This paper aims to improve the metallurgical properties of vanadium-titanium magnetite (VTM) concentrates pellets by applying solid waste containing B2O3. Thus, the effects of adding B2O3 on the drop strength, compressive strength, pores area ratio, high-temperature metallurgical properties, and microstructure of VTM pellets were studied through pelletizing and roasting experiments. Results show that the addition of B2O3 reagent is not conducive to the increase of the drop strength of the green pellets. Nevertheless, the compressive strength and fracture toughness of the roasted pellets can be improved by adding more B2O3 during the pelletizing. The reduction degree of VTM pellets is firstly decreased and then increased with the added B2O3 amount. It is possible to improve the compaction degree and restrain the reduction-pulverization degree of the pellet by a low amount of additive (B2O3). The reduction-expansion performance of VTM pellets, in turn, can be raised by adding B2O3.
Vanadium-titanium magnetite (VTM), which is found in rich reserves, has been widely used in the steel-making industry in recent years. Currently, VTM has been processed by the blast furnace-converter route, where the vanadium is extracted from the vanadium-rich slags [
Metallurgical properties of the pellets can be improved by the addition of solid fuels, basic fluxes, and boron-containing oxides. Wastes containing B2O3, such as boric mud and boric acid waste liquor (produced during the extraction process of boron from boron ore), can cause considerable environmental pollution. These byproducts are also enriched with magnesium, silicon, and boron elements. Therefore, using these materials in the pelleting process may enhance pellet quality and also eliminate related waste. Akberdin et al. [
Yu et al. [
For the previous investigations mentioned above, it was found that the performance of the pellets with magnetite can be improved by adding boron slime, boron acid, and boron-containing iron concentrates. Intending to further develop the vanadium-titanium pellets, the present study studied the effects of B2O3 on pellets (made of vanadium-titanium concentrates) by an experimental approach. The B2O3 analytical reagent is used to systematically study the mechanism of forming and roasting consolidation of VTM concentrates pellets. Before pelletizing, the VTM concentrates and bentonite were added to the raw materials at a particular proportion. The effects of the B2O3 addition on the green strength, the roasting process, phase changes, and microstructures of the pellets, as well as the strength of the roasted pellets, were studied with an experimental device. The results can provide theoretical guidance for the possible use of boron minerals in the production of pellets and a new solution for the recycling of boron solid waste.
The iron ore used in this study is vanadium-titanium concentrates obtained from southwest of China. This was used together with bentonite and B2O3 (≥98%). The chemical composition of the sintering raw materials is shown in Tables
Chemical composition of bentonite (wt%).
Name | SiO2 | Al2O3 | CaO | P | MgO | Fe2O3 | K2O | Na2O | MnO |
Bentonite | 61.68 | 13.80 | 2.83 | 0.03 | 5.10 | 2.41 | 0.06 | <0.01 | 0.01 |
Chemical composition of VTM concentrates for the experiments (wt%).
Name | TFe | SiO2 | CaO | Al2O3 | MgO | TiO2 | MnO | FeO | P | S |
VTM | 55.78 | 4.33 | 0.69 | 3.86 | 2.78 | 9.08 | 0.36 | 30.50 | 0.09 | 0.54 |
Pellets were made on a disc pelletizer following the experimental batching scheme. The pellets experiment process is shown in Figure
Flowchart of pellets experiment.
The mineral phase of roasted pellets was studied by microscope and image analysis software to estimate the pore area ratio. HV-1000B Vickers hardness tester was used to conduct five groups of microhardness experiments on the roasted pellets, and the average values of the results were taken. The Vickers rhombic pressure cone was selected for the analysis. The objective lens of the Vickers hardness tester is ×40, the numerical aperture is 0.85, and the Vickers diamond cone is selected. The test load was 300 g and the time of action of the pressure cone was 10 s. Ten randomly measured points on the microscopic surface of each pellet were selected for the microhardness test and average value were taken after measuring the points. The fracture toughness results calculated by the microhardness test are shown in Figure
Microhardness indentation of pellet.
According to the national standards of the People’s Republic of China (GB/T 13241-2017 Iron Ores-Determination of Reducibility) [
According to the national standards of the People’s Republic of China (GB/T 13242-2017 Iron Ores-Low-Temperature Disintegration Test-method Using Cold Tumbling after Static Reduction) [
According to the national standards of the People’s Republic of China (GB/T 13240-91 Iron Ore Pellets-Method for Measuring Relative Free Swelling Index) [
Flowchart of pellets reduction swelling experiment.
The main mechanical properties of pellets are presented in Table
Main mechanical properties of pellets (wt%).
B2O3% (wt%) | Drop strength of green pellets (T/P) | Compressive strength of green pellets (N) | Compressive strength of roasted pellets (N) |
0 | 15.0 | 7.2 | 1087 |
0.2 | 7.5 | 7.2 | 1136 |
0.4 | 9.8 | 7.1 | 1033 |
0.6 | 5.5 | 7.5 | 849 |
0.8 | 5.6 | 7.0 | 903 |
1.0 | 6.4 | 4.7 | 1302 |
1.2 | 4.8 | 5.6 | 1501 |
1.4 | 5.6 | 5.7 | 1701 |
1.6 | 5.8 | 5.3 | 1095 |
1.8 | 4.5 | 5.5 | 1181 |
2.0 | 4.1 | 7.2 | 1045 |
Drop strength and compressive strength of vanadium-titanium pellets with B2O3 addition.
The compressive strength of the vanadium-titanium roasted pellets without the addition of B2O3 was 1100 N at 1200°C. The compressive strength test of roasted pellets used pressure testing machine WDS-10QT; With a maximum force is 10000 N. When 0.6% of B2O3 was added (lower panel of Figure
Thus, an appropriate amount of added B2O3 may improve the compressive strength of vanadium-titanium pellets. Since the B2O3 has a low melting point, it can form low-melting binding phases in the pellets which increases the strength of the pellets. The pellets bonding force produced by B2O3 can be complementary to the oxidative recrystallization and bonding force of Fe3O4. However, if more B2O3 was added (than the optimal amount), the binder phases become excessive. This prevents the direct connection of solid-phase particles and also the permeation of the liquid phase along the grain boundaries. As a result, the consolidation of the large crystal clusters is weakened. The glass phase is also brittle, which reduces the strength of the pellets.
With the addition of B2O3, the area of the pores of the pellets has decreased (Figure
Pores area ratio and average fracture toughness of pellets with different B2O3 addition.
SEM images of pellets with different B2O3 addition.
For the pellets used in the blast furnace, the reducing agent (CO, H2, and carbon) makes them undergo a stepwise reduction reaction, finally yielding metallic iron. During this reduction process, many complex physical and chemical changes occur in the pellets, including an increase of the volume of the pellets. High-temperature reduction properties of pellets with different B2O3 addition are shown in Figure
Metallurgical properties of pellets with different B2O3 addition.
The reasons for the volume expansion of different pellets during the reduction process depend on differences in mineral and chemical composition and internal structure. Along with the addition of B2O3 to the raw material, the reduction swelling rate is gradually decreased. The reason may be that the B2O3 activates the consolidation reaction during the roasting process. This also can strengthen the intermolecular forces, which suppresses volume changes of the pellets.
Possible chemical reactions by which B2O3 reacts with some oxides in VTM are listed in Table
Main reactions of B2O3 and oxides.
Number | Chemical reaction equation |
(3-1) | 2Al2O3 + B2O3 = 2Al2O3·B2O3 |
(3-2) | MgO + 2B2O3 = MgO·2B2O3 |
(3-3) | 9Al2O3 + 2B2O3 = 9Al2O3·2B2O3 |
(3-4) | 2MgO + B2O3 = 2MgO·B2O3 |
(3-5) | CaO + B2O3 = CaO·B2O3 |
(3-6) | CaO + B2O3 + 2SiO2 = CaO·B2O3·2SiO2 |
(3-7) | CaO + Al2O3 + B2O3 = CaO·B2O3·Al2O3 |
(3-8) | 3MgO + B2O3 = 3MgO·B2O3 |
(3-9) | CaO + 2B2O3 = CaO·2B2O3 |
(3-10) | 2CaO + B2O3 = 2CaO·B2O3 |
(3-11) | 2CaO + B2O3 + SiO2 = 2CaO·B2O3·SiO2 |
(3-12) | 2CaO + Al2O3 + B2O3 = 2CaO·B2O3·Al2O3 |
(3-13) | 3CaO + B2O3 = 3CaO·B2O3 |
(3-14) | 11CaO + B2O3 + 4SiO2 = 11CaO·B2O3·4SiO2 |
Gibbs free energy of B2O3 and oxides and liquid content of the pellets at different B2O3 addition.
Adding B2O3 promotes the rearrangement of the particles of the pellets during the roasting process. As a result, the compactness of the particles is increased. Adding a small amount of B2O3 can promote the recrystallization development of hematite, increase the liquid phase formation, and reduce the area of the pores. As the amount gradually increases, large-diameter pores appear in the pellets. The reason is the liquid phase, the appearance of which is promoted by B2O3 addition, and that the pellets shrink during the cooling process, which is illustrated in Figure
SEM images of pellets with different B2O3 addition.
When more than 2% B2O3 was added to the raw material mix, the pores in the pellets gradually disappeared. The shape of the hematite was changed from bone-like to tightly connected granular, as shown in Figure
SEM-EDS images of pellet at 2% B2O3 addition.
The effects of adding B2O3 to the raw material mix before pelletizing on the drop strength, compressive strength, pores area ratio, high-temperature reduction properties, and microstructure of vanadium-titanium magnetite (VTM) concentrates pellets have been studied through pelletizing and roasting experiments. The following main conclusions can be drawn based on the findings: Although the addition of the B2O3 reagent did not increase the drop strength of the green pellets, the compressive strength and fracture toughness of the roasted pellets were improved to some extent with the increase of B2O3 addition. Under the conditions of the experiments undertaken in this work, the optimal amount of B2O3 added was found to be between 1.4% and 1.6%. The reduction degree of VTM pellets firstly decreased and then increased with the addition of B2O3. The addition can increase the amount of liquid phase in the pellets, enhance the compactness, and reduce the reduction pulverization if the added amount is low. Also, the reduction expansion performance of the VTM pellets may improve by adding B2O3, but gradually yielding holes with growing size in the pellets, which reduces the strength of pellets.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Hao Liu, Ke Zhang, and Xiaoyan Xiang contributed to performing the experiments, material characterization, data analysis, and paper writing; Henrik Saxén, Weiqiang Liu, and Yuelin Qin revised the paper and refined the language; and Hao Liu and Yuelin Qin contributed to the design of the experiment.
This work was supported by the National Natural Science Foundation of China (Grant no. 51974054); Youth project of Science and Technology Research Program of Chongqing Education Commission of China (no. KJQN201901); and China Scholarship Council (201802075005).