Acid mist (such as hydrochloric acid mist, sulfuric acid mist, chromic acid mist, etc.) and alkaline mist (such as sodium hydroxide mist) generated during the electroplating process are common air pollutants and need to be effectively treated to avoid harming workers' health and the surrounding environment.
Author: Marisa
Acid mist (such as hydrochloric acid mist, sulfuric acid mist, chromic acid mist, etc.) and alkaline mist (such as sodium hydroxide mist) generated during the electroplating process are common air pollutants and need to be effectively treated to avoid harming workers' health and the surrounding environment.
The following are commonly used treatment methods and technical points:
1. Acid mist treatment technology
Physical interception method
Baffle/filter: filter acid mist particles through multiple layers of plastic balls, PP fillers or glass fibers, suitable for primary treatment of large particle acid mist.
Disadvantages: limited effect on small particle size droplets (such as chromic acid mist), need to be combined with other methods.
Chemical absorption method
Alkali spray neutralization: use alkaline solutions such as NaOH, Na₂CO₃ to spray and absorb acidic gases (such as HCl, H₂SO₄).
Typical reaction:
HCl+NaOH→NaCl+H2O
Equipment: packed tower, spray tower (empty tower spray efficiency is low, need to optimize nozzle design).
Special treatment of chromic acid mist:
Add a reducing agent (such as sodium sulfite) to reduce Cr(VI) to Cr(III), and then remove it by precipitation.
Or use chromic acid mist inhibitors (such as F-53 inhibitors) to reduce droplet dispersion.
Electrostatic demisting method
High voltage static electricity charges the acid mist particles and then adsorbs them by the electrode, which is suitable for high-concentration chromic acid mist (efficiency can reach more than 95%).
Advantages: low energy consumption, suitable for fine processing; Disadvantages: high equipment investment, need to clean the plate regularly.
Condensation recovery method
For high-temperature acid mist (such as chrome plating tank), condense first and then absorb, and some acid can be recovered.
2. Alkali mist treatment technology
Acid spray neutralization
Use dilute sulfuric acid or hydrochloric acid solution to spray and absorb NaOH mist, and react to generate neutral salt solution:
NaOH+HCl→NaCl+H2O
Note: Control the pH in the range of 6~9 to avoid secondary pollution.
Wet scrubber
Combined with a packed tower to increase the gas-liquid contact area, the removal rate of alkali mist can reach more than 90%.
3. Combined process recommendation
Acid mist treatment process
Source control (tank side exhaust) → physical interception (baffle demisting) → chemical absorption (spray tower) → electrostatic demisting (optional) → standard discharge
Case: A certain electroplating plant uses "PP packing tower + alkali spray" to treat hydrochloric acid mist, and the emission concentration is <20mg/m³.
Alkali mist treatment process
Gas hood collection → acid spray tower → demister → discharge
4. Key design parameters
Air volume calculation: determined according to the tank area, liquid temperature and volatilization rate (usually 0.5~1.5m/s cross-sectional wind speed).
Liquid-gas ratio (L/G): The spray tower is generally 2~10 L/m³, which needs to be adjusted according to the pollutant concentration.
Packing selection: PP ball ring, ceramic Raschig ring, etc., with a specific surface area ≥200m²/m³.
5. Precautions
Anti-corrosion materials: The equipment needs to use PP, FRP or titanium alloy, and the nozzle should be made of silicon carbide.
Sludge disposal: The heavy metal-containing sludge generated by neutralization is treated as hazardous waste (such as HW17).
Operation and maintenance: Regularly clean the blocked fillers and monitor the pH and concentration of the spray liquid.
VI. Emerging technologies
Biological treatment: The use of microorganisms such as Thiobacillus to degrade acidic gases is still in the experimental stage.
Membrane separation technology: It has the potential to recover high-value acids (such as hydrofluoric acid).