I. Project Background and Production Line Specifications
This case study involves a hot-dip galvanizing production line designed for large-scale steel structural components, with a zinc kettle having internal dimensions of 13 meters in length, 1.8 meters in width, and 3.5 meters in depth. These dimensions correspond to a rated zinc capacity of 420 tons and a maximum hourly output of approximately 18 tons, classifying this as a heavy-duty continuous-operation production line. The line is designed for direct natural gas firing and is configured for a 380V/50Hz three-phase power supply, establishing a different baseline from the 60Hz project destined for South America in the previous case.
The tank system adopts a concrete substrate with fiberglass-reinforced plastic (FRP) anti-corrosion lining. The core process tanks – pickling, rinsing, fluxing, cooling, and passivation – share unified external dimensions of 13 meters in length, 1.8 meters in width, and 3 meters in depth, creating a 0.5-meter depth differential from the zinc kettle that must be accounted for in hoist travel path planning.
II. Heating System and Gas Supply Parameters
The heating energy source is piped natural gas, with the calorific value required to fall within the range of 8,400 to 8,600 multiplied by 4.18 kJ/m³, inlet pressure maintained between 100 and 200 kPa, and gas node pressure controlled between 90 and 80 kPa. The combustion section is equipped with six high-speed pulse burners, model TJ300, each with an instantaneous gas consumption of approximately 70 m³/h, bringing the total instantaneous consumption to 420 m³/h. The gas supply pipeline adopts a dual-main-inlet configuration with one unit in service and one on standby, ensuring uninterrupted supply during continuous production.
The furnace operates under a micro-positive pressure combustion mode. At full load production, the design gas consumption is estimated at approximately 12.5 cubic meters per ton of workpieces. Temperature control employs a 3-stage PID regulation loop, with zinc bath temperature acquired via thermocouples and displayed on a large screen. The target control range is 438 to 450°C, with the system automatically adjusting burner output based on deviation from setpoint. Flame detection uses ultraviolet sensors, and the ignition system supports both automatic and manual high-low dual modes.
III. Tank Construction Materials and Anti-Corrosion Approach
In contrast to the steel-structure-with-steel-plate construction used in the previous case, this production line utilizes concrete tanks with FRP anti-corrosion linings for pre-treatment and post-treatment sections. A total of six tanks – for pickling, rinsing, fluxing, cooling, and passivation – share unified external dimensions of 13 meters in length, 1.8 meters in width, and 3 meters in depth. The concrete tank construction is the buyer's responsibility, with the seller providing only the FRP lining installation. This division of work is suitable for scenarios where the buyer already has existing tank pits or prefers heavy-duty fixed-tank configurations.
The pickling section comprises three enclosed pickling tanks. The immersion time per tank is approximately 6 minutes under normal rust conditions and can be extended to 30 minutes for heavily rusted workpieces, depending on surface condition. The pickling room operates under negative pressure, with the negative pressure setpoint at 5 to 10 Pa – 5 Pa on clear days and adjusted to 10 Pa on rainy days to compensate for atmospheric pressure variations. The variable-frequency fan automatically reduces frequency upon reaching the set pressure. Ground rail carts are used for material loading and unloading to ensure acid mist does not diffuse into the workshop.
There is one fluxing tank, with the operating temperature maintained between 50 and 70°C and pH controlled at approximately 5. An automatic iron removal device limits the ferrous ion concentration to within 1 mg/L, and filtered sludge is sent to the sludge drying oven. The cooling and passivation sections each consist of one tank, with the passivation process being chrome-free.
IV. Acid Mist and Zinc Fume Treatment System Configuration
The enclosed pickling room measures 17,150 mm in length, 15,000 mm in width, and 6,350 mm in height, with the external steel structure measuring 17,150 mm by 15,600 mm by 8,450 mm. The acid mist first passes through a dilution stage – the dilution water is also used for preparing fresh acid, reducing alkali consumption – and then enters the acid mist absorption neutralization tower. The tower is driven by a 45 kW variable-frequency induced draft fan, with pH detection probes installed at both inlet and outlet to monitor neutralization efficiency in real time.
Zinc fume treatment employs two independent collection logics. One is a mobile cover hood installed above the zinc kettle, utilizing the natural rising tendency of smoke for capture. The other is a double-sided suction capture system on both sides of the kettle. Both approaches use variable-frequency control – starting at 50 Hz for 2 to 3 minutes before workpieces are lowered into the kettle, then reducing to 10 Hz during non-galvanizing periods to minimize power consumption.
The captured zinc fumes pass through a bag filter for particulate removal, then through a water-curtain deodorization stage, and are finally discharged by a 160 kW variable-frequency induced draft fan. The dust collector is designed to stop during high-speed blowing and operate at low speed to ensure the filter bags are cleaned effectively.
V. Cascaded Waste Heat Utilization System (Core Technology Module)
The waste heat utilization scheme for this production line is more comprehensive than the previous case, encompassing six stages. Flue gas temperatures progressively decrease from approximately 500°C at the furnace outlet to near ambient conditions.
Stage 1: Fluxing liquid heating. The flue gas exits the furnace at approximately 500°C and passes through a stainless steel heat exchanger to heat circulating water to approximately 90°C. This hot water is then sent to a titanium alloy heat exchange coil inside the fluxing tank, raising the fluxing liquid to the process setpoint of 50 to 70°C. Titanium alloy is a mandatory material choice due to the corrosive nature of zinc chloride and ammonium chloride in the flux solution.
Stage 2: Workpiece drying. After the first-stage heat exchange, the flue gas enters the drying pit, which measures 14 meters in length, 5 meters in width, and 3 meters in depth. The flue gas passes through a heat exchanger, and a high-temperature blower delivers hot air to the workpiece surface for rapid and uniform drying. The drying effect combines radiant heat from the pit structure itself with convective hot air flow.
Stage 3: Combustion air preheating (patented). As the flue gas continues to cool to approximately 200°C, it enters the combustion air heat exchanger, preheating the combustion air required by the burners from ambient temperature to 50 to 150°C. This preheated combustion air, when delivered to the furnace, increases the theoretical combustion temperature. The design estimates a gas saving of approximately 2 to 3 m³ per ton of galvanized parts – actual savings depend on site-specific operating conditions.
Stage 4: Sludge drying (patented). A 316L stainless steel sludge drying tank measuring 6,000 mm in length, 750 mm in width, and 375 mm in height is installed above the flue gas duct of the combustion air heat exchanger. It utilizes flue gas at approximately 150 to 200°C to dry filter press sludge from approximately 80% moisture content down to approximately 10%, significantly reducing off-site disposal weight.
Stage 5: Pot-edge waste heat recovery (patented). The zinc kettle edge temperature reaches approximately 200°C. A stainless steel double-layer coil placed along the kettle edge heats circulating water for bathing, acid preheating, or workshop heating.
Stage 6: Flue gas workshop heating. The low-grade waste heat remaining before final discharge is released directly into the workshop through steel pipes, with burn protection measures implemented.
Additionally, hot water from the cooling section is circulated through heat exchange pipes with the cold acid in the pickling tank, simultaneously achieving cooling water temperature reduction and acid liquid preheating – a dual-purpose design that reduces cooling tower investment requirements.
VI. Lifting Logistics and Online Weighing
Workpiece lifting employs a 5-ton plus 5-ton dual-hoist configuration. Four sets of hoists of this specification are installed in the pickling area, mounted on the exterior of the enclosed room roof, with hooks extending into the room for operation via wireless remote control. Workpieces are manually attached to large racks using a lifting frame, raised to a fixed height, then transported by ground rail cart into the pickling room, where overhead cranes sequentially process them through each tank station.
This production line is equipped with an online weighing system that reads both the raw workpiece weight before pickling and the finished weight after galvanizing. By calculating the difference, it monitors iron loss during pickling and zinc coating weight gain, enabling real-time zinc consumption tracking and process adjustment – a feature not present in the previous case.
The logistics solution provides two automation options for the buyer: a gantry electric hoist with remote control, or a ring-rail electric hoist with RGV carts for PLC-programmed control. The programmed control approach precisely controls immersion time to prevent operators from extending immersion time based on individual experience, thereby avoiding excessively thick zinc coatings.
VII. Electrical Load Distribution
The total installed electrical capacity of the entire production line is approximately 530 kW. Major loads are distributed as follows: zinc fume induced draft fan at 160 kW, acid mist induced draft fan at 45 kW, waste acid treatment system at 80 kW, overhead crane motor at 15 kW, iron removal equipment at 15 kW, air compressor at 7.5 kW, spray pumps totaling 26 kW (including two 7.5 kW units and one 11 kW unit), hoist motors totaling 40 kW, cooling tower at 4 kW, lighting at 20 kW, and lift station at 22.5 kW. A 30 kW reserve is allocated for future expansion.
VIII. Optional Module: Waste Acid Regeneration System
This proposal includes an optional module – a waste acid treatment line employing the sulfuric acid displacement method, with a designed processing capacity of 5 to 10 tons per day. The waste acid must have a ferrous ion concentration greater than 120 g/L, ferric ion concentration less than 5 g/L, and free acid concentration greater than 5%. The process uses freeze crystallization to precipitate ferrous chloride, which is then reacted with concentrated sulfuric acid to produce ferrous sulfate and hydrogen chloride gas. The hydrogen chloride gas is recovered as regenerated hydrochloric acid and returned to the pickling section, while ferrous sulfate is sold as a byproduct in the form of monohydrate ferrous sulfate.
The equipment has a total power rating of approximately 200 kW, with power consumption of about 40 kWh per ton of waste acid treated. The system occupies approximately 150 square meters, with equipment height of 7 meters and a required installation and operation clearance of 9 meters. Process specifications require that the regenerated acid contains less than 4% iron residue and achieves a regenerated acid concentration of 15% to 22%. The byproduct ferrous sulfate must have a purity greater than 90%, with impurity limits of titanium below 0.75%, arsenic below 0.0002%, lead below 0.0001%, cadmium below 0.0001%, mercury below 0.00002%, chromium below 0.001%, free acid below 1%, and insoluble matter below 0.5%.
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