Problems with bottom-blown furnaces

1 Introduction: The Mizuguchiyama Lead Industry Development Company began production in August 2005, using the SKS lead smelting method with independent intellectual property rights (oxygen bottom-blown melting – blast furnace reduction lead smelting method), thereby successfully addressing the environmental pollution problem caused by lead smelting emissions. In the early stages of furnace operation, due to various reasons, the rate of primary lead deposition is quite low. After more than a year of production practice, the single lead sinking rate can reach or exceed the design level. 2 Basic principle of oxygen bottom-blown melting The basic principle of oxygen bottom-blown melting is as follows: The PbS in lead concentrate is oxidized by O2, which is highly dispersed in the molten pool in the form of bubbles, to produce metallic Pb and PbO. These substances, along with the oxidized FeO and other components that contribute to slag formation, melt together to yield crude lead, a slag with a high lead content (commonly known as high-lead slag), and flue gas containing SO2. The main chemical reaction is: PbS + 2PbO = 3Pb + SO2↑ Schuhmann and others drew the equilibrium phase diagram of the Pb—S—O system at pSo2 = 105 Pa (Figure 1). The temperature at point y in Figure 1 is the lowest equilibrium temperature at which PbS transforms into liquid lead. When pSO2 changes, the equilibrium temperature and equilibrium oxygen pressure indicated by point Y also change accordingly. For example: pSO2 = 1×105 Pa, T = 960°C, pO2 = –4.5 Pa; pSO2 = 0.1×105 Pa, T = 860°C, pO2 = –5.7 Pa; pSO2 = 0.01×105 Pa, T = 830°C, pO2 = –6.3 Pa. During the production process, the SO2 concentration in the flue gas is between 10% and 15%, while pSO2 is between 0.1 and 0.15×105 Pa; therefore, the equilibrium temperature for the formation of liquid lead is approximately 900°C. In Figure 1, the equilibrium diagram of the Pb–S–O system at pso2 = 105 Pa shows that within the furnace, PbS primarily reacts according to the following equation: PbS(l) + 2PbO(l) = 3Pb(l) + SO2. The equilibrium constant K is given by K = aPb3 × pso2 / aPbS × aPbO2. Assuming that crude lead constitutes a dilute solution, aPb = 1; aPbS is expressed in terms of the sulfur content in the lead melt. Thus, the equilibrium constant K can be written as K1 = pso2 × S(wt%) × aPbO2. This indicates that, at a certain temperature and pso2 level, the sulfur content in the lead melt is inversely proportional to the square of aPbO in the slag. Therefore, a low level of aPbO in the slag leads to an increase in sulfur content in the crude lead as well as significant volatilization of PbS; in such cases, the conversion of PbS to metallic Pb is incomplete. Therefore, the lead content in high-lead slag cannot be reduced to very low levels; it generally does not fall below 35%. 3 Analysis of factors affecting the rate of lead sinking per cycle 3.1 Furnace temperature During normal production, the slag temperature ranges from 1000~1100°C, and sometimes even reaches 1150°C, which is much higher than the equilibrium temperature required for liquid lead to form. From a thermodynamic perspective, such a temperature meets the conditions for the formation of liquid lead ； From a kinetic perspective, the main reactions within the furnace are liquid-solid reactions. The two factors that affect their reaction rate are temperature and the diffusion rate of substances. Under high temperatures and strong stirring conditions in the furnace, the reactions can proceed rapidly, allowing liquid lead to be formed quickly. Furthermore, the high temperature inside the furnace effectively reduces the viscosity of the high-lead slag, increasing its fluidity, which facilitates the sedimentation and separation of slag from lead. This helps to minimize the loss of metallic lead mixed within the high-lead slag, thereby improving the rate of primary lead separation. On the contrary, an excessively low slag temperature leads to a decrease in the primary lead settlement rate. However, excessively high temperatures can cause a significant increase in the soot ratio, which in turn leads to a decrease in the primary lead deposition rate. Therefore, both excessively high and low furnace temperatures lead to a decrease in the primary lead sinking rate. Therefore, an appropriate furnace temperature needs to be selected during the production process. Based on production experience over the past two years, it is appropriate to set the slag temperature at 1000–1100°C and the flue gas temperature at 850–900°C. 3.2 Particle ore lead grade: Based on production experience, the main factor affecting the primary lead precipitation rate is the grade of the particle ore. The calculation method for the primary lead sinking rate is as follows: Primary lead sinking rate = Lead content in primary crude lead / Lead content in the lead sand used / 100%. In the case of a closed-loop circulation of ash, the lead content in primary crude lead equals the lead content in the lead sand used – the lead content in the high-lead slag – plus any unaccounted losses of lead. Therefore, in production, in order to increase the primary lead precipitation rate, it is necessary to reduce the lead content in the high-lead slag, while increasing the lead content and output of the primary crude lead. Based on the above analysis, the PbO content in high-lead slag cannot be reduced to very low levels. When the lead grade of the granular ore is low, this leads to an increase in the amount of high-lead slag per unit volume, which in turn results in an increased amount of lead carried away by the high-lead slag, thereby reducing the primary lead precipitation rate. Based on production experience over the past two years, when the lead content in the ore is below 45%, it is basically impossible to produce crude lead in a single batch ； However, when the lead content in the ore is below 48%, the yield of primary crude lead is significantly affected, and the rate of lead precipitation drops considerably. Therefore, the lead content in the granular ore should be maintained at over 48%, and ideally at over 52%. 3.3 High-lead slag type: During the production process, a slag type with a low melting point and low viscosity should be selected to facilitate better separation of lead from the slag. According to the liquid phase diagram of the FeO—SiO2—CaO system, 2FeO·SiO2, which is near ferroolivine, has a relatively low melting point of about 1200°C; the addition of FeO lowers this melting point to around 1100°C. Based on an analysis of the melting point and viscosity of slags, slags in the vicinity of the FeO·SiO2—2FeO·SiO2 composition have a lower melting point and lower viscosity. Adding too much FeO on this basis can still reduce viscosity, but it also raises the melting point; increasing the amount of SiO2 is even more detrimental, as it not only raises the melting point but also increases viscosity. Therefore, based on past practical experience, it is appropriate to choose a slag composition with FeO/SiO2 = 1.8–2.0 and CaO/SiO2 = 0.45–0.55. 3.4 Oxygen-to-fuel ratio The oxygen-to-fuel ratio is a very important control parameter in bottom-blown furnace production; an excessively high value will cause a large amount of metallic lead to oxidize and enter the slag, which undoubtedly leads to an increase in the lead content in the slag. At the same time, it also causes more FeO to oxidize into Fe3O4, a substance with a high melting point that is harmful to production ； Excessive control can result in insufficient heat input into the furnace and poor slag fluidity, leading to inadequate separation of lead from the slag and an increase in the metal lead content in the slag. Therefore, the control of the oxygen-to-feed ratio is very important. Theoretically, this ratio should be determined through calculations, but due to the instability in the composition of the raw materials and the inaccuracy in oxygen measurement during our production process, the ratio calculated theoretically cannot be used to guide production. Hence, the oxygen-to-feed ratio should be determined based on the fluidity of the high-lead slag at the time of slag discharge: reduce the ratio when the slag is too thin, and increase it when it is too viscous. 3.5 Relative height between the lead dam and the slag dam: Based on practical production experience, an appropriate height for the lead dam is an important measure for reducing the amount of metallic lead in the slag and increasing the rate of lead precipitation in the first stage. The height of a lead dam can be calculated using the following method: Calculation basis: a) The specific gravity of liquid lead is 11.0 g/cm3, while that of liquid slag is 5.5 g/cm3. b. When the level of the slag surface drops to the center of the slag outlet, the height of the lead liquid level in the siphon channel is equal to the height of the lead dam. c. The lead thickness at the bottom of the reaction section is 300 mm. Given that the center height of the slag outlet is 925 mm, the height of the lead dam should be: (300×11 + 625×5.5) ÷ 11 = 612.5 mm. 3.6 Other factors Other factors that contribute to increasing the rate of lead precipitation include: reducing dust levels, keeping the siphon channels unobstructed, improving standardized operational procedures, enhancing production efficiency, and controlling appropriate levels of Zn, Cu, and S in the raw materials. 4. Conclusion The conditions for increasing the primary lead precipitation rate in the SKS lead smelting method are: a) maintaining the slag temperature at 1000–1100°C and the flue gas temperature at 850–900℃ ； b. The grade of the ore concentrate is over 52% ； c. The slag type for high-lead slag is selected as FeO/SiO2=1.8~2.0, CaO/SiO2=0.45~0.55 ； d. Determine the appropriate oxygen-to-feed ratio based on the fluidity of the high-lead slag during slag discharge ； e. The relative height between the lead dam and the slag dam should be maintained at an appropriate level ； f. Reduce dust emission, maintain the unobstructed flow in the siphon channels, standardize operational procedures, increase production efficiency, and control appropriate levels of Zn, Cu, S, and other elements in the raw materials.

I’ve benefited a lot, thank you! There is another question: if the ore contains too much sulfur (around 20%), 56% lead and 15% iron, with normal levels of other impurities, will this have an impact on the rate of lead extraction in the first pass?

The raw materials you mentioned have little impact on the primary lead precipitation rate, but the processing volume probably cannot be too high. High sulfur, high iron, and excessive calorific value. Ah, it would be great if it could be combined with our materials. You can’t be Yuteng, can you?

Can copper oxide ore be used in a bottom-blown furnace to produce crude copper?

What is the smallest specification for a bottom-blown furnace?

Oxygen bottom-blown furnaces are primarily used for oxidation reactions; therefore, copper oxide ore cannot be used to produce crude copper. Copper oxide ore is primarily used in reduction reactions to produce crude copper.

Currently, bottom-blown furnaces in China generally have a diameter of 3.8 meters and a length of 11.5 meters. Only the Mizuguchi Mountain one is thicker, with a diameter of 4.1 meters and a length of 14 meters. Copper oxide ore can be used in combination with copper sulfide ore to produce matte; if the copper content in the ore is above 40%, it can be used together with matte to smelt crude copper, otherwise smelting crude copper will certainly not be economically viable.

It seems everyone here is an expert. Who can tell me on QQ, guys?

The flue gas temperature is between 850 and 900℃； Is it the temperature at the inlet of the waste heat boiler?

It is the temperature at the inlet of the vertical flue.