Effect of TiO 2 on the Sintering Behavior of Chromium-Bearing Vanadium – Titanium Magnetite

The sintering pot test was used to investigate the effect of TiO2 on the sintering behavior of chromium-bearing vanadium–titanium magnetite (CVTM) sinter. The main characterization methods of X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy-energy disperse spectroscopy (SEM-EDS), and metallographic microscopy were employed. In this study, yield, tumbler index (TI), vertical sintering speed, productivity, reduction degradation index (RDI), and reduction index (RI) were tested and calculated. The yield first increases from 82.87% to 84.37% and then decreases to 83.65%, vertical sintering speed first increases from 17.00 mm·min−1 to 23.45 mm·min−1 and then decreases to 20.61 mm·min−1, and productivity first increases from 2.33 t·m−2·h−1 to 3.14 t·m−2·h−1 and then decreases to 2.69 t·m−2·h−1 with increasing TiO2 content. The TI increases from 45.81% to 52.09%, and RDI increases from 74.99% to 96.74%, while RI decreases from 67.92% to 47.15% with increasing TiO2 content.


Introduction
Chromium-bearing vanadium-titanium magnetite (CVTM) iron ore containing Fe, Ti, Cr, and V is a special and valuable iron ore as known by the exploitation of vanadium-titanium magnetite (VTM), ilmenite, and chromite [1,2].As a kind of valuable iron ore resource, Hongge CVTM is found in the Panzhihua area in China.Similar types of ores are mainly distributed in Russia, Canada, Australia and other places [3,4].As it is known, many studies have been done for the ordinary VTM, while the studies on CVTM with complex phase compositions and different properties are scarce [5,6].Owing to the immature production technology of CVTM in the blast furnace (BF) process, the abundant quantity of such CVTM has not been exploited and efficiently used on a large scale.Therefore, it is of great importance to study the utilization of the special iron ore.
In order to use CVTM efficiently, the sintering behaviors of CVTM have been proposed by the authors' laboratory [7][8][9].In the BF process, the CVTM sinter with different basicity, coke ratios, and MgO contents have been studied.Yang [7,8] studied the effects of coke ratios and MgO on the sintering behaviors and the metallurgical properties of CVTM.Yang [9] surveyed the effect of basicity on the mineral compositions and elements migration of CVTM sinters.Particularly, titanium is one of the main valuable elements in CVTM, and titanium has an important influence on sintering properties.However, the contents of TiO 2 in these studies were lower than 2 wt %, and the sintering behaviors with the relatively higher content of TiO 2 have not been studied comprehensively in the past.Therefore, the Minerals 2018, 8, 263 2 of 14 sintering behaviors of CVTM with high TiO 2 contents are necessary to be researched, and it is of great importance to carry out in this work.
As part of continuing work to study the utilization of CVTM in the BF process, the effect of TiO 2 on sintering behavior of CVTM was explored in this paper.First, the yield, tumbler index (TI), vertical sintering speed, productivity, reduction degradation index (RDI), and reduction index (RI) were tested and calculated.The mineral compositions and microstructures of CVTM sinter with different TiO 2 contents were then studied.These results will provide theoretical and technical bases for the effective production of the CVTM sinter and facilitate its exploitation.

Raw Materials and Characterisation Methods
Hongge CVTM originates from the Panzhihua area in China.Ordinary magnetite (OM), gas-ash (GA), magnetic powder (MP), return mine (RM), lime, coke, and coal were supplied by Chengde Jianlong Iron and Steel Group Company (Chengde, China).
The X-ray fluorescence (XRF, ZSXPrimus II; Rigaku, Tokyo, Japan) was used to test the chemical compositions of raw materials.The X-ray diffraction (XRD, X' Pert Pro; PANalytical, Almelo, The Netherlands) with Cu Kα radiation (wavelength = 1.5406Å) at a setting of 40 kV and 40 mA was used to analyze the mineral phases of CVTM and CVTM sinters.The scanned range was 2θ = 5-90 • with a step of 2θ = 0.17 • and 1 s•step −1 .All of the tested powders were ground to lower than 200 µm.The scanning electron microscope (SEM, Ultra Plus; Carl Zeiss GmbH, Jena, Germany) was used to detect the microstructure of reductive CVTM sinters, and backscattering detector (BSE) was used to investigate sinter surface, and energy disperse spectroscopy (EDS) was used to acquire mineral compositions.Sinter sample was heat mounted in resin and polished by mirror finish for microstructure and mineralogy analysis.A metallographic microscope (Leica DM1750M; Leica, Cambridge, UK) was used to detect the mineralogy of CVTM sinter.

Experiments
Figure 1 shows the granulating process of CVTM sinter materials.Figure 2 shows the schematic of sinter pot test equipment including operation cabinet, gas supply, gas burner, sinter pot, data acquisition, dust extraction, and draft fan.Table 1 shows the sintering parameters of sintering pot test.Table 2 shows the experimental scheme and ingredient of sinter materials.The TiO 2 content changes with the addition of ilmenite.First, 100 kg raw materials with 4.05% fuel (coke:coal = 1) are mixed in a mixer and granulated in a granulator for 10 min, and rotation angle of granulator is in the range of ±45 • at the process of granulation as shown in Figure 1.Then, the pelletized sinter materials are filled in sinter pot with a diameter of 320 mm and a height of 700 mm, and 20 g coke is sprinkled evenly on the surface of sinter materials for ignition.Then, liquefied natural gas is lit and used to ignite the sinter materials at an ignition temperature of 1050 • C, and ignition suction is 8.00 kPa.At the end of ignition, the extraction fan was started up to make sinter suction at 12.00 kPa.Finally, the temperature of flue gas reaches the peak value, and sinter temperature begins downturn.When the temperature of flue gas is below 100 • C, sintering pot test finishes, and dust extraction fan starts up to eliminate the dust in sinter broken process.The properties of TI, particle size distribution, RDI, and RI were determined according to ISO-3271, ISO-4701, ISO-4696, and ISO-7215, respectively.  Ordinary magnetite. 2Return mine. 3 Gas-ash. 4Magnetic powder.

Productivity
The productivity of sinter is an important indicator to measure the production capacity of sintering production.The calculation equation of productivity is given in Equation (1):    1 Ordinary magnetite. 2Return mine. 3 Gas-ash. 4Magnetic powder.

Productivity
The productivity of sinter is an important indicator to measure the production capacity of sintering production.The calculation equation of productivity is given in Equation (1): where P is the unit productivity (t•m −2 •h −1 ), M is the mass of product sinters (t), Y is the yield of product sinters (%), A is the effective sintering area of sinter pot (m 2 ), and t is the sintering time (h).

Reduction Degradation Index
The RDI is a measure of disintegration property of iron ore sinter on the reduction reaction with CO at 500 • C. The RDI reflects the BF in the upper stack regions where it is mildly reducing, and temperatures are low [10].The particle sizes (+6.3 mm, +3.15 mm, −0.5 mm) of sinter after reduction are used to calculate the RDI.The standard of ISO-4696, RDI +6.3 represents the reduction intensity of iron ore sinter, and RDI +3.15 represents the RDI, and RDI −0.5 reflects the attrition strength of iron ore sinter after reduction.The equations of RDI are given in Equations ( 2)-( 4): where m D0 is the sinter weight after the reduction (g), m D1 is the weight of sinter on the 6.3 mm sieve (g), m D2 is the weight of sinter on the 3.15 mm sieve (g), and m D3 is the weight of sinter on the 0.5 mm sieve (g).

Reduction Index
The RI of sinter is based on the standard ISO-7215, and it shows the reduction conditions of sinter in BF at 900 • C. The RI of sinter is calculated via Equation (5): where X t is the RI of CVTM sinter at time t (%), ω(FeO) and ω(TFe) are the mass of FeO and total iron of CVTM sinter before reduction (%); m 1 and m 2 are the mass of CVTM sinter before reduction (g).

Chemical Assays and Characterisation
The chemical compositions of raw materials are shown in Table 3.The coke and coal breeze and chemical compositions are shown in Table 4. Table 5 shows the size distribution of CVTM, and it indicates the particle size lower than 75 µm occupies 72.47%, which indicates the CVTM ore is appropriate for granulation.Figure 3 shows the XRD pattern of CVTM, and the CVTM primarily consists of magnetite (Fe3O4), titanomagnetite (Fe2.75Ti0.25O4),ilmenite (FeTiO3), coulsonite (Fe2VO4) and chromite (FeCr2O4).The inorganic crystal structure database (ICSD) card numbers of phases in CVTM are shown in Table 6.

Effect of TiO2 on the Properties of CVTM Sinters
Figure 4 shows the XRD patterns of CVTM sinters with different TiO2 content.The main phases of CVTM sinter are magnetite, hematite (Fe2O3), perovskite (CaTiO3), coulsonite, and chromite.The peak intensity of perovskite becomes stronger with increasing TiO2 content, where the kirschsteinite (CaFeSiO4) generates with a TiO2 content higher than 7.35% and transforms to fayalite (Fe2SiO4) when the TiO2 content increases to 11.85%.Meanwhile, the magnesium ferrite (MgFe2O4) generates with a TiO2 content higher than 7.35% and generates magnesium titanate (MgTiO3) with a TiO2 content higher than 10.35%.The ionic radii of Mg 2+ (0.78 Å) and Fe 2+ (0.83 Å), as well as Ca 2+ (0.99 Å) and Mg 2+ (0.78 Å) are similar.Therefore, Mg 2+ enters the Fe3O4 lattice to generate MgFe2O4, and Mg 2+ enters the CaTiO3 lattice to generate MgTiO3.The ICSD card numbers of phases in CVTM sinters are shown in Table 6.The properties of CVTM sinter with different TiO2 content are shown in Figure 5.The vertical sinter speed of CVTM sinter first increases and then decreases with increasing TiO2 from 5.85% to 11.85%.The maximum value of sinter speed is 23.45 mm•min −1 when TiO2 content reaches 10.35%, as shown in Figure 5.In the sintering process, the oxidation reaction of magnetite and formation of liquid phase take place at low temperatures, while the ilmenite and titanomagnetite react with CaO, and generate perovskite with the addition of ilmenite under high temperature.By thermomechanical analysis, perovskite has an earlier generation than liquid phase of calcium ferrite.
Figure 5 shows the effect of TiO2 on yield and TI of CVTM sinter.It shows that the yield increases from 82.87% to 84.37%, which then drops to 83.65%, and TI increases from 45.81% to 52.09% with increasing TiO2 content.However, the results of yield and TI are different from previous findings where the TiO2 has a negative effect on the crystal boundaries [11].The productivity increases with increasing TiO2 content and reaches a peak value of 3.14 t•m −2 •h −1 .From the microstructural analysis of the CVTM sinter as shown in Figure 6, the relatively concentrated silicate bonding phases exist as connection phases among crystals of magnetite and hematite, and perovskite phases exist in silicate phase as the form of crystallization [12,13].Moreover, the hematite phases develop as crystals, and some detached hematite and magnetite phases are connected by the silicate and calcium ferrite.Hence, the silicate phases play essential roles in the bonding phase of CVTM sinter.Additionally, perovskite has a thermodynamic generation advantage compared with calcium ferrite, which is the initial melt in iron ore sintering process [14], and calcium ferrite is beneficial to the compressive strength of sinters [13].When the TiO2 content increases to 11.85%, the calcium ferrite phases exist on The properties of CVTM sinter with different TiO 2 content are shown in Figure 5.The vertical sinter speed of CVTM sinter first increases and then decreases with increasing TiO 2 from 5.85% to 11.85%.The maximum value of sinter speed is 23.45 mm•min −1 when TiO 2 content reaches 10.35%, as shown in Figure 5.In the sintering process, the oxidation reaction of magnetite and formation of liquid phase take place at low temperatures, while the ilmenite and titanomagnetite react with CaO, and generate perovskite with the addition of ilmenite under high temperature.By thermomechanical analysis, perovskite has an earlier generation than liquid phase of calcium ferrite.
Figure 5 shows the effect of TiO 2 on yield and TI of CVTM sinter.It shows that the yield increases from 82.87% to 84.37%, which then drops to 83.65%, and TI increases from 45.81% to 52.09% with increasing TiO 2 content.However, the results of yield and TI are different from previous findings where the TiO 2 has a negative effect on the crystal boundaries [11].The productivity increases with increasing TiO 2 content and reaches a peak value of 3.14 t•m −2 •h −1 .From the microstructural analysis of the CVTM sinter as shown in Figure 6, the relatively concentrated silicate bonding phases exist as connection phases among crystals of magnetite and hematite, and perovskite phases exist in silicate phase as the form of crystallization [12,13].Moreover, the hematite phases develop as crystals, and some detached hematite and magnetite phases are connected by the silicate and calcium ferrite.Hence, the silicate phases play essential roles in the bonding phase of CVTM sinter.Additionally, perovskite has a thermodynamic generation advantage compared with calcium ferrite, which is the initial melt in iron ore sintering process [14], and calcium ferrite is beneficial to the compressive strength of sinters [13].When the TiO 2 content increases to 11.85%, the calcium ferrite phases exist on the silicate phase, and the high strength of calcium ferrite enhances the strength of silicate bonding phases.With the analysis of the mineral content of CVTM sinter, the stable magnetite increases, and the glass phase decreases with increasing TiO 2 content.Combined with the XRD pattern of CVTM sinter, the volume expansion of sinter alleviates the cooling process due to the stable phases magnetite and magnesium ferrite.the silicate phase, and the high strength of calcium ferrite enhances the strength of silicate bonding phases.With the analysis of the mineral content of CVTM sinter, the stable magnetite increases, and the glass phase decreases with increasing TiO2 content.Combined with the XRD pattern of CVTM sinter, the volume expansion of sinter alleviates the cooling process due to the stable phases magnetite and magnesium ferrite.the silicate phase, and the high strength of calcium ferrite enhances the strength of silicate bonding phases.With the analysis of the mineral content of CVTM sinter, the stable magnetite increases, and the glass phase decreases with increasing TiO2 content.Combined with the XRD pattern of CVTM sinter, the volume expansion of sinter alleviates the cooling process due to the stable phases magnetite and magnesium ferrite.Figure 7 shows the particle size distribution of CVTM sinters with different TiO 2 content.When TiO 2 content is 5.85%, the particle size distribution of sinter focuses on the size less than 16 mm and occupies 62%.When TiO 2 content increases from 7.35% to 11.85%, the particle size distribution of sinter focuses on the size greater than 16 mm and occupies over 63%.The characteristics of particle size distribution of CVTM sinter indicates the improvement of sinter strength with increasing TiO 2 content.Although the perovskite has low compression strength in iron ore sinters, perovskite has high wear resistance and hardness [15].Therefore, the particle size distribution of CVTM sinter focuses on large particles.
Minerals 2018, 8, x FOR PEER REVIEW 8 of 13 Figure 7 shows the particle size distribution of CVTM sinters with different TiO2 content.When TiO2 content is 5.85%, the particle size distribution of sinter focuses on the size less than 16 mm and occupies 62%.When TiO2 content increases from 7.35% to 11.85%, the particle size distribution of sinter focuses on the size greater than 16 mm and occupies over 63%.The characteristics of particle size distribution of CVTM sinter indicates the improvement of sinter strength with increasing TiO2 content.Although the perovskite has low compression strength in iron ore sinters, perovskite has high wear resistance and hardness [15].Therefore, the particle size distribution of CVTM sinter focuses on large particles.The RDI and reduction intensity (RDI+6.3)increase with increasing TiO2 content, while the attrition strength (RDI−0.5)decreases as shown in Figure 8.In the upper stack regions of BF, the principal reduction reaction is the phase transformation of hematite to magnetite in sinter at 400 °C-600 °C.The primary cause of RDI is thought to be a crystalline transformation from rhombohedral hematite to cubic magnetite [16].The anisotropic dimension varies because the change of crystal form results in severe stresses in specific planes and cracks in the brittle matrix [17].Bristow et al. [18] found that TiO2-containing glass phase in sinter structure had further increased the severity of crack propagation during reduction, and resulted in higher RDI of sinter.From the analysis of the microstructure of the CVTM sinter with different TiO2 content, the structural stress decreases due to the homogeneous phase structure.With the decreasing content of the hematite and glass phases, the variation of the stress decreases in the low reduction temperature.Meanwhile, the crystallization of perovskite phases mainly exists in the silicate phases as the form of solid-solution, and the perovskite has little effect on bonding phase of silicate and crystal strength between hematite and magnetite.Moreover, the magnesium ferrite exists between the intervals of hematite and magnetite, and magnesium ferrite has a suppressing effect on the reduction of hematite [19].Hence, the RDI of CVTM sinter is well improved, and the permeability of lumpy zones could be improved with the application of CVTM sinter on BF.The RDI and reduction intensity (RDI +6.3 ) increase with increasing TiO 2 content, while the attrition strength (RDI −0.5 ) decreases as shown in Figure 8.In the upper stack regions of BF, the principal reduction reaction is the phase transformation of hematite to magnetite in sinter at 400 • C-600 • C. The primary cause of RDI is thought to be a crystalline transformation from rhombohedral hematite to cubic magnetite [16].The anisotropic dimension varies because the change of crystal form results in severe stresses in specific planes and cracks in the brittle matrix [17].Bristow et al. [18] found that TiO 2 -containing glass phase in sinter structure had further increased the severity of crack propagation during reduction, and resulted in higher RDI of sinter.From the analysis of the microstructure of the CVTM sinter with different TiO 2 content, the structural stress decreases due to the homogeneous phase structure.With the decreasing content of the hematite and glass phases, the variation of the stress decreases in the low reduction temperature.Meanwhile, the crystallization of perovskite phases mainly exists in the silicate phases as the form of solid-solution, and the perovskite has little effect on bonding phase of silicate and crystal strength between hematite and magnetite.Moreover, the magnesium ferrite exists between the intervals of hematite and magnetite, and magnesium ferrite has a suppressing effect on the reduction of hematite [19].Hence, the RDI of CVTM sinter is well improved, and the permeability of lumpy zones could be improved with the application of CVTM sinter on BF.
Figure 9 shows the XRD patterns of reductive CVTM sinters with different TiO 2 content at 900 • C. The main phases of reductive CVTM sinter are metallic iron (Fe), wustite (FeO), maghemite (γ-Fe 2 O 3 ), perovskite, coulsonite, magnesium ferrite, and titanium monoxide (TiO).The peak intensity of perovskite, wustite, and titanium monoxide become stronger with increasing TiO 2 content, where the hematite disappears while magnetite generates with TiO 2 higher than 7.35%.A new spinel phase Mg 2 TiO 4 forms with TiO 2 higher than 10.35%, and new phases titanomagnetite (Fe 2.5 Ti 0.5 O 4 ) and titanomaghemite (Fe 0.23 (Fe 1.95 Ti 0.42 )O 4 ) form when TiO 2 content increases to 11.85%.The ICSD card numbers of phases in reductive CVTM sinters are shown in Table 6. Figure 9 shows the XRD patterns of reductive CVTM sinters with different TiO2 content at 900 °C.The main phases of reductive CVTM sinter are metallic iron (Fe), wustite (FeO), maghemite (γ-Fe2O3), perovskite, coulsonite, magnesium ferrite, and titanium monoxide (TiO).The peak intensity of perovskite, wustite, and titanium monoxide become stronger with increasing TiO2 content, where the hematite disappears while magnetite generates with TiO2 higher than 7.35%.A new spinel phase Mg2TiO4 forms with TiO2 higher than 10.35%, and new phases titanomagnetite (Fe2.5Ti0.5O4)and titanomaghemite (Fe0.23(Fe1.95Ti0.42)O4)form when TiO2 content increases to 11.85%.The ICSD card numbers of phases in reductive CVTM sinters are shown in Table 6.The RI of sinter is influenced by phase compositions.As shown in Figure 10, the tendency of RI decreases with increasing TiO2 content.Based on the XRD analysis, the reduction of iron oxides and Ti-bearing iron oxides are carried out in the reduction process.Hence, the reduction reactions of iron oxides and titanium oxides are obstructed due to the nonreactive minerals with increasing TiO2 content.With the increase of TiO2 content, the iron grade and the quantity of iron-bearing minerals decrease while gangue minerals of perovskite and Ti-bearing spinel Mg2TiO4 increase.Moreover, under the same reduction condition, the increasing gangue minerals decrease the RI of sinter.Furthermore, combined with the mineral contents of the CVTM sinter with increasing TiO2 content,  Figure 9 shows the XRD patterns of reductive CVTM sinters with different TiO2 content at 900 °C.The main phases of reductive CVTM sinter are metallic iron (Fe), wustite (FeO), maghemite (γ-Fe2O3), perovskite, coulsonite, magnesium ferrite, and titanium monoxide (TiO).The peak intensity of perovskite, wustite, and titanium monoxide become stronger with increasing TiO2 content, where the hematite disappears while magnetite generates with TiO2 higher than 7.35%.A new spinel phase Mg2TiO4 forms with TiO2 higher than 10.35%, and new phases titanomagnetite (Fe2.5Ti0.5O4)and titanomaghemite (Fe0.23(Fe1.95Ti0.42)O4)form when TiO2 content increases to 11.85%.The ICSD card numbers of phases in reductive CVTM sinters are shown in Table 6.The RI of sinter is influenced by phase compositions.As shown in Figure 10, the tendency of RI decreases with increasing TiO2 content.Based on the XRD analysis, the reduction of iron oxides and Ti-bearing iron oxides are carried out in the reduction process.Hence, the reduction reactions of iron oxides and titanium oxides are obstructed due to the nonreactive minerals with increasing TiO2 content.With the increase of TiO2 content, the iron grade and the quantity of iron-bearing minerals decrease while gangue minerals of perovskite and Ti-bearing spinel Mg2TiO4 increase.Moreover, under the same reduction condition, the increasing gangue minerals decrease the RI of sinter.Furthermore, combined with the mineral contents of the CVTM sinter with increasing TiO2 content, The RI of sinter is influenced by phase compositions.As shown in Figure 10, the tendency of RI decreases with increasing TiO 2 content.Based on the XRD analysis, the reduction of iron oxides and Ti-bearing iron oxides are carried out in the reduction process.Hence, the reduction reactions of iron oxides and titanium oxides are obstructed due to the nonreactive minerals with increasing TiO 2 content.With the increase of TiO 2 content, the iron grade and the quantity of iron-bearing minerals decrease while gangue minerals of perovskite and Ti-bearing spinel Mg 2 TiO 4 increase.Moreover, under the same reduction condition, the increasing gangue minerals decrease the RI of sinter.Furthermore, combined with the mineral contents of the CVTM sinter with increasing TiO 2 content, the increase of difficultly reductive magnetite, and the decrease of easily reductive hematite and calcium ferrite have negative effects to the RI of CVTM sinter.
the increase of difficultly reductive magnetite, and the decrease of easily reductive hematite and calcium ferrite have negative effects to the RI of CVTM sinter.The reduction path of CVTM sinter with CO at 900 °C is shown in Figure 11.The main changes of phases are Fe3O4, Fe2.75Ti0.25O4,and FeTiO3.The phases of CVTM sinter are complicated, and common phases of CVTM sinter before and after reduction are Fe3O4, Fe2O3, MgFe2O4, CaTiO3, and Fe2VO4.The reduction products of CVTM sinter are Fe, FeO, Mg2TiO4, TiO, Fe2.5Ti0.5O4,and Fe2.18Ti0.42O4.The main gangue mineral CaTiO3 increases with the increase of TiO2 content in the CVTM sinter due to the reaction between FeTiO3 and CaO in the sintering process of CVTM.The Fe3O4 occupies most of the iron oxides in CVTM sinter, and RDI improves because magnesium ferrite has a suppressing effect on the reduction of hematite with increasing TiO2 content.The increasing gangue minerals CaTiO3 and Mg2TiO4 decrease the RI of sinter with increasing TiO2 content.Hence, TiO2 addition has favorable effects on low reduction temperature while TiO2 addition has unfavorable effects on high reduction temperature.The main component difference between VTM and CVTM is the content of Cr2O3, and the contents of V2O5 and TiO2 also have differences in the different types of VTM and CVTM.The main differences between sinters of VTM and CVTM are mineral contents of Cr-bearing phase and CaTiO3 which can affect the quality and performance of the sinter.Compared with the sintering behavior of VTM in our laboratory [9], the RDI of CVTM sinter is superior to VTM sinter due to the suppressing effect of magnesium ferrite, while the RI and TI of CVTM sinter are inferior to VTM sinter due to the more gangue minerals CaTiO3 in sinter.The main gangue mineral CaTiO 3 increases with the increase of TiO 2 content in the CVTM sinter due to the reaction between FeTiO 3 and CaO in the sintering process of CVTM.The Fe 3 O 4 occupies most of the iron oxides in CVTM sinter, and RDI improves because magnesium ferrite has a suppressing effect on the reduction of hematite with increasing TiO 2 content.The increasing gangue minerals CaTiO 3 and Mg 2 TiO 4 decrease the RI of sinter with increasing TiO 2 content.Hence, TiO 2 addition has favorable effects on low reduction temperature while TiO 2 addition has unfavorable effects on high reduction temperature.The main component difference between VTM and CVTM is the content of Cr 2 O 3 , and the contents of V 2 O 5 and TiO 2 also have differences in the different types of VTM and CVTM.The main differences between sinters of VTM and CVTM are mineral contents of Cr-bearing phase and CaTiO 3 which can affect the quality and performance of the sinter.Compared with the sintering behavior of VTM in our laboratory [9], the RDI of CVTM sinter is superior to VTM sinter due to the suppressing effect of magnesium ferrite, while the RI and TI of CVTM sinter are inferior to VTM sinter due to the more gangue minerals CaTiO 3 in sinter.
the increase of difficultly reductive magnetite, and the decrease of easily reductive hematite and calcium ferrite have negative effects to the RI of CVTM sinter.The reduction path of CVTM sinter with CO at 900 °C is shown in Figure 11.The main changes of phases are Fe3O4, Fe2.75Ti0.25O4,and FeTiO3.The phases of CVTM sinter are complicated, and common phases of CVTM sinter before and after reduction are Fe3O4, Fe2O3, MgFe2O4, CaTiO3, and Fe2VO4.The reduction products of CVTM sinter are Fe, FeO, Mg2TiO4, TiO, Fe2.5Ti0.5O4,and Fe2.18Ti0.42O4.The main gangue mineral CaTiO3 increases with the increase of TiO2 content in the CVTM sinter due to the reaction between FeTiO3 and CaO in the sintering process of CVTM.The Fe3O4 occupies most of the iron oxides in CVTM sinter, and RDI improves because magnesium ferrite has a suppressing effect on the reduction of hematite with increasing TiO2 content.The increasing gangue minerals CaTiO3 and Mg2TiO4 decrease the RI of sinter with increasing TiO2 content.Hence, TiO2 addition has favorable effects on low reduction temperature while TiO2 addition has unfavorable effects on high reduction temperature.The main component difference between VTM and CVTM is the content of Cr2O3, and the contents of V2O5 and TiO2 also have differences in the different types of VTM and CVTM.The main differences between sinters of VTM and CVTM are mineral contents of Cr-bearing phase and CaTiO3 which can affect the quality and performance of the sinter.Compared with the sintering behavior of VTM in our laboratory [9], the RDI of CVTM sinter is superior to VTM sinter due to the suppressing effect of magnesium ferrite, while the RI and TI of CVTM sinter are inferior to VTM sinter due to the more gangue minerals CaTiO3 in sinter.

Microstructure
The investigation of microstructure with different TiO 2 content on reductive CVTM sinter was analyzed, as shown in Figure 12. Figure 13 presents the corresponding EDS analyses of points A, B, C, D, and E. It can be seen that there are different phases after each reduction, and the different phase boundaries are also easily recognized.An obvious iron phase is observed in light point A in the microstructure, and the contents of O in point A are relatively lower than those in a light grey point C. The silicate phases are also observed in dark grey points B and E. The perovskite phase is observed in a grey point D. With TiO 2 content in the range of 5.85-7.35%, the subhedral wustite phase concentrates, and perovskite and silicate phase exist among the grains of subhedral wustite and euhedral metallic iron, where the clear boundaries are surrounded by wustite crystals and silicate phase.Meanwhile, pores exist in iron and wustite phases due to the reduction of iron oxides and the aggregation of iron, and many micropores exist in wustite phases, and the porous structure is beneficial in improving the reducing property of the CVTM sinter although this structure is adverse to the sinter strength and RDI.With TiO 2 content higher than 8.85%, the wustite phase exists dispersively in silicate and perovskite phases.The silicate and perovskite phases obstruct the reduction reactions between reduction gas and wustite.Therefore, the reduction of CVTM sinter decreases with increasing TiO 2 content.Nevertheless, the relatively uniform structure is beneficial to enhance the strength and RDI of the CVTM sinter in low reduction temperatures.

Microstructure
The investigation of microstructure with different TiO2 content on reductive CVTM sinter was analyzed, as shown in Figure 12. Figure 13 presents the corresponding EDS analyses of points A, B, C, D, and E. It can be seen that there are different phases after each reduction, and the different phase boundaries are also easily recognized.An obvious iron phase is observed in light point A in the microstructure, and the contents of O in point A are relatively lower than those in a light grey point C. The silicate phases are also observed in dark grey points B and E. The perovskite phase is observed in a grey point D. With TiO2 content in the range of 5.85-7.35%, the subhedral wustite phase concentrates, and perovskite and silicate phase exist among the grains of subhedral wustite and euhedral metallic iron, where the clear boundaries are surrounded by wustite crystals and silicate phase.Meanwhile, pores exist in iron and wustite phases due to the reduction of iron oxides and the aggregation of iron, and many micropores exist in wustite phases, and the porous structure is beneficial in improving the reducing property of the CVTM sinter although this structure is adverse to the sinter strength and RDI.With TiO2 content higher than 8.85%, the wustite phase exists dispersively in silicate and perovskite phases.The silicate and perovskite phases obstruct the reduction reactions between reduction gas and wustite.Therefore, the reduction of CVTM sinter decreases with increasing TiO2 content.Nevertheless, the relatively uniform structure is beneficial to enhance the strength and RDI of the CVTM sinter in low reduction temperatures.

Figure 5 .
Figure 5. Properties and strength of CVTM sinter with different TiO2 content.

Figure 5 .
Figure 5. Properties and strength of CVTM sinter with different TiO 2 content.

Figure 5 .
Figure 5. Properties and strength of CVTM sinter with different TiO2 content.

Table 1 .
Parameters of CVTM sintering pot test.

Table 1 .
Parameters of CVTM sintering pot test.

Table 4 .
Industrial analysis of coke breeze and chemical compositions of the ash (wt %).

Table 4 .
Industrial analysis of coke breeze and chemical compositions of the ash (wt %).

Table 6 .
ICSD card numbers of phases in CVTM.

Table 6 .
ICSD card numbers of phases in CVTM.