The Iron Oxide Red Method for LFP Preparation Process part 2
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Last blog we introduced half of the the Iron Oxide Red Method for High-Density LFP Preparation Process, here is the part 2 of the process
Process Step 2: Wet Milling (Sand Milling)
Objective: To thoroughly blend iron oxide red (Fe₂O₃), lithium dihydrogen phosphate (LiH₂PO₄), and glucose through wet ultrafine milling, reducing particle size to the submicron or nanoscale level. This achieves homogeneous mixing of all components at the molecular level, providing an ideal reactive interface for the subsequent carbothermal reduction reaction.
Operating Procedure:
Pre-mixed slurry is pumped from the premixing tank into the coarse-grinding agitation tank for further homogenization.
The slurry is pumped into the primary sand mill for coarse grinding, after which it is returned to the coarse-grinding tank for additional mixing.
The coarse-ground slurry is transferred to the fine-grinding agitation tank, then fed into the secondary nano sand mill for precision milling.
The milled slurry passes through an iron remover to eliminate metallic iron contaminants introduced by equipment wear, before entering the finished slurry storage tank.
Milling Equipment: High-efficiency pin-type (bead mill) sand mills. Coarse grinding employs φ0.6–0.8 mm zirconia beads; fine grinding employs φ0.3–0.5 mm yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) beads. Grinding chamber bead fill rate is approximately 80%. Since Fe₂O₃ is already in its highest oxidation state, nitrogen blanketing for oxidation prevention is not required during milling.
Critical Process Parameters:
Post-fine-milling particle size is critical to the uniformity of the subsequent carbothermal reduction reaction. A D50 ≤ 0.5 μm is the baseline requirement for ensuring adequate contact between iron oxide red, lithium source, and carbon source, thereby enabling uniform reduction kinetics.
Recirculating milling is employed to ensure all slurry passes through the milling zone and achieves the target fineness.
Milling temperature must be maintained below 40°C to prevent excessive water evaporation and unintended changes in slurry solid content.
Post-milling slurry must pass through an iron remover to eliminate wear-induced metallic iron contamination.
Process Step 3: Spray Drying and Granulation
Objective: To convert the milled slurry into uniformly spherical precursor powder via instantaneous spray drying, yielding a precursor with good flowability and uniform bulk density for the subsequent sintering step.
Operating Procedure:
Finished slurry is pumped from the storage tank to the atomizer of the spray dryer.
The slurry is atomized into micron-scale droplets via a rotary atomizing disc or spray nozzle.
Contact with heated air at 200–300°C causes instantaneous evaporation of the aqueous phase, forming dry spherical precursor particles.
Dried material is collected by cyclone separators and bag-house filters, then conveyed automatically to the pre-sintering buffer silo.
Process Notes: The iron oxide red route uses deionized water as the dispersion medium, enabling an open-loop spray drying system (air heating is sufficient). This contrasts with the ferrous oxalate route, which requires a closed-loop system with methanol as solvent, resulting in significantly lower capital expenditure, reduced operating costs, and improved operational safety. Drying temperature must be controlled within an appropriate range — excessively high temperatures may cause premature carbonization or thermal decomposition of glucose, adversely affecting subsequent sintering outcomes.
Process Step 4: High-Temperature Sintering (Carbothermal Reduction — Core Process Step)
Objective: Under inert atmosphere protection, to reduce Fe³⁺ to Fe²⁺ via carbothermal reduction while simultaneously synthesizing olivine-type lithium iron phosphate (LiFePO₄) crystals, with concurrent in-situ formation of a conductive carbon coating layer. This constitutes the core technical stage of the iron oxide red route.
Operating Procedure:
Dried precursor powder is automatically loaded into cordierite/mullite saggers.
Saggers enter an atmosphere-controlled roller kiln and are sintered according to a programmed temperature profile under N₂ atmosphere (purity ≥ 99.999%).
Following sintering, the material is slowly cooled to below 60°C under continuous N₂ protection.
Saggers are automatically unloaded and material advances to the next process step.
Sintering Protocol (Two-Stage Sintering):
The carbothermal reduction sintering in the iron oxide red route typically employs a two-stage sintering profile:
Sintering Mechanism:
During the pre-sintering stage (300–450°C), glucose undergoes carbonization pyrolysis, with the resulting amorphous carbon distributing homogeneously throughout the powder mixture. Concurrently, lithium dihydrogen phosphate begins to decompose, generating highly reactive phosphate intermediates.
During the high-temperature stage (650–750°C), the following core reactions occur:
Carbothermal Reduction: Carbon and CO reduce Fe³⁺ to highly reactive Fe²⁺ (as FeO), which is the pivotal step of the entire process. CO gas generated by pyrolysis of the carbon source establishes a localized reducing atmosphere within the reaction system, simultaneously promoting the reduction reaction and preventing re-oxidation of the nascent Fe²⁺ by trace oxygen.
Solid-State Synthesis: Highly reactive FeO reacts with phosphate and lithium salts via solid-state diffusion at elevated temperature to form LFP nuclei. As temperature increases and the isothermal hold progresses, nuclei grow into well-crystallized, structurally stable olivine-type LiFePO₄.
Carbon Encapsulation: Residual carbon derived from pyrolysis of the organic carbon source forms an in-situ conductive carbon network on the surface of LFP particles, significantly enhancing the electronic conductivity of the material.
Critical Process Control Parameters:
Atmosphere control is paramount: Oxygen content within the furnace must be strictly maintained at ≤ 10 ppm. A reducing micro-environment (CO atmosphere) must be established through complete pyrolysis of the carbon source, which is essential for full reduction of Fe³⁺. Any oxygen ingress will result in re-oxidation of the generated Fe²⁺.
Carbon content and sintering temperature matching: The carbon source addition level must be precisely matched to sintering temperature and isothermal hold duration to ensure complete reduction of Fe³⁺ while achieving a final product carbon content within specification (1.5–2.5 wt%).
Temperature uniformity: Temperature uniformity within each thermal zone must be controlled within ±5°C, as carbothermal reduction kinetics are highly temperature-sensitive.
Sintering temperature selection: Lower temperatures (650°C) with extended hold times favor grain refinement but risk incomplete reaction; higher temperatures (750°C) promote crystallinity but require control of excessive grain growth and inter-particle sintering.
Process Step 5: Jet Milling and Classification
Objective: To comminute sintered agglomerated LiFePO₄ material to the target particle size and remove oversized particles, yielding a product with a narrow, uniform particle size distribution.
Operating Procedure:
Cooled sintered material is pneumatically conveyed to the pre-milling feed silo.
Material enters a jet mill (fluid energy mill), where high-pressure gas streams drive inter-particle impact comminution (media-free milling, eliminating risk of secondary contamination).
Milled material is classified by an air classifier; on-specification material advances to the downstream silo.
Oversized coarse fraction is recycled back for further milling (closed-loop circuit).
Process Step 6: Sieving / Magnetic Separation / Batch Blending / Packaging
Objective: To remove oversized particles and magnetic foreign matter, homogenize batch-to-batch product variation, and complete final product packaging.
Operating Procedure:
Milled material is screened through an ultrasonic vibrating sieve (300–400 mesh) to remove oversized particles.
Dry-powder electromagnetic separators remove magnetic impurities; magnetic field strength ≥ 12,000 Gauss, target magnetic impurity content ≤ 50 ppb.
Multiple batches are blended in a ribbon or paddle blender for 2–3 hours to ensure inter-batch consistency.
After sampling and quality verification, material is vacuum- or nitrogen-packed in a low dew-point environment (≤ −30°C).
Core Technical Challenges of the Iron Oxide Red Route
The fundamental production bottleneck of the iron oxide red route lies in the difficulty of sourcing iron oxide red of sufficient purity, morphology, and consistency to meet the stringent requirements of high-performance LiFePO₄ cathode materials. Unlike standard industrial pigment-grade iron oxide red, battery-grade iron oxide red demands:
Exceptionally high purity: Fe₂O₃ content must reach 99.6–99.9% or above; trace elements detrimental to battery performance (Na, Mg, Ni, Cr, Cu, Zn, etc.) must be controlled to ppm or even ppb levels.
Superior morphology: Crystal habit should be uniformly ellipsoidal or near-spherical with virtually no secondary crystalline phases and highly consistent particle shape — in contrast to the acicular, platy, or irregular morphologies typical of standard iron red pigments.
Controlled particle size: Primary particle size must be maintained at the nanoscale (80–100 nm) with a narrow size distribution and stable specific surface area.
Robust batch-to-batch consistency: Control precision must reach the ppm (1/1,000,000) to ppb (1/1,000,000,000) level.
