How to precisely control the iron to phosphorus ratio of lithium iron phosphate ?

jiangyaoyao0501
Sep 1
5 min read

The iron to phosphorus ratio is the critical factor in the quality control of lithium iron phosphate production. Precise control of the iron-to-phosphorus ratio isn't a single point of control, but rather a systematic process throughout the entire production process . It tests a company's depth of understanding of the process and the sophistication of its process control.

Below I will divide it into two parts: "Precise Control Method" and "Deviation Solution", and explain in detail how to control each process.

I. Iron to Phosphorus Ratio Precise Control System

The control of the iron-to-phosphorus ratio follows the principle of "prevention over correction" , with the core focus on the stability of upstream processes rather than downstream remedies.

Control links, control method principles and purposes

1. Source control: precise feeding

This is the most basic and critical step.

1.1. Precise Analysis: Accurately measure the effective content of raw materials (ferrous sulfate and monoammonium phosphate) using precise methods (e.g., titrimetric method for iron, quinoline phosphomolybdate gravimetric method for phosphorus), rather than simply relying on labeled content for calculations.

1.2. Molar Ratio Feeding: Strictly calculate and feed materials based on a molar ratio slightly lower than the theoretical 1:1 (e.g., Fe:P = 0.98:1.00). The design of a slight phosphorus excess ensures complete iron precipitation, preventing iron excess at the source.

1.3. Concentration and Flow Calibration: Regularly calibrate the level gauges of solution storage tanks and the flow meters of transfer pumps to ensure accurate feeding volumes.

Objective: To ensure correctness from the starting point of reaction stoichiometry, laying a solid foundation for the entire process. If the source is incorrect, all subsequent processes cannot correct it.

2. Process Control: Precipitation Reaction Control Methods

2.1. Precise pH Control: Use high-precision online pH probes and automated dosing systems to stabilize the reaction process within the optimal pH window (e.g., 1.8-2.2). Fluctuations in pH can significantly affect the completeness of the precipitation reaction and coprecipitation behavior.

2.2. Parallel Flow Dosing: Ensure that iron source and phosphorus source solutions are fed into the reactor in a parallel, uniform, and slow manner to avoid local supersaturation or instantaneous excess of any component.

2.3. Reaction Endpoint Determination: Upon reaction completion, a small amount of slurry may be centrifuged to test the residual iron ion content in the supernatant, ensuring the reaction is substantially complete.

Objective: To create a stable and homogeneous reaction environment, ensuring the theoretical molar ratio of the feed is maximally converted into the product's stoichiometric ratio and guaranteeing a high reaction conversion rate.

3. Core Correction: Thorough Washing

This is the most critical measure for controlling the iron-to-phosphorus ratio, particularly for reducing phosphorus content.

3.1. Washing Process: Employ multi-stage counter-current washing. The use of hot water (60-80°C) enhances the solubility of impurities.

3.2. Endpoint Determination: Monitoring the conductivity of the wash liquor is the most effective method. Stabilization of conductivity at a low value (close to that of the washing water) indicates that soluble ionic impurities (e.g., NH₄⁺, SO₄²⁻, Na⁺) have been effectively removed, meaning residual phosphate salts are also eliminated.

3.3. Verification Metrics: Monitor the pH and phosphate concentration of the wash liquor (e.g., using ammonium molybdate spectrophotometry).

Objective: To completely remove soluble ammonium phosphate salts (such as NH₄H₂PO₄) adsorbed on the surface of iron phosphate particles and entrained within the filter cake. This is essential for reducing the apparent phosphorus content and preventing an excessively low Fe/P ratio.

4. Final Safeguard: Sintering Stabilization

4.1. Atmosphere Control: Sintering must be conducted in an air atmosphere or weakly oxidizing atmosphere to prevent the reduction of Fe³⁺ to Fe²⁺ (which can cause fluctuations in measured iron content and lead to the formation of impurity phases).

4.2. Temperature Uniformity: Ensure uniform temperature distribution within the kiln to avoid localized over-sintering or under-sintering, guaranteeing that all material undergoes consistent dehydration and crystal phase transformation.

Objective: To stabilize the final phase composition of the product and prevent the introduction of new stoichiometric deviations due to sintering process variations.

II. Iron-to-Phosphorus Ratio Abnormality: Root Cause Analysis and Solutions

When abnormal Fe/P ratio is detected, it is essential to approach it like a medical diagnosis: identify the root cause before prescribing a targeted solution.

Case 1:Iron to phosphorus ratio is too high (Fe/P > 0.99, for example 1.02)

Meaning: Iron is relatively excessive and impurities such as Fe₂O₃ may be present in the production .
Root causes and solutions:

Possible Causes:	Analysis and Diagnosis	Solutions (By Process Stage)
1. Incorrect feed calculation	Review feeding records to confirm whether an excess of phosphorus source was used.	Batching Process:Immediately correct the calculation error.
2. Insufficient washing (most common cause!)	Test the conductivity and phosphate concentration (PO₄³⁻) of the wash water. If the final wash water exhibits high conductivity or a significant concentration of phosphate ions can be detected, this conclusively identifies inadequate washing as the cause. Concurrently, the Loss on Ignition (LOI) of the product will typically also be abnormally high.	Washing Process: Increase the number of washing stages or the volume of wash water. Increase the temperature of the wash water (e.g., from 60°C to 80°C). Optimize the reslurrying conditions of the filter cake to break up channels and ensure more uniform washing. Extend the washing time (e.g., by reducing the feed rate to the filter). Check the filter cloth for damage. A torn cloth can allow fine particles to pass through (fines passage). Note: Loss of iron-containing fines through the filter can ironically cause a falsely high measured Fe/P ratio and requires comprehensive analysis to diagnose.
3. Incomplete aging	Insufficient aging time or temperature results in loose particle structure, excessively high specific surface area, and overly strong adsorption capacity, making impurities difficult to remove during washing.	Aging Process:Optimize the aging process parameters to improve crystallinity and reduce the surface activity of the particles, making them easier to wash.
4. Analytical error	As with the issue of a "Fe/P ratio that is too high", the first step is to rule out analytical error.	Quality Control Process:Strictly adhere to the standard analytical procedures to prevent measurement errors.

III.Summary and Workflow Suggestions

1. Retest for Confirmation: First, rule out analytical error by re-testing the sample.

2. Trace Feeding Records: Verify the raw material test reports and feeding calculation records for the affected batch.

3. Review Process Records: Retrieve trend curves (e.g., pH, temperature, flow rate, conductivity) from the precipitation, aging, and washing processes to identify abnormal fluctuations.

4. Pinpoint the Diagnosis:

l If the Fe/P ratio is too high, focus on investigating precipitation reaction efficiency (check iron content in the supernatant).

l If the Fe/P ratio is too low, prioritize evaluating washing effectiveness (check wash water conductivity and product Loss on Ignition).

5. Implement Corrections: Make adjustments in the corresponding process stage and track data over several subsequent batches to verify the effectiveness of the corrective measures.

Precise control of the iron-to-phosphorus ratio reflects refinement in process control and systematization in quality management. It requires that every single step of the process be measurable, monitorable, and traceable.