The solid-state method is currently one of the most mature and widely adopted processes for manufacturing lithium iron phosphate (LiFePO4) cathode materials. Utilizing iron phosphate and lithium carbonate as primary raw materials, this process achieves large-scale production through precise batching, wet milling, high-temperature reductive sintering, and subsequent post-processing. The detailed manufacturing workflow and key control points for each stage are outlined below:
Mixing and Batching
After inspection and storage, raw materials such as iron phosphate, lithium carbonate, glucose, and additives (conductive agents) are temporarily stored in the raw material warehouse. Iron phosphate and lithium carbonate are supplied in ton-bags. First, the ton-bags of iron phosphate and lithium carbonate are placed on the top of the silo. Using internal silo cutting tools, the bottom of the ton-bag is cut to allow the material to fall into the silo. The temporarily stored materials in the silo are weighed according to the formula in a batching tank. Glucose and conductive agents are directly added into the batching tank.
During the unloading of materials like iron phosphate and lithium carbonate, dust is generated. When feeding, the exhaust port is activated to collect dust generated at the material outlet. The collected dust is returned to the batching process, and purified exhaust gas settles in the workshop.
Stirring Mill and Grinding
A wet process is used to mix the materials. After batching iron phosphate, lithium carbonate, glucose, pure water, and conductive agents in the batching tank, the materials form a slurry, which is pumped into a stirring mill. Oxidized zirconia balls are used for grinding and mixing to reduce the particle size to below 50 mesh. The slurry is then transferred to a sand mill for further grinding until the particle size is below 100 mesh.
During production, ice water is used to cool the stirring mill and sand mill to keep the material at a low temperature. The ice water is returned to a circulating cooling water pool and cooling tower, cooled, and reused without discharge. The ice water is further cooled by a refrigeration system for reuse. As the stirring mill and sand mill operate in a closed system with wet materials, no dust is generated. However, stirring, grinding, and cooling tower operations do produce some noise.
Spray Drying
The ground slurry is pumped into a spray dryer. At the top of the tower, a high-speed centrifugal atomizer sprays the slurry into fine droplets, which contact hot air in a very short time, drying into semi-finished powder. The dryer uses natural gas as the heat source. The hot air is heated in the dryer and enters the air distributor at the top of the drying chamber. The hot air enters the drying chamber in a spiral pattern to achieve a temperature of 320°C.
All semi-finished materials are continuously discharged from the bottom of the drying tower and the cyclone dust collector. Exhaust gas is drawn by a fan to a bag filter for dust removal. Dust collected by the bag filter is returned to the drying process. Purified exhaust gas is discharged through a 25 m high chimney. The bag filter has a pore size <0.1 μm. The exhaust gas inlet and outlet temperature is about 100°C; water vapor does not condense into droplets and does not affect the bag filter. The spray dryer generates noise during operation.
Sintering
Material collected by the spray drying dust collector is conveyed through a sealed pipeline into the sintering process using vacuum feeding, producing no dust. The material is sintered in a sealed roller kiln. The sintering temperatures are set in different zones according to the sintering process, typically 700–800°C (roller kiln uses electric heating). Materials are loaded in graphite crucibles (no dust generation) on the roller. The rotation of the roller moves the crucibles forward, completing sintering.
During sintering, trivalent iron is reduced to ferrous iron. High-purity nitrogen, prepared by a nitrogen generator, is introduced into the sintering furnace to provide an inert atmosphere. LiFePO₄ is synthesized at high temperature with a reaction conversion of 99.9% (based on LiFePO₄), and the LiFePO₄ yield is 99.5%.
The main reaction mechanism is as follows:
- Decomposition of lithium carbonate releasing CO₂:
Li₂CO₃ → Li₂O + CO₂ - Glucose decomposes into carbon and water under inert atmosphere:
C₆H₁₂O₆ → 6C + 6H₂O - Iron phosphate reacts with lithium in the presence of carbon to synthesize LiFePO₄:
2FePO₄ + Li₂O + 6C → 2LiFePO₄ + 5C + CO
Overall reaction:
2FePO₄ + Li₂CO₃ + C₆H₁₂O₆ → 2LiFePO₄ + 5C + CO₂ + CO + 6H₂O
After sintering, the material is cooled and sent to the crushing and sınıflandırma section. The post-sintering section of the roller kiln uses a water-circulating jacket and air cooling. Circulating water is cooled by a fiberglass cooling tower and reused.
During sintering, side reactions occur due to glucose decomposition:
C₆H₁₂O₆ → 6C + 6H₂O
C + CO₂ → 2CO
The nitrogen generator uses air separation technology, taking air as raw material and carbon molecular sieves as adsorbents. It operates via pressure swing adsorption, selectively adsorbing oxygen and nitrogen to separate them. Noise is generated during nitrogen generator operation.
Sintering reactions produce large amounts of water vapor, CO, CO₂, and minor volatile products from incomplete glucose decomposition, which enter the sintering exhaust incineration system. The exhaust is ignited with natural gas and discharged through a 15 m high chimney. Combustion exhaust pollutants include dust, SO₂, and NOx. Waste heat from combustion is recovered for air preheating in the spray drying process.
Jet Frezeleme
Post-sintered material is conveyed to the jet milling process for pulverization. A fluidized-bed jet mill is used. Compressed air is accelerated through four Laval nozzles surrounding the grinding chamber to form supersonic airflow. In the grinding zone, particles collide at nozzle intersection points, resulting in pulverization.
Ground material is carried by the rising airflow into the classification zone. A high-speed sınıflandırıcı wheel separates fine particles, which are collected by cyclone separators and bag filters. Coarse particles return to the grinding zone for further milling. Exhaust gases are filtered and recirculated via the fan turbine to the mill; gases are reused and not discharged. Material collected by cyclone and bag filters is conveyed to the next process. The jet milling process operates in a closed loop, producing no dust emissions.
Magnetic Separation and Screening
Electromagnetic dry powder separators and screens are installed below the jet mill. Material collected by cyclone and bag filters falls into these devices for iron removal and screening to eliminate magnetic impurities. The iron removal container is sealed. After screening and iron removal, the material enters a vacuum packaging machine. This process is fully enclosed, producing no dust except for small amounts of iron slag, which are collected and recycled.
Vacuum Packaging
Powdered product is automatically fed into a vacuum packaging machine for vacuum sealing. The air inside the packaging bag is removed, carrying a small amount of powder. Dust generated during vacuum packaging is collected by the built-in bag filter in the enclosed packaging room. After treatment, exhaust settles in the workshop.
Çözüm
Overall, from precise batching and mixing to final vacuum packaging, every step of the solid-state lithium iron phosphate production is closely interlinked and strictly controlled. By precisely managing critical nodes such as milling particle size, sintering atmosphere, and magnetic iron removal, the quality and purity of the final cathode material are effectively guaranteed.
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