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Dross Formation in the Galvanizing Kettle By Bernardo Duran
Goal of Presentation Discuss why dross forms in the galvanizing kettle and methods to decrease its formation.
Overview • Definition of dross • Why dross formation matters • How dross forms • How to reduce dross formation and its negative effects on the finished product
Definition of Dross Byproduct of the galvanizing process which consists of loose iron particles (iron salts) that have metallurgically reacted with zinc in the galvanizing kettle. Dross can contain more than 94% zinc (6% iron).
Why Dross Formation Matters • Aesthetic concerns from dross pimples. • Steel can be rejected for gross dross inclusions. • Dross formation accounts for approximately 15% of the zinc usage in the galvanizing process. • Dross formation can be reduced.
How Dross Forms Free iron particles in the galvanizing kettle metallurgically react with zinc to create dross particles rather than the zinc reacting with the steel to create a galvanized coating.
Types of Dross • Floating Dross: free particles of dross that can float throughout the kettle and usually consist of long intermetallic spikes interwoven in clumps; hollow-like structures • Bottom Dross: dross particles that aggregate (settle) to the bottom of the galvanizing kettle
Where the Free Iron Particles Come From • Iron salts formed by a reaction between pickling agents and steel • Iron salts formed by reaction between flux and steel • Zinc/iron alloys formed by a direct reaction between steel and molten zinc (loose iron particles on the steel)
Iron Salts from Pickling Agents • Pickling agents react with iron oxide and the steel to create iron salts. • Creates loose iron salts that can then be transferred to subsequent tanks if not rinsed properly.
Iron Salts from Reactions with Flux Fluxing agents such as ammonium chloride and zinc chloride can form iron salts when reacting with the steel which can then be carried over to the galvanizing kettle.
Free Iron from Zinc/Steel Reaction • Loose particles of iron from the iron or steel that comes off while in the kettle can go on to create dross particles. • Reactive steels can develop excess zeta layer formation which can flake off crystals that create free iron particles.
New Galvanizing Kettles Dross formation can be high in new kettles until a layer of intermetallic layers form on the kettle walls, which then usually inhibits further dross formation.
Reducing Dross Formation • Limiting formation of iron salts during pickling operation • Adequately rinsing iron salts after pickling operation • Monitoring iron levels in pickling and flux baths, and galvanizing kettle • Elemental additions to galvanizing kettle • Avoiding large temperature swings in galvanizing kettle
Limiting Iron Salt Formation During Pickling • Inhibitors can be used which limit the attack of the acid on the steel while not affecting dissolving action of the iron oxide and mill scale on the steel • Monitor pickling time to ensure steel is not over pickled
Adequate Rinsing of Pickling Salts • Allow enough time for pickling salts to drain before transferring the steel to the rinse tank • Allow enough time in rinse tank • Implement a second rinse tank when possible/practical
Monitoring Iron Levels in Pickling and Flux Solutions • Frequently check iron levels and pH in pickling tank and rinse tanks • Frequently check iron levels in flux (should be no greater than 0.5%) • Aim for an iron level of about 0.1% in the galvanizing kettle (iron solubility will vary with kettle temperature)
Properly Maintain Flux Solutions • Dross formation can be higher in wet flux method than dry method • Agitation of the flux in the dry process can help to convert excess iron to sludge • Work with flux supplier to find optimal chemical levels and filtering strategies
Elemental Additions to the Galvanizing Kettle • Small additions of lead (1%) can reduce dross formation, but the industry is moving away from lead use in kettles • Nickel additions (0.04 to 0.09%) can reduce floating dross (however, nickel decreases the solubility of iron and can increase bottom dross formation); (see iron solubility on next slide) • Adding elements to kettle in smaller quantities more frequently is preferred over bigger quantities less frequently
Maintaining Consistent Temperatures in the Galvanizing Kettle • Iron solubility increases with increases in kettle temperature (see graph on next slide) • When temperature drops, the iron precipitates out of solution which is then available to form dross • Uppermost level of zinc can be cooler than lower levels of zinc and have lower iron solubility
Removing Dross from Kettle • Bottom dross should be removed on a scheduled basis, whether by time or steel volume throughput • Nitrogen can be bubbled in zinc (ensure bottom dross is not disturbed) to float dross to the surface where it can then be scooped out
Conclusion Reducing dross formation: • Increases profits because less zinc is consumed in the form of dross • Makes for happier customers since there are less dross inclusions on the steel
Additional Resources • Galvanizing Note: Skimmings & Dross • Troubleshooting Guideline: Reducing Dross Pimples • AGA Resource Library • Your pickling and flux solution suppliers