Effect of CaO on the Phase Evolution of Vanadium Slag during Crystallization and Roasting–Leaching Processes for Selective Extraction of Vanadium

Abstract

In this paper, the effects of CaO on the phase evolution mechanism of vanadium slag during slagging, direct roasting, and (NH₄)₂CO₃ leaching processes are investigated. Results indicate that with the increase in CaO content, vanadium is always concentrated as (Fe, Mn, Mg)V₂O₄ in spinels, part of titanium is concentrated and transformed into CaTiO₃, and phosphorus is concentrated in 3CaO·P₂O₅ (C₃P) and transformed into n·2CaO·SiO₂-3CaO·P₂O₅ (nC₂S-C₃P). During the direct roasting process, a part of the vanadium-containing spinel phase oxidizes and reacts with Ca₂SiO₄ to produce calcium vanadate (Ca₃V₂O₈, Ca₁₀V₆O₂₅, and Ca₂V₂O₇), which is soluble in (NH₄)₂CO₃ aqueous solution. However, a part of the vanadium-containing spinel phase is oxidized and decomposed to vanadium oxides (V₂O₅ and V₆O₁₃), which are insoluble in (NH₄)₂CO₃ aqueous solution. This is not beneficial for vanadium extraction using (NH₄)₂CO₃ aqueous solution. In addition, (NH₄)₂CO₃ aqueous solution can restrain the leaching of C₃P from the nC₂S-C₃P solid solution in the directly roasted vanadium slag with high CaO content.

1. Introduction

Owing to the excellent physical properties such as high tensile strength, hardness, and fatigue resistance of vanadium alloys, vanadium plays an extraordinarily important role in steelmaking, chemical industries, and aviation fields [1,2,3]. However, there is no single recoverable vanadium mineral, and the distribution of vanadium is fragmented. It is mainly associated with patronite, vanadinite, carnotite, and vanadium–titanium magnetite [4]. Among them, vanadium–titanium magnetite is a major raw material used to extract vanadium. The current process to extract vanadium from V-Ti magnetite is shown by the solid lines in Figure 1. In this process, vanadium–titanium magnetite is smelted in a blast furnace to produce vanadium-containing hot metal and, subsequently, oxidized in a converter to obtain vanadium slag and semi-steel. The vanadium slag usually contains 5–20 wt.% of V₂O₃ in the form of FeV₂O₄ spinels and is the source for vanadium extraction [5].

Vanadium can be extracted from vanadium slag via a sodium roasting–water leaching process, which requires CaO-free vanadium slag to avoid the formation of water-insoluble calcium vanadate [6,7,8]. However, the CaO-free vanadium slag leads to poor dephosphorization ability, which causes a heavy dephosphorization burden in the subsequent steelmaking process. To achieve the goal of dephosphorization and vanadium extraction simultaneously, CaO is added to the converter during the production of vanadium slag. The flowsheet of this novel process is shown by the dotted lines in Figure 1. Due to the increase in CaO content during slagging, the melting properties and phase compositions of vanadium slag change. The hemisphere point temperature (the melting temperature of slag) decreases significantly with the increase in CaO content, which can reduce the precipitation temperature of spinels through thermodynamic calculations [9]. Zhou et al. [10] employed the crystal size distribution (CSD) theory to analyze the non-isothermal crystallization kinetics of spinel crystals in vanadium slags containing high CaO content. The results indicate that a low cooling rate and high CaO content benefit the growth of spinel crystals. Many studies [11,12,13,14,15,16] suggest that dicalcium silicate (C₂S) and tricalcium phosphate (C₃P) form a solid solution at the steelmaking or hot metal dephosphorization temperature. The mineral phases in phosphorus-containing vanadium slag were investigated by Chen et al. [17]. The results indicate that vanadium was present in the slag only as spinels, and phosphorus was still present in the form of calcium phosphate eutectic in calcium silicate. Some studies [18,19,20,21] reported the mineral phases in calcification of roasted vanadium slag, and the results indicate that the main vanadium-containing phases in the roasted vanadium slag were FeVO₄, Ca₃(VO₄)₂, CaV₂O₅, CaV₂O₆, Ca₂V₂O₇, and Ca₃V₂O₈.

However, the effect of CaO on the phase evolution of vanadium slag during slagging and roasting–leaching processes was seldom reported and is still not well-defined, especially for the evolution of vanadium- and phosphorus-containing phases. To selectively extract vanadium into liquor from roasted vanadium slag but maintain phosphorus in the solid phase, it is essential to focus on the phase evolution of vanadium slag during the vanadium extraction process. Hence, in this work, we aim to investigate the effect of CaO on the phase evolution mechanism of vanadium slag during slagging and roasting–leaching processes.

2. Experimental Section

2.1. Smelting of Vanadium Slag with Different CaO Contents

The chemical composition of vanadium slags (Ca0–Ca3) is listed in

Table 1. Chemical composition (wt.%) of vanadium slags.

	FeO	SiO2	CaO	V2O3	TiO2	MgO	MnO	P2O5	Cr2O3	Al2O3
Ca0	39.12	12.45	1.63	13.05	16.90	3.52	8.01	0.12	2.07	3.13
Ca1	30.00	13.48	17.52	13.00	10.00	3.00	8.00	2.00	-	-
Ca2	29.00	13.00	26.00	11.00	8.00	3.50	5.50	2.00	-	-
Ca3	19.28	12.70	31.56	9.84	11.42	5.56	3.38	2.10	1.96	2.20

. Chemical compositions of samples were determined via X-ray fluorescence (XRF, Shimadzu XRF-1800) or inductively coupled plasma–atomic emission spectroscopy (ICP-AES, PerkinElmer Optima-4300DV). CaO content changed from 1.63 to 31.56 wt.%, and P₂O₅ content changed from 0.12 to 2.1 wt.%. Ca0 slag was obtained in the process of smelting vanadium-containing hot metal in a converter without lime at Pan-steel. Ca3 slag was obtained from the industrial test of adding lime to a vanadium extraction converter at Pan-steel. Ca1 and Ca2 were experimentally synthesized slags. Initial materials were chemicals with purities of FeC₂O₄·2H₂O (≥98.0%), SiO₂ (≥99.0%), V₂O₃ (≥99.0%), TiO₂ (≥99.0%), MnCO₃ (≥99.0%), MgO (≥98.0%), CaO (≥98.0%), and Cr₂O₃ (≥99.0%), supplied by Chengdu Kelong Chemical Co. Ltd. The reagent of CaO was calcined at 1373 K for 12 h before weighing and mixing. A mixture of the initial materials was placed in a 10 kW box-type electric resistance furnace with a proportional–integral–differential (PID) controller (±2 K) for temperature control.

A total of 100 g of the mixture was charged into an iron crucible (inner diameter: 46 mm; height: 120 mm), which was subsequently placed in another corundum crucible (inner diameter: 53 mm; height: 140 mm). The crucibles were placed in the furnace and heated to 1373 K at the rate of 5 K/min to allow the reagents of FeC₂O₄·2H₂O and MnCO₃ to decompose into FeO (totally assumed as FeO) and MnO, respectively. Subsequently, the mixture was heated and held at 1773 K for 30 min to ensure homogeneity. The mixture was then cooled down and held at 1473 K for 1 h, and further cooled to about 873 K at the rate of 5 K/min to ensure complete precipitation of minerals in the slag sample. Finally, the sample was taken out and air-cooled.

Phase compositions of solid samples (Ca0-Ca3) were identified via X-ray diffraction analysis (XRD, Rigaku D/MAX 2500PC) using Cu Kα radiation. Microscopic observation and analysis of element distribution in samples were conducted via scanning electron microscopy (SEM, TESCAN VEGA III) together with energy-disperse X-ray spectrometry (EDS or EDX, INCA Energy 350).

2.2. Roasting–Leaching Processes

Ca0 chemical composition is typical of ordinary vanadium slag. Its phase evolution during roasting–leaching processes has been reported by some researchers [22,23,24,25]. As shown in Figure 2, the phase composition of Ca3 vanadium slag is basically the same as Ca2 vanadium slag. Therefore, Ca1 and Ca2 vanadium slags were used to investigate the phase evolution during roasting–leaching processes.

In the stage of directly roasting, the vanadium slags were milled, and the portion in particle size <74 μm was collected. The collected portion of vanadium slags was put into a ceramic evaporation dish and heated in a muffle furnace at 1173 K for 2 h. The directly roasted vanadium slags were cooled down to room temperature by air.

In the stage of (NH₄)₂CO₃ leaching, the directly roasted vanadium slags were milled, and the portion in particle size <74 μm was collected. The roasted slags were leached with 500 g/L of (NH₄)₂CO₃ solution at a solid/liquid ratio of 1:20 (g/mL). The leaching experiments were performed at atmospheric pressure in a three-neck flask with a plug to maintain concentrations of reactants and products. The reaction mixture was stirred with a magnetic stirrer at 100 rpm and heated in a water bath at 353 K for 1 h in a commercial magnetic stirring water bath pot (Type DF-101, Gongyi Electric Equipment Corp., Zhengzhouy, Henan, China).

The phase composition of the roasted slags and leaching residues was identified with X-ray diffraction analysis (XRD, Rigaku D/MAX 2500PC) using Cu Kα radiation.

3. Results and Discussion

3.1. Effects of CaO on the Phase Evolution of Vanadium Slag during Slagging Process

Phase composition has a great influence on the physicochemical properties of vanadium slag, and it is instructive to determine the phase composition of vanadium slag for the selection of the subsequent roasting–leaching process. Thus, the effects of CaO on the phase evolution of vanadium slag during the slagging process were investigated using XRD. The results are shown in Figure 2. The spinel phases always consisted of (Fe, Mn, Mg)(Fe, V, Cr)₂O₄, and (Fe, Mg)₂TiO₄ in all the slags, although other phases changed with CaO content. This indicates that vanadium-containing spinels are the main existing form of vanadium in all the slags. When the CaO content was 1.63 wt.%, the slag was mainly composed of spinels and olivines. This agrees with the reported results [26,27]. With the increase in CaO content from 1.63 to 17.52 wt.%, the diffraction peaks of olivine (Fe₂SiO₄) disappeared, and those of kirschsteinite (CaFeSiO₄) appeared, indicating that Fe₂SiO₄ transformed to CaFeSiO₄. With further increase in CaO content, 2CaO·SiO₂ formed in the silicate phases. In addition, the diffraction peaks of perovskite (CaTiO₃) and akermanite (Ca₂MgSi₂O₇) appeared, while the CaO content increased to 17.52 wt.%. The diffraction peak intensity of CaTiO₃ increased significantly with the increase in CaO content from 17.52 to 31.56 wt.%. This means that titanium spinels transformed to perovskite with the increase in CaO content. For the slag with 17.52 wt.% of CaO, the phosphorus-containing phase was nCa₂SiO₄-Ca₃(PO₄)₂ (nC₂S-C₃P), and the value of n increased from 2 to 6 with an increase in the CaO content from 17.52 to 31.56 wt.%. It was concluded that vanadium always exists in the form of (Fe, Mn, Mg)V₂O₄ spinels, and phosphorus element is concentrated in C₃P and transformed into nC₂S-C₃P.

3.2. Effect of CaO on Element Distribution Laws in Vanadium Slag

To reveal the effect of CaO on the evolution of element distribution laws in vanadium slags with CaO contents, SEM characterization was conducted. SEM images and spot energy disperse spectrum (EDS) analysis results are shown in Figure 3 and

Table 2. Spot analysis (in wt.%) of the slag samples as shown in the representative SEM images.

Point	Phase	O	Mg	Al	Si	P	Ca	Ti	V	Cr	Mn	Fe
A1	Spinel	24.5	1.1	1.4	-	-	-	13.8	13.9	2.8	5.2	37.4
A2	Olivine	44.1	6.7	-	16.6	-	-	-	-	-	6.6	26.0
A3	Augite	54.2	0.8	6.2	25.9	-	5.5	-	-	-	1.3	6.1
B1	Spinel	29.3	3.2	7.8	-	-	-	7.6	16.4	-	6.0	29.6
B2	Silicates	44.3	-	7.8	14.5	1.5	24.8	1.0	-	-	1.3	4.9
C1	Spinel	24.0	-	-	-	-	-	7.5	29.3	3.6	7.6	28.2
C2	Perovskite	38.8	-	-	-	-	31.4	29.9	-	-	-	-
C3	C2S-C3P	48.0	-	-	4.3	15.1	32.5	-	-	-	-	-

, respectively. As shown in Figure 3a, the main mineral phases included spinel and olivine when the CaO content was 1.63 wt.%, which agrees with the results of XRD. As shown in Figure 3b,c olivines (Fe, Mn, Mg)₂SiO₄ disappeared with the increase in CaO content. When the CaO content was 31.56 wt.%, there were three kinds of minerals in the slag, and the corresponding colors of the minerals are white, gray, and dark gray. The results of EDS and XRD indicate that the three kinds of minerals were spinel, perovskite, and nC₂S-C₃P solid solution. It can be seen that V and Ti were concentrated in the spinel phase, Fe in the olivine and spinel phase, and Ca in the augite phase when the CaO content was 1.63 wt.%. With the increase in CaO content from 1.63 to 17.52 wt.%, Fe, V, and Ti were mainly concentrated in the spinel phase, while Ca, Si, and P were in the silicate phase. According to the results of XRD, when the CaO content increased to 31.56 wt.%, the coexistent area of Ca and Ti was perovskite, and the co-enrichment area of Ca, Si, and P was the nC₂S-C₃P solid solution. In conclusion, V is always concentrated in the spinel phase, part of Ti is concentrated and transformed into CaTiO₃ with the increase in CaO content, and P is concentrated in the silicate phase.

According to the CaO-FeO_x-SiO₂ ternary phase diagram [28], with the increase in CaO content, the olivine of Fe₂SiO₄ transformed to the intermediate of CaFeSiO₄, and then CaFeSiO₄ to Ca₂SiO₄. As shown in Figure 3, the olivine (Fe₂SiO₄) transformed to Ca₂SiO₄, which is consistent with the results of the CaO-FeO_x-SiO₂ ternary phase diagram [28]. In addition, Figure 3 also shows that the nC₂S-C₃P solid solution and CaTiO₃ (perovskite) formed in silicates with the increase in CaO content, which are consistent with the results of the CaO-FeO_x-TiO₂ and CaO-P₂O₅-SiO₂ ternary phase diagrams [28]. The results of CaTiO₃ formation are also consistent with the research of Fang et al. [9], which indicated that CaTiO₃ is formed with the increase in CaO content to 14.94 wt.% in the vanadium slag.

3.3. Phase Evolution during Direct Roasting and (NH₄)₂CO₃ Leaching Processes

The phase evolution of vanadium slag with high CaO content during direct roasting and (NH₄)₂CO₃ leaching processes reveals the mechanism of selective extraction of vanadium, which provides a theoretical foundation for optimization of the roasting–leaching process. The directly roasted slags and (NH₄)₂CO₃ leaching residues were investigated via XRD, and the patterns are shown in Figure 4. It can be seen that vanadium existed as calcium vanadate (Ca₃V₂O₈, Ca₁₀V₆O₂₅, and Ca₂V₂O₇) and vanadium oxide (V₂O₅ and V₆O₁₃); titanium existed as CaTiO₃, Fe₂TiO₅, and Mg₂TiO₄; and phosphorus existed as nC₂S-C₃P in the directly roasted vanadium slag. Moreover, the diffraction peaks of SiO₂ and Fe₂O₃ appeared during the direct roasting process, among them, the crystal system and space groups of SiO₂, which are monoclinic and P_c (space group number: 7), respectively. After the (NH₄)₂CO₃ leaching process, the diffraction peaks of calcium vanadate disappeared, while those of calcium carbonate appeared. However, the diffraction peaks of vanadium oxide and nC₂S-C₃P solid solution continued to exist.

In order to visualize the new phase formation of the slag during the direct roasting process, the Ca1 slag sample was directly roasted in a muffle furnace at a temperature of 1173 K for 30 min, and then the metallography of the slag sample was analyzed using SEM/EDS. SEM images and spot energy-dispersive spectrum analysis results are shown in Figure 5 and

Table 3. Spot analysis (in wt.%) of the Ca1 directly roasted slag sample as shown in the SEM image.

Point	O	Mg	Si	P	Ca	Ti	V	Mn	Fe
1	49.4	1.6	-	-	8.6	-	12.1	22.3	6.2
2	38.8	4.0	-	-	3.8	-	7.7	41.6	4.1
3	35.7	0.6	-	-	9.6	-	12.9	23.7	17.6
4	47.0	0.7	7.6	1.8	23.4	9.7	-	2.6	7.3

, respectively. Figure 5 shows that the spinel phase in vanadium slag was obviously destroyed during the direct roasting process. Combined with the EDS analysis results in Table 3, it can be seen that calcium element appeared in the spinel phase (before roasting) area during the direct roasting process, i.e., the calcium originally existed in the silicate phase diffused to the spinel phase area via the reaction to form calcium vanadate during the direct roasting process. As shown in Figure 2, calcium existed in the silicate phase of Ca1 slag in the form of CaTiO₃, CaFeSiO₄, C₂S, and 2C₂S-C₃P, and combined with the XRD results of Ca1-roasted slag in Figure 4a, it was concluded that the calcium element that reacts with the vanadium in the spinel phase comes from the CaFeSiO₄ and C₂S phases. Moreover, phosphorus always exists in the silicate phase.

In order to compare the phase evolution of Ca1 and Ca2 vanadium slags during the direct roasting and (NH₄)₂CO₃ leaching processes, the main phase composition of the slag samples is presented in

Table 4. The main phase composition of vanadium slag, roasted slag, and leaching residue.

		Vanadium Slag	Roasted Slag	Leaching Residue
Main Phase Composition	Ca1	FeV2O4	Ca3V2O8 Ca10V6O25 V2O5	V2O5 CaCO3
		Fe2TiO4	Fe2O3	Fe2O3
		CaTiO3	Fe2TiO5 CaTiO3	Fe2TiO5 CaTiO3
		CaFeSiO4	SiO2 Ca2SiO4	SiO2 Ca2SiO4
		Ca2MgSi2O7	Ca2MgSi2O7	Ca2MgSi2O7
		2C2S-C3P	2C2S-C3P	2C2S-C3P
	Ca2	FeV2O4	Ca3V2O8 Ca2V2O7 V6O13	V6O13 CaCO3
		Fe2TiO4	Fe2O3	Fe2O3
		CaTiO3	Fe2TiO5 CaTiO3	Fe2TiO5 CaTiO3
		Ca2SiO4	SiO2 Ca2SiO4	SiO2 Ca2SiO4
		6C2S-C3P	6C2S-C3P	6C2S-C3P

. According to the transformation of phases during the roasting–leaching process, the phase evolution mechanism can be deduced.

The results of phase evolution during direct roasting indicate that a part of the spinel phase FeV₂O₄ oxidized and decomposed to Fe₂O₃ and vanadium oxides, as shown in reactions (1) and (2). The results of other researchers [29,30] also indicated that FeV₂O₄ oxidized to V₂O₅ and Fe₂O₃ during the roasting process.

The following are examples of the equations used in this process:

$${4\mathrm{FeV}}_2{\mathrm{O}}_4{+5\mathrm{O}}_2{ =2\mathrm{Fe}}_2{\mathrm{O}}_3{+4\mathrm{V}}_2{\mathrm{O}}_5$$

(1)

$${12\mathrm{FeV}}_2{\mathrm{O}}_4{+11\mathrm{O}}_2{ =6\mathrm{Fe}}_2{\mathrm{O}}_3{+4\mathrm{V}}_6{\mathrm{O}}_{13}$$

(2)

A part of the spinel phase FeV₂O₄ oxidized and reacted with Ca₂SiO₄ to produce calcium vanadate and SiO₂. V³⁺ ions in the spinels diffused away from the spinel lattice and oxidized to produce Ca₃V₂O₈, Ca₁₀V₆O₂₅, and Ca₂V₂O₇, as shown in reactions (3)–(5).

$${4\mathrm{FeV}}_2{\mathrm{O}}_4{+6\mathrm{Ca}}_2{\mathrm{SiO}}_4{+5\mathrm{O}}_2{ =2\mathrm{Fe}}_2{\mathrm{O}}_3{+4\mathrm{Ca}}_3{\mathrm{V}}_2{\mathrm{O}}_8{+6\mathrm{SiO}}_2$$

(3)

$${12\mathrm{FeV}}_2{\mathrm{O}}_4{+20\mathrm{Ca}}_2{\mathrm{SiO}}_4{+15\mathrm{O}}_2{ =6\mathrm{Fe}}_2{\mathrm{O}}_3{+4\mathrm{Ca}}_{10}{\mathrm{V}}_6{\mathrm{O}}_{25}{+20\mathrm{SiO}}_2$$

(4)

$${4\mathrm{FeV}}_2{\mathrm{O}}_4{+4\mathrm{Ca}}_2{\mathrm{SiO}}_4{+5\mathrm{O}}_2{ =2\mathrm{Fe}}_2{\mathrm{O}}_3{+4\mathrm{Ca}}_2{\mathrm{V}}_2{\mathrm{O}}_7{+4\mathrm{SiO}}_2$$

(5)

Spinel phase Fe₂TiO₄ oxidized to produce Fe₂TiO₅, as shown in reaction (6).

$${2\mathrm{Fe}}_2{\mathrm{TiO}}_4{+\mathrm{O}}_2{ =2\mathrm{Fe}}_2{\mathrm{TiO}}_5$$

(6)

Kirschsteinite phase CaFeSiO₄ oxidized and decomposed to Fe₂O₃, Ca₂SiO₄, and SiO₂, as shown in reaction (7).

$${4\mathrm{CaFeSiO}}_4{+5\mathrm{O}}_2{ =2\mathrm{Fe}}_2{\mathrm{O}}_3{+2\mathrm{Ca}}_2{\mathrm{SiO}}_4{+2\mathrm{SiO}}_2$$

(7)

In addition, phases CaTiO₃, Mg₂TiO₄, Ca₂MgSi₂O₇, and nC2S-C3P did not transform into other phases during the direct roasting process. Based on the above analyses of the phase evolution mechanism, it can be seen that vanadium-containing spinels transformed into calcium vanadate and vanadium oxide, and the phosphorus-containing phase remained as nC2S-C3P without transforming during the direct roasting process.

The results of the phase evolution during (NH₄)₂CO₃ leaching indicate that calcium vanadate reacted with (NH₄)₂CO₃ to produce CaCO₃ and NH₄VO₃, as shown in reactions (8)–(10).

$${\mathrm{Ca}}_3{\mathrm{V}}_2{\mathrm{O}}_8+3{\left({\mathrm{NH}}_4\right)}_2{\mathrm{CO}}_3{+2\mathrm{H}}_2{\mathrm{O}=3\mathrm{CaCO}}_3{+2\mathrm{NH}}_4{\mathrm{VO}}_3{+4\mathrm{NH}}_3\cdot {\mathrm{H}}_2\mathrm{O}$$

(8)

$${\mathrm{Ca}}_{10}{\mathrm{V}}_6{\mathrm{O}}_{25}+10{\left({\mathrm{NH}}_4\right)}_2{\mathrm{CO}}_3{+7\mathrm{H}}_2{\mathrm{O}=10\mathrm{CaCO}}_3{+6\mathrm{NH}}_4{\mathrm{VO}}_3{+14\mathrm{NH}}_3\cdot {\mathrm{H}}_2\mathrm{O}$$

(9)

$${\mathrm{Ca}}_2{\mathrm{V}}_2{\mathrm{O}}_7+2{\left({\mathrm{NH}}_4\right)}_2{\mathrm{CO}}_3{+\mathrm{H}}_2{\mathrm{O}=2\mathrm{CaCO}}_3{+2\mathrm{NH}}_4{\mathrm{VO}}_3{+2\mathrm{NH}}_3\cdot {\mathrm{H}}_2\mathrm{O}$$

(10)

The phases Fe₂TiO₅, CaTiO₃, Mg₂TiO₄, Ca₂MgSi₂O₇, Ca₂SiO₄, SiO₂, Fe₂O₃, V₂O₅, V₆O₁₃, and nC2S-C3P still existed in the leaching residues of Ca1 and Ca2 vanadium slags directly roasted at 900 °C. This means that these phases cannot be leached using (NH₄)₂CO₃ aqueous solution. Among these phases, vanadium oxides insoluble in (NH₄)₂CO₃ aqueous solution produced by the direct roasting of vanadium slag with high CaO content led to a large amount of loss of valuable vanadium. During leaching with (NH₄)₂CO₃ aqueous solution, C₃P reacted with (NH₄)₂CO₃, as shown in reaction (11).

$${\mathrm{Ca}}_3{({\mathrm{PO}}_4)}_2 (\mathrm{s})+3{\mathrm{CO}}_3^{2-} (\mathrm{aq})=3\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3 (\mathrm{s})+2{\mathrm{PO}}_4^{3-} (\mathrm{aq})$$

(11)

The equilibrium constant of reaction (11) can be expressed as

$$K=\frac{c_{{\mathrm{PO}}_4^{3-}}^2}{c_{{\mathrm{CO}}_3^{2-}}^3}=\frac{K_{sp}({\mathrm{Ca}}_3{({\mathrm{PO}}_4)}_2)/c_{{\mathrm{Ca}}^{2-}}^3}{{(K_{sp}({\mathrm{CaCO}}_3)/c_{{\mathrm{Ca}}^{2-}})}^3}=\frac{K_{sp}({\mathrm{Ca}}_3{({\mathrm{PO}}_4)}_2)}{K_{sp}^3({\mathrm{CaCO}}_3)}$$

(12)

where the solubility product constant (K_sp) of Ca₃(PO₄)₂ K_sp(Ca₃(PO₄)₂) = 2.0 × 10⁻²⁹ at 25 °C and that of CaCO₃ K_sp(CaCO₃) = 2.8 × 10⁻⁹ at 25 °C. The value of the equilibrium constant (K) of reaction (11) was 9.11 × 10⁻⁴, which means that the Gibbs free energy of the reaction was larger than zero. Hence, insoluble C₃P could not react with (NH₄)₂CO₃ to produce soluble ammonium phosphate, which is consistent with the study by Li et al. [5].

In order to investigate the effect of CaO content on the evolution of the weight fractions of the different phases during the direct roasting process, the phase compositions of Ca1 and Ca2 roasted slags were quantitatively analyzed via the reference intensity ratio (RIR) method [31]. The mass fraction of component X in the roasted slags can be calculated by

$${\omega }_X=\frac{I_X}{K_A^X{\displaystyle \sum _{i=A}^N\frac{I_i}{K_A^i}}}$$

(13)

where ω_X is the mass fraction of component X in the slag; I_X is the integral intensity of X phase in the XRD pattern of the slag; $K_A^X$ is the RIR value of component X, while component A is used as the internal standard.

$K_A^X$ can be expressed as

$$K_A^X=\frac{K_{{\mathrm{Al}}_2{\mathrm{O}}_3}^X}{K_{{\mathrm{Al}}_2{\mathrm{O}}_3}^A}$$

(14)

where $K_{\mathrm{A}{\mathrm{l}}_2{O}_3}^X$ is the RIR value of component X, while pure α-Al₂O₃ is used as the internal standard; it can be found in the Powder Diffraction File (PDF) card from International Center for Diffraction Data (ICDD).

Based on the RIR method, the results of quantitative phase analysis of Ca1 and Ca2 roasted slags are shown in

Table 5. The phase contents (wt.%) of Ca1 and Ca2 roasted slags.

	Fe2O3	CaTiO3	Fe2TiO5	MgTi2O4	SiO2	Ca2(SiO4)2	Ca2MgSi2O7	2C2S-C3P	Ca3(VO4)2	Ca10V6O25	V2O5
Ca1 roasted slag	32.11	9.76	3.91	3.44	9.60	6.98	1.24	7.54	7.30	12.89	5.22
	Fe2O3	CaTiO3	Fe2TiO5	MgTi2O4	SiO2	Ca2(SiO4)2	/	6C2S-C3P	Ca3(VO4)2	Ca2V2O7	V6O13
Ca2 roasted slag	31.46	11.88	2.22	2.83	7.59	9.44	/	15.73	7.39	6.26	5.20

. It can be seen that Fe₂O₃, SiO₂, Ca₃V₂O₈, Ca₁₀V₆O₂₅, and Ca₂V₂O₇ were the main phases (>6 wt.%) formed during the roasting process, whereas V₂O₅, V₆O₁₃, and Fe₂TiO₅ were the minor phases formed in roasted slag.

According to the results of the phase evolution during (NH₄)₂CO₃ leaching, calcium vanadate (Ca₃V₂O₈, Ca₁₀V₆O₂₅, and Ca₂V₂O₇) was soluble in (NH₄)₂CO₃ aqueous solution. Therefore, the vanadium recovery in vanadium slag with different CaO contents was calculated by the ratio of the oxidation of vanadium-containing spinels to calcium vanadate, which can be expressed as

$$V_{\mathrm{leaching} \mathrm{efficiency}}=\frac{{\omega }_{{\mathrm{V}}_2{\mathrm{O}}_3(\mathrm{in} \mathrm{calcium} \mathrm{vanadate})}}{{\omega }_{{\mathrm{V}}_2{\mathrm{O}}_3(\mathrm{in} \mathrm{vanadium} \mathrm{slag})}}\times 100\%$$

(15)

where ${\omega }_{{\mathrm{V}}_2{\mathrm{O}}_3\left(\mathrm{in} \mathrm{calcium} \mathrm{vanadate}\right)}$ is the mass fraction of V₂O₃ in the calcium vanadate; ${\omega }_{{\mathrm{V}}_2{\mathrm{O}}_3\left(\mathrm{in} \mathrm{vanadium} \mathrm{slag}\right)}$ is the mass fraction of V₂O₃ in the initial vanadium slag.

Thus, the vanadium recovery of Ca1 slag with 17.52 wt.% of CaO content during direct roasting and (NH₄)₂CO₃ leaching processes was 64.4%, and the vanadium recovery in Ca2 slag with 26.0 wt.% of CaO content was 57.8%. It means that although the CaO content was higher than 26.0 wt.%, it was not conducive to the formation of calcium vanadate, and the vanadium recovery would decrease with the increase in CaO content.

Based on the above analyses of the phase evolution mechanism during the (NH₄)₂CO₃ leaching process, it can be seen that, in the roasted vanadium slag, vanadium in the form of calcium vanadate could be extracted with (NH₄)₂CO₃ solution, and phosphorus in the form of nC₂S-C₃P could not enter into the leaching liquor. This is beneficial to leach vanadium selectively into the liquor from the roasted vanadium slag while maintaining phosphorus in the leaching residue. However, vanadium oxide still existed in the leaching residue, which indicates that a part of vanadium-containing phases in vanadium slag could not transform into calcium vanadate during direct roasting, and vanadium loss occurred during the (NH₄)₂CO₃ leaching process. Moreover, while the CaO content was higher than 26.0 wt.%, the vanadium recovery decreased with the increase in CaO content. Therefore, the roasting process of vanadium slag with high CaO contents still requires further investigation to promote the maximum transformation of vanadium-containing phases into extractable phases.

4. Conclusions

In this work, the results of the phase composition of vanadium slags with different CaO contents indicate that CaFeSiO₄, Ca₂MgSi₂O₇, Ca₂SiO₄, CaTiO₃, and nC₂S-C₃P could be formed by increasing CaO content according to XRD and SEM/EDS analyses. With the increase in CaO content from 1.63 to 31.56 wt.%, vanadium concentrated as (Fe, Mn, Mg)V₂O₄ in the spinel phase in vanadium slags, while a part of titanium concentrated and transformed into CaTiO₃. The main existence form of phosphorus element was Ca₃(PO₄)₂, which subsequently reacted with Ca₂SiO₄ and formed into nC₂S-C₃P solid solution, and the value of n increased from 2 to 6 with the increase in CaO content from 17.52 to 31.56 wt.%. Vanadium and phosphorus were concentrated in different phases, which is beneficial to separate phosphorus from the vanadium slag with high CaO content in the subsequent process.

The results of phase evolution during direct roasting and (NH₄)₂CO₃ leaching processes indicate that Fe₂TiO₅, SiO₂, Ca₃V₂O₈, Ca₁₀V₆O₂₅, Ca₂V₂O₇, Fe₂O₃, V₂O₅, V₆O₁₃, and CaCO₃ could be formed in the slags. Fe₂O₃, SiO₂, Ca₃V₂O₈, Ca₁₀V₆O₂₅, and Ca₂V₂O₇ were the main phases (>6 wt.%) formed during the roasting process, whereas V₂O₅, V₆O₁₃, and Fe₂TiO₅ were the minor phases formed in roasted slag. A part of spinel phase FeV₂O₄ oxidized and reacted with Ca₂SiO₄ to produce calcium vanadate (Ca₃V₂O₈, Ca₁₀V₆O₂₅, and Ca₂V₂O₇), which is soluble in (NH₄)₂CO₃ aqueous solution. Moreover, although the CaO content was higher than 26.0 wt.%, it was not conducive to the formation of calcium vanadate, and the vanadium recovery decreased with the increase in CaO content. In addition, (NH₄)₂CO₃ aqueous solution can restrain the leaching of C₃P from the nC₂S-C₃P solid solution in the directly roasted vanadium slags with high CaO content.