Coupled Effect of Valuable Components in High-Chromium Vanadium-bearing Titanomagnetite during Oxidization Roasting

Abstract

TiO₂, V₂O₅, and Cr₂O₃ are the three characteristic and valuable components of high-chromium vanadium-bearing titanomagnetite. The coupled effect of the three components during oxidation roasting is investigated in the work. The ordinary iron ore together with analytically pure TiO₂, V₂O₅, and Cr₂O₃ is used to prepare pellet samples and two typical on-site iron ores are used to verify the analyses. TiO₂, V₂O₅, and Cr₂O₃ in one-component pellet form iron titanium solid solution, single V₂O₃, and iron chromium solid solution respectively to destroy the integrity of hematite recrystallization and restrain a wide range of connected crystal. TiO₂ is conductive to form high melting point slag and increase the amount of liquid slag. As increased TiO₂, V₂O₅, and Cr₂O₃, the compressive strength (CS) of pellet decreases. The reduction swelling index (RSI) of pellet is improved by TiO₂ but worsened by V₂O₅ and Cr₂O₃. The decrease of CS is aggravated seriously by the coupled effect of TiO₂&V₂O₅, and RSI of TiO₂&V₂O₅ pellet is higher than TiO₂ pellet but lower than V₂O₅ pellet. CS of TiO₂&V₂O₅&Cr₂O₃ pellets is worse than one-component TiO₂, V₂O₅, and Cr₂O₃ pellet but better than TiO₂&V₂O₅ pellet, which is attributed to the new generated vanadium chromium solid solution cutting down the destroy of single V₂O₃ grains and remitting the decrease of CS more or less. The verified analyses of two typical on-site iron ores indicate that the obtained coupled effects of valuable components in high-chromium vanadium-bearing titanomagnetite during oxidation roasting are credible.

1. Introduction

Vanadium–bearing titanomagnetite (VT) is a complex iron ore mainly concentrated in Russia, China, North Africa, and America etc.¹⁾ According to the content of Cr₂O₃, VT is divided into two major types of ordinary VT and high-chromium vanadium–bearing titanomagnetite (HCVT). HCVT has a high comprehensive utilization value for bearing titanium, vanadium and chromium.²⁾ In China, Hongge mineral deposit with HCVT’s reserve of 3.55 billion tons is also widely recognized as the biggest chromium-bearing deposit in China, the reserve of chromium is 900 million tons accounting for 68% chromium reserve in China.^3,4)

At present, HCVT is smelt in blast furnace (BF),⁵⁾ meanwhile, gas-based reduction with rapid reduction rate and clean production applied to dispose HCVT has attracted much attention.^6,7,8) Lots of studies on HCVT have been conducted by many researchers.^9,10,11) Xuewei et al.⁹⁾ discussed the effect of Cr₂O₃ on viscous properties of Ti-bearing BF slag. Gongjin et al.¹⁰⁾ gained the non-isothermal reduction kinetic of HCVT pellet by simulating BF lumpy zone. A new gas-based reduction-melting process for HCVT is proposed by authors’ team, through which, the Ti-bearing slag together with the iron containing V and Cr is obtained.^6,12,13)

No matter BF or gas-based shaft furnace, pellet as one major burden presents many advantages, such as uniform size, high physical strength, and low degradation. Metallurgical performances of pellet play a vital role in increasing productivity, therefore, many researchers pay much attention to them such as compressive strength (CS) and reduction swelling index (RSI) affected by inner structure and chemical composition.^14,15,16) Sirinivas et al.¹⁴⁾ pointed that magnesioferrite (MgO·Fe₂O₃) could be attributed to the improved performance of pellet. Sharma et al.¹⁵⁾ reported that CaO, MgO, SiO₂, and Al₂O₃ have significant effects on the swelling of pellet.

In the present researches on HCVT, many efforts have been made to understand slag viscous properties, thermodynamic and kinetic.^{6,7,8,9,10,11,12,13)} TiO₂, V₂O₅, and Cr₂O₃ are considered as three characteristic components of HCVT, but few descriptions are aimed at its coupled effects during HCVT oxidation roasting. So in this work, the coupled mechanisms of TiO₂, V₂O₅, and Cr₂O₃ on CS, RSI, inner structure, and phase composition of HVCT pellet are investigated.

2. Experimental

2.1. Raw Materials

The base ordinary iron ore together with analytically pure TiO₂, V₂O₅, and Cr₂O₃ is applied to prepare pellet sample. Bentonite is added with 1.5 wt%. Two typical on-site VT from Panzhihua Steel (Panzhihua VT) and HCVT from Jianlong Steel (Jianlong HCVT) are used to verify the analyses. Main chemical composition of iron ores and bentonite are listed in Tables 1 and 2, respectively. The total value of bentonite composition is about 92%. The residual 8% is a small amount of adsorbed water and burning loss. All the raw materials are screened and the particle size is less than 74 μm.

Table 1. Main chemical composition of iron ore (wt%).

Iron ore	TFe	FeO	CaO	SiO2	MgO	TiO2	V2O5	Cr2O3
Base	65.02	23.90	0.29	8.34	0.29	–	–	–
Panzhihua VT	56.21	26.21	0.65	5.17	1.14	12.36	0.42	–
Jianlong HCVT	62.45	27.29	0.21	2.69	0.71	5.05	1.03	0.58

Table 2. Main chemical composition of bentonite (wt%).

SiO2	MgO	Al2O3	CaO	Na2O	K2O
67.45	4.61	14.47	2.47	1.68	1.19

2.2. Experimental Scheme

In order to ensure the experiment reliability, the chemical composition of VT from typical mineral deposits and steel plants are surveyed, it is found that the content (wt%) of TiO₂, V₂O₅, and Cr₂O₃ is 0–16%, 0–1.0%, and 0–0.8%, respectively. Meanwhile, considering the chemical compositions of Panzhuhua VT and Jianlong HCVT, the one-component (TiO₂, V₂O₅, Cr₂O₃), two-component (TiO₂&V₂O₅), and three-component (TiO₂&V₂O₅&Cr₂O₃) tests are carried out, the batching scheme is listed in Table 3.

Table 3. Batching scheme in the work (wt%).

One	TiO2				V2O5			Cr2O3
	2.50 5.00 7.50 10.00 12.50 15.00 16.00				0.25	0.50		0.20		0.40
					0.75	1.00		0.60		0.80
Two	TiO2		V2O5		TiO2	V2O5		TiO2		V2O5
	5.00		0.25		10.00	0.25		12.50		0.25
	5.00		0.50		10.00	0.75		12.50		0.50
	5.00		0.75		10.00	0.5		12.50		0.75
	5.00		1.00		10.00	1.00		12.50		1.00
Three	TiO2	V2O5		Cr2O3	TiO2		V2O5		Cr2O3
	12.50	0.50		0.20	5.00		1.00		0.20
	12.50	0.50		0.40	5.00		1.00		0.40
	12.50	0.50		0.60	5.00		1.00		0.60
	12.50	0.50		0.80	5.00		1.00		0.80

Green pellets are prepared by a laboratory balling disc with a diameter of 1000 mm, an edge height of 200 mm, and a tilting angle of 45° at 18 rpm. The moisture in green pellet is around 8%. The green pellets with diameter of 10–12.5 mm are roasted in air using a muffle furnace. Through the preparatory test, it is known that the pellet without reagent has a relatively sufficient oxidation consolidation when roasting for 25 min at 1250°C. So the green pellets are roasted at those conditions to prepare oxidized pellets. The pellet sample after roasting was air-cooled to ambient temperature. CS and RSI of oxidized pellets are detected by ISO4700 and ISO4689: 2007 respectively. Pellet samples are analyzed by FACTSAGE 7.0 package, Qwin of Leica optical microscope, XRD, and SEM-EDS.

3. Results and Discussion

3.1. Effect of TiO₂

Effects of TiO₂ on CS and RSI are described in Fig. 1. As TiO₂ increased, the decreasing tendency of CS is divided into two sections (I and II). The CS of base pellet is high to 4913 N, but it sharply decreases to 3666 N when TiO₂ is 2.5%. Further increasing TiO₂ to 16.0%, CS is 2287 N. A 1.0% increase in TiO₂ will decrease 499 N and 102 N of CS corresponding to section I and II. However, RSI is improved by TiO₂ and a 1.0% increase will decrease 0.29% of RSI.

Fig. 1.

Effects of TiO₂ on the CS and RSI.

XRD analyses of TiO₂ pellets are shown in Fig. 2. The main phase in base pellet is hematite (Fe₂O₃) (Fig. 2(a)). In Figs. 2(b) and 2(c), Ti is observed to be present in the forms of hematite solution Fe₉TiO₁₅ (4Fe₂O₃·FeTiO₃), ilmenite (FeTiO₃), and geikielite (MgTiO₃). According to early literature,¹⁷⁾ the FeO–Fe₂O₃–TiO₂ system contains three major solid solution series of orthorhombic ferropseudobrookite-pseudobrookite (FeTi₂O₅–Fe₂TiO₅), rhombohedral illmenite-hematite (FeTiO₃–Fe₂O₃) and spinel ulvospinel-magnetite (Fe₂TiO₄–Fe₃O₄). The illmenite-hematite series develops a miscibility gap below 800°C and the two stoichiometric compounds Fe₂O₃ and FeTiO₃ can potentially form below 400°C. Therefore, reactions during oxidation should proceed as Eqs. (1), (2), (3), (4).   

$$4\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+{\text{O}}_2=6{\text{Fe}}_2{\text{O}}_3$$(1)

  

$${\text{Fe}}_{\text{3}}{\text{O}}_{\text{4}}+{\text{TiO}}_{\text{2}}={\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}+{\text{FeTiO}}_{\text{3}}$$(2)

  

$${\text{4Fe}}_{\text{2}}{\text{O}}_{\text{3}}+{\text{FeTiO}}_{\text{3}}={\text{Fe}}_{\text{9}}{\text{TiO}}_{\text{15}}$$(3)

  

$$\text{MgO}+{\text{TiO}}_{\text{2}}\hspace{0.17em}={\text{MgTiO}}_{\text{3}}$$(4)

Fig. 2.

XRD patterns of TiO₂ pellets.

Image analysis studies by Leica QWin of base pellet and TiO₂ pellets reveal that pure hematite and iron titanium solid solution (Fe–Ti solid solution), and silicate melt are the major phases in pellets. As Fig. 3 shown, the amount of pure hematite decreases with increasing TiO₂ but Fe–Ti solid solution increases, meanwhile, porosity becomes more, which are bad for continuous hematite recrystallization resulting in terrible CS.

Fig. 3.

Distribution of different phases in TiO₂ pellets.

Empirical equation between CS and porosity is given as Eq. (5).¹⁸⁾ It is evident that increasing porosity will lead to decreasing CS.   

$$CS=K\cdot d^{-\alpha }\cdot exp(-\beta p)$$(5)

Where, K, α, and β are coefficients, –; d is grain radius, cm; p is porosity, %.

On the other hand, according to Griffith microcrack theory,¹⁹⁾ the relationship between critical fracture stress (σ) and elasticity modulus (E) is shown as Eq. (6), the empirical equation of E and porosity (p) is given as Eq. (7). When p is small, the part of K₂p² can be omitted and a linear relationship between E and p is obvious. As p increased, E of pellet is weakened accordingly. In addition, pores usually exist at polycrystalline interfacial boundary leading to stress concentration and then cracks form easily, which both result in increasing half-length of crack (C). Therefore, the critical fracture stress (σ) of pellet decreases, CS of pellet becomes bad.   

$$\sigma =Y\sqrt{E\gamma /C}$$(6)

  

$$E/E_0=1-K_{1}p+K_2{p}^2$$(7)

Where, σ is critical fracture stress, MPa; Y, K₁, and K₂ are coefficient, –; E is elasticity modulus, GPa; E₀ is elasticity modulus without pore, GPa; γ is surface energy, J; C is half-length of crack, m; p is porosity, %.

To further verify the effect of TiO₂ on CS, pellets are analyzed by SEM-EDS. In base pellet (Fig. 4(a)), there are only two main phases, the white (Point A) is pure hematite and its grain is round and connected with each other tightly, and the dark gray (Point B) is silicate slag acting as bonding phase. The inner structure of base pellet is compact, and the oxidation of Fe₃O₄ to Fe₂O₃ is sufficient which avoids generating the fayalite (2FeO·SiO₂) with a low melting point. So CS of base pellet is good. When TiO₂ is 2.5% (Fig. 4(b)), pores in pellet are obviously more and bigger than base pellet, the shape of hematite grain is changed from round to whisker, and the structure of pellet is loose. Therefore, CS decreases dramatically. Further increasing TiO₂ (Figs. 4(c) and 4(d)), pores with increasing amount and diameter distribute unevenly. On the other hand, the gray white phase of Fe–Ti solid solution (Point C) appears and embeds into hematite grain, which destroys the integrity of hematite, and hematite cannot form a wide range of crystal bridge connection. Especially, the amount of gray white phase increases as increased TiO₂. As a result, CS is worsened by TiO₂. The distributions of elements in 15%TiO₂ pellet are shown in Fig. 5. It is further revealed that Fe–Ti solid solution really forms and disperses in the hematite grain.

Fig. 4.

SEM-EDS analyses of TiO₂ pellets.

Fig. 5.

Distribution of elements in 15%TiO₂ pellet.

FACTSAGE 7.0 package is applied to further evaluate the changes of RSI in air atmosphere. FToxid and FactPs database are used. The total pressure is one bar and the equilibrated atmosphere partial pressure of oxygen is 0.21 bar. The calculated temperature is the experimental roasting temperature of 1250°C. The possible species of solid phases are considered, such as spinel, monoxide, clinopyroxene, wollastonite, melilite, olivine, rutile, ilmenite, pseudobrookite, perovskite, M₂O₃ (corundum) and so on.

In acid pellets, reduction is accompanied by the reaction between Fe²⁺ and SiO₂ to form low melting point phase, fayalite (2FeO·SiO₂) that melts at 1188°C. High swelling can be attributed to the plastic or mobile nature of low melting point slag that provides a medium for absorbing reduction stresses by increased distances between particles.¹⁶⁾ But TiO₂ increases the slag melting point as described in Fig. 6. And high melting point slag will produce sufficient bonding strength to restrain swelling behaviors.²⁰⁾ So RSI of pellet decreases with the increase of TiO₂. In addition, pellets having more amount of liquid phase generally have higher RSI, similar observations are made by Frazer et al.²¹⁾ From Fig. 7, it is evident that the liquids appears to decrease as TiO₂ increased, which also demonstrates that RSI is improved by TiO₂.

Fig. 6.

Effect of TiO₂ on slag melting point.

Fig. 7.

Effect of TiO₂ on liquid slag amount.

3.2. Effect of V₂O₅

Effects of V₂O₅ on CS and RSI of pellet are shown in Fig. 8. With the increase of V₂O₅ from base to 0.25%, CS has a sharp decrease from 4913 N to 3667 N. Further increasing V₂O₅ to 1.00%, CS decreases to 2582 N. The CS change tendency of V₂O₅ pellet is also divided into two sections. A 1.00% increase in V₂O₅ will decrease 4984 N and 1447 N of CS corresponding to section I and II. RSI is deteriorated by V₂O₅ and a 1% increase will increase 5.14% of RSI.

Fig. 8.

Effects of V₂O₅ on the CS and RSI.

XRD patterns of V₂O₅ pellets are given in Fig. 9. V is seen to be presented in the form of V₂O₃. The reaction should proceed as Eq. (8). Similar result has been found by Y Wang.²²⁾ Single V₂O₃ grains disperse in pellet which restrains the diffusion among lattices and the growth of grains, and then CS decreases. From the distribution of pores in V₂O₅ pellets, it is obvious that porosity increases with the increasing V₂O₅. According to Eqs. (5), (6), and (7), CS goes down concomitantly.   

$${\text{4Fe}}_{\text{3}}{\text{O}}_{\text{4}}+\hspace{0.17em}{\text{V}}_{\text{2}}{\text{O}}_{\text{5}}={\text{6Fe}}_{\text{2}}{\text{O}}_{\text{3}}+\hspace{0.17em}{\text{V}}_{\text{2}}{\text{O}}_{\text{3}}$$(8)

Fig. 9.

XRD patterns of V₂O₅ pellets.

SEM-EDS analyses of V₂O₅ pellets are described in Fig. 10. Similar to the results from optical microscope (Fig. 10), the amounts and diameter of pores increase sharply when increasing V₂O₅. The structure is loose. Meanwhile, hematite can’t form connected crystal and many single grains with different diameters diffuse in the pellet. All these will lead to a terrible CS. From Figs. 10(a) and 10(b), the structures of pellets with 0.25% and 0.50% V₂O₅ are similar, so CS has no obvious changes, conforming to the results of Fig. 8. Through the EDS analysis of Point A, it is initially speculated as a mixture of Fe₂O₃, FeO, SiO₂, and V₂O₃. In addition, reduction of pellet is the process of formation and growth of metallic particle. According to the kinetic of grain growth,²³⁾ the small boundary curvature and big excess free energy can result in the big crystal active. The hematite grains in V₂O₅ pellets are not round and have many sharp corners leading to big crystal active and heterogeneous surface activity. Then the metallic iron forms partly and grows directionally due to the heterogeneous activity and different nucleation ability of grain surface during the reduction. Therefore RSI of V₂O₅ pellet is worsened. Distribution of elements in 1.00%V₂O₅ pellet is shown in Fig. 11 and V disperses in the whole pellet including iron oxides and slag.

Fig. 10.

SEM-EDS analyses of V₂O₅ pellets.

Fig. 11.

Distribution of elements in 1.00%V₂O₅ pellet.

3.3. Effect of Cr₂O₃

Effects of Cr₂O₃ on the CS and RSI of pellet are given in Fig. 12. CS of base pellet is high to 4913 N, but it decreases to 3524 N obviously when Cr₂O₃ is 0.2%. CS has few changes when Cr₂O₃ is 0.2%–0.6%. A 1.0% increase in Cr₂O₃ will decrease 6945 N and 1767 N of CS corresponding to section I and II. RSI is deteriorated by Cr₂O₃ and a 1% increase will increase 2.29% of RSI.

Fig. 12.

Effects of Cr₂O₃ on the CS and RSI.

XRD analyses of Cr₂O₃ pellets are seen in Fig. 13. During oxidation, Cr₂O₃ and Fe₂O₃ form a hematite solid solution of (Fe_0.6Cr_0.4)₂O₃, and the reaction should proceed as Eq. (9). From the distribution of pores in Cr₂O₃ pellets, it is obvious that the amount of pores increases as increased Cr₂O₃, which has terrible influences on CS.   

$$2/5\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3+3/5\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3={\left(\mathrm{F}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{r}}_{0.4}\right)}_2{\mathrm{O}}_3$$(9)

Fig. 13.

XRD patterns of Cr₂O₃ pellets.

SEM-EDS analyses of Cr₂O₃ pellets are given in Fig. 14. According to the EDS analysis of Point A_, it is speculated as a mixture of Fe₂O₃, FeO, SiO₂, and (Fe_0.6Cr_0.4)₂O₃. When Cr₂O₃ is 0.2% (Fig. 14(a)), the connection force between grains is weakened, the amount as well as diameter of pores increases, and the structure is not compact at all. Additionally, the slag (Point B) distributes in segregation and a part of slag unites with each other resulting in heterogeneous inner structure. The two combined actions are bad for CS. The structure of 0.4%Cr₂O₃ pellet has no obvious changes corresponding to the slight decrease of CS. Further increasing Cr₂O₃ to 0.8%, the hematite grains with burr shape boundary generate, the connected crystal bridges are destroyed, so CS dramatically decreases. From the distribution of elements in Cr₂O₃ pellets (Fig. 15), it is evident that most of Cr combines with Fe.

Fig. 14.

SEM-EDS analyses of Cr₂O₃ pellets.

Fig. 15.

Distribution of elements in 0.8%Cr₂O₃ pellet.

Effect of Cr₂O₃ on liquids is evaluated by FACTSAGE 7.0 (Fig. 16). As said before, pellets having more liquid phase generally have higher RSI. The liquid phase increases as Cr₂O₃ increased from 0% to 0.2%, so RSI is worsened. Further increasing Cr₂O₃, the liquid phase has no obvious changes but RSI is deteriorated more, it may because of the un-sufficient oxidation and incomplete hematite recrystallization by the function of increasing Cr₂O₃.

Fig. 16.

Effect of Cr₂O₃ on the liquid slag amount.

Above all, effects of single factorial TiO₂, V₂O₅ and Cr₂O₃ on CS and RSI are summarized in Tables 4 and 5, respectively. All CS changes are divided into two sections. TO₂ and Cr₂O₃ have the smallest and biggest effects on decreasing CS separately. RSI is improved by TiO₂ but worsened by V₂O₅ and Cr₂O_3. The effect of Cr₂O₃ on RSI is the most significant.

Table 4. Effects of TiO₂, V₂O₅ and Cr₂O₃ on CS.

	Effect	Section I		Section II
		Content/%	CS change rate*/N	Content/%	CS change rate/N
TiO2	Decrease	0–2.5	499	2.5–16.0	102
V2O5		0–0.25	4984	0.25–1.00	1447
Cr2O3		0–0.2	6945	0.2–0.8	1767

Note: CS (RSI) change rate = CS (RSI) change/Content change

Table 5. Effects of TiO₂, V₂O₅ and Cr₂O₃ on RSI.

	Effect	Content/%	RSI change rate*/%
TiO2	Decrease	0–16%	0.29
V2O5	Increase	0–1.00%	5.14
Cr2O3	Increase	0–0.8%	2.29

3.4. Coupled Effect of TiO₂&V₂O₅

Effects of TiO₂&V₂O₅ on the CS of pellet are given in Fig. 17. The CS of TiO₂&V₂O₅ pellet is lower than base pellet, TiO₂ pellet, and V₂O₅ pellet. The decrease of CS is further aggravated by the coupled effect of TiO₂&V₂O₅. And 12.5%TiO₂&1.0%V₂O₅ pellet owns the worst CS under experimental ranges.

Fig. 17.

Effects of TiO₂&V₂O₅ on the CS.

Effects of TiO₂&V₂O₅ on the RSI of pellet are described in Fig. 18. RSI of TiO₂&V₂O₅ pellet is higher than TiO₂ pellet but lower than V₂O₅ pellet, which further illustrates the one-component test result that RSI is improved by TiO₂ but aggravated by V₂O₅. RSI of 12.5%TiO₂&V₂O₅ pellet and TiO₂&0.25%V₂O₅ pellet are lower than base pellet, which suggests that the improvement of RSI by TiO₂ is more significant than the aggravation by V₂O₅ under these conditions.

Fig. 18.

Effects of TiO₂&V₂O₅ on the RSI.

The 12.5%TiO₂&1.0%V₂O₅ pellet is analyzed by XRD, shown in Fig. 19. Compared with TiO₂ pellet and V₂O₅ pellet, a new iron titanium solid solution of pseudobrookite Fe₂TiO₅ forms and the reaction should proceed as Eq. (10).   

$$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{T}\mathrm{i}{\mathrm{O}}_2=\mathrm{F}{\mathrm{e}}_2\mathrm{T}\mathrm{i}{\mathrm{O}}_5$$(10)

Fig. 19.

XRD patterns of TiO₂&V₂O₅ pellets.

SEM-EDS analyses of TiO₂&V₂O₅ pellets are described in Fig. 20. The bright white (Point C) is pure hematite with a little Ti and V, and the light gray (Point A) is Fe–Ti solid solution with a little V. Base on the EDS analysis of Point A, it is initially speculated as a mixture of FeTiO₃, MgTiO₃, Fe₉TiO₁₅, Fe₂TiO₅, and V₂O₃. Compared with base pellet, the hematite recrystallization is un-sufficient and the wide range of connected crystal can’t form in TiO₂&V₂O₅ pellets, meanwhile, the inner structures are heterogeneous and loose, resulting in bad CS. Contrastively analyzing (a) and (d), (b) and (e), (c) and (f), the amounts of single and fine grains increase seriously as increased V₂O₅. As for (a), (b), (c) and (d), (e), (f), the light gray (Point A) marked as Fe–Ti solid solution increases obviously which prevents hematite from glomerocryst and growth. In addition, distribution of elements in 12.5%TiO₂&1.0%V₂O₅ pellet is given as Fig. 21, it is known that most of V is together with Ti. Fe–Ti solid solutions connecting with single V₂O₃ grains embed into hematite. Naturally, pellet strength destroy is enhanced by TiO₂&V₂O₅.

Fig. 20.

SEM-EDS analyses of TiO₂&V₂O₅ pellets.

Fig. 21.

Distribution of elements in 12.5%TiO₂&1.0%V₂O₅ pellet.

3.5. Coupled Effect of TiO₂&V₂O₅&Cr₂O₃

Two series of TiO₂&V₂O₅&Cr₂O₃ pellets are investigated to evaluate the coupled effect by three components. The content of TiO₂ and V₂O₅ is similar to the Panzhihua VT (12.5%TiO₂&0.5%V₂O₅) and Jianlong HCVT (5.0%TiO₂&1.0%V₂O₅). The results of CS and RSI are given in Figs. 22(a) and 22(b), respectively. When Cr₂O₃ is less than 0.6%, CS of 5.0%TiO₂&1.0%V₂O₅&Cr₂O₃ pellet is more than 12.5%TiO₂&0.5%V₂O₅&Cr₂O₃ pellet, further increasing Cr₂O₃, CS of 12.5%TiO₂&0.5%V₂O₅&Cr₂O₃ pellet has a higher CS instead.

Fig. 22.

CS (a) and RSI (b) of TiO₂&V₂O₅&Cr₂O₃ pellets.

Additionally, compared with the results of TiO₂&V₂O₅ pellets, it is known that CS of 12.5%TiO₂&0.5%V₂O₅&Cr₂O₃ pellet is higher than 12.5%TiO₂&0.5%V₂O₅ pellet, the reason can be speculated that a new vanadium chromium solid solution (V–Cr solid solution) may generate when V and Cr is existing and it should weaken the destroy of single V₂O₃ grains and restrain the decrease of CS more or less. On the other hand, when Cr₂O₃ is less than 0.4%, CS of 5.0%TiO₂&1.0%V₂O₅&Cr₂O₃ pellet is more than 5.0%TiO₂&1.0%V₂O₅ pellet, further increasing Cr₂O₃, 5.0%TiO₂&1.0%V₂O₅ pellet owns a higher CS instead, the excessive Cr₂O₃ starts to play an biggest destroy on CS (said in Table 4). Meanwhile, it is worth mentioning that CS of TiO₂&V₂O₅&Cr₂O₃ pellets are all worse than single factorial TiO₂, V₂O₅, Cr₂O₃ pellet.

RSI of all the TiO₂&V₂O₅&Cr₂O₃ pellets shows an increasing tendency and Cr₂O₃ pellet has a medium RSI. From the results from single factorial tests, it is seen that TiO₂ and V₂O₅ has a positive and negative effect on RSI respectively. So 12.5%TiO₂&0.5%V₂O₅&Cr₂O₃ pellet has a better RSI.

XRD analyses of base pellet and two typical TiO₂&V₂O₅&Cr₂O₃ pellets are seen in Fig. 23. A new V–Cr solid solution of (Cr_0.15V_0.85)₂O₃ forms and the reaction should proceed as Eq. (11).   

$$3/20\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_3+17/20{\mathrm{V}}_2{\mathrm{O}}_3={\left(\mathrm{C}{\mathrm{r}}_{0.15}{\mathrm{V}}_{0.85}\right)}_2{\mathrm{O}}_3$$(11)

Fig. 23.

XRD patterns of TiO₂&V₂O₅&Cr₂O₃ pellets.

SEM-EDS analyses of TiO₂&V₂O₅&Cr₂O₃ pellets are seen in Fig. 24. The structure of TiO₂&V₂O₅&Cr₂O₃ pellets are apparently loose and plenty of single grains with different diameter disperse disorderly, leading to a bad CS. Additionally, it is obvious that four main phases are in TiO₂&V₂O₅&Cr₂O₃ pellets: hematite with a little Ti, V, and Cr (Point A), Fe–Ti solid solution with a little V and Cr (Point B), silicate slag with a little V and Cr (Point C). Combined with XRD and SEM-EDS analyses, the new phase of (Cr_0.15V_0.85)₂O₃ really forms and may exist in the slag which should increase the melting point of slag, enhance the crystal connection, and weaken the destroy of single V₂O₃ grains, remitting the decrease of CS more or less. Distribution of elements in 5%TiO₂&1.0%V₂O₅&0.8%Cr₂O₃ pellet are shown as Fig. 25. Most of V is still combined with Ti, and the others are connected with Cr which gives another evidence for the existence of V–Cr solid solution.

Fig. 24.

SEM-EDS analyses of 5.0%TiO₂&1.0%V₂O₅&0.8%Cr₂O₃ pellet (a) and12.5%TiO₂&0.5%V₂O₅&0.8%Cr₂O₃ pellet (b).

Fig. 25.

Distribution of elements in 5%TiO₂&1.0%V₂O₅&0.8%Cr₂O₃ pellet.

Considering the CS changes, XRD, SEM-EDS, and map analyses, the illustration on destroying pellet strength is summarized as Fig. 26. In base pellet, the structure is compact and hematite recrystallization is sufficient. In the TiO₂, V₂O₅, and Cr₂O₃ pellet, Fe–Ti solid solution, single V₂O₃, and Fe–Cr solid solution destroy the intact hematite recrystallization and restrain the wide range of connected crystal, the hematite grains are burr shape, the amounts of pore increase, and all these have terrible effects on CS. In TiO₂&V₂O₅ pellets, Fe–Ti solid solution connecting with single V₂O₃ grain embeds into hematite, naturally, pellet strength destroying is enhanced by TiO₂&V₂O₅. In TiO₂&V₂O₅&Cr₂O₃ pellet, the structure is loose and many single grains disperse in pellet, CS is seriously aggravated by TiO₂&V₂O₅&Cr₂O₃. But the new V–Cr solid solution will increase slag melting point and weaken the destroying by single V₂O₃ grains, which should restrain the decrease of CS more or less.

Fig. 26.

Illustration on destroying pellet strength by TiO₂, V₂O₃, Cr₂O₃, TiO₂&V₂O₅, and TiO₂&V₂O₅&Cr₂O₃.

3.6. Verification Experiment

Two typical on-site iron ores are used to verify the analyses above. The content of TiO₂ and V₂O₅ in Panzhuhua VT is 12.36% and 0.42% respectively. The content of TiO₂, V₂O₅, and Cr₂O₃ in Jianlong HCVT is 5.05%, 1.03%, and 0.58% respectively. CS and RSI of the two pellets are shown as Fig. 27. Due to the existing of Cr, a V–Cr solid solution forms to enhance the connection of grains and cut down the destroying of single V₂O₃ grains, so CS of Jianlong HCVT pellet (2351 N) is a little higher than Panzhihua VT pellet (2018 N), And the result agrees well with the CS changes of 5.0%TiO₂&1.0%V₂O₅&0.6%Cr₂O₃ pellet (2485 N) and 12.5TiO₂&0.5%V₂O₅ pellet (2200 N). Owing to much more TiO₂ of 12.36% and less V₂O₅ of 0.42% in Panzhihua VT, furthermore RSI is improved by TiO₂ but worsened by V₂O₅, so RSI of Panzhihua VT pellet (7.53%) is better than Jianlong HCVT pellet (16.29%), and the result is also consistent with the RSI changes of 12.5TiO₂&0.5%V₂O₅ pellet (6.69%) and 5.0%TiO₂&1.0%V₂O₅&0.6%Cr₂O₃ pellet (15.06%).

Fig. 27.

CS (a) and RSI (b) of Panzhihua VT pellet and Jianlong HCVT pellet.

XRD patterns and SEM-EDS analyses of Panzhihua VT pellet and Jianlong HCVT pellet are presented in Figs. 28 and 29, respectively. Ti is observed to be present in the form of Fe₉TiO₁₅, FeTiO₃, Fe₂TiO₅ and MgTiO₃. V and Cr generate solid solutions of (Cr_0.15V_0.85)₂O₃ and (Cr_1.3Fe_0.7O₃). The other V is in the form of V₂O₃. All the phases can be also found in 12.5%TiO₂&0.5%V₂O₅ pellet and 5.0%TiO₂&1.0%V₂O₅&0.6%Cr₂O₃ pellet. In addition, it is seen that the structure of Jianlong HCVT pellet is more compact than that of Panzhihua VT pellet resulting in a better CS. Similarly, the structure of 5.0%TiO₂&1.0%V₂O₅&0.6%Cr₂O₃ pellet (Fig. 24(a)) is also tighter than that of 12.5%TiO₂&0.5%V₂O₅ pellet (Fig. 20(c)). Meanwhile, the slag bearing a little V and Cr (Point F) is in Jianlong HCVT pellet, which is also detected in 5.0%TiO₂&1.0%V₂O₅&0.6%Cr₂O₃ pellet.

Fig. 28.

XRD patterns of Panzhihua VT pellet (a) and Jianlong HCVT pellet (b).

Fig. 29.

SEM-EDS analyses of Panzhihua VT pellet ((a) and (c)) and Jianlong HCVT pellet ((b) and (d)).

Generally, the results on the verifications of Panzhihua VT and Jianlong HCVT, including changes of CS and RSI, main phases by XRD analysis and inner structure by SEM-EDS analysis, agree well with the previous designed experiments and illustrate that the obtained coupled effects by TiO₂, V₂O₅ and Cr₂O₃ during oxidation roasting are credible.

4. Conclusions

(1) TiO₂, V₂O₅, and Cr₂O₃ in one-component pellet form Fe–Ti solid solution, single V₂O₃, and Fe–Cr solid solution respectively, which destroy the integrity of hematite recrystallization and restrain the wide range of connected crystal. CS decreases as TiO₂, V₂O₅, and Cr₂O₃ increased. TO₂ and Cr₂O₃ have the smallest and biggest effects on decreasing CS separately.

(2) With the increasing TiO₂, the slag melting point increases and the amount of liquid phase appears to decrease. RSI is improved by TiO₂ but worsened by V₂O₅ and Cr₂O₃, and the effect of Cr₂O₃ on RSI is the most significant.

(3) The decrease of CS is aggravated seriously by the coupled effect of TiO₂&V₂O₅, and 12.5%TiO₂&1% V₂O₅ pellet owns the worst CS under experimental ranges. RSI of TiO₂&V₂O₅ pellet is higher than that of TiO₂ pellet but lower than that of V₂O₅ pellet.

(4) CS of TiO₂&V₂O₅&Cr₂O₃ pellets is worse than that of TiO₂ pellet, V₂O₅ pellet and Cr₂O₃ pellet, but it is better than that of TiO₂&V₂O₅ pellet which is attributed to the new generated V–Cr solid solution increasing slag melting point, weakening the destroying by single V₂O₃, and restraining the decrease of CS more or less.

(5) The verified analyses of two typical on-site Panzhihua VT and Jianlong HCVT can agree well with the designed experiments. The obtained coupled effects by valuable components in high-chromium vanadium-bearing titanomagnetite during oxidation roasting are credible.

Acknowledgments

The authors are especially grateful to National Natural Science Foundation of China (Grant No. 51574067).

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