Manganese Ore Decomposition and Carbon Reduction in Steelmaking

Experimental materials and preparation

The starting materials were metallurgical manganese ore powder, manganese oxides of reagent grade, manganese carbonates and silicates, reducing agents (coke powder, anthracite coal, and graphite), and converter slag. The starting materials were crushed, milled, and screened to pass through a 0.074-mm sieve and oven dried at 200 °C for 24 h.

The manganese ore contained 42.8 % TMn, 11.6 % TFe, 7.47 % Si, 0.13 % P, and 0.016 % S. Figure 1 shows the powder X-ray diffraction pattern and major mineral phases of the manganese ore: 14 % 3Mn₂O₃·MnSiO₃, 32 % 3Mn₂O₃·MnSiO₃, 15 % MnO₂, 23 % MnCO₃, and 16 % Fe₃O₄. The initial and final converter slag used in the experiments contained, respectively, 36.30 % and 44.30 % CaO, 20.33 % and 14.35 % SiO₂, 10.73 % and 11.37 % MgO, 20.33 % and 14.60 % TFe, 2.20 % and 3.80 % P₂O₅, and the slag basicity was 1.78 and 3.09.

Figure 1:

Powder X-ray diffraction pattern of the manganese ore.

Experimental equipment and method

A synchronous thermal analyzer and resistance furnace were used in the experiments. The analyzer simultaneously performs thermogravimetric (TG) and differential scanning thermal analysis. The samples (6–10 mg) were placed in alumina or platinum crucibles, which were then loaded in the thermal analyzer and heated from 25 °C to 1,600 °C at 10 K/min in Ar gas atmosphere with a flow velocity of 50 ml/min. The reducing agent was graphite, anthracite, and coke. The carbon content in the raw materials was 20 %.

The samples were examined with a scanning electron microscope to obtain the slag phase composition.

Thermal decomposition of manganese ore at 1,600 °C

Figure 2 shows the decomposition of manganese ore from room temperature to 1,600 °C. The total mass loss was 16.31 % and the DSC (Differential Scanning Calorimetry) data can be divided into five regions. Based on the relevant literature reports [6] and thermodynamic analysis, the following reactions may occur in the five regions of the DSC curve. Region 1 between 200 °C and 400 °C includes an endothermic and an exothermic peak. The endothermic peak is attributed to the evaporation of the adsorbed water, while the exothermic peak is due to the conversion of α-MnO₂ to β-MnO₂. The decomposition of MnO₂ and MnCO₃ occurs in region (2), with measured mass loss of 9.43 % close to the theoretical value of 9.2 % and measured enthalpy change of 25.31 kJ/mol compared with the calculated value of 28.20 kJ/mol. The decomposition of Mn₂O₃ occurs in region (3), with measured mass loss of 3.2 % close to the theoretical value of 3.18 % and measured enthalpy change of 29.5 kJ/mol compared with the calculated value of 28.18 kJ/mol. The decomposition of 3Mn₂O₃·MnSiO₃ and Mn₃O₄ occurs in region (4); the measured mass loss is 1.85 %, much less than the theoretical loss of 6.99 %, and the measured enthalpy is 52.46 kJ/mol, much higher than the theoretical enthalpy change of 23.73 kJ/mol. The decomposition of Fe₃O₄ occurs in region (5); the measured mass loss is 1 %, much less than the theoretical value of 6.89 %. The remaining material is a mixture of MnO (43 %), MnSiO₃ (40 %), and FeO (17 %).

Figure 2:

Decomposition of Mn ore and reduction of Mn ore and graphite.

Carbon reduction of manganese ore

Assuming that MnO₂, MnCO₃, Mn₂O₃, and Fe₃O₄ completely decomposed at 1,400 °C (in contrast to MnSiO₃), the carbon loss was 50 % and the theoretical mass loss of the manganese ore mixture was 44.2 %. Figure 2 shows that the manganese mass loss is 34.39 % at 1,400 °C. The curve in Figure 2(b) can be divided into four regions: (1) the water evaporation region, (2) the reaction of MnO₂ and carbon to produce MnO and CO, (3) the melting of MnSiO₃, and (4) the decomposition of Mn₃O₄ and the reduction of manganese oxide to Mn, i. e., C+MnO=Mn+CO. The remaining material was a mixture of metal (30.8 % Mn₇C₃ and 16.1 % FeC₃) and slag (2.5 % FeO, 5.1 % SiO₂, and 18.8 % MnO).

Figure 3(a) shows the effects of the different reducing agents on the reduction of manganese ore at 1,200 °C. It is seen from Figure 3(b) that the mass loss (ΔTG) of anthracite, coke, and graphite mass loss is 28.85 %, 26.7 %, and 24.31 %, respectively. After deducting the volatiles in anthracite and graphite, the largest mass loss in this mixture is that of coke, followed by that of anthracite and graphite. Clearly, coke is the best reducing agent of the three.

Figure 3:

Comparison of the TG and DTA curves of manganese ore and carbon mixtures.

Decomposition products of manganese ore at 1,600 °C

Manganese ore without a reducing agent was heated at 1,600 °C under argon atmosphere for 30 min. Manganese ore (80 %) and graphite (20 %) were subsequently reacted at 1,600 °C under argon atmosphere for 30 min. In both cases, the slag samples were cooled and observed using an optical microscope and scanning electron microscopy (SEM). The results are shown in Figures 4(a) and 4(b).

Figure 4:

Slag after manganese ore decomposition and carbon reduction at 1,600 °C. (a) Slag after manganese ore decomposition at 1,600 °C. (b) Slag after carbon reduction of manganese ore at 1,600 °C.

In Figure 4(a), the slag samples contain two phases in equal proportions. The chemical composition of each phase is given in Table 1. The results suggest that phase A comprises MnO, FeO, Al₂O₃, and SiO₂. The alumina is attributed to the alumina crucible. Phase A is calculated to comprise 68.4 % MnSiO₃, 18.8 % MnO, and 12.8 % FeO. Phase B comprises FeO and MnO.

In Figure 4(b), the slag samples from the carbon reduction of the manganese ore contain three phases. The third phase is the gel material for preparing the samples. The chemical composition of each phase is given in Table 1. Point A represents the carbon-rich ferromanganese phase produced by the reduction of manganese ore. Point B is a ferromanganese phase that also contains Al₂O₃, SiO₂, MnO, and FeO. Point C denotes the excess graphite after the reduction of the manganese ore. Point D mainly comprises Al₂O₃, SiO₂, and MnO.

Based on the determination of XRD in samples, semiquantitative results were obtained. The carbon reduction products of the manganese ore were Mn₇C₃ (27.8 %) and FeC₃ (9.4 %) in the metal phase and SiO₂ (14.7 %) and MnO (43 %) in the slag.

Decomposition of manganese ore at 1,600 °C

The decomposition of pure MnO₂ is shown in Figure 5(a). There are mainly three similarities with the DTA (Differential Thermal Analysis) of the manganese ore. In pure MnO₂, the decomposition of MnO₂ corresponds to peak 1, the decomposition of Mn₂O₃ corresponds to peak 2, and the decomposition of Mn₃O₄ corresponds to peak 4. The decomposition of manganese ore with 15 % MnO₂ is associated with a mass loss of 1.35 % at 550–680 °C (peak 1), mass loss of 0.45 % at 950–1,020 °C (peak 2), and mass loss of 0.63 % at 1,400–1,550 °C (peak 3). The mass losses at peaks 1, 2, and 4 are lower than the corresponding losses in the decomposition of manganese ore. Thus, MnO₂ contributes to the decomposition of the manganese ore.

Figure 5:

DTA of pure MnO_2, MnCO₃, and MnSiO₃. (a) Decomposition of pure MnO₂, (b) decomposition of pure MnCO₃, (c) decomposition of pure MnSiO_3.

The high-temperature decomposition of pure MnCO₃ is shown in Figure 5(b). Compared with the DTA of the manganese ore decomposition, the main similarity is in peak 1 that is associated with the reaction MnCO₃=MnO+CO₂. Manganese ore with 23 % MnCO₃ undergoes 8.25 % mass loss at 400–600 °C (peak 1), which equals the sum of the mass loss of pure MnO₂ and the decomposed manganese ore.

The high-temperature decomposition of MnSiO₃ is shown in Figure 5(c). The main similarity with the DTA of the manganese ore is in peak 1, where MnSiO₃ melting occurs and heat is absorbed. There is little mass loss because there are only few impurities.

The comparison of the DTA of the pure substances can help decipher the decomposition of manganese ore. In the manganese ore, MnO₂, MnCO₃, Mn₂O₃, and Mn₃O₄ decompose below 1,200 °C and produce MnO that accounts for 49 % of the manganese. Fe₃O₄ decomposes to FeO at 1,400 °C, whereas the MnSiO₃ (35 %) in the manganese ore is difficult to break down. Improving this step in the reduction of the manganese ore will maximize the yield of manganese.

Effect of converter slag on carbon reduction of manganese ore

BOF (Basic Oxygen Furnace) slag from the early and interim blowing stage [18] was added to the manganese ore. Figure 6 shows the DTA of mixtures with 80 % manganese ore, 20 % graphite, and 10 % initial or endpoint BOF slag. The addition of initial or endpoint BOF slag has little effect on the manganese ore reduction.

Figure 6:

DTA of Mn ore, graphite, and converter slag mixture.

Manganese ore (80 %), graphite (20 %), and converter slag (10 %) were heated at 1,600 °C under argon atmosphere for 30 min. The slag sample was then cooled and observed using an optical microscope and SEM. The results are shown as Figure 7.

Figure 7:

Slag of manganese ore after carbon reduction with converter slag at 1,600 °C.

In Figure 7, the slag samples of the carbon reduction of manganese ore are divided into two phases. The second phase is the gel material for preparing the samples. The chemical composition of each phase is given in Table 2. Point A represents carbon ferromanganese and ferrosilicon phases. The composition of point B is mainly that of a ferromanganese phase with Al₂O₃, SiO₂, MnO, FeO, and CaO. Point C is mainly characterized by the presence of Al₂O₃, SiO₂, MnO, and CaO. Finally, point D represents the excess graphite after the carbon reduction of the manganese ore.

Improving the reduction of manganese in manganese ore

In the experiments, the achieved reduction rate of manganese ore was only 66 %. MnSiO₃ must be decomposed to increase the reduction rate of manganese ore. Figure 8 shows the DTA of the MnSiO₃ decomposition and carbon reduction in 1:1 carbon and manganese ore mixtures. The MnSiO₃ decomposition is associated with 4.19 % mass loss, and some additional minor mass loss is due to impurities. The mass loss of the mixture of MnSiO₃ and graphite is 17.81 % at 1,400 °C. Based on calculations, if the carbon loss is 50 %, approximately 25 % of MnSiO₃ is reduced by graphite. Therefore, the carbon reduction of MnSiO₃ is critical to the direct alloying of the manganese ore.

Figure 8:

The TG and DSC curves of MnSiO₃, graphite and Mn ore, and graphite and CaO or MgO. (a) Manganese silicate and graphite, (b) graphite and Mn ore, and graphite and CaO or MgO.

The results in [19] suggest that adding calcium oxide or magnesium oxide to the manganese ore contributes to the reduction of manganese ore. The added calcium or magnesium substitute for manganese in the manganese silicate to produce manganese metal.

According to thermodynamic calculations, the Gibbs-free energy of reactions (1) and (2) at 0–1,400 °C is less than 1. Therefore, the CaSiO₃ and MgSiO₃ phases in reactions (1) and (2) are relatively stable and can contribute to the reduction of manganese.

Figure 8(b) shows the thermal analysis of mixtures of 72.7 % manganese ore, 18.2 % graphite, and 9.1 % calcium oxide or magnesium oxide. The mass loss in the samples with CaO or MgO is higher and suggests that adding calcium oxide or magnesium oxide to manganese ore promotes the carbon reduction of manganese ore. Magnesium oxide has the most positive effect.