Development and Innovative Application of Main Equipment for Zinc Electrowinning

The latest development and application of zinc electrowinning equipment in China are introduced. The characteristics and innovative applications of main equipment for zinc electrowinning, such as cathode and anode, electrolytic cell, cathode and anode multifunctional crane, zinc stripping and cathode washing machine, zinc sheet conveyor, anode temper mill and solution cooling tower, are analyzed. It is pointed out that the application of advanced equipment characterized by large-scale and automation will dominate the upgrading and transformation of existing zinc electrowinning capacity.


Introduction
Zinc hydrometallurgy was put into industrial production in 1916, and developed rapidly, soon surpassing pyrometallurgy.Since 1980s, zinc hydrometallurgy has reached more than 80% of the world's zinc production [1] .Zinc electrowinning is an important link in zinc hydrometallurgy process, which is to extract metal zinc from zinc sulfate solution by electrowinning, including electrowinning, electrode plate loading and trough, cathode stripping and washing, anode leveling, solution cooling and circulation, etc.Because of the complexity of zinc electrowinning operation, the equipment level of zinc electrowinning represents the equipment level of zinc hydrometallurgy to a great extent.In 2022, Chinese zinc output was 6.802 million tons, accounting for more than 40% of the world's zinc output, ranking first in the world for more than 20 consecutive years.With the improvement of zinc smelting capacity and quality, in recent 10 years, some newly-built zinc hydrometallurgy projects in China have successively adopted advanced equipment such as large plate, large electrolyzer and automatic zinc stripping, and achieved good results, which has positive reference significance for promoting the transformation and upgrading of traditional zinc hydrometallurgy production system and improving the overall equipment level of zinc smelting industry.

Cathode
Cathode is usually composed of electrode plate, conductive rod, conductive head and insulating strip.The electrode plate is pure aluminum plate, and its hardness and flatness have great influence on the service life and production operation of the electrode plate.The conductive rod is made of aluminum, the conductive head is made of pure copper or copper-aluminum composite material, and the insulating strip is generally made of PP and other insulating materials.In addition, in order to improve the rate of mechanical zinc stripping, some cathodes with mechanical zinc stripping are provided with zinc stripping opening and insulating tape at the liquid level line on the electrode plate [2] 。 The development trend of cathode is the continuous enlargement of the effective area of cathode, the reduction of the number of cathode sheets and the operation of stripping zinc.Cathode effective area refers to the area where zinc metal can be deposited on a single cathode, ranging from 1.13 m 2 to 1.6 m 2 , 2.6 m 2 , 3.2 m 2 , 3.6 m 2 .Due to the weight of the plate, below to 1.6 m 2 cathodes can be manually stripped of zinc, and above 2.6 m 2 cathodes generally need to be mechanically stripped of zinc.See Table 1 for typical cathode specification series and production operation parameters.

Anode
Anodes are generally composed of plates, conductive rods and insulators.The electrode plate is usually lead-silver alloy plate containing 0.75% ~1% silver, and ternary alloy plate of lead, silver and calcium or multi-element alloy plate can also be used to reduce the amount of silver [3] .The plate is produced by calendering or casting, and the plate surface is mesh or flat.The mesh helps the anode slime to precipitate on the plate and take it out of the cell, thus reducing the frequency of digging anode slime from the electrolytic cell.The conductive rod is pure copper, and the insulator is PP and other insulating materials.
Anode and cathode are used together, and they are also developing towards large scale.The number of anodes per tank is generally one more than that of cathodes to ensure the maximum deposition area.Typical anode specification parameters are shown in Table 2.

Electrolytic cell
The development of electrolyzer is mainly reflected in two aspects: large-scale specifications and diversified materials.
With the application of large plates and mechanical zinc stripping, the specifications of electrolytic cells gradually increase, the length develops from 4m to about 10m, and the number of plates accommodated in each cell increases from dozens to more than 100 groups, which effectively reduces the number of electrolytic cells and the occupied area of electrolytic cells under the same production scale.The concentration polarization phenomenon in large electrolyzer can be avoided by increasing the circulating amount of solution and other measures.See Table 3 for typical electrolyzer specifications.Electrolytic cell materials mainly include concrete lining anticorrosive materials, steel frame lining plastic, glass fiber reinforced plastic, resin concrete and so on.Resin concrete monolithic electrolyzer has developed rapidly in recent years.Because of its long service life, maintenance-free and high dimensional accuracy, it has been applied in many large-scale zinc hydrometallurgy projects such as lead-zinc smelting project of Jiangxi Copper Group and zinc-oxygen pressure leaching project of Western mining industry.

Cathode and anode multifunctional crane
In traditional zinc electrowinning production in China, bridge insulated cranes operated by cab are mostly used to discharge and load cathode and anode into the tank.Because of the acid mist released from the electrolyzer in production, the working environment of crane drivers is very bad.
In recent years, some newly-built zinc hydrometallurgy projects have used cathode and anode multifunctional cranes, which can remotely operate the control room, and realize accurate positioning and automatic loading and unloading trough.A zinc electrowinning workshop with a production scale of 100,000 t/a, if 3.2 m 2 is used Large plate process and 48-hour cathode deposition cycle, equipped with two cathode and anode multifunctional cranes can meet the needs of production operation.The cathode and anode multifunctional crane has two sets of gripping and lifting systems, which are used to lift cathode and anode respectively, lifting 1/2 groove cathode or 1/3 groove anode at a time [4] 。

Zinc stripping and cathode washing equipment
Because of the large number of electrode plates and short electrodeposition period, zinc stripping and cathode washing have become the main production operations in zinc electrode workshop.The electrowinning period of manual stripping zinc is generally 24h, and the small plate process is mostly used, so the labor intensity of stripping zinc is very high.Taking a 100,000 t/a zinc electrowinning workshop as an example, use 1.13m 2 cathode and deposition cycle of 24 hours, need to strip 26,000 cathodes and 300t metal zinc every day on average.Mechanical zinc stripping greatly improves labor efficiency and working environment.In 2005, China successfully realized mechanical zinc stripping for the first time, and successively adopted mechanical zinc stripping equipment in zinc hydrometallurgy projects such as Zhuzhou Smelter Zinc System Reconstruction Project, Danxia Smelter Zinc Oxygen Pressure Leaching Project, Jiangxi Copper Group Lead Zinc Smelting Project and Western Mining Zinc Oxygen Pressure Leaching Project.Mechanical zinc stripping is mostly 3.2 m 2 Large plate, 36h~48h long period electrowinning process, the capacity of 100 kt/a needs to be equipped with two sets of zinc stripping units with a capacity of 200 cathodes/h and one set of cathode washing units with a capacity of 400 cathodes/h.CINF has creatively developed large, medium and small zinc electrolysis plates (effective area 3.2m 2 , 2.6m 2 , 1.6m 2 , 1.4m 2 or 1.13m 2 ) and a full range of intelligent complete sets of equipment (including a full range of The area of electrolytic cell required for 100,000 t/a electric zinc, m 2 /(100,000 t/a galvanic zinc) 3600 3700 2500 1900 zinc stripping units, intelligent driving units, cathode washing units and anode treatment units), and put into use in new projects and renovation projects to provide high-quality technical services for enterprises.

Zinc sheet conveying equipment
The zinc sheets stripped from the cathode need to be stacked neatly, and then sent to the zinc melting and casting workshop to be cast into zinc ingots.CINF has developed intelligent forward and reverse flat stacking technology, which is higher, more uniform and faster than the prior art (Figure 1), and brings great convenience to the transportation of zinc stacks [5] .
Forklift trucks are traditionally used to transport zinc sheets in China.The lead-zinc smelting project of Jiangxi Copper Group completed by CINF has realized the "non-landing" transportation of zinc sheets for the first time, that is, the zinc sheets stripped by the zinc stripping machine are directly sent from the zinc electrowinning workshop to the zinc melting and casting workshop through the automatic zinc sheet conveying line and hoist, and pushed into the zinc melting furnace through the automatic feeder, thus avoiding secondary transportation and reducing manual operation.
Fig. 1 The zinc stack (left: use intelligent forward and reverse flat stacking technology, right: not use )

Anode leveling equipment
Anode mechanical flattening is an operation corresponding to cathode mechanical stripping of zinc, which is an improvement on manual flattening.Manganese dioxide and other solid deposits are produced on the surface of anode plate during electrodeposition, which must be cleaned regularly, and the cleaning period is about 20 days.The cleaning and leveling of anode has great influence on the quality and current efficiency of zinc sheet.
The mechanical leveling process is as follows: the anode is lifted out of the electrolytic cell by the cathode and anode multifunctional crane and put on the anode trolley, and the anode plate is transported to the automatic flattening machine by the anode trolley, and the high-pressure water washing and flattening treatment are carried out on the anode flattening machine.When the anode trolley returns to the electrolysis area, the cathode and anode multifunctional crane hoists the patted and cleaned anode plate back to the electrolysis cell [6] .In the application of many projects in China, the improvement and optimization of anode flattening equipment are mainly reflected in reducing water consumption, reducing water consumption and installed power of high-pressure water pumps by adding accumulators in high-pressure water systems [7] .

Solution cooling tower
The electrothermal effect of zinc electrowinning process leads to the increasing temperature of electrolyte.In order to ensure the stability of electrolysis temperature, it is necessary to circulate the electrolyte with a cooling range of 6~8 ℃ to maintain the heat balance of electrolyzer.In industrial production, the solution cooling methods such as heat transfer in tank, vacuum evaporation and air cooling tower have been used.Heat transfer tank will reduce the utilization of space in tank.The main disadvantages of vacuum evaporative cooling are high energy consumption and difficult maintenance.Practice has proved that air cooling tower is the appropriate way to cool zinc electrowinning solution.At present, air cooling tower is basically used in zinc electrowinning production in China.
The cooling area of air cooling tower has been developed from small to large, and different specifications have been developed: 12.5m 2 , 24m 2 , 30m 2 , 40m 2 , 50m 2 , 60m 2 , 72m 2 etc.With the expansion of zinc smelting scale, 50m 2 or larger cooling towers is widely used at present, the large-scale equipment reduces the number of equipment used.The capacity of electrolyte cooling equipment required by zinc smelting project is closely related to the climatic conditions in the area where it is located.In general, 100,000 t/a zinc electrowinning workshop needs to be equipped with 10 50 m 2 cooling towers or 7 72 m 2 cooling towers in china.Cooling towers need more in hot and humid areas, whereas less cooling towers can be selected.
Using cooling tower to ventilate and remove acid mist in zinc electrowinning workshop is an ideal measure to improve the operating environment of zinc electrowinning workshop.Traditional zinc electrowinning workshop ventilation is mostly natural ventilation, and the ventilation effect is not good.The fan of electrolyte cooling tower is used to ventilate and remove acid mist, which can cool the solution and realize ventilation in the workshop.The acid mist is also brought into the cooling tower and purified by the droplet catching layer, thus reducing the unorganized discharge of acid mist and achieving the effect of achieving multiple goals at one stroke。

Conclusion
With the vigorous development of domestic zinc smelting industry, advanced zinc electrowinning equipment has been applied to many new projects, which promotes the large-scale zinc electrowinning equipment, mechanization and automation of production, reduces labor intensity, and improves the level of safety, hygiene, energy conservation and environmental protection.However, there is still a long way to go to improve the overall level of zinc electrowinning equipment in China, and the penetration rate of automation equipment such as zinc stripping machine and multifunctional crane is less than 1/4 of the domestic zinc smelting capacity.With a series of key equipment gradually completing the process of introduction, digestion, absorption, improvement and re-innovation, and gradually realizing localization, in the future, the improvement of equipment level will dominate the upgrading of existing zinc electrowinning capacity.

Table 2
Typical specifications and parameters of the anodes

Table 3
Typical specifications and parameters of the cells