Effect of Co₂O₃ on Oxidation Induration and Reduction Swelling of Chromium-Bearing Vanadium Titanomagnetite Pellets with Simulated Coke Oven Gas

Abstract

This study discusses the oxidation induration and swelling behavior of chromium-bearing vanadium titanomagnetite pellets (CVTP) with Co₂O₃ additions. The reduction swelling index (RSI) and compressive strength of reduced CVTP (CSRC) were investigated with simulated coke oven gas (COG). The results show that the compressive strength (CS) of CVTP decreases from 2448 to 1983 N and the porosity of CVTP increases from 14.86 to 22.49% with increasing Co₂O₃ additions. The Co₂O₃ mainly exists in the form of CoFe₂O₄ in both of CVTP and reduced CVTP, and the CoFe₂O₄ is hard to be reduced by thermodynamic calculation. The Co-bearing phase mainly distributes on gap edges and among adjacent hematite grains. Many cracks and pores distribute along the grain boundaries and damage the connection of hematite grains. The CSRC decreases from 901 to 376 N, and RSI of reduced CVTP increases from 5.87 to 9.05% with increasing Co₂O₃ additions. The Co₂O₃ addition facilitates the aggregation and diffusion of metallic iron particles, and the aggregations of metallic iron thicken the lamellar crystals. The pores and interval of grains enlarge with increasing Co₂O₃ additions. This study can supply the theoretical and technical basis for the utilization of CVTP and waste residue-bearing cobalt with COG recyclable technology.

1. Introduction

The blast furnace (BF) is widely used in the ironmaking process and has been the primary ironmaking method. Figure 1 shows the timeline of the smelting process and development of vanadium titanomagnetite (VTM) with major events in China. Before 1970, the manufacturing process of VTM is BF smelting with the BF slag containing 8–12 wt % TiO₂. Since the studies on the smelting of high-titanium VTM in the 1970s, the smelting of VTM became easy in BF with the content of TiO₂ BF slag higher than 16 wt %. Meanwhile, to decrease the yield of Ti-beaing BF slag, the Non-BF technologies were studied since 1978, and the technologies mainly include the gas-based and coal-based process [1,2]. The BF process is still the main technique due to its high production efficiency. Nevertheless, the massive utilization of coke and coal in BF generates plenty of carbon monoxide (CO) and carbon dioxide (CO₂). The CO₂ emission from iron and steel industry is equivalent to about 15% of the total CO₂ emissions in China. At present, the idle coke oven gas (COG) is more than 65 million m³ every year in a big iron and steel industry. The average calorific value of COG is 18,500 kJ/m³; hence, COG is a kind of valuable fuel to decrease the usage of coal and coke in BF. Besides, the COG contains 60.7% H₂, and COG injection is recognised as one of the viable methods to decrease CO₂ emission and for low-carbon ironmaking in BF.

The chromium-bearing vanadium titanomagnetite (CVTM) in the Sichuan-Panzhihua area in China is a large reserve resource, and the reserves are more than 3.5 billion tons. The CVTM is one of the largest VTM mineral resources which have Fe, Ti, V, Cr, and many other rare elements [3,4]. Similar types of ores are mainly distributed in Russia, Canada, Australia, and other places in the world [5,6]. The reserve of Co reaches 1.4 million tonnes in China, and almost half of the reserves of Co in China exist in the Panzhihua VTM, and the cobalt oxides are mainly used to manufacture the cemented carbide. Meanwhile, a mass of waste residue-bearing cobalt about 2.0–5.0 wt % is low-cost in China. Hence, the Co can be recycled in the smelting of CVTM to manufacture heat resistant alloys, tool steels, hard alloys, and magnetic materials. To further study the production process of cobalt-ferroally, the studies on pelletizing and reduction of Co-bearing raw materials are important in the early stage before smelting. Meanwhile, in order to strength the smelting of CVTM and decrease the CO₂ emission, the hydrogen smelting technology using COG was studied in the reduction swelling of pellets. To study the single variable of cobalt, the pure reagent was selected.

Mousa [7] studied the influence of COG injection on the BF with isothermal and nonisothermal reduction of sinter under different gas compositions and temperatures, and the reduction rate can be increased with COG injection under isothermal condition. Liu [8] considered that the operation of COG injection into BF could increase hot metal productivity and decrease coal ratio, coke ratio, and carbon emission of BF. Wang [9] investigated the mathematical simulation on BF operation of COG injection, and the indirect reduction degree increases due to the involvement of hydrogen in the cohesive zone with COG injection. Further, the productivity and CO utilization efficiency increase, and CO₂ emission and energy consumption decrease, with COG injection. Nishioka [10] considered that the COG was available and stable as a kind of hydrogen source in the industry. Meanwhile, COG has been successfully carried out in BF in the COURSE50 project. Mousa [11] studied that the reduction rate of pellets increased sharply with COG.

Mohan [12] studied the properties of mixed spinel CoFe_2−xCr_xO₄ (x = 0–2) which covers the cobalt ferrite (CoFe₂O₄) to cobalt chromite (CoCr₂O₄), and the CoCr₂O₄ is a normal spinel with a canted ferrimagnetic structure. Li [13] studied the Co₂O₃ addition on the properties of ceramic, and the Co₂O₃ decomposes to CoO when the sintering temperature is higher than 900 °C. Aleksandrov [14] investigated the thermodynamics of oxygen solutions of Fe-Co-Cr melts, and the higher Co content enhanced the oxygen concentration of melts. Landolt [15] studied the oxidation mechanism of Co-Cr alloys, and the low Cr content led to the intermediate phase CoCr₂O₄ due to the Co₂O₃ is thermodynamically more stable. Li [16,17] studied the effect of V₂O₅ on the oxidation induration and reduction process of vanadium titanomagnetite and the V₂O₅ decreases the CS of pellets and accelerates the reduction of pellets with the increase of porosity with simulated shaft furnace gas. Li [18,19,20] researched that effects of TiO₂ and Cr₂O₃ on the oxidation induration and reduction process of vanadium titanomagnetite, and the TiO₂ and Cr₂O₃ decrease the CS of pellets and suppress the growth of iron whiskers. Moreover, the production of the CVTP after smelting can be used as raw material to manufacture the biocompatible metal including stainless steels, cobalt-chromium alloys, and titanium alloys [21,22].

In this study, first, the effect of Co₂O₃ addition on the phase compositions, compressive strength, porosity, microstructure, and oxidation induration of CVTP were first analysed. Further, the impact of Co₂O₃ addition on reduction swelling behavior of CVTP with simulated COG was investigated. These results will provide theoretical and technical bases for the production of CVTP and waste residue-bearing cobalt with COG recyclable technology.

2. Materials and Methods

2.1. Materials

The CVTM stems from the Hongge (Sichuan, China). The chemical compositions of the CVTM and bentonite are listed in

Table 1. Chemical compositions of raw materials (wt %).

Sample	FeTotal	FeO	TiO2	Co2O3	V2O5	Cr2O3	CaO	SiO2	MgO	Al2O3	P	S
CVTM	53.35	26.91	11.60	0.02	0.57	0.81	0.96	4.71	3.33	2.82	0.02	0.26
Bentonite	-	-	-	-	-	-	2.19	68.28	3.56	13.45	-	-

. Figure 2 shows the XRD pattern of the CVTM. The main minerals of the CVTM are Fe₃O₄, FeTiO₃, and FeCr₂O₄. The Co₂O₃ was of analytical grade purchased from Sinopharm Chemical Reagent Co. (Shanghai, China).

2.2. Apparatus and Procedure

The pelletizing process includes mixing, balling, drying, oxidation induration, and cooling. The main parameters of pelletizing process include 8.0% moisture of mixing materials, 30 min of pelletizing time, 10–12 mm size of green pellets, 105 °C of drying temperature for 5 h, 900 °C of preheating temperature for 20 min, and 1200 °C of roasting temperature for 20 min with 1.5 L/min blowed air. When the oxidation process was finished, the CVTP was taken out of the muffle furnace with temperature lower than 900 °C and cooled to ambient temperature.

The swelling behavior of the CVTP was studied in a comprehensive metallurgical measuring apparatus, as shown in Figure 3. First, the 18 CVTP with average size of 10–12.5 mm were placed into the constant temperature zone of the apparatus and heated to the target temperature in the N₂ atmosphere with 3L/min. Then, middle COG injection (150 m³/tHM, CO-H₂-CO₂-N₂ = 40–15–10–35 Vol.%) was purged into the apparatus with 15 L/min. Finally, the reactor was removed from the apparatus and cooled in the N₂ atmosphere when the reduction was completed. The reduction swelling index (RSI) and compressive strength of reduced CVTP (CSRC) were measured. The reduction temperature was 900 °C and time was 60 min.

2.3. Definition of Parameters

The RSI is given in Equation (1).

$$RSI=\frac{V_{\mathrm{t}}-V_0}{V_0}\times 100\%$$

(1)

where the V₀ and V_t are the volumes of original CVTP and reduced CVTP, respectively, mm³. The diameters of the CVTP and reduced CVTP were measured through an electronic vernier calliper.

2.4. Analytical Methods

The X-ray fluorescence (XRF, ZSXPrimus II; Rigaku, Japan) was used to test the chemical compositions of raw materials. The X-ray diffraction (XRD, X’ Pert Pro; PANalytical, Almelo, Netherlands) with Cu Kα radiation (wavelength = 1.5406 Å) at a setting of 40 kV and 40 mA was used to analyze the mineral phases of CVTM and CVTP. The scanned range was 2θ = 5–90° with a step of 2θ = 0.17° and 1 s·step⁻¹. The scanning electron microscope (SEM, Ultra Plus; Carl Zeiss GmbH, Jena, Germany) was used to detect the microstructure of CVTP with backscattering detector (BSE) and energy disperse spectroscopy (EDS). The CVTP was heat mounted in resin and polished by mirror finish for microstructure analysis. The porosity and pore size distribution of CVTP were tested by mercury injection apparatus (Micromeritics Instrument Corporation, Autopore IV 9500, Norcross, GA, USA). The compressive strength (CS) of CVTP was referred to ISO4700.

3. Results and Discussion

3.1. Oxidation Induration of CVTP

3.1.1. Phase Composition

The oxidation induration process of CVTP includes a series of physical-chemical reactions. Figure 4 shows the primary phases of CVTP with different Co₂O₃ additions. It is revealed that the primary phases of CVTP without Co₂O₃ addition are Fe₂O₃ and Fe₂TiO₅. As the Co₂O₃ addition increases from 2 to 6 wt %, the peak intensity of CoFe₂O₄ increases continuously and peak intensities of Fe₂O₃ and Fe₂TiO₅ decrease slightly, hence, the content of CoFe₂O₄ increases with increasing Co₂O₃ additions. Further, the additive Co₂O₃ decomposes to CoO first and then react with Fe₂O₃ to generate CoFe₂O₄ [13,23]. The main oxidation reactions and phase transformation of Fe₃O₄, FeTiO₃, and Co₂O₃ in CVTP can be written as

$${4\mathrm{Fe}}_3{\mathrm{O}}_4+{\mathrm{O}}_2=6{\mathrm{Fe}}_2{\mathrm{O}}_3$$

(2)

$${2\mathrm{Fe}}_2{\mathrm{O}}_3+4{\mathrm{FeTiO}}_3+{\mathrm{O}}_2=4{\mathrm{Fe}}_2{\mathrm{TiO}}_5$$

(3)

$${2\mathrm{Co}}_2{\mathrm{O}}_3=4\mathrm{CoO}+{\mathrm{O}}_2$$

(4)

$$\mathrm{CoO}+{\mathrm{Fe}}_2{\mathrm{O}}_3={\mathrm{CoFe}}_2{\mathrm{O}}_4$$

(5)

3.1.2. Compressive Strength and Porosity

Figure 5 shows the changes of CS and porosity of CVTP with different Co₂O₃ additions. The CS decreases from 2448 to 1983 N and porosity increases from 14.86 to 22.49% with increasing Co₂O₃ additions. It is observed that Co₂O₃ addition decreases the CS obviously, and the high CS with low porosity is obtained for CVTP without the addition of Co₂O₃. Figure 6 shows the relationship between porosity and CS, and it can be seen that the porosity increases while the CS decreases. Further, they exhibit a negative correlation relationship, and experimental linear regression equation between porosity and CS of CVTP is

$$C=-62.48P+3382.14$$

(6)

where the C is the compressive strength and P is the porosity of the CVTP.

According to the histograms, Figure 7 shows the effect of different Co₂O₃ additions on the pore size distribution of CVTP. The microsize pore size distribution of CVTP mainly distributes between 0 to 5 μm when Co₂O₃ addition is lower than 2 wt %, and the proportion of microsize pore size distribution of CVTP decreases obviously with increasing Co₂O₃ additions, reflecting the incremental intrusion of mercury in microsize pore size decreases continuously. Meanwhile, the distribution of histogram data becomes sparse, and the pore size increases markedly in a large range from 5 to 30 μm with increasing Co₂O₃ additions, which corresponds to the increasing porosity from 14.86 to 22.49%. Based on the decreasing CS of CVTP, the oxidation induration of CVTP weakens with Co₂O₃ additions. Hence, the porosity of CVTP increases due to the increasing size of pores with increasing Co₂O₃ additions.

3.1.3. Microscopic Structure

Based on the XRD analysis of CVTP with Co₂O₃ addition, the Co element mainly exists as the form of CoFe₂O₄ on the phase composition of CVTP during the oxidation induration process. Hence, the microstructure of CVTP with different Co₂O₃ additions should be investigated to study the effect of cobalt on the structure of CVTP, and the microstructures of CVTP detected by SEM are shown in Figure 8. Figure 8a shows the microstructure of CVTP without Co₂O₃ addition and interval of grains are distinct with little-closed pores which correspond to the low porosity, and the grain size distribution is relatively uniform. Meanwhile, the gangue mineral exists among the individual hematite grains. Further, the silicate phases mainly act as binder phases among the hematite grain boundaries to form a continuous structure and decrease cracks, corresponding to the relatively high CS of CVTP without Co₂O₃ addition. When Co₂O₃ addition increases to 2 wt %, the small hematite grains, granular Ti-bearing phases, strip-shaped silicate phases, and micropores mix together, as shown in Figure 8b. The porous mineral structure and microcracks are adverse to the strength of grains. The Figure 8e shows the EDS analysis of point A in Figure 8b. Based on the element concentration analysis of point A, this area mixes Fe-V oxide, Fe-Ti oxide, CoFe₂O₄, and silicate phases. It is obvious that the formation of micropores and microcracks structure due to these inhomogeneous phases with low bonding strength. Hence, the CS of CVTP decreases, and porosity of CVTP increases with 2 wt % Co₂O₃ additions. Further, the hematite grains connect, and similar phases aggregate together gradually when the Co₂O₃ addition increases to 4 wt %. Figure 8c shows the large hematite grains, covered Ti-bearing phase, and granulated Co-bearing grains. The silicate phases are mainly mixed with granulated Co-bearing and small hematite grains. The Co-bearing spinels mainly distribute on gap edges and among adjacent hematite grains. Based on the element concentration analysis of point B, this area mixes silicate, hematite, and Co-bearing phases, and the content of Co is 11.80 wt %. The loose structure is harmful to the connection of hematite grains. Hence, the CS of CVTP deteriorates sequentially with 4 wt % Co₂O₃ addition. When the Co₂O₃ addition increases to 6 wt %, the microstructure of CVTP is loose and porous with dispersive and individual hematite grains. Many cracks and pores distribute along the grain boundaries and damage the connection of hematite grains. Hence, the recrystallization of hematite is slow and limited.

Figure 9 shows the EDS analysis of CVTP with 6 wt % Co₂O₃ addition. From point A to point C, the contents of Co₂O₃ are 16.92, 3.40, and 3.80 wt %, respectively. Based on the EDS of point A, the light grey area is hematite, Co-bearing phase, and silicate phase. Some micropores distribute on these phases. According to the EDS of point B, the light grey area mixes silicate phase and little Fe-V and Co-bearing phases. These fine grains are connected by the silicate phase, and the structure is porous and loose. With the analysis of point C, the grey area mixes Fe-Ti, Fe-V, and Co-bearing phases on the interval of hematite grains. The Co₂O₃ addition leads to the porous structures which distribute on small hematite grains and grain boundaries, which restrains the recrystallization of hematite in the oxidation induration of CVTP. Hence, the CS decreases sequentially, while these structures increase the porosity of CVTP.

Figure 10 shows the X-ray element mapping of CVTP with 6 wt % Co₂O₃. It can be observed that the elemental distributions of Fe, Co, Ti, Si, and Ca in Figure 10. From Figure 10, it indicates that Co mainly exists on the porous and loose grains and some individually tiny particles in interval of mineral grains, and silicate phases fill with the gaps of loose grains. A part of Ti-bearing and silicate phases distribute on the interval of large hematite grains. According to the above analysis, the addition of Co₂O₃ damages the microstructure and recrystallization of hematite grains and increases the porous structure of CVTP during oxidation induration, and finally results in the decreasing CS of CVTP.

3.2. Induration Mechanism

According to the above results, the Co₂O₃ addition has an inhibiting effect on the recrystallization of hematite grains and changes the morphology of hematite and increases the loose structure in the interval of mineral grains due to the generation of CoFe₂O₄ phase. Figure 11 shows the schematic diagrams of oxidation induration mechanism of CVTP. The hematite grains are large-grained and interconnected and distribute relatively uniform without Co₂O₃ addition. Further, the hematite grains fully recrystallize and connect, and bonding phases exist at the interval of grains. Nevertheless, the loose and porous of CoFe₂O₄ phase fills with the interval of hematite grains with Co₂O₃ addition. The CoO mainly reacts with hematite to generate CoFe₂O₄ phase on the interval and boundary of hematite grains. The CoFe₂O₄ phase increases the porous structure and decreases the recrystallization of hematite grains. Hence, Co₂O₃ addition is disadvantageous to the oxidation induration of CVTP and decreases the CS of CVTP.

3.3. Swelling Behavior of CVTP

3.3.1. Phase Composition

Figure 12 shows the XRD patterns of CVTP reduced for 60 min with different Co₂O₃ additions. The XRD pattern shows the gradual change in the peak with Co₂O₃ addition, indicating the effect of Co₂O₃ addition on the phase composition of reduced CVTP. The primary phases of reduced CVTP with Co₂O₃ addition are Fe, FeO, Fe₂TiO₄, and CoFe₂O₄. Further, the peak intensities of Fe and FeO phases strengthen with an increase of Co₂O₃ addition. Nevertheless, the Co and Cr₂O₃ were not detected might be because of the lower detectable content by XRD or the crystal factors. Liu [24] studied that the Fe₂TiO₄ was one of the reduced products of iron titanium with H₂. Meanwhile, the peak intensities of Fe and FeO increase obviously with increasing Co₂O₃ additions. On the one hand, the H₂ accelerates the reduction of iron oxides. On the other hand, the increasing porosity of CVTP enhances the reduction rate of pellets.

In the reduction of CVTP with simulated COG, the main possible reactions can be classified as the reduction of Fe₂O₃, Fe₂TiO₅, and CoFe₂O₄, and side reactions of the products. The possible chemical reactions during the reduction of CVTP with simulated COG are simply expressed in Figure 13. Equations (1–4) and Equations (7–10) are the possible reduction reactions of hematite by CO and H₂, respectively. Equations (5–6) and Equations (11–12) are the possible reduction reactions of Fe₂TiO₅ and CoFe₂O₄, respectively. Figure 13 shows the relation between ΔG^Θ and T for CVTP possible reduction reactions. According to the calculated results, the whole ∆G^Θ lines of Equations (6) and (12) are above zero line which means that the CoFe₂O₄ and CO and H₂ cannot react to generate CoO at the reduction temperature 900 °C. The thermodynamic analysis of reductions of CoFe₂O₄ verifies the CoFe₂O₄ in reduced CVTP. The whole ∆G^Θ lines of Equations (5) and (11) are below zero line which means that the Fe₂TiO₅ and CO and H₂ can react to generate Fe₂TiO₄ at the reduction temperature 900 °C. According to the above analysis, the increasing XRD peak intensities of Fe and FeO are mainly affected by the increasing porosity of CVTP due to the loose and porous structure of CoFe₂O₄ phase.

3.3.2. Effect of Time and Co₂O₃ Addition on RSI

The physical stability of CVTP in BF is an essential factor in the reduction process. The RSI and CS are the important physical characteristics of reduced CVTP. The swelling behavior with simulated COG at 900 °C with different time was studied, and results are shown in Figure 14. Figure 14 shows the different reduction stages in the swelling behavior of reduced CVTP. The RSI of CVTP can be separated into three stages which include initial stage, intermediate stage, and the final stage. In the initial stage, the RSI increases rapidly with increasing time and reach the peak value of 8.45% at 15 min. Moreover, the RSI decreases gradually in the intermediate stage with increasing time. Further, the RSI decreases relaxedly as a smooth curve in the final stage with increasing time. Finally, the RSI reaches 5.87% at 60 min. The intermediate stage and final stage indicate the shrinkage of CVTP after 15 min which is believed to be the sintering of iron whiskers and grains. Hence, the maximum swelling of CVTP is 15 min at the reduction temperature of 900 °C. Sharma [25] investigated that the reasons including sintering of iron whiskers and pores and formation of silicate phases most possibly leads to the shrinkage. These phenomena restrict the growth and crystallization of iron whiskers in the structure of reduced pellet and result in the shrinkage of pellets.

Figure 15 indicates the effect of Co₂O₃ addition on the RSI and CSRC of CVTP with simulated COG injection into BF. The RSI increases from 5.87 to 9.05% while the CSRC decreases from 901 to 376 N with increasing Co₂O₃ additions. Sharma [26] studied that the effect of oxidation induration on the reduction swelling behavior of pellets, and the increasing CS and decreasing porosity of pellets result in the decreasing RSI. Hence, the CS and porosity of pellets influence the RSI of pellets. The low reduction swelling pellet is mainly due to the presence of slag bonds which are not able to push the adjacent grains mechanically and leads to the decreasing volume [27]. Moreover, the generation of iron whiskers during reduction leads to more stresses in pellets and makes the higher reduction swelling and less CSRC of CVTP. The initial high porosity of pellets indicates the fast reduction and less reduction swelling [28]. In the previous studies, the reduction swelling of pellets usually reaches a maximum value at about 900 °C by CO at the temperature range of 800–1100 °C because of the formation of lots of whiskers at about 900 °C [29]. Yi [30] studied that the addition of H₂ on reduction atmosphere decreased the reduction swelling and expansion characteristic of pellets which showed higher CSRC of pellets because the pellets passed the wüstite stage rapidly and the bonding of pellets was remarkably improved.

3.3.3. Microscopic Structure

To reveal the mechanism of swelling behavior and CSRC of CVTP, the microstructures of reduced CVTP with different Co₂O₃ additions were detected, and the results are shown in Figure 16. It can be indicated that the Co₂O₃ addition has obvious effect on the microstructures of reduced CVTP. As shown in Figure 16a, many small metallic iron whiskers exist on the surface of lamellar microstructure, and the lamellar mineral grains are gauzy and concentrated. Hence, the small metallic iron whiskers result in the slight swelling of CVTP. Figure 16b shows that the shorter metallic iron whiskers generate on the surfaces of lamellar grains as the formation of the cellular structure when the Co₂O₃ addition is 2 wt %. The lamellar microstructure of CVTP with 2 wt % Co₂O₃ becomes thicker than the CVTP without Co₂O₃ addition, and the interval of grains become large gradually. When the Co₂O₃ addition increases to 4 wt %, the metallic iron whiskers and round dot of metallic iron grow gradually, and lamellar crystals thicken by degrees. Further, the interval of grains and the pores among mineral grains become larger than the CVTP with 2 wt % addition, and some metallic iron particles aggregate together gradually due to the mixture of adjacent and long iron whiskers. Moreover, the swelling speed of CVTP increases quickly when the Co₂O₃ addition is higher than 2 wt %, as shown in Figure 14. From the partial enlarged images of Figure 16d, the interval of grains and pores enlarge continuously, and some neighboring lamellar grains assemble to thicker lamellar minerals. The short and dense metallic iron particles aggregate together on the surface of lamellar minerals, thus making the lamellar minerals thicker. Hence, the interval of grains and pores of the adjacent lamellar minerals become large when Co₂O₃ addition increases to 6 wt %. Therefore, the RSI of CVTP increases gradually with increasing Co₂O₃ addition, as shown in Figure 15. The generation of metallic iron whiskers is restrained, and the short metallic irons thicken the lamellar minerals with increasing Co₂O₃ addition. As is shown in the microstructure of reduced CVTP, a large amount of small metallic iron whiskers are detected without Co₂O₃ addition. When the Co₂O₃ addition is higher than 2 wt %, the microstructure of reduced CVTP become loose, and the aggregation and diffusion of metallic iron enhance gradually with increasing Co₂O₃ addition in the reduction process. Hence, the swelling behavior of CVTP strengthens with the increase of Co₂O₃ addition.

According to the above analysis, Figure 17 shows the evolutive schematic diagrams of swelling behavior of reduced CVTP with Co₂O₃ addition during the reduction process with COG injection. The lamellar crystals are dense and thin, and the interval of lamellar crystals are small when CVTP without Co₂O₃ addition. In this case, the CS of reduced CVTP is high, and the RSI of reduced CVTP is low with the minimal swelling. Much short metallic iron forms on the surface of lamellar crystals and thicken the lamellar crystals with high Co₂O₃ addition. At this condition, the CS of reduced CVTP is low, and the RSI of reduced CVTP is high with the most swelling behaviors. When the Co₂O₃ addition is high, the aggregation and diffusion of metallic iron enhance the interval and thickness of lamellar crystals leading to the further swelling behavior. Wang [29] studied the swelling behavior of iron ore during the reduction process, and the metal iron whisker as crucial factor caused the swelling. Hence, the Co₂O₃ addition facilitates the swelling of CVTP and restrains the increasing CS of CVTP.

4. Conclusions

• The primary phases of CVTP with Co₂O₃ addition are Fe₂O₃, Fe₂TiO₅, and CoFe₂O₄. The CS of CVTP decreases from 2448 to 1983 N and porosity increases from 14.86 to 22.49% with increasing Co₂O₃ additions.
• The Co-bearing phase mainly distributes on gap edges and among adjacent hematite grains. Many cracks and pores distribute along the grain boundaries and damage the connection of hematite grains.
• The primary phases of reduced CVTP with simulated COG are Fe, FeO, CoFe₂O₄, and Fe₂TiO₄. The RSI of reduced CVTP increases rapidly with increasing time and reach the peak value of 8.45% at 15 min, then RSI decreases to 5.87% as a smooth curve with increasing time. The RSI of reduced CVTP increases from 5.87 to 9.05% while the CSRC decreases from 901 to 376 N with increasing Co₂O₃ additions.
• The Co₂O₃ addition facilitates the aggregation and diffusion of metallic iron particles, and the aggregations of metallic iron thicken the lamellar crystals in reduced CVTP. The pores and interval of grains enlarge with increasing Co₂O₃ additions.