Application of Oxygen-enriched Side-blowing Technology in Treating Zinc Leaching Residue in China

Zinc hydrometallurgy is the main process of zinc smelting in the world. The leaching residue produced by different hydrometallurgical processes have different components, but they contain valuable metals such as zinc, silver, gold, indium and germanium, and heavy metals such as lead, cadmium and arsenic. In China, leaching residue is defined as hazardous waste, which requires harmless treatment and recovery of valuable metals. At present, China’s main zinc leaching residue treatment technologies include Welz kiln, fuming furnace and Top-submerged lance Smelting furnace. These technologies have been applied for many years and are mature and reliable, but they need further improvement in energy conservation and environmental protection. In recent years, with the continuous progress of China’s smelting technology, the oxygen-enriched side-blowing technology has been successfully applied to the treatment of zinc leaching residue, mainly including the Side-submerged combustion smelting technology of China ENFI and the Vanukov side-blowing technology. At present, the above two side blowing technologies have been put into industrial practice in many zinc smelters in China, which are characterized by low energy consumption, high recovery rate of valuable metals and high sulfur recovery rate. This paper introduces the application of the above two side blowing technologies in China’s zinc smelters, and compares them from furnace structure, production operation, and technical indicators. Oxygen-enriched side-blowing technology will become the mainstream treatment process of zinc residue in China and even in the world in the future.


Current status of zinc leaching residue treatment technology
The zinc smelting process is mainly divided into two types: pyrometallurgy and hydrometallur gy, with hydrometallurgy accounting for over 80% of the total zinc smelting capacity worldwide.The leaching residues produced by the zinc hydrometallurgy process contains heavy metals such as lead, arsenic, and cadmium.Zinc leaching residue has been classified as hazardous waste in China (waste codes 321-004, 005, 006, 007-48).Before 2020, some zinc smelting factories constructed hazardous waste disposal sites to reduce production costs.In 2020, the Ministry of Industry and Informatio n Technology of China issued the "Standard Conditions for the Lead and Zinc Industry", which stipulated that the zinc hydrometallurgy process must be equipped with a harmless treatment system for leaching residue and sulfur residue treatment facilities [1] .Therefore, the new zinc factories need to be equipped with harmless disposal facilities for zinc leaching residue.For existing zinc factories that use hazardous waste disposal sites to store leaching residues, limited by the strict environmental policies, they will also build harmless treatment facilities for zinc leaching residues.The selection of harmless disposal technology for zinc leaching residue has become a top priority for the sustainable development of zinc factories.
The treatment of zinc leaching residue usually adopts the high-temperature volatilization process by pyrometallurgy, which not only achieves harmless treatment of the residue but also achieves the recovery of valuable metals.At present, the main technology of zinc residues treatment in China is Waelz kiln, box fuming furnace,top-submerged lance smelting furnace (TSL) and side-blowing furnace & box fuming furnace.

Waelz kiln
Waelz kiln is the most widely used in the world.Its advantages are mature and reliable technology, and simple operation.However, the coke rate of this technology is as high as 40%~50%, and it needs to use coke or high-quality anthracite, which has high processing costs.At the same time, there are also problems such as low silver recovery rate, large amount of flue gas, low concentration of sulfur dioxide in the flue gas, and easy formation of kiln deposits in the kiln [2] .

Box fuming furnace
Box fuming furnace was first applied in China at the beginning of this century and is currently being used by two factories.Its advantage is that the fuel can be bituminous coal, with a low price.But this technology can only use air or low oxygen enriched air for smelting, with low energy efficie nc y and coal consumption of up to 50%~60%.Due to the intermittent feeding process in its smelting process, the fluctuation of sulfur dioxide in the flue gas is significant, resulting in high flue gas treatment costs [3] .This technology is suitable for lead-zinc joint smelting factory, which can use lead smelting hot slag to treat zinc leaching residue.

TSL
TSL is an imported technology that was first applied in China in 2015.At present, only one factory in China is using it.Its advantage is the use of oxygen rich smelting, which has a high smelting intens ity.However, this technology has drawbacks such as high coal consumption, frequent replacement of lance, high construction investment, low sulfur dioxide concentration in flue gas [4] .Due to its drawbacks of this technology, this technology has not been widely applied in China.

Side-blowing furnace & box fuming furnace
In view of the problems existing in the above technologies, ENFI developed the technology of treating zinc leaching residue with side-submerged combustion furnace & box fuming furnace, and it was successfully put into production in Chihong Zn & Ge Huize Smelter in July 2019.This device is the world's first production line to use side blowing furnace to treat zinc leaching residue.With the successful application of this technology, ENFI has promoted the process to Gansu Baiyin Nonferrous and Sichuan Shengtun Mining, and has made continuous optimization and improvement in technology.The above two factories were successfully put into operation at the end of 2021 and achieved production and standard in 2022, with all indicators exceeding the design values.
At the same time, a design institute in Northwest China designed a 200000 ton Jarosite residue treatment system in Hanzhong Zinc Industry in September 2020, using a Vanukov type side-blowing furnace & box fuming furnace.
At present, this technology has been applied in multiple zinc smelting factories in China, with the advantages of using high oxygen rich smelting, high smelting intensity, low coal consumption, high recovery rate of precious metals such as gold and silver, high sulfur dioxide concentration in sideblowing furnace flue gas, which can directly produce acid and achieve sulfur resource utilization at low cost.

Summary
Practice has proven that side-blowing furnace & box fuming furnace technology has significa nt advantages, such as simple operation, low energy consumption, high recovery rate of valuable metals, and good environmental protection.The following text introduces the application of this technology and its application in zinc smelting factories in China, and analyzes and compares the two side blowing technologies from aspects such as furnace structure, production operation, and technical indicators.

Reaction principle of zinc residues smelting
Zinc in zinc residues mainly exists in the form of ZnO•Fe2O3, ZnO, ZnS, ZnSO4, and ZnO•SiO2, with ZnO•Fe2O3 and ZnO accounting for 90% of the total zinc in the residue.Pb is mainly PbSO4.
The compounds of lead and zinc are mainly oxides and sulfates, both of which are non heating materials.The boiling point of zinc is 907℃, and lead and its compounds are easily volatile at high temperatures.This technology utilizes the volatile characteristics of lead and zinc metals at high temperatures to achieve the reduction and volatilization.The lead and zinc vapor is re oxidized in the gas phase zone in contact with oxygen to generate small particles of zinc oxide and lead oxide, which enter the dust collection facility with the flue gas and are collected.The main chemical reactions are shown in equations (1) to (7) [5] .
ZnSO4= ZnO+ SO2(g)+1/2O2(g) △H= +333 kJ/mol (3) The smelting of zinc leaching residue is mainly divided into two stages: residue melting and reduction volatilization.The melting stage deals with the cold material of zinc leaching residue, which absorbs a large amount of heat during the melting and decomposition of various substances and consumes a large amount of heat.The reduction and volatilization stage deals with hot slag, and the reduction and volatilization of lead and zinc require some heat, which accounts for a relatively low proportion of the total energy consumption.Therefore, during the residue melting stage, the sideblowing furnace adopts high oxygen rich combustion to achieve the goal of saving energy consumptio n.
The reduction and volatilization stage adopts a box fuming furnace, which provides strong stirring and controls a strong reducing atmosphere to achieve the reduction and volatilization of lead and zinc.Zinc leaching residue is melted in a side-blowing furnace and reduced and volatilized in a box fuming furnace.Elements such as zinc, lead, silver, indium, germanium, and gold in the slag evaporate into the flue gas at high temperatures and are collected through boilers and dust collectors.The purified flue gas is sent for acid production and desulfurization.The tailings are general solid waste, stored or sold out.

Zinc residue melting in side-blowing furnace
As mentioned earlier, the main energy consumption and the removal of sulfur elements are also in the melting stage.The side-blowing furnace is a fixed type furnace with straight elliptical or rectangular edges, which is heated by a spray gun or crushed coal is added as fuel from the top of the furnace.Oxygen rich air is sprayed into the side spray gun or tuyere of the furnace to support combustion, and the concentration of oxygen rich air injected into the furnace is 60%~90%.The high concentration of oxygen rich air reduces the amount of gas generated during combustion, thereby reducing the heat carried away by the flue gas and effectively reducing fuel efficiency.The smelting temperature of the side-blowing furnace is controlled between 1200℃ and 1500℃.The sulfate in the zinc leaching residue decomposes to produce sulfur dioxide, and some zinc and lead volatilize into the flue gas.The slag is discharged into the box fuming furnace through a chute.The flue gas is cooled by a waste heat boiler and purified by a dust collector to obtain zinc oxide dust.The purified flue gas is sent to the acid production system to produce sulfuric acid.

Reduction and volatilization in box fuming furnace
The box fuming furnace is a rectangular fixed furnace type, where air and pulverized coal are injected through the side tuyere.The coal is burned to provide heat to the molten pool, while the excess coal plays a reducing role.The excess oxygen coefficient in the molten pool area is~0.6 [6] .The reduction and volatilization of zinc in the molten pool must ensure dynamic conditions.Therefore, the box fuming furnace uses air to support combustion, with a large gas volume and sufficient stirring strength.The temperature of the melting pool is controlled to 1300℃.After most of the lead and zinc are reduced and volatilized into the flue gas, they are oxidized again in the gas and ultimately collected in the form of dust.The tailings contains<2.0%zinc and<0.2%lead.The flue gas is cooled by a waste heat boiler and purified by a dust collector to obtain zinc oxide dust.The purified flue gas is sent for desulfurization.
The process flow chart [7] is as follows: (1) Raw materials The amount of leaching residue and oxidized ore processed by side-submerged combustion furnace is about 20t/h dry basis.The components of leaching residue and oxidized ore are shown in Table 1.(3) Consumption The fuel consumption for processing each ton of residue is equivalent to 310kg~330kg of standard coal.
The equivalent oxygen consumption per ton of residue is about 300Nm 3 /t to 330Nm 3 /t.

Case B
The factory has built one Vanukov type side-blowing furnace, one insulation furnace, and two box fuming furnaces.The function of the insulation furnace is to distribute slag to two box fuming furnaces.The project was completed and put into industrial production at the end of September 2020, with a side-blowing furnace processing 600 tons of residue on a dry basis per day.
(1) Raw materials The amount of leaching residue processed by side-blowing furnace is about 24t/h dry basis.The components of leaching residue is shown in Table 4. (2) Output The composition of tailing is shown in table 5.
The volatilization rates of Zn,Pb, Ag are shown in the table 6. (3) Consumption The fuel consumption for processing each ton of residue is equivalent to 320kg~340kg of standard coal.
The equivalent oxygen consumption per ton of residue is about 350Nm

Case C
The factory has built one side-submerged combustion furnace, one box fuming furnaces.The project was completed and put into industrial production at the end of December 2021, with a sideblowing furnace processing 500 tons of residue on a dry basis per day.
(1) Raw materials The amount of leaching residue processed by side-submerged combustion furnace is about 20t/h dry basis.The components of leaching residue is shown in Table 7.  (3) Consumption The fuel consumption for processing each ton of residue is equivalent to 270kg~290kg of standard coal.
The equivalent oxygen consumption per ton of residue is about 270Nm 3 /t to 300Nm 3 /t.

Case D
The factory has built one Vanukov type side-blowing furnace, one box fuming furnaces.The project was completed and put into industrial production at the end of December 2021.After nearly half a year of production in the factory, there are problems with temperature control difficulty, high energy consumption, large fluctuations in flue gas sulfur dioxide, unstable acid production, and high start-up costs in the side-blowing furnace.In May 2022, ENFI transformed it into side-submerged combustion.At present, the side-blowing furnace processes approximately 600 tons of residue on a dry basis every day.
(1) Raw materials Before the renovation, the treatment capacity of residue was 20t/h dry basis, and after the renovation, the treatment capacity was increased to 25t/h dry basis.The components of leaching residue is shown in table 10.  (3) Consumption Before the renovation, the fuel consumption for processing each ton of residue is equivalent to 300kg~320kg of standard coal; after the renovation, the fuel consumption for processing each ton of residue is equivalent to 270kg~290kg of standard coal.
Before the renovation, the equivalent oxygen consumption per ton of residue is about 320Nm 3 /h to 350Nm 3 /h; after the renovation, the equivalent oxygen consumption per ton of residue is about 270Nm 3 /t to 300Nm 3 /t.

Summary
Based on the data from the above cases, make a summary as shown in table 13.

Comparison of side-blowing technology for treating zinc leaching residue
At present, there are two types of side-blowing furnaces for treating zinc leaching residue in China, with significant differences in furnace structure, production operation, and technical indicators.The following text compares the two types of furnaces.(2)Blowing method The Vanukov type side blowing furnace is a single channel tuyere that only passes through oxygen rich air, and the valve group is simple.The side-submerged combustion furnace is a multi-cha nne l spray gun that injects coal powder, oxygen rich air.The valve group is relatively complex and requires a matching coal powder injection device.
The distance between the tuyeres of the Vanukov type side blowing furnace is small and there are many tuyeres.It is necessary to change the tuyeres according to the furnace condition, and 50% of the tuyeres should be used normally.The distance between the spray guns of the side-submerged combustion furnace is large and the quantity is small.Generally, more than 70% of the spray guns are used.
(3) Heating method Vanukov type side blowing furnace uses high calorific value granular coal (washed coal or Anthracite) as fuel and reducing agent.After being metered together with zinc leaching residue in the bin, it is fed into the furnace from the furnace top through the belt, and then the oxygen enriched air is blown through the side tuyere to support combustion and supplement heat.By adding crushed coal from the furnace top for combustion and heating, the temperature of the slag is uneven, and the temperature of the upper part of the molten pool reaches 1400℃ or even higher.Meanwhile, due to the high smelting temperature, a large amount of nitrogen oxide is produced.
The side-submerged combustion furnace uses a small amount of granular coal (with no special requirements for coal quality) as a reducing agent, measured together with zinc leaching residue in the bin, and then added to the furnace through a belt from the furnace top.The side spray gun injects powder coal and oxygen rich air to support combustion, and powder coal is used as the main fuel for heating.Due to the fact that only powder coal is injected from the middle of the molten pool for combustion and heating, the temperature of the molten pool is uniform and the slag temperature is low, with a temperature of less than 1300℃.

Production operation (1) Oven
The Vanukov type side blowing furnace is operated by inserting multiple oxygen+natural gas burners from the tuyere for heating, and requires a separate natural gas drying system.
The side-submerged combustion furnace is mainly dried by inserting natural gas burner into the slag discharge port and the upper burner hole of the furnace.
(2) Furnace startup The Vanukov type side blowing furnace does not have a burner and adopts the traditional blowing furnace opening method.It requires workers to add a large amount of wood, block coke, block slag, lead plates to create a molten pool.Taking a 13m 2 side blowing furnace as an example, 60 to 70 tons of materials are manually added to each melting pool.
The method of creating a molten pool in a side-submerged combustion furnace is relatively simple.In the initial stage of the molten pool, a small amount of wood and crushed coke needs to be added.
After the wood and coke are fully burned, automatic heating operation is carried out through powder coal spray gun.
(3) Tuyere operation During normal production, the operation of the side blowing furnace mainly includes ingredie nt control and blast control.Both types of furnaces are automatically adjusted through DCS.However, due to the differences in tuyere and heating methods, the main difference in operation between the two types of furnaces lies in the tuyere operation.
The Vanukov type side blowing furnace uses tuyere to directly blow oxygen-rich air into the molten pool.However, due to the form of the tuyere, manual tuyere blowing operations need to be carried out on a regular basis for each shift to prevent tuyere blockage and observe the slag situatio n, which is labor-intensive and risky.
The side-submerged combustion furnace adopts a multi-channel spray gun to directly inject powder coal and oxygen-enriched air into the molten pool, without the need for normal operation.The gas flow and pressure of all spray guns, as well as the injection amount of powder coal, are monitored in real-time online, achieving fully automated operation of the spray gun.

Technical indicators
Based on the production practice of each factory, the following analysis and comparison were conducted on the indicators of using different side blowing furnaces to treat zinc leaching residue.The results are shown in table 14. (2) Side-submerged combustion furnace, injecting powder coal effectively improves slag properties, lowers melting temperature, and reduces the generation of nitrogen oxides.
(4) Vanukov side blowing furnace needs to use high-quality anthracite, and the production cost is high.

Conclusion
In recent years, the continuous progress of China's smelting technology, the treatment of zinc leaching residue using oxygen rich side blowing technology has been put into industria l practice in multiple zinc smelting factories in China, gradually replacing other zinc leaching residue treatment technologies.Practice has proven that the oxygen rich side blowing technology has the following advantages in treating zinc leaching residue.
(1) The melting of zinc leaching residue adopts high concentration oxygen rich side blowing smelting, and the actual standard coal consumption per ton of residue is 270~340kg, which reduces the standard coal consumption by more than 30% compared to other existing pyrometallurgical residue treatment processes.The energy-saving and carbon reduction effect is significant.
(2) The recovery rate of zinc, lead, silver and germanium is high, up to 90~98%, especially the recovery rate of silver is far higher than that of Welz kiln.
(3) The flue gas from the side blowing furnace has a high SO2 content, which can directly produce acid.The flue gas treatment cost is low, achieving the resource utilization of sulfur.
(4) The side blowing furnace+fuming furnace process, using high-temperature melting, truly realizes the harmless treatment of zinc leaching residue, and has obvious environmental advantages.
At the same time, this article introduces the application of Vanukov type side blowing technology and side-submerged combustion technology in zinc smelting factories in China, and analyzes and compares them from the aspects of furnace structure, production operation, and technical indicators.
Based on the above comparison, it can be seen that the side-submerged combustion technology developed by ENFI has advantages such as low energy consumption, controllable melting temperature, simple operation, high automation, low start-up cost, stable flue gas, and low nitrogen oxide content.ENFI's side-submerged combustion technology has been successfully applied to many factories in the treatment of zinc leaching residue, such as Chihong Zn & Ge Huize Smelter, Gansu Baiyin Nonferrous and Sichuan Shengtun Mining, and will be widely used in global zinc smelting factories in the future.Through continuous engineering practice and continuous optimization and improve me nt of this technology, the zinc leaching residue treatment technology will be elevated to a higher level.
The 10th International Conference on Lead and Zinc Processing (Lead-Zinc 2023)

4. 1
Furnace structure (1)Furnace type The distance between the tuyeres of the Vanukov type side blowing furnace is 1.5−2 meters; The distance between the tuyeres of the side-submerged combustion furnace is about 2−2.5 meters.The lower part of the two types of furnaces is equipped with a shallow hearth; Copper water jackets are The 10th International Conference on Lead and Zinc Processing (Lead-Zinc 2023) Journal of Physics: Conference Series 2738 (2024) 012023 IOP Publishing doi:10.1088/1742-6596/2738/1/0120239 used in the upper melting pool area of the furnace hearth, which are protected by hanging slag.

Table 1
The composition of tailing is shown in table 2.The volatilization rates of Zn,Pb, Ag and Ge are shown in the table 3. 6

Table 7
The volatilization rates of Zn,Pb, Ag are shown in the table 9.
The volatilization rates of Zn,Pb, Ag and Ge are shown in the table 12.
The volatilization rates of lead, zinc, and precious metals in the two side blowing technology are basically the same.