Basic principle of smelting manganese-rich slag

The process of smelting manganese- rich slag is the enrichment process of manganese in slag, including the decomposition of ore crystallization water at high temperature, the decomposition of carbonate, the reduction of manganese high-valent oxide to low-oxide oxide, and the reduction atmosphere. The selective reduction of iron and phosphorus in China. The most fundamental of these is the selective reduction of iron and phosphorus.
The theoretical basis of manganese-rich smelting is the selective reduction of oxides in ore by controlling heat and slagging processes in accordance with thermodynamics and kinetics.
(1) Reduction of oxides in manganese-rich slag smelting MnO ₂ , Mn ₂ O ₃ , Mn ₃ O ₄ , Fe ₂ O ₃ , P ₂ O ₅ and other oxides in manganese ore are easily reduced by CO or H ₂ MnO, and FeO, MnO and FeO but C is further reduced to the metal, which is somewhat different conditions, temperature, and the heat required for reduction of MnO much higher. The reaction equation is as follows:

It can be seen from the above reaction equation that the reduction temperature of iron and phosphorus is lower, the heat required is less, so it is easy to reduce, and the reduction temperature of manganese is high, the heat consumption is large, and the reduction is difficult. Therefore, under the proper conditions of the reducing agent, the smelting temperature is controlled below 1350 ° C, iron and phosphorus are preferentially reduced, and manganese is enriched in the slag in the form of MnO.
The manganese ore containing silica, manganese, iron, and phosphorus oxide is reduced by coke , and if the amount of coke used is different from the temperature, different products are obtained (Table 1).

Under blast furnace smelting conditions, the order of reduction and reduction of each element is different. The reason for these differences is that the reduction conditions required for each element are different, that is, the composition of the reducing agent, temperature and pressure that can be created in the blast furnace. The degree of difficulty required to achieve a balance in the reduction reaction varies. [next]
The ease with which an oxide is reduced depends on the affinity of the element for oxygen, that is, the size of the oxide decomposition pressure, which can be measured by the oxide equilibrium decomposition pressure Po ₂ (see Table 2) and (Figure 1). . The affinity for oxygen is large, and the reduction of the element having a small decomposition pressure of the oxide is difficult, and the oxide is relatively stable. vice versa.

It can be seen from the graph that the lower the temperature, the smaller the decomposition pressure of pure oxide, the larger the pressure difference between various pure oxides, and the smaller the degree of reduction of oxides in the slag, between various oxides. The greater the difference in reduction. On the contrary, the higher the temperature, the smaller the decomposition pressure difference, the greater the degree of reduction of the oxide in the slag, and the smaller the difference in the reduction degree of various oxides.
It can also be seen that Cu ₂ O, NiO and FeO are more easily reduced under blast furnace conditions, so almost all of them are reduced to metal in the blast furnace; and Cr ₂ O ₃ , MnO, SiO ₂ and TiO ₂ are difficult. Reduced oxides; therefore only a portion of the blast furnace can be reduced. Al ₂ O ₃ , CaO, and MgO cannot be reduced in the blast furnace, but all enter the slag. [next]
The manganese in manganese ore is mostly in the form of MnO ₂ , Mn ₂ O ₃ , Mn ₃ O ₄ , MnO, etc. The high valence oxide of manganese is not as stable as the low-valent oxide, so the first three oxides are easily smelted in sintering or blast furnace. In the process, it is reduced to a low-oxide by burning. The reduction process of manganese oxide is carried out in the same manner as the reduction of iron oxide, in the order of higher oxide to lower oxide. The reaction equation in the blast furnace is as follows:
2MnO ₂ +CO===Mn ₂ O ₃ +CO ₂ +226840kJ (1)
3Mn ₂ O ₃ +CO===2Mn ₃ O ₄ +CO ₂ +170240kJ (2)
Mn ₃ O ₄ +CO===3MnO+CO ₂ +51920KJ (3)
The reactions (1) and (2) are irreversible, and the reaction proceeds easily under blast furnace pressure and a reducing atmosphere. Although the reaction (3) is reversible, when the equilibrium is actually reached, the concentration of CO in the gas phase is small, so that Mn ₃ O ₄ is also easily reduced in the blast furnace.

MnO is a relatively stable oxide, and it is very difficult to reduce MnO with CO (Fig. 2). At 1400 ° C, MnO is reduced with CO, and the CO ₂ concentration in the equilibrium phase is 0.03%. The reduction of MnO with CO can only be carried out under the condition that a large amount of solid carbon exists and continuously reacts with CO2, so that the reaction is actually direct reduction, and the reaction formula is as follows:

MnO+CO===Mn+CO2-121590kJ

                                        C+CO2===2CO-157890kJ       

                                       MnO+C===Mn+CO-279480kJ

Since MnO has entered the slag before the reaction, the reaction is actually carried out between the solid phase and the liquid phase. In order to suppress the reduction of manganese under blast furnace conditions, it is necessary to reduce the partial pressure of CO and reduce the activity of MnO. The most influential of these is temperature and slag basicity.
(2) Selection of smelting temperature The smelting of manganese-rich slag should inhibit the reduction of manganese, which is actually the reduction condition of MnO in the slag. The direct reduction reaction of MnO MnO+C=Mn+CO is an endothermic reaction. The partial pressure of CO in the equilibrium gas phase increases as the temperature rises. That is, as the smelting temperature increases, the MnO reduction is intensified. Therefore, controlling the smelting temperature is the key measure to control the reduction of MnO and improve the grade of manganese-rich slag. Figure 3 is a graph showing the relationship between the smelting temperature and the degree of reduction of MnO and MnSiO ₃ . Figure 4 is a graph showing the relationship between the slag temperature and the MnO content in the slag.

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Manganese-rich slag smelting treatment of manganese-poor ore, the amount of SiO _{2 in the} slag is relatively high. In the presence of sufficient SiO ₂ , when the temperature in the blast furnace is 1170 ° C, almost all of MnO combines with SiO ₂ to form slag. Reducing Mn from slag is much more difficult than reducing from a separate phase. The experiment indicates that MnSiO _{3 is} only reduced by 3% at 1300 °C. On the other hand, the reduction of iron is easier to carry out. The reduction of iron FeO+C=Fe+CO starts from 685 °C, while the high-valent iron oxide is reduced to Low-cost iron oxide (FeO) is completed at 900~1000 °C. When the temperature reaches 1250 °C, iron silicate (Fe ₂ SiO ₄ ) is also reduced in a large amount. Therefore, from the perspective of ensuring the full reduction of iron and the reduction of manganese, the smelting temperature of manganese-rich slag is controlled at 1280~1350 °C. At this temperature, the fluidity of the slag is also guaranteed.
(3) Selection of alkalinity of slag Alkaline oxides CaO and MgO have greater affinity for SiO ₂ than MnO, so MnO is displaced from silicate to form free MnO, and MnO activity increases. , reducing the starting temperature of MnO reduction and promoting the reduction of manganese. Its reaction formula is

MnSiO3+CaO===MnO+CaSiO3+59030KJ

                                         MnO+C===Mn+CO-279470KJ           

                                     MnSiO3+CaO+C===Mn+CO+CaSiO3-220440KJ

This is unfavorable for smelting of manganese-rich slag, so slag basicity must be controlled in smelting of manganese-rich slag. General manganese-rich smelting The ratio is controlled below 0.4. The alkalinity of the lean manganese ore itself is very low, so the natural alkalinity without adding a flux is usually used in the smelting operation.

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