Production Practice of Lead Oxygen-enriched Side-blown Furnace Commissioning

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Production Practice of Starting Up a Lead Oxygen-enriched Side-blown Furnace Hu Weiwen, Xu Xudong, Ouyang Kun (Hunan Shuikoushan Non-ferrous Metals Group Co., Ltd., Hengyang 421500, China) Abstract: This paper provides a detailed description of the trial production process and technical parameters for the largest oxygen-enriched side-blown reduction furnaces currently in use in China, which employ smokeless granular coal as a reducing agent. Industrial production practice has shown that this side-blown reduction furnace features advanced technology, low investment costs, stable operation, low comprehensive energy consumption per ton of lead produced, and a favorable working environment. Keywords: lead ； side-blown furnace ； Production practice ； Oxygen-enriched smelting: The side-blown furnace at a certain factory was designed by Xi’an Non-ferrous Metallurgy Design Institute. The furnace bed area is 12.15 m2, making it the largest oxygen-enriched side-blown reduction furnace in China to date that uses granular coal as a reducing agent. Its designed capacity is 100,000 tons of crude lead per year. Construction began in November 2013, and trial production started in October 2014, with the project progressing smoothly. The construction contents of the liquid high-lead slag reduction plant mainly include: 1) a side-blown reduction furnace capable of processing 230,000 tons of liquid high-lead slag per year ； 2) Flue gas desulfurization furnace for treating 129,000 tons of side-blown reduction furnace slag per year ； 3) Waste heat power generation system ； 4) Desulfurization system. The process flow diagram of a side-blown reduction furnace in a certain factory is shown in Figure 1. The side-blown reduction furnace includes systems such as a waste heat boiler, surface cooler, bag filter, waste heat power generation, and desulfurization. The basic task of oxygen-enriched side-blown reduction smelting is to reduce the liquid high-lead slag produced by the bottom-blown furnace, thereby yielding crude lead, zinc-containing slag, and sulfur-containing flue gas. The zinc-containing slag is sent to a flue-gas desulfurization furnace for further treatment, while the sulfur-containing flue gas is sent to a dust collection system before entering the desulfurization system. 1. For furnace construction and heating operations, magnesium-chromium bricks are currently primarily used as the lining material for the furnace shaft in oxygen-enriched side-blown furnaces that employ smokeless granular coal as a reducing agent. Using this type of building material has the following disadvantages: 1) The body of the side-blown reduction furnace is composed of water jackets, and once these water jackets leak, the magnesia-chromia bricks will react with water and disintegrate ； 2) During the construction of magnesia-chromium bricks, gaps between the bricks remain, and lead has a particularly strong ability to penetrate. During production, lead seeps into these gaps, damaging the bricks that make up the furnace bottom; it can even cause those bricks to lift off, thereby affecting the lifespan of the furnace chamber ； 3) The construction of magnesia-chromia bricks takes a long time; the entire construction process requires 2 to 3 weeks. A factory made comprehensive changes to the original design for constructing the furnace bowl; the refractory materials used for this bowl were replaced with aluminum-chromium slag ramming material and aluminum-chromium slag bricks. The entire construction process involved using aluminum-chromium slag ramming material for ramming, allowing for one-time formation of the structure. The use of this type of refractory material and construction method has the following advantages: 1) The aluminum-chromium slag refractory material exhibits good resistance to erosion and thermal shock ； 2) The hearth area of the side-blown furnace is compacted as a whole using aluminum-chromium slag compaction material, achieving one-time molding; this prevents lead from seeping to the furnace bottom and effectively extends the furnace’s service life ； 3) The furnace construction time is short; the entire furnace chamber can be built in less than 1 week. The heating of a side-blown furnace must meet the temperature rise requirements for the refractory materials in the furnace bowl, while also taking into account the regulations regarding the temperature rise of the waste heat boiler. The designed drying time for this furnace is 15 days, with three constant temperature levels of 200 ℃, 400 ℃, and 800 ℃. The specific steps are as follows: 1) Use a resistance wire furnace to heat the furnace at a low temperature; after reaching 200 ℃ according to the heating curve, maintain that temperature for 72 hours to ensure that the moisture on the surface of the chromium-aluminum slag packing material in the furnace slowly evaporates. 2) After maintaining the temperature at 72 °C for 72 hours, the temperature was increased further according to the heating curve (20 °C/h) to 400 °C, where it was maintained for another 72 hours. During this period, the rate of temperature increase must be controlled; it should not be too rapid. This is necessary to ensure that the moisture in the aluminum-chromium slag ramming material located in the middle and bottom parts of the furnace chamber evaporates gradually. If the temperature rises too quickly, a large amount of moisture in the ramming material on the surface and in the middle of the furnace chamber will evaporate, resulting in numerous cracks in that ramming material and significantly reducing the lifespan of the furnace chamber. 3) After maintaining the temperature at 72 hours, the resistance wire was removed, a steel plate was placed inside the furnace, and an oil gun was installed to heat the furnace. The temperature was raised to 800 °C following a specified heating curve, and the temperature was maintained at that level for 48 hours. The purpose of placing the steel plate was to contain the flame from the oil gun and ensure even heating of the refractory materials inside the furnace. Once all systems are ready for trial production, the furnace hearth temperature is raised to 1,200 °C in accordance with the heating curve before starting production with feedstock. 2. Once all the equipment in the side-blown reduction furnace for pilot production has been adjusted and tested, and the coordinated testing was carried out successfully without any faults; once the relevant auxiliary facilities are in place, with normal supply of water, electricity, gas, and oil; and once all the materials required for starting up the furnace are ready, with their quantities and quality meeting the requirements for operation, then it is possible to begin the process of feeding materials into the side-blown reduction furnace. Boiler startup procedure: 1) Lay 3 layers of wood inside the boiler. The purpose of laying wood is to start a fire and maintain the temperature in the furnace. 2) Light the wood, and once the fire is burning well, add coke to the furnace evenly. During coke feeding, control the process parameters of the side-blown reduction furnace to ensure thorough combustion of the coke inside the furnace. 3) Once the coke in the furnace has burned completely, bottom lead is added. The amount of lead added at the bottom is determined based on the size of the molten pool in the furnace; in a certain plant, the height of the lead at the bottom of the side-blown reduction furnace ranges from 600 to 650 mm. The rate at which the bottom lead is added must be adjusted according to changes in the temperature inside the furnace; during this process, a small amount of coke can be added to raise the temperature. All of the bottom lead used must be added within 1.5 hours. 4) Once all the lead has been added, the siphon opening is submerged in the lead pool, and the side-blown furnace begins to transfer the hot slag from the bottom-blown furnace into the first refining stage. During this period, the side-blown reduction furnace sets appropriate process parameters such as the amount of primary air, the number of primary air nozzles, the amount of secondary air, and the amount of coal fed, based on the characteristics of the high-lead slag in the bottom-blown furnace, including its volume and temperature. It is essential to maintain the temperature of the molten pool throughout this refining stage (with the slag temperature ranging from 1,200 to 1,300 °C and the lead liquid temperature ranging from 600 to 800 °C), in order to prevent the formation of a barrier at the interface between the hot slag and the lead pool, thus avoiding accidents involving lead reaching the furnace platform. Due to the large amount of unburned coke in the furnace during this period, the coal feeding rate can be adjusted appropriately during this blowing phase to ensure that all the coke in the furnace is burned up. 5) After one furnace cycle of blowing is completed, the side-blown reduction furnace returns to normal production operation. The composition (% ) of typical high-lead slag produced over 3 months during the trial operation of a side-blown reduction furnace in a certain factory was as follows: Pb 39.81, Zn 10.20, Cu 2.01, S 0.50, SiO2 9.88, FeO 17.79, CaO 3.56, Au 2.7 g/t, Ag 20.20 g/t. The slag type of high-lead slag in bottom-blown furnaces has a significant impact on the operation of top-blown furnaces; therefore, it is essential to strictly control the slag type of high-lead slag in bottom-blown furnaces. The normal production process is carried out in three stages: slag feeding, reduction, and slag discharge. Since the melting temperature in a bottom-blown furnace is between 1,000 and 1,050 °C, it does not meet the requirement of 1,200 °C for side-blown reduction furnaces. During the slag feeding stage, in order to achieve an appropriate slag composition suitable for the side-blown reduction furnace and the smelting furnace processes, a certain amount of white stone must be added to form the slag. The melting of white stone, the decomposition reaction of CaCO3, and the reduction reactions of metals such as lead and zinc are all endothermic reactions; therefore, to maintain the temperature in the side-blown reduction furnace, it is crucial to control the oxygen-to-carbon ratio during these three stages. The blowing times and the oxygen excess coefficient α vary for each stage, with the specific values being as follows: the blowing times for slag feeding, reduction, and slag discharge are 40–50 minutes, 30–40 minutes, and 20–30 minutes respectively ； The α values are 0.6~0.7, 0.4~0.5, and 0.85~0.9 respectively. The side-blown reduction furnace can appropriately extend or shorten the heating time for slag feeding and slag removal depending on the furnace conditions. To coordinate with the production processes in the bottom-blown furnace and the smelting furnace, the refining cycle of the side-blown reduction furnace is 2 hours per furnace; this cycle can be further shortened by adjusting the times for slag feeding, reduction, and slag discharge. Operation data of the crude lead smelting system at this plant in November 2014: feed rate to the bottom-blown furnace was 39–45 t/h; the number of batches processed per day in the side-blown reduction furnace was 12; coal consumption was 2.86 t/h; the amount of white stone added was 1.4 t/h; the dust emission rate from the side-blown reduction furnace was 8.5% (relative to the initial slag); the primary air supply volume was 3,000–4,000 m3/h (under standard conditions); the oxygen enrichment level was 54.85%; and the secondary air supply volume was 2,700–3,500 m3/h. The inlet temperature of the waste heat boiler is 1,100–1,200 °C; the steam output (at 4.0 MPa saturated steam pressure) is 12–16.5 t/h. The outlet temperature of the waste heat boiler ( economizer) is around 300 °C, the temperature at the siphon point is 700–900 °C, and the slag temperature is around 1,200 °C. 3 Production technology indicators: As of January 2015, the side-blown reduction furnace had been in operation for continuous trial production for 3 months. During this period, the furnace operated well, with stable and continuous production; the number of furnaces in use per day remained between 10 and 12. The amount of high-lead slag fed into the furnace was over 17,000 tons per month, while the output of crude lead was between 7,000 and 8,000 tons per month. The average lead content in the slag per month was ≤2.0%. The output of crude lead could be increased further by raising the lead content in the high-lead slag and increasing the processing capacity of the side-blown reduction furnace. The technical and economic indicators of the side-blown reduction furnace are compared with those of the blast furnace lead smelting process in Table 1. Table 1 Comparison of technical indicators between side-blown reduction furnaces and blast furnaces. Table 1: Index comparison of side-blown furnaces and blast furnaces. Technical and economic indicators: Side-blown reduction furnace; Blast furnace. Total lead recovery rate: 98% vs. 97%; Gold recovery rate: 99% vs. 98%; Silver recovery rate: 98% vs. 97%; Copper recovery rate: 80% vs. 60%. Comprehensive energy consumption per ton of lead: 270 kgce vs. 470 kgce. Monthly production capacity of crude lead: 7,100 t vs. 5,000 t. Direct smelting cost of crude lead: 1,065 yuan vs. 1,386 yuan. Average lead content in slag: 2.0% vs. 3.0%; Average copper content in slag: 0.25% vs. 0.40%; Average silver content in slag: 22 g/t vs. 39 g/t. As can be seen from Table 1, the metal recovery rates using side-blown reduction furnace technology are higher than those using blast furnaces, with a particularly significant increase in copper recovery rate – about 20% higher compared to blast furnaces. The comprehensive energy consumption per ton of lead is reduced by over 40% compared to the traditional lead smelting process, and further reductions are possible in this aspect as the plant is still in the trial production phase. As can be seen from the comparison of technical indicators, the side-blown reduction furnace offers significant advantages in terms of efficiency; the direct smelting cost per ton of crude lead is reduced by 321 yuan. The annual comprehensive benefits generated by this project exceed 20 million yuan. Its advantages in energy conservation, emission reduction, and environmental protection are particularly notable, as it enables clean production and greatly improves the adaptability of the bottom-blown furnace to various raw materials. 4 Conclusion: The trial production run of the side-blown reduction furnace was successful. Over the past 3 months of operation, all technical and economic indicators have met the expected goals; it features low comprehensive energy consumption and significant environmental benefits. The technology of integral ramming molding using aluminum-chromium aggregates is a first in China, and it has shown good performance.