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14

2024

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03

Effect of Humic Acid Binder on the Preparation of Oxidized Pellets from Vanadium-Bearing Titanomagnetite Concentrate (1)

Keywords:

vanadium-bearing titanomagnetite,oxidized pellets preparation,binder, bentonite

Source: internal company


1. Introduction
Under the background of carbon peak and neutrality goals, how to achieve carbon emission reduction has become the focus in China's steel industry. Replacing the high proportion of sinter by the pellets with better smelting performance but less pollution for blast furnace operation is considered to be one of the important ways. In 2022, China's total pellet production reached 230 million tons, with new production capacity of 8 million tons. According to the plan of steel industry, the proportion of pellet use will be increased from the current 17% to about 30% in the next five years, corresponding to carbon emission reduction of 40 million tons.


Binder is an important auxiliary raw material for oxidized pellets production, and its performance directly determines the quality of pellets. At present, bentonite is commonly used as binder in most domestic pellet plants. The dosage is always within 1.5%~2.0%, sometimes higher than 3.0%, which is much higher than the average level (0.6%~0.8%) of pellets from abroad. Bentonite use inevitably reduces the iron grade of pellets, thereby increasing fuel consumption of the blast furnace. Organic binders have been developed to avoid this defect. Organic binders can be burnt out during the high temperature roasting process with almost no residue left, which would not affect the iron grade of pellets. However, the use of organic binder always brings about strength drop on sintered pellets, due to the higher porosity and less bonding phase between the iron oxide particles. Based on this, a HA binder (a new type of binder composed of organic and inorganic components) was invented by our research group. It has the advantages of both high bonding properties and low residue, and, thus, has good application prospects in the production of iron ore pellets.


As a typical polymetallic ore in China, vanadium-bearing titanomagnetite (VTM), with a relatively low iron grade (57%~60%), is always difficult to process. To make its oxidized pellets always requires more bentonite, higher temperatures, and a longer duration compared to that of ordinary magnetite concentrates. Therefore, applying the HA composite binder to VTM pelletizing is of great significance for the upgrading of VTM pellet production. This work studied the effects of HA binder on the balling, preheating, and roasting behaviors of VTM systematically by comparing them with bentonite. The morphology evolution and interaction features of the mineral phases during high temperature processes were deeply investigated by XRD, optical microscopy, and SEM–EDS measures to explain the induration mechanism of VTM-oxidized pellets.


2. Experimental Materials and Method
2.1. Materials
The compositions of vanadium-bearing titanomagnetite, bentonite, and limestone used in this work are given in Table 1. The VTM concentrated with natural basicity (CaO/SiO2) of 0.17 was mainly composed of 55.45% total iron and 11.17% TiO2. The bentonite with 59.66% of SiO2, 12.43% of Al2O3, and 2.68% of Na2O was a typical sodium-based bentonite. Limestone with 49.92% of CaO and 2.21% of MgO was used as an additive for basicity regulation of oxidized pellets.


Table 1. Chemical composition of VTM concentrate and bentonite/wt%.

Component Fe Total FeO TiO2 V2O5 SiO2 CaO MgO Al2O3 K2O Na2O LOI
VTM 55.45 32.36 11.17 0.61 4.39 0.75 3.16 3.05 0.02 0.11 −1.33
Bentonite - - - - 59.66 4.60 3.40 12.43 0.94 2.68 14.08
Limestone - - - - 2.35 49.92 2.21 0.37 0.11 0.05 42.77


VTM was identified as a titanomagnetite concentrate with minor traces of ilmenite and trace of hortonolite, based on the XRD pattern (Figure 1a). The SEM image (Figure 1b) shows that the VTM has irregular particles with size distributed between 10~100 μm. The coarse particles are mostly magnetite (white) and gangue (light gray), while the fine particles are almost entirely pure magnetite.

Figure 1. XRD pattern (a) and SEM image (b) of VTM.


The particle size distribution of the VTM concentrate, bentonite, and limestone are shown in Figure 2. The VTM with D50 of 35.22 μm is a suitable raw material for pellet-making. A raw material containing the appropriate amount of fine particles can improve the strength of green balls and the consolidation of pellets. For the bentonite and limestone with similar particle size distribution, their D50 reaches as fine as 7.75 μm and 6.75 μm, respectively. In general, fine particle size facilitates the dispersion and mixing of additives (bentonite and limestone) in VTM concentrate and, therefore, its pelletizing process.

 

Figure 2. Particle size distribution of VTM concentrate (a), bentonite (b), and limestone (c).


Proximate analysis of the HA-based binder used in this work and the chemical composition of HA binder ash are shown in Table 2. The moisture, volatiles, and fixed carbon of the HA binder are 15.62%, 18.21% and 26.66%, respectively, which is burnt out during the pellet-sintering process. About 39.51% of ash could remain in sintered pellets, which are mainly composed of 56.90% of SiO2, 24.69% of Al2O3, and 6.80% of Na2O.


Table 2. Proximate analysis (ad) of HA binder and chemical composition of binder ash/wt%.

Component Moisture Volatiles Ash Fixed Carbon Chemical Composition of Ash
Fe2O3 SiO2 CaO MgO Al2O3 Na2O
Mass/wt% 15.62 18.21 39.51 26.66 5.86 56.90 0.58 0.70 24.69 6.80


2.2. Method
Balling: VTM concentrate were fully mixed with a certain amount of bentonite, HA binder, or limestone. The mixture was pelletized to green balls by a disc pelletizer (Φ 1 m, with dip angle of 45° and rotation speed of 23 rpm) in 10~12 min with moisture of ~8.0%. Then, the drop strength, compressive strength, and decrepitation temperature of the green balls (size: 10~16 mm) were tested.


Sintering: To simulate the thermal regulation of industrial grate–kiln roasting process, pellets were sintered in two stages in an experimental tube furnace: (1) preheating, dried green balls were roasted at 900~1000 °C for 12~18 min; (2) oxidized roasting, the preheated pellets were roasted at 1200~1300 °C for 8~12 min. Then, the compressive strength of the sintered pellets was measured with a universal material testing machine (KL-WS) as they cooled to room temperature.


Characterization: The mineral phase composition of the oxidized pellet was identified by X-ray diffraction (Rigaku, Japan, D/max2500). Morphology and dissemination features of minerals in oxidized pellets were observed by optical microscopy (Leica, Germany, DMRXP) and scanning electron microscope (FEI, Netherlands, Quanta-200) equipped with EDS.

 

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