Working Principle and Applicable Conditions of Tubular Wet Electrostatic Precipitators in Aluminum Anodizing Acid Mist Control

Introduction

In the daily operation of aluminum anodizing workshops, acid mist control is a continuous engineering challenge. The acid mist concentration at the outlet of a scrubber may occasionally exceed emission limits, or a visible white plume may appear at the stack during humid weather. These phenomena often indicate that the capture capability of conventional spray scrubbing for submicron acid mist droplets has reached its limit. The tubular wet electrostatic precipitator (WESP) is a terminal polishing unit designed for this operating condition. This article explains its working principle and applicable conditions to help engineers and procurement personnel evaluate whether this equipment suits their production scenario.

Sources of Acid Mist in Aluminum Anodizing and Treatment Challenges

The gases generated by the aluminum anodizing process itself are mainly hydrogen at the cathode and oxygen at the anode. However, in pre-treatment stages such as chemical degreasing, alkaline etching, chemical polishing, anodizing, and electrolytic coloring, volatile acid solutions such as sulfuric acid, nitric acid, and hydrofluoric acid release acid mist. Sulfuric acid mist is a key pollutant in the emission control of aluminum anodizing workshops.

According to the Electroplating Pollutant Emission Standard (GB21900-2008), the maximum allowable emission concentration is 30 mg/m³ for sulfuric acid mist and 7 mg/m³ for fluoride. Conventional alkali spray scrubbers typically achieve acid mist removal efficiency above 90% and can meet routine emission requirements in many operating conditions. However, as environmental standards become stricter and some projects require further reduced emission concentrations, the capture capability of spray scrubbers for submicron acid mist droplets in the 0.1–1 μm range is limited, and outlet concentrations may fluctuate. The tubular wet electrostatic precipitator serves as a terminal polishing unit in this stage, capturing fine acid mist droplets and aerosol particles that are difficult for spray scrubbers to remove.

Working Principle of Tubular Wet Electrostatic Precipitators

Charging and Collection Process

The core mechanism of a wet electrostatic precipitator is charge-driven particle collection. Under a high-voltage DC field of tens of kilovolts, the corona discharge electrode continuously releases electrons, ionizing the gas between electrodes and generating a large number of positive and negative ions. Acid mist droplets and fine dust particles collide with these ions and become charged, then move toward the oppositely charged electrode under the Coulomb force of the high-voltage electrostatic field. Positively charged particles move toward the corona electrode, while negatively charged particles move toward the collection electrode.

The tubular structure is characterized by a precipitation electrode composed of multiple parallel circular or polygonal metal tubes, with a corona wire positioned at the center of each tube. The acid-mist-laden gas flows vertically through the tubes, completing charging and collection within the electric field inside each tube. Acid mist droplets reaching the inner wall of the collection tube lose their charge and flow down the wall by gravity to the bottom of the unit for discharge.

Flushing and Cleaning Mechanism

Unlike dry electrostatic precipitators, tubular wet electrostatic precipitators remove particles attached to the collection electrode and corona electrode through continuous or periodic water film flushing. Spray devices arranged between the gas distribution plate and the electric field can continuously inject water mist to condition the gas entering the electric field and reduce particle resistivity. The flushing device at the upper part of the electric field periodically cleans the collection electrode and corona electrode according to a set cycle, suppressing re-entrainment and back corona. The flush water can be collected by a water collection tray and recirculated through membrane filtration, or used as process makeup water for a desulfurization/scrubbing tower.

Key Technical Parameters Reference

Based on industry technical data, typical performance values of tubular wet electrostatic precipitators in acid mist control are as follows:

Note: The above data are typical reference values within the public industry range. Actual performance depends on inlet concentration, gas temperature, acid mist particle size distribution, and equipment configuration parameters.

Applicable Conditions for Tubular Wet Electrostatic Precipitators

Applicable Process Stages

Tubular wet electrostatic precipitators are mainly applicable in the following scenarios in aluminum anodizing workshops:

As a terminal polishing unit after a scrubber. When the acid mist concentration at the outlet of an alkali spray scrubber approaches or occasionally exceeds the emission limit, a tubular wet electrostatic precipitator connected in series after the scrubber can deeply capture residual submicron acid mist droplets.

Acid mist control for chemical polishing and anodizing tanks. These two processes involve higher acid concentrations and greater acid mist generation. After collection by side suction or top suction at the tank, the gas is pre-treated by a scrubber and then enters the wet electrostatic precipitator for polishing.

Acid mist control for electrolytic coloring and sealing processes. Tanks containing nickel salt or other metal salts may generate mist containing metal salts during heating. Wet electrostatic precipitators can co-capture these salt-containing droplets.

Inlet Conditions and Selection Reference

When evaluating whether a tubular wet electrostatic precipitator is suitable for a specific project, the following inlet conditions need to be confirmed:

Inlet acid mist concentration. Wet electrostatic precipitators are usually used as a secondary or tertiary treatment unit. The inlet acid mist concentration should preferably be controlled at the scrubber outlet level (generally below 50 mg/m³). Excessively high inlet concentrations increase the electric field load and flushing frequency.

Gas temperature and humidity. Tubular wet electrostatic precipitators are suitable for saturated wet flue gas conditions. The gas temperature should preferably be below 60°C. Acid mist exhaust from aluminum anodizing workshops is usually near saturation humidity after scrubber treatment, making it suitable for entering a wet electrostatic precipitator.

Air volume and tube diameter matching. The inner diameter of collection tubes in tubular structures is generally in the range of φ200–400 mm, and the gas velocity inside the tubes should preferably be controlled at 1.0–2.5 m/s. When the air volume is too large, the number of collection tubes can be increased through modular combination to match.

Material selection. The materials of the collection tubes and corona wires need to be determined based on acid mist composition and concentration. Commonly used materials include conductive FRP, duplex stainless steel, and lead-antimony alloy. For acid mist environments containing fluoride, special attention should be paid to the fluoride corrosion resistance of the materials.

Common Operating Problems and Troubleshooting Approaches

Decreased Mist Removal Efficiency

Phenomenon: Visible acid mist at the stack or increased droplet carryover.

Possible causes and troubleshooting directions:

• Insufficient flush water flow or clogged nozzles, resulting in incomplete water film on the collection electrode surface. Check the operating status of the flush pump and the flow of nozzles.

• Electric field voltage or current deviates from the set value. Check the rectifier output parameters and whether the corona wire has scale buildup.

• Inlet acid mist concentration exceeds the design range. Verify the operating status of the upstream scrubber.

Frequent High-Voltage Tripping

Phenomenon: The electric field frequently alarms and exits operation.

Possible causes and troubleshooting directions:

• Discharge short circuit between the corona electrode and collection electrode, possibly caused by electrode scale, fallen debris, or uneven flush water distribution.

• Contamination or moisture on the insulator surface. Check the sealing and heating status of the insulator compartment.

• Spark rate set too tightly. Verify whether the spark rate setting of the automatic voltage control (AVC) matches the current operating condition.

Summary

The tubular wet electrostatic precipitator is positioned as a terminal polishing unit in aluminum anodizing acid mist control. Its core value lies in capturing submicron acid mist droplets that are difficult for spray scrubbers to remove. When evaluating applicability, engineers need to focus on four dimensions: inlet acid mist concentration, gas temperature and humidity, air volume matching, and material selection. The combined process of this equipment with an alkali spray scrubber can provide a complete acid mist control path for aluminum anodizing workshops, from coarse treatment to fine treatment.

To evaluate the acid mist control solution for your anodizing workshop, please contact us. You can provide the following information, and our engineers will assist with a preliminary selection evaluation:

• Type and quantity of acid mist tanks (chemical polishing, anodizing, electrolytic coloring, etc.)

• Current exhaust gas treatment process and emission test data

• Exhaust air volume (m³/h) and stack dimensions

• Local emission standard requirements

After receiving the information, we will provide preliminary configuration recommendations based on equipment parameters and operating conditions.