Hot-dip galvanising line

The chemical preparation tanks are a steel structure with a special external and internal coating. All tanks except the cooling tank have an automatic heating system with a boiler: a temperature controller, thermocouples in housings, heat exchangers.

In the section the temperature, level and concentration of the solutions are monitored. The working level is kept 250–300 mm below the top of the tank, the concentration is checked daily by the plant’s chemical laboratory and the process chemist adjusts the solutions.

The drying oven dries the articles after fluxing and preheats them before immersion in molten zinc. It is heated by the combustion gases of the galvanising furnace; there is an additional natural gas burner and the temperature is regulated automatically. A chain conveyor moves the cross-beams along the oven.

The horizontal furnace heats the melt and keeps it at working temperature. It consists of the furnace, the galvanising kettle, equipment for measuring and regulating temperature, a white-fume collection plant and a plant for recovering the waste gases.

The kettle is designed for molten zinc up to 480 °C: it is welded from sections of U-shaped steel profile with a double-sided weld, of steel with low carbon and silicon content 50 mm thick. The kettle is 12.5×1.6 m, 3 m deep and weighs 47 t. It is built into the furnace; the melt temperature is measured by thermocouples and regulated automatically both in the combustion chamber and in the kettle itself.

Double-trolley overhead cranes move the cross-beams through the whole cycle, and rail carriages move them between bays. The process tooling (cross-beams, hooks, wire loops, stands for hanging and unhanging, a hydraulic lift for heavy parts) is inspected regularly, registered and checked for wear.

Each process operation is described in detail in the article “Process Operations on a Hot-dip Galvanising Line”.

This page is about hot-dip galvanising by immersion in molten zinc, not electroplating or cold galvanising; the difference is explained in Hot-dip, Electro- or Cold Galvanising: Which to Choose. We offer two main types of hot-dip galvanising equipment: a line for long structures (its units are described above) and a unit for fasteners and small parts.

The most obvious difference is the furnace. The line has a gas-fired or electric furnace with a steel kettle. The unit for small parts is different: a gas-fired reverberatory furnace with a ceramic kettle. The zinc touches no metal parts, so the melt can be run at 460–480 °C, and the parts need less cleaning after centrifuging.

On the line, structures hang on crossbeams and overhead cranes carry them. Small parts go through in motorised polypropylene drums that turn as they are lowered and lifted, so that the liquids drain evenly. The unit’s preparation section is sized for both drums and material on crossbeams.

The unit’s preparation tanks have a load-bearing steel frame with an inner polypropylene tank 10 mm thick, and the pickling acid is 15–17% hydrochloric. The additives in the solutions are compatible with each other, so there is no rinse between degreasing and pickling.

After galvanising, baskets 500 mm in diameter and up to 500 mm high (up to 200 kg per basket) go into a centrifuge with a hydraulic lid and a system for collecting zinc drops. A tipper and a cooling cradle unload the parts into boxes once they have cooled in water.

Different units limit the output of the two layouts. On the line it is the hourly load on the melt, and its limit is given in the line’s specifications on this page. For the unit, the benchmark is the centrifuge: up to 2000 kg/h.

More on both layouts is in Hot-dip Galvanising Plant Equipment, and the temperatures and times of each operation on the line are in Process Operations on a Hot-dip Galvanising Line.