the peak intensity of V₂O₅ continuously attenuated with the increase of temperature to 973.15 K, which indicated that the reaction between V₂O₅ and MgO basically completed within the temperature range of 873.15 ~ 973.15 K. In addition, the intensity of the main peak of MgV₂O₆ increased and shifted, indicating that the change of magnesium vanadate crystal form.

The peaks shifted again at 1023.15 K. The intensity of the main peak of MgV₂O₆ became weakly at the temperature of 1073.15 K, and the phase peaks of Mg₂V₂O₇ and Mg₂V₆O₁₇ appeared, indicating that MgV₂O₆ partially decomposed to insoluble Mg₂V₂O₇ and Mg₂V₆O₁₇ t at temperature from 1023.15 to 1073.15 K. The experimental results were consistent with the conclusions of R.C. Kerby (Kerby & Wilson 1973).

Comprehensive thermal analysis

Figure 4b shows that the first weight loss section appeared due to the removal of adsorbed water from room temperature up to 473.15K. The second weight loss section appears due to deoxygenation from 673.15 K to 873.15 K.

The differential thermal analysis (DTA) figure 4b shows that the endothermic peak near the temperature of 443.15 K was due to the dehydration reaction, and the exothermic peak appears around the temperature of 743.15 K was due to the formation of MgV₂O₆ and crystal transformation of V₂O₅. The endothermic peak near 923.15 K was due to the crystal transition of MgV₂O₆. In addition, the endothermic peak near 1033.15 K was due to the transition from MgV₂O₆ to Mg₂V₂O₇ and Mg₂V₆O₁₇.

SEM analysis

Figure 5 shows that the sample was in the form of irregular powder melts and sticks at the temperature of 573.15 K. The sample consisting of clastic particles was collected with fine powders at the temperature of 673.15 K. Combined with XRD analysis, the sample was mainly composed of V₂O₅, MgO, and a few VO₂ below 673.15 K.

Figure 5
The SEM images of sample roasted at different temperatures for 1 hour with the molar ratio of 1:1 between V₂O₅ and MgO.

At the temperature of 773.15 K, the sample consisted of massive particles with the attachment of abundant fine powders. The sample roasted at 873.15 K consisted of ovoid and irregular sphere particles with a few filamentous particles and clear surface. Combined with XRD analysis, the sample roasted at 773.15 K was mainly composed of V₂O₅, MgO, a small quantity of MgV₂O₆ and Mg₂V₂O₇, while the sample roasted at 873.15 K was mainly composed of MgV₂O₆, a few V₂O₅, and MgO.

Samples at 973.15 K mainly consisted of massive and ribbon-like particles with different particle sizes. The sample was composed of massive particles at 1023.15 K, where the large particles were breakable and could be further made into small particles. The sample at 1073.15 K was composed of different sized massive particles that were aggregated to form larger clusters. Combined with XRD analysis, the sample roasted at 973.15 K and 1023.15 K mainly consisted of MgV₂O₆ and a small amount of V₂O₅. In addition, the sample roasted at 1073.15 K consisted of water-insoluble Mg₂V₂O₇ and Mg₂V₆O₁₇.

CONCLUSION

V₂O₅ in the sample did not react with MgO below 673.15 K, however reacted at 773.15 K to produce MgV₂O₆, which was decomposed into insoluble Mg₂V₂O₇ and Mg₂V₆O₁₇ at 1023.15 K to 1073.15 K.

From room temperature to 473.15K, the weight loss was mainly caused by dehydration, and the weight loss in the temperature range of 473.15 ~ 873.15 K was caused by the deoxidation reaction. The endothermic peak was observed at 444.11 K due to the dehydration reaction, and the exothermic reaction peak around 744.09 K was observed to the reaction of MgV₂O₆ generating and crystal transformation of V₂O₅. Furthermore, the endothermic peak around 927.92 K was due to the crystal transition of MgV₂O₆, and the endothermic peak at around 1024.34 K was observed to the conversion of MgV₂O₆ to Mg₂V₂O₇ and Mg₂V₆O₁₇.

The samples began to melt at 573.15 K and reacted at 773 K, and the shape of the particles changed from block to ovoid or irregular sphere. As the reaction temperature increased from 973.15 K to 1073.15 K, the particle shape gradually returned to block shape.

The conversion rate of vanadium is 99.4% with the addition of 1.65% MgO under optimal conditions.

ACKNOWLEGMENTS

This work was supported by a grant from the National Natural Science Foundation (U1360204); National 973 Project (2013CB632603); National 863 project (2012AA062304) and Fundamental scientific research funds (N120602003).

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Publication Dates

• Publication in this collection
  06 July 2020
• Date of issue
  2020

History

• Received
  6 Dec 2017
• Accepted
  10 Sept 2019