Design and production practice of large fluidized roasting furnace

With the expansion of the scale of zinc smelting enterprises and the progress of new equipment and technology, 152 m2 fluidized roasting furnace has been developed and applied to industrial production, with a single unit capacity of 150,000 to 170,000 tons of electrolytic zinc per year. This paper analyzes and introduces an engineering example of zinc concentrate treatment by a dual-series fluidized roasting furnace. The project was designed by China ENFI Engineering Corporation and put into operation in Hunan province of China in 2018. This paper firstly describes the process flow and main equipment of the preparation system, roasting furnace system, waste heat utilization system, dust collection system from a design perspective. Then, based on production practice, it analyzes and compares aspects such as furnace operation, actual production data, major technical and economic indicators, and the characteristics and optimization of the roasting system equipment. Finally, it is concluded that the dual-series large-scale fluidized roasting furnace has stable production, good product quality, less investment, and fewer labor requirements, which brings good economic benefits to the enterprise. Its operational experience has accumulated parameters and experience for the development of larger roasting furnaces in the future.


Introduction
Modern zinc smelting production processes are mainly divided into two categories: pyrometallurgical zinc smelting and hydrometallurgical zinc smelting.The production of hydrometallurgical zinc smelting has accounted for more than 85% of the world's zinc production.Hydrometallurgical zinc smelting is mainly composed of roasting, leaching, purification, electrowinning, and casting processes.Roasting, as the first process of hydrometallurgical zinc smelting, has a significant impact on the hydrometallurgical process due to its roasting capacity and the quality of roasted ore.In recent years, China, as a major zinc-producing country, has developed a 152m 2 fluidized roasting furnace through continuous exploration of new equipment and technologies, with a single unit capacity reaching 150,000 to 170,000 tons of electrolytic zinc per year.
Hunan Zhuye Nonferrous Metals Co., Ltd. has constructed a zinc and zinc alloy factory with an annual production capacity of 300,000 to 340,000 tons in Songbai Town, Changning City.The roasting system adopts two 152m 2 fluidized roasting furnaces.The project was completed and put into operation in December 2018 and has been running for 5 years .

Roasting process flow
The roasting system mainly consists of two parts: the material preparation section and the roasting section.The material preparation section includes: storage of zinc concentrate, drying, crushing and screening processes.
In addition, there are waste heat boiler system, dust collection system and other auxiliary production systems.
The process flow chart of the roasting system is shown in Figure 1.

Figure 1.
The process flow chart of the roasting system.

Material preparation system
The concentrate warehouse is a semi-underground warehouse.Zinc concentrate is transported to the warehouse by trucks or trains.The concentrate is fed into the proportioning bins through a bridge-type grab crane and a variable speed disc feeder.After the concentrate is weighed by the electronic belt scales, it is conveyed to the concentrate drying section by a conveyor belt.Depending on the incoming material in the concentrate warehouse, the drying section is divided into two routes.One route is for the concentrate that needs to be dried, which is sent to the drying kiln through a conveyor belt for drying and then to the screening and crushing section.The other route is for the concentrate that does not need to be dried, which is directly sent to the screening and crushing section by the conveyor belt without going through the drying kiln.
The screening and crushing system consists of a crusher and two high-efficiency vibrating screens.The material is first fed into the vibrating screen.The undersized material is sent to the feeding conveyor belt, while the oversized material enters the crusher for crushing.After the second screening, the oversized material is manually sorted to remove impurities, the undersized material is transported back to the conveyor belt by a bucket elevator.The screened material from both screenings is collected and sent to the pre-furnace storage bin in the roasting plant.
The entire plant is equipped with one set of drying system, one set of screening and crushing system, and one set of feeding system to provide raw materials for the two roasting furnaces.

Roasting furnace system
The two roasting workshops are arranged in a north-south mirror relationship on the general drawing.Each workshop is equipped with a 152m 2 fluidized roasting furnace.A bin is set in front of each roasting furnace to store concentrate for four hours.the zinc concentrate is measured by the double belt quantitative scales at the bottom of the bin and then thrown to the roasting furnace at a speed of 15 ~ 18 m/s by the throwing machine.
Under the action of the distribution plate airflow, the zinc concentrate is in a boiling state.Eight sets of cooling coils are set up in the boiling layer to take away the remaining heat and maintain a reaction temperature of about 930℃ in the furnace.Under this condition, a series of physical and chemical reactions such as roasting desulfurization occur in the zinc concentrate.The produced zinc calcine flows out through the overflow port on the furnace side and is cooled to around 150 ℃ after two stages of cooling, then the calcine is transported by a buried scraper conveyor to the calcine ball milling workshop for grinding treatment.After grinding, the calcine is relayed to the leaching workshop by the buried scraper conveyors and bucket elevators.
The gas produced by the fluidized roasting furnace first enters the waste heat boiler to recover the waste heat, and then goes through the two-stage cyclone dust collector and the electrostatic precipitator to collect the dust, and finally is sent to the acid production system by the exhaust fan.The collected dust is sent to the leaching workshop.
Figure 2 shows the external view of a 152m 2 fluidized roasting furnace.The diameter of the fluidized bed is 13.9m, the diameter of the expansion section is 17.58m, and the height of the furnace chamber is 18.5m.The height of the fluidized bed is approximately 1m.The furnace wall materials, from inside to outside, consist of refractory bricks, insulation bricks, steel shell, and external insulation coating.The furnace structure, from bottom to top, includes the air box, distribution plate, fluidized bed, conical furnace chamber, expanded furnace chamber and furnace top.

Waste heat utilization system
The gas flow rate of a single roasting furnace is 79185 Nm 3 /h, with a temperature of 950℃, a dust concentration of 195 mg/Nm 3 , and a SO2 content of 9.97% by volume.The steam production of a single boiler is approximately 43t/h, which is used for power generation and supplying the wet process production.
Each roasting furnace is equipped with a waste heat boiler, and the inlet of the waste heat boiler is connected to the gas outlet of the roasting furnace through a flexible fabric compensator.The waste heat boiler consists of a horizontal straight-through flue, a radiation chamber, and a convection section, and operates in a forced circulation mode with an outdoor layout.The furnace wall adopts a membrane water-cooled wall structure, with the boiler shell covered by an external insulation structure.The hightemperature gas passes through the radiation chamber horizontally and is cooled to 700℃.After the radiation cooling chamber is the convection section, which is equipped with condensing and convection tube bundles.Both the condensing and convection tube bundles are made of Ф38×5 boiler steel tubes and arranged in series.The gas exits the waste heat boiler at a temperature of around 360℃ and enters the dust collection system.
The waste heat boiler is equipped with 77 sets of elastic hammer-type ash cleaning devices and 24 explosion ash cleaning points, which can timely and effectively remove the accumulated ash on the heating surface, ensuring the normal operation of the waste heat boiler.The lower part of the ash hopper of the waste heat boiler is equipped with a buried scraper conveyor.The settled dust in the waste heat boiler and the ash residue shaken off by the ash cleaning device are discharged from the boiler and transported to the calcine cooling cylinder by the buried scraper conveyor.

Dust collection system
The gas generated by the roasting furnace has the characteristics of high SO2 content, high dust content, corrosiveness, high viscosity of dust, fine particle size, and high specific resistance, making it difficult to recover.The design adopts a multi-stage dust collection system, with the following process: roasting furnace → waste heat boiler → primary cyclone dust collector → secondary cyclone dust collector → electrostatic precipitator → fan → acid production.
The gas enters the dust collection system at a temperature of 360℃.The gas is first passed through two stages of cyclone dust collectors to recover larger particles of dust, and then purified by an electrostatic precipitator to recover fine particles of dust that are difficult to recover.The purified gas is then sent to the acid production system by a high-temperature fan.The dust collected by the cyclone dust collectors and electrostatic precipitator is sent to the leaching workshop without ball milling.
The high-temperature fan is located at the outlet of the electrostatic precipitator, which reduces the wear of the fan impeller by the dust in the gas.It also allows the electrostatic precipitator to operate under negative pressure, improving the operating and working conditions of the electrostatic precipitator.The high-temperature fan adopts variable frequency speed regulation to adjust the air volume and pressure of the system.
A bypass duct is set at the outlet of the second-stage cyclone dust collector, which allows the flue gas generated during drying and firing to be directly sent to the desulfurization system for firing gas treatment.
Each dust collection system consists of two φ2600×4 cyclone dust collectors and one 130m 2 fivefield electrostatic precipitator.Two sets of dust collection systems are configured for the two roasting furnaces.

Roasting furnace preheating and start-up
After the construction of the roasting furnace, it needs to undergo a drying process.The main purpose of furnace drying is to remove free water and bound water from the refractory materials.During the drying process, a series of physical and chemical changes occur in the refractory materials of the roasting furnace, so that the furnace can obtain high temperature stability, high temperature strength, and longer service life.
The furnace drying process follows a temperature ramp-up curve.Initially, wood is used for drying, followed by diesel burners.The drying process follows the principles of multi-point heating, even heating, and slow drying.The initial drying cycle takes about 20-30 days, with a diesel consumption of approximately 150 tons.

Roasting furnace production indicators
The zinc concentrate used in the production consists of 50% imported ore and 50% domestic ore, with a particle size of less than 260um and a moisture content of 14%.The main components of the zinc concentrate are shown in Table 1.
The design includes one φ3x22m drying kiln, with a processing capacity of 120t/h.The natural gas consumption is 3.5Nm3 for per ton of ore, and the moisture content of the dried concentrate is reduced to 7%.The drying process helps reduce the gas volume and increase the processing capacity of the roasting furnace.The design feed rate of the roasting furnace is 42t/h, while the actual feed rate on-site is 47-50t/h.The design and actual specifications for the sulfur content of dust and calcine are detailed in Table 2 and Table 3.The mixture of calcine and dust collectively referred to as roasted ore, which contains approximately 1.8% soluble sulfur, 0.5-1.0%insoluble sulfur, and the zinc content varies depending on the zinc content of the concentrate, generally around 55%.The roasted ore is sent to the wet process for leaching, and the resulting acid leaching residue contains about 16% zinc, which is better than the design value of 18%, indicating a higher zinc leaching rate.
The main production technical indicators are shown in Table 4.According to the table, the bed capacity of the roasting furnace can reach a maximum of 7.8t/m 2 •d, and the operating rate can reach a maximum of 345 days, both of which are better than the design value.Therefore, it has the ability to increase production by 20%.
The main production technical indicators are shown in Table 4.

Operation control of the roasting furnace
During normal production, the furnace temperature is relatively stable.Typically, the temperature of the boiling layer is controlled at around 930°C and can reach a maximum of 1000°C.There are a total of 8 sets of cooling coils, but currently only 3 sets are needed to maintain thermal balance.The temperature of the boiling layer is adjusted by adjusting the feeding amount and the air volume on-site.Based on on-site experience, the blower of the roasting furnace is relatively large and not easy to adjust.Therefore, once the air volume is adjusted properly during the initial production stage, it is generally not further adjusted.
When there is a high concentration of SO4 2-in the wet system, the feeding amount is increased to raise the furnace temperature, leading to the decomposition of sulfates and a reduction in residual sulfur in the roasted ore.

Roasting furnace system equipment
In previous projects, two outlets from the front bin of the furnace were connected to two short belt conveyors, one belt conveyor, one metering belt conveyor and one distribution disc, totaling five conveying devices to transport the concentrate to the roasting furnace.The multiple transfer points resulted in multiple dust points, and the distribution disc hanging below the platform was inconvenient for maintenance.In this project, after preliminary analysis and investigation, the distribution disc was eliminated, and two double-belt quantitative scales were used instead of the five devices to achieve the functions of feeding, metering, and distribution.Practice has proven that this approach is effective, saving investment, reducing equipment footprint, and reducing the height of the plant, which can be promoted in future projects.
For this project, one non-frequency-controlled domestic fan was used.Pipe elbows are fewer ,silencers were installed at both the inlet and outlet to keep the on-site noise below 85 decibels.The original design of the furnace bottom air box inlet was a vertical inlet, but in this project, three tangential inlets were used, and the wind caps on the distribution plate were designed as straightthrough type to ensure uniform and stable air distribution inside the furnace.
The roasting furnace is equipped with two bottom discharge ports, which are connected to the bottom discharge bin.The outer wall of the bin is made of steel, and the interior is lined with refractory castable to ensure that the temperature of the outer wall is below 80°C.Two water-cooled buried scraper conveyors are connected to the outlet of the discharge bin to handle the bottom discharge.The bottom discharge system is not used during normal production.When planned shutdown is required, the feed inlet and overflow outlet are closed, the feeding machine stops adding material, and the fan increases the air volume to discharge the materials from the bottom discharge port into the bottom discharge bin.The design of the bottom discharge device greatly reduces the labor intensity of manual furnace scraping in high-temperature and high-dust environments.

Site retrofit and optimization measures
The following improvements have been made on-site: (1) The buried scraper conveyor transporting from the cooling cylinder to the ball mill has been improved by increasing the height of the baffles and upgrading the motor, after the modification, the occurrence of motor overload protection and shutdown due to sudden large material volume was avoided.Therefore, when selecting equipment, the instantaneous maximun material flow rate should be fully considered.
(2) An additional outdoor feeding device was installed in both roasting workshops.In the event of a sudden failure in the feeding system that cannot be repaired in time, the outdoor feeding device can be used to supply the roasting furnace, ensuring uninterrupted production.
(3) During the initial operation of the roasting furnace, the temperature of the furnace shell was found to be excessively high, reaching around 350℃.It was discovered that the external insulation layer was too thick, preventing the dissipation of heat from the furnace shell.The insulation layer was subsequently thinned and the furnace shell temperature is controlled at around 200℃ now.

Conclusions
(1) In the feasibility study and design phase, the project initially planned to use three sets of 109m 2 fluidized roasting furnace.However, after many times negotiation, it was decided to use two sets of 152m 2 fluidized roasting furnace, which proved to be the correct decision.The two roasters share one set of raw material preparation system, resulting in a simple configuration, smaller overall footprint, fewer equipment units, lower construction costs, and a shorter design and construction period.The two roasting systems are arranged in a completely mirrored layout, making them easy to manage.The labor force required is reduced by 30% to 40% compared to the three sets of 109m 2 roasters.The overall economic benefits of the dual-series 152m 2 fluidized roasting furnace scheme are much higher than the three sets of 109m 2 roasters scheme.
(2) Currently, there are a total of four 152m 2 fluidized roasting furnaces in China.In chronological order of production, they are: one at the Baiyin Nonferrous Group Co., Ltd.Northwest Lead-Zinc Smelter (2016), two at the Hunan Zhuye Nonferrous Metals Co., Ltd (2018), and one at the Henan Jinli Gold-Zinc Co., Ltd (2022).The two 152m 2 roasters in this project have made some improvements based on the first roasting furnace.The project has achieved production and exceeded production in a short period of time.The high annual operating rate and low-sulfur calcine ore indicating that the design technology of the 152m 2 fluidized roasting furnace is advanced and the onsite operational level is high.The whole operation production experience of the 152m 2 roasters have accumulated rich practical data for the development of larger roasters.
(3) With technological advancements, zinc smelting will have the following development trends in the future: weakly oxidizing roasting, intelligent control, large-scale production, and cleaner processes.Considering the significant potential for increasing capacity in roasting furnaces, future design of roasting furnace systems should take into account the oxygen consumption, and the supporting equipment should be matched with the increased production capacity.

Figure 2 .
Figure 2. Schematic diagram of the roasting furnace.

Table 1 .
Composition of zinc concentrate fed into the furnace ( dry basis)( wt %).

Table 2 .
Comparison of design and actual value for sulfur content of calcine (wt %).

Table 3 .
Comparison of design and actual value for sulfur content of dust (wt %).

Table 4 .
Main production technical indicators.