What are the optional processes for ammonium chloride evaporation and crystallization equipment?

Currently, industrial ammonium chloride evaporation-crystallization processes can be broadly categorized into two types: "evaporation and concentration followed by crystallization and separation." Each route offers various equipment combinations and can be further subdivided based on steam utilization and crystallization methods.

1.By Steam Utilization Method: MVR (Mechanical Vapor Recompression) Forced Circulation Evaporator + Cooling Crystallizer

This method utilizes a compressor to pressurize and heat secondary steam for reuse, resulting in the lowest energy consumption. It is suitable for plants with high steam unit prices or a scale of 3 t/h or higher. It is often paired with an OSLO or DTB cooling crystallizer, enabling closed-loop circulation of the mother liquor. Crystallization particle size is 120-180 µm, and operating costs can be reduced by more than 60% compared to triple-effect evaporation.

Multi-effect evaporation (triple/quadruple effect) + flash evaporation/cooling crystallization: Uses primary live steam as a heat source, utilizing waste heat through 3-4 effects in series. Investment is lower than MVR, but steam consumption is approximately 0.3-0.4 tons per ton of water. The concentrated liquid from the last effect then enters a flash tank or cooling crystallizer for cooling and salt precipitation. Suitable for fertilizer and coal chemical projects where steam is cheap or low-pressure steam is a byproduct.

TVR (Thermal Recompression) + Multi-effect evaporation: Adds a steam jet pump between any two effects, using high-pressure steam to inject secondary steam. This can save approximately one effect. Operating costs are between MVR and pure multi-effect evaporation. The system is simpler but requires a continuous supply of high-pressure steam.

2.By Crystallization Method

Cooling Crystallization (OSLO/DTB): Utilizing the significant decrease in NH4Cl solubility with decreasing temperature (approximately 40 g/100 g water decreases from 100℃ to 20℃), the concentrated solution at 80-90℃ is cooled to 30-40℃ using a tube-and-plate heat exchanger. Supersaturation is controllable, resulting in large crystal particles with low water content. The mother liquor is directly pumped back to the evaporator, making the entire process continuous.

Vacuum Flash Crystallization: The concentrated solution undergoes adiabatic flash evaporation at 60-80 kPa absolute pressure, simultaneously cooling and evaporating. It combines concentration and crystallization functions, offering compact equipment but requiring high-performance vacuum systems and condensers. It is often combined with a DTB crystallizer for low-flow, high-purity applications.

Integrated Evaporation-Crystallization (Triple-Effect "Salt-Bearing Box" Crystallizer)

The lower part of the third-effect evaporation chamber is equipped with a salt collection box and a vortex thickener, allowing crystals to grow directly within the evaporator. This eliminates the need for a separate crystallizer, resulting in low heat loss and space saving. It is suitable for applications with high raw material concentration and moderate particle size requirements.

3.Key Points for Process Selection

For boiling point elevation ≤12℃, MVR is preferred; if >12℃ or with high fluorine content or COD, multi-effect crystallization with cooling can be used to distribute the load.

Due to the strong corrosiveness of chloride ions, the evaporator material should be selected in the order of "titanium > 2205 duplex steel > 316L". For the high-vacuum, low-temperature section, 2205 steel or titanium-lined composite plates can be used to reduce costs.

For particle size requirements ≥150 µm, the cooling rate should be controlled at 1-2℃/min, the residence time ≥30 min, and 50-100 µm seed crystals should be added.

In summary, the core combination of optional processes for ammonium chloride evaporation crystallization equipment is: "MVR forced circulation" or "multi-effect/TVR forced circulation" for concentration, and "OSLO/DTB cooling crystallization" or "flash crystallization" for particle size control. Then, the two routes can be weighed based on steam price, electricity price, product particle size, and investment budget.