Basic principles of oxygen bottom-blown melting

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Basic principle of oxygen bottom-blown melting: Oxygen bottom-blown melting is a type of melting process in which a mixture prepared by batching and granulation is fed from the top of the furnace, while oxygen is blown in from the bottom to stir the melt. High-pressure oxygen and nitrogen are ejected at supersonic speeds from the nozzle, and the small bubbles, driven by their kinetic energy, cause the high-temperature melt to move upward and circulate on both sides, thereby ensuring thorough mixing between the melt and the bubbles. This results in the surface of the melt bulging slightly and taking on a conical shape with a smaller base and a larger top, similar to that of medium-pressure hot springs. The metal sulfides in the mixture undergo oxidation, slag formation, and lead precipitation reactions in the slag layer, producing metal oxides, sulfur dioxide, and liquid lead droplets. A large amount of heat is released, sustaining these oxidation reactions. At the same time, some crude lead and low-melting-point lead slag are formed, with the crude lead flowing toward the lead precipitation area due to gravity. Based on the oxygen potential-sulfur potential phase diagram of the Pb-O-S system at 1200°C, it is possible to determine the locations of the sintering and roasting areas, the blast furnace reduction area, and the direct lead smelting area. The stable regions for PbS, PbO, PbSO4·2PbO, PbSO4·PbO, PbSO4, and Pb can also be identified. For the slag produced in direct lead smelting to reach the level of blast furnace slag (<3%), the free oxygen potential in the slag discharged must be 10-5 Pa. Therefore, in direct lead smelting, lead is discharged when the oxygen potential is high, while slag is discharged when the oxygen potential is low ； Under melt control at temperatures of 900–1050°C, the lead content in the initial slag is 30–40%. During actual melting, the melt is controlled at a temperature of 1000–1150°C; such conditions are favorable for increasing the rate of primary lead precipitation. However, at higher temperatures, the vapor pressures of PbS and PbO increase – the vapor pressure of PbS ranges from 8 to 101.325 KPa at 1074–1281°C, while the vapor pressure of lead oxide ranges from 1.3 to 13 KPa at the same temperature range, which leads to an increase in the ash content. Due to the high zinc content in the raw materials, sufficient iron is required to combine with zinc to form stable zinc ferrite, thereby reducing the melting point of the high-lead slag. It is necessary to keep the zinc-to-iron ratio in the raw materials between 0.58 and 1.0; some of the iron reacts with PbO, SiO2, and CaO to form a slag solid solution that helps keep the slag’s melting point low. This approach allows for melting at temperatures between 1000 and 1150°C, while also enabling the lead matte to dissolve in the crude lead at higher temperatures without forming barriers that could block the siphon effect. In addition to displacing PbO from the slag, the role of CaO is also to break the Fe-O-Zn bonds, thereby facilitating the reduction of zinc oxide. The actual results of melting show that the lead content in the slag ranges between 40% and 48%. Of course, there are differences between the actual thermodynamic and kinetic conditions in the melt inside the furnace and those assumed in theory, but a lead content of 30–40% in the initial slag is the asymptote to which actual control practices converge. The main reaction equations are as follows: Main reactions for lead precipitation in bottom-blown furnaces: 2PbS + 3O2 = 2PbO + 2SO2↑, PbS + O2 = Pb + SO2↑, PbS + 2PbO = 3Pb + SO2↑, PbS + PbSO4 = 2Pb + 2SO2↑. Slag formation reactions: 2FeS + 3O2 + SiO2 = 2FeO·SiO2 + 2SO2↑, CaCO3 = CaO + CO2↑; 2FeO·SiO2 + CaO + 2PbO = 2PbO·CaO·2FeO·SiO2; FeO + ZnO + SiO2 + CaO = ZnO·FeO·SiO2·CaO; ZnO + Fe2O3 = Zn(FeO2)2 (zinc ferrite). Reactions involving other impurities: 2ZnS + 3O2 = 2ZnO + 2SO2↑