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How to control the product quality of melamine

Melamine powder is a widely used organic chemical intermediate that appears as a white, odorless, and low-toxicity powder. Its appearance quality has almost no visual difference, so strict quality grading and process control are crucial to ensure compliance with industrial standards and final performance. This article summarizes mature methods for quality of melamine control based on long-term industrial production experience, covering key influencing factors and operational guidelines.

Why is the quality of melamine important?

Melamine powder comes in various grades and applications, and non-compliant products are often affected by excessive moisture, high ash content, discoloration, high turbidity, or impurities, resulting in economic losses. Stable, high-quality melamine directly enhances product competitiveness and reduces waste.

Low ash content and turbidity can prevent defects in transparent coatings and molded products.

Controlled Color (APHA) is crucial for white or light colored finished products.

The appropriate pH value can affect storage stability and compatibility with other chemicals.

Unqualified quality leads to:

  • Poor resin fluidity and gel time change
  • Final product discoloration or turbidity
  • Customer complaints and contract loss
  • Reduce prices to achieve

Quality indicators of melamine powder

ItemPremium GradeFirst Class
Melamine Content, % ≥99.899.0
Moisture, % ≤0.10.2
Ash Content, % ≤0.030.05
Color (APHA) ≤2030
Turbidity (Kaolin) ≤2030
pH Value7.5–9.57.5–9.5
AppearanceWhite free‑flowing powderWhite free‑flowing powder

8 key factors affecting the quality of melamine

1.Purity of urea

In the production of melamine powder, if there is too much oil and metal ions in the urea solution, it can cause the activated carbon bed in the carbon bed reactor to be penetrated, resulting in excessive product color. The oil content in the urea solution must be strictly controlled.

2.Process Circulating Water Volume

To ensure sufficient process circulation water volume corresponding to the reactor load, control the OAT at a lower level than the water volume. The optimal circulation amount must consider two factors: the melamine content in the solution of the rapid cooling section and the consumption of utilities. The optimal melamine concentration in the feed to the gas stripping tower is approximately 8% to 9%.

3.Ammonia dosage for purification system

It is necessary to strictly control the ammonia ratio in the purification system to maintain it at 12% to 14% to ensure the ammonia concentration in the refining system. If the ammonia concentration is too low, some undecomposed impurities will precipitate during crystallization, resulting in unqualified melamine. Excessive concentration can cause overpressure in the crystallizer.

4.Working status of online filters and carbon bed reactors

The functions of online filters and carbon bed reactors are to remove impurities and colored substances from melamine solutions. If performance decreases or the carbon rod is damaged, it will result in excessive chromaticity and produce unqualified melamine. Therefore, it is necessary to regularly inspect and replace the carbon rod.

5.Temperature of dryer

If the dryer temperature is too low, the product’s moisture content will be too high. The dryer temperature must be maintained above 80℃.

6.Quantity of CO₂ added to the second crystallizer of OAT

Adding carbon dioxide to the second crystallizer of OAT adjusts the solution’s pH. The optimal pH value for the feed is 7-7.5, which can be achieved by adjusting the CO₂ flow rate injected into the crystallizer.  

7.Situation of filtrate in OAT filter

The turbidity of the filtrate indicates that the filtration effect of OAT is poor. There will be a large amount of OAT in the solution that has not been filtered out, leading to a high OAT content in the circulating process water and excessive turbidity in the product, resulting in unqualified products.  

8.Non-process factors

If the airflow blower filter is misaligned or fails, impurities in the air will enter the system, producing small black spots that affect the appearance quality.

In addition, non-standard operations in packaging positions may also result in unqualified product inspections upon arrival. Melamine powder has hygroscopicity. Exposure to humid air can increase moisture content (over 0.1%), which later leads to clumping and poor flow.

How to control the product quality of melamine

  1. The bottom temperature of the carbon dioxide stripping tower should be strictly controlled above 158 ℃ to ensure that all carbon dioxide is fully extracted. This is beneficial for the operation of the ammonia recovery system and ensures that OAT has high solubility under rich ammonia conditions in the refining and crystallizer systems.
  2. The bottom temperature of the ammonia stripping tower must be strictly controlled. Under normal pressure, it should be strictly controlled above 205 ℃ to ensure sufficient ammonia stripping, allowing the OAT to crystallize and precipitate fully in the OAT crystallizer and be removed in the filter. The OAT content in the circulating water during the process should be kept at a low level.
  3. It is necessary to strengthen the monitoring of the ammonia recovery system to avoid prolonged overheating at the top of the ammonia tower, which may cause ammonia pump gasification. Once there is a shortage of ammonia supply to the refining system, the reactor load and system circulation should be immediately reduced to ensure that the refined ammonia is qualified. If it cannot be guaranteed, the reactor should be immediately cut off, and the ammonia should be fed again after it returns to normal to avoid turbidity exceeding the standard and producing unqualified products.
  4. Pay attention to the changes in the pressure difference of the melamine online filter. The service life of the carbon bed reactor is two years, and when the time comes
    Timely replacement is necessary to prevent damage to the carbon rod, which may result in substandard product quality.
  5. Always pay attention to the content of OAT in the mother liquor tank, usually below 0.1%, which can reflect the quality changes of the process circulating water. Once an increase is found, the system should be checked and restored as soon as possible; there is a risk of product turbidity exceeding the allowed limit.
  6. When packaging, check whether there is any debris inside the packaging bag, whether it is clean, and whether the seams are neat; Outsourcing should be clean and free of dirt. In case of rainy weather, attention should be paid to preventing dirt from splashing onto the product packaging. If sludge is found, it should be cleaned up promptly. The packaging must be packed in a moisture-proof bag (multi-layer polyethylene or lined kraft paper). It should be stored in a cool and dry warehouse, away from acids and oxidants. The packaging position should maintain detailed records of the team’s packaging status to improve product traceability.

conclusion

Controlling the quality of melamine products is not a single action, but a systematic approach. By strictly controlling the purity of urea and ammonia/CO₂, as well as temperature, filtration, and packaging, high-quality melamine powder that meets international requirements can be consistently produced.

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