In high-pressure melamine production, molten salt serves as a critical heat-transfer medium, delivering the necessary heat for the endothermic reaction of urea converting to melamine (380℃, 8.0 MPa). However, long-term operation leads to gradual deterioration of the molten salt, characterized by increased melting point, excessive alkalinity, and component imbalance, which risks production efficiency and equipment safety.
This article analyzes the root causes of molten salt deterioration, interprets component change data, and provides actionable maintenance strategies—essential for melamine manufacturers aiming to extend service life and reduce costs.
In the production of melamine powder, the reaction of urea to melamine is an endothermic reaction, and the heat required for the endothermic reaction in the melamine reactor is obtained by burning natural gas in a molten salt furnace. In this molten salt furnace, heat is transferred to the circulating molten salt, which then transfers this heat to the reaction medium in the melamine reactor for reaction. The composition of molten salt is as follows: NaNO2, 40% (mass fraction); NaNO3， 7% (quality score); KNO3， 53% (quality score); The melting point of molten salt is 142 ℃.
It is a low-eutectic mixture with very low corrosiveness and sufficient thermal stability over the temperature range of 160-550 ℃. Especially at temperatures below 480 ℃, the thermal stability is quite good. Between 480 ℃ and 550 ℃, the molten salt slowly decomposes, and sodium nitrite decomposes into sodium nitrate, generating sodium oxide and releasing nitrogen gas. When the molten salt system is exposed to air and directly in contact with oxygen, the same reaction described above will occur at 430 ℃.
Therefore, to prevent air from entering the molten salt system, nitrogen gas is introduced into the molten salt tank to protect it, preventing direct contact between the molten salt and oxygen. During the production process, the molten salt system is equipped with a high-temperature interlock protection device, and the temperature is strictly controlled during actual operation to prevent overheating.
Melamine Molten Salt Deterioration is a cumulative process driven by operational conditions and system interactions. Key causes include:
Below 480℃: Molten salt remains stable; above 480℃, NaNO₂ decomposes into NaNO₃, releasing nitrogen and forming Na₂O.
Air Exposure: Even at 430℃, contact with oxygen (from air leakage) accelerates NaNO₂ decomposition—breaking the original component balance.
During decomposition, Na₂O reacts with trace moisture or CO₂ to form Na₂CO₃, thereby increasing the molten salt’s alkalinity.
Alkalinity exceeding 0.3% (the critical threshold) indicates severe deterioration, as seen in a 1047-day service sample that reached 0.36%.
Moisture ingress (from air, nitrogen pipeline condensation, or system leakage) causes two issues:
Rapid vaporization during heating (200–400℃) poses an overpressure risk.
Accelerates hydrolysis of nitrate/nitrite, promoting Na₂CO₃ formation and alkalinity spikes.
Organic contact (oil, paper, wood, cotton) triggers violent combustion or even explosions—strictly prohibited.
Metal contact (aluminum, magnesium, or copper) can cause chemical reactions, contaminating the molten salt and corroding equipment.
Chloride ion (Cl⁻) accumulation (e.g., up to 100×10⁻⁶ in long-term use) degrades thermal stability.
Over-temperature (exceeding 600℃) induces direct reactions between molten salt and steel equipment, accelerating deterioration.
Pressure fluctuations in the reactor or molten salt system can cause ammonia to leak into the molten salt, disrupting component balance.
To mitigate deterioration and extend service life (target: >1000 days), implement the following targeted measures:
Maintain nitrogen purity ≥98% and pressure >0.3 MPa in the molten salt tank—form an inert gas blanket to prevent oxygen from entering.
Regularly drain condensation from nitrogen pipelines (critical in winter to prevent water ingress) and verify there is no cross-connection between nitrogen and air pipelines.
After nitrogen purging/leak testing, promptly relieve pipeline pressure to prevent water backflow.
Limit molten salt temperature to ≤480℃ (avoid exceeding 550℃) with high-temperature interlocks.
Stabilize reactor conditions: pressure = 7.5–8.0 MPa, temperature = 380℃—prevent leakage caused by drastic fluctuations.
Control molten salt system pressure at 0.02–0.03 MPa; promptly inspect abnormal vent valve openings.
Dry molten salt thoroughly before use; heat slowly (200–400℃) to remove residual moisture.
Prohibit contact with organics (oil, paper, wood) and incompatible metals (Al, Mg, Cu) in the system.
Regularly clean molten salt pipelines and equipment to reduce Cl⁻ and impurity accumulation.
Test molten salt components (NaNO₂, NaNO₃, KNO₃, Na₂CO₃, alkalinity, Cl⁻) every 3–6 months.
Replace molten salt when: alkalinity >0.3%, NaNO₂ <40%, or melting point rises by >10℃.
In case of molten salt ignition: Use dry sand, dry powder, or CO₂ extinguishers—NEVER use water (risk of splashing and scalding).
Avoid welding or flame-heating molten salt pipelines/equipment (to prevent local overheating and decomposition).
Melamine Molten Salt Deterioration is primarily driven by thermal decomposition, air/moisture ingress, and operational deviations. By monitoring component changes (especially NaNO₂ content and alkalinity), implementing strict nitrogen protection, and controlling temperature/pressure, manufacturers can extend molten salt service life to over 1000 days.
Proactive maintenance not only reduces replacement costs (46 tons per batch) but also minimizes unplanned downtime—critical for ensuring stable, efficient melamine production. As a core heat transfer medium, molten salt’s health directly impacts product quality and operational safety—making its proper maintenance a top priority for high-pressure melamine powder plants.
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