One sentence conclusion: With proper pre-treatment, material matching, and matching gas-liquid separation, low-temperature evaporators can achieve stable reduction of hazardous waste; But it is only a physical concentration and cannot eliminate heavy metals. The concentrated mother liquor still belongs to hazardous waste and must be disposed of in compliance with regulations by outsourcing.

1、 Why can it stabilize heavy metal retention

Heavy metals (nickel, copper, zinc, chromium, etc.) have extremely high boiling points and will not evaporate with water vapor under vacuum low-temperature conditions of 30-40 ℃. Under normal conditions, they are all trapped in the concentrated solution; The heavy metal index of condensate water is naturally easy to meet the standard and can be reused in the cleaning process.

Exception: Waste liquids containing mercury and some organic metals pose a risk of volatilization and are not suitable for direct entry into ordinary low-temperature evaporators. Additional gas-phase adsorption units are required.

2、 To achieve long-term stability, it is necessary to cross four risk points (which are also where many projects fail)

1. Foam running=maximum compliance hazard

If surfactant and emulsifier are entrained in the high salt waste liquid, it is very easy to foam when it is concentrated to high multiples. The foam entrained droplets pass through the demister, and heavy metals and salts directly run into the condensate, which immediately exceeds the standard.   ✅ Countermeasure: front-end oil separation sedimentation; Equipped with efficient foam remover and mechanical defoamer; Set a concentration limit and do not blindly pursue a 95% ultra-high reduction rate; If necessary, it should be equipped with automatic defoaming and dosing.

2. Corrosion risk caused by chloride ions (high-frequency pitting)

Most high salt waste liquids contain chloride ions, and ordinary 316L can experience pitting and leakage during long-term operation. Once leaked, heavy metal waste liquid leakage is a major environmental risk.   ✅ Countermeasure: Select materials based on chloride ion concentration: choose 2205/2507 duplex steel and titanium materials for medium to high chloride working conditions; All overcurrent components such as flanges, nozzles, sealing rings, and heat exchange plates should be checked for their materials uniformly, and large parts should not be used with good materials while small parts should be reduced.

3. High salt continues to precipitate and scale, and the evaporation rate continues to decline

Salt continues to concentrate, saturate, and precipitate, adhering to the heat exchange surface, resulting in a decrease in heat transfer efficiency and a decrease in evaporation rate. Frequent shutdown and acid washing are necessary.   ✅ Countermeasure:

Pre treatment of hardness and reduction of calcium and magnesium in the front-end;

Priority should be given to scraper type and forced circulation models for high salt crystallization conditions;

Configure online spray cleaning and set a scheduled cleaning program.

4. Preprocessing is missing, and suspended solids are directly fed into the host

Waste residue and metal debris directly enter the evaporator, causing wear on the pump body, blocking the flow channel, and inducing the adhesion of scale layers.   ✅ Countermeasure: Pre precipitation+precision filtration to separate solid impurities first.

3、 Recommended process route (electroplating, surface treatment high salt heavy metal waste liquid)

Raw waste liquid → pH adjustment+chemical precipitation (converting heavy metals into stable hydroxides, prioritizing precipitation) → oil separation/filtration pretreatment → low-temperature evaporator → 👉  Gas phase: foam removal+exhaust gas collection and adsorption 👉  Produced water: condensate water, tested and approved for reuse in the workshop 👉  Concentrated solution: Heavy metal high salt concentrated solution, collected in a closed manner, recorded in a ledger, and handed over to qualified units for hazardous waste disposal

Important reminder: Chemical precipitation is a pre selection method, and not all heavy metal raw materials are directly fed into the evaporator. Pre precipitation can reduce the load on the evaporator, decrease the consumption of chemicals, and reduce the risk of foaming.

4、 Boundary: Which situations are not suitable?

Waste liquid containing mercury and volatile organic heavy metals;

There are a large number of azeotropic VOCs in the waste liquid, and the cost of gas-phase treatment is extremely high;

High salt content, extremely high crystallization volume, and large amount of waste liquid (more suitable for MVR/crystallization systems);

The water quality fluctuates greatly and there is no stable pre-treatment support.