Hot-Dip Galvanizing Kettle

Overview

A hot-dip galvanizing kettle is a large steel pot of molten zinc, held near 450C, into which cleaned steel parts are dipped so they come out sheathed in a tough zinc coating. The coating is metallurgically bonded: the iron in the steel reacts with the zinc to form a stack of iron-zinc alloy layers under a pure zinc outer skin, which is what makes hot-dip galvanizing last for decades outdoors. The kettle is the centre of a galvanizing plant that also cleans the steel, fluxes it, lowers it into the zinc, lifts it out, and captures the fume that comes off.

The pot itself is the Kettle Vessel, heated by the Furnace around it. Steel is prepared in the Pretreatment Line and lowered by the Dipping Hoist. The Dross and Ash System keeps the bath clean, the Fume System catches the smoke, the Zinc Feed tops the bath up, and the Control Cabinet holds the temperature steady.

The kettle and zinc bath

The Kettle Wall and Kettle Bottom are welded from special low-silicon, low-carbon boiler plate. Steel composition matters here because molten zinc slowly dissolves iron, and high silicon accelerates that attack, so a kettle made from the wrong plate can wear through in a year. A well-built kettle lasts eight to twelve years. The wall is braced with a Stiffener Rib grid so the static head of zinc, which can run to several hundred tonnes, does not bulge the plate at temperature.

The Zinc Bath is mostly special high-grade zinc with small additions: aluminium at a few hundredths of a percent brightens the coat and slows oxidation of the surface, and nickel suppresses the excessive alloy growth that reactive, high-silicon steels would otherwise cause. Bath temperature is read by a Bath Thermocouple set and the depth by a Level Probe, so the operator knows when to add zinc.

Heating the bath

The kettle sits inside the Furnace, a refractory chamber that heats the kettle wall from outside rather than putting flame on the zinc. The Furnace Shell holds a Refractory Lining of ceramic fibre and firebrick, and several Gas Burner units fire along the walls. The burners are high-velocity so their jets sweep the kettle face evenly and avoid local hot spots that would thin the steel. A Burner Train meters the gas and proves each flame with a Flame Rod before it keeps the gas on.

Because the kettle runs continuously, fuel is a major cost, so a Recuperator in the Flue Stack reclaims heat from the exhaust to preheat the combustion air fed by the Combustion Fan. That can cut gas use by a fifth.

Cleaning and fluxing

Zinc will not bond to dirty or scaled steel, so every part passes through the Pretreatment Line first. The Degrease Tank strips oil and paint in hot caustic, the Pickle Tank dissolves mill scale and rust in hydrochloric acid, and the Rinse Tank washes the acid off so it does not poison the flux. The steel then goes through the Flux Tank, a warm zinc ammonium chloride solution that leaves a film keeping the steel active until it meets the zinc. A Preflux Dryer drives off the water, because wet steel entering molten zinc flashes to steam and throws zinc out of the kettle.

Dipping and withdrawal

Parts are racked on a Dip Jig and lowered into the zinc by the Dipping Hoist. The Crane Bridge carries a Hoist Trolley whose Lift Drum, driven through the Hoist Gearbox, winds the Lift Cable runs. The jig is dipped at an angle so zinc fills hollow sections and air escapes, then held for one to six minutes until the part reaches bath temperature and the alloy layers form. On the way out the Tilt Frame tips the load so excess zinc drains back to the bath and the coat does not run.

Keeping the bath clean

Two byproducts have to be managed every shift. Iron that dissolves off the steel sinks as a heavy iron-zinc dross to the kettle floor, where it is lifted out with the Dross Grab or drawn off by the Dross Pump into a side Dross Kettle to cool for sale back to refiners. On the surface, zinc oxidises to a powdery ash that the Ash Skimmer drags off along the Skim Rail just before each dip so it does not print onto the coating.

Fume, feed, and control

When fluxed steel hits the zinc the flux burns off as a dense white zinc oxide plume, captured by the Fume Hood and pulled through the Baghouse by the Induced-Draft Fan, where a Filter Bag array traps the dust. Zinc consumed by the coating and the dross is replaced through the Zinc Feed, which slides fresh ingots down the Charge Ramp so they melt without splashing, while the Alloy Feeder trims the chemistry. Over it all, the Temperature Controller modulates the burners to hold the bath in a tight band, and the Burner Management keeps the firing safe. Kettles vary mainly in length, from short job-shop pots for fasteners and brackets to long pots that take full structural beams and lighting columns in one dip.