Minimizing Zinc Consumption
Where can I minimize Zinc Consumption?
Zinc is the largest material cost factor in the hot-dip galvanizing process. Do you know where all your plant's zinc goes? According to process surveys conducted by the AGA in 2003 and 2007, a typical consumption rate of zinc for batch hot-dip galvanizing is around 5.2% - 5.5% lbs of zinc per lbs of steel production. If you don't already track your zinc consumption rate, I would strongly encourage you to do so. This industry statistic can be useful to identify whether your plant has a relatively low, average, or high consumption rate. Over time you can identify process changes that affect consumption rate or identify general areas for improvement. If you need assistance tracking your zinc usage, zinc companies can provide material balance worksheets or you can refer to the AGA Galvanizing note Calculating Zinc Consumption Rate (with included spreadsheet and examples).
Upon assessing your plant's zinc usage, analysis of plant procedures can help identify areas of zinc consumption. Generally, there are three major areas were zinc is lost:
- Final Product Coating
- By-products: skimmings and dross
- Non-Recoverable Zinc: Fixtures, splashes, chains, wire
The below recommendations address minimizing zinc consumption in these three major areas.
The Final Product Coating
Most of your zinc will end up on the parts as the final product to be sold, but there is no need to give away free zinc to the customer beyond the minimum specification requirements. However, reactive and/or thick steels can be problematic as zinc pickup is often high or excessive for these materials. To optimize coating thickness, you should address steel chemistry, the cocoon effect, withdraw rates, and the use of bath alloying elements.
If you are able to obtain material test reports (MTRs) from the customer, look up the steel grade's composition requirements, document your experience with different steels, or run a test piece. Then, with this information you have an opportunity to implement a plan for controlling reactive steel coating growth. Various measures include minimizing overall immersion time, lowing the bath temperature for a shift to galvanize large quantities of reactive steel, or by blast cleaning reactive steel prior to HDG.
Next, the hot-dip galvanizing of thick steels can present issues with excessive immersion times, often the result of a cocoon effect which slows the heating of the steel. Especially when steels are both thick and reactive, excessive immersion times can result in a high zinc consumption and thick, brittle coatings that have a tendency to flake off the steel. The most effective practice to minimize coating thickness for thick steels is to agitate and slowly move the articles within the kettle utilizing overhead cranes to lessen the cocoon effect. If the thick steel is also of reactive steel chemistry, coating thickness can be further minimized by blast cleaning the steel prior to galvanizing.
Withdrawal speed from the galvanizing kettle is another factor which affects coating thickness. The more quickly steel is withdrawn from the kettle, the less zinc is able to drain away before solidifying. As a result, faster withdrawal speeds create a thicker eta (free-zinc) layer, thereby increasing coating thickness. Withdrawal at maximum crane speed has the potential to increase the coating weight by over 50%, adding up to a lot of zinc over time. Optimum withdrawal rates may vary from 1.5 3 ft/min (0.5 1.0 m/min) as long as a nice, smooth, and slow withdrawal is obtained.
A final option for reducing coating thickness is with bath element additions. Nickel reduces the growth of the intermetallic layers in steels with chemistry that falls into the Sandelin Range. However, with nickel it may be difficult to reach required minimums on smooth, non-reactive steels with silicon less than 0.15%. Other bath elements, such as lead and bismuth (and aluminum) will reduce the surface tension of zinc and improve zinc drainage, therefore reducing runs, clogs, drips, and overall thickness of the coatings eta (free-zinc) layer. For detailed information on the use of these bath additions, refer to AGA's Galvanizing Note entitled The Zinc Bath.
Skimmings & Dross
Zinc is consumed not only when reacting with steel to form hot dip galvanized coatings, but also through the formation of skimmings and dross formed in the kettle when the molten zinc reacts with air and impurities/chemicals that are transported into the kettle from earlier stages of the process.
The AGA developed a detailed Galvanizing Note entitled Skimmings & Dross to provide details on methods used to control the formation of skimmings and dross which lead to increased zinc consumption. Should you experience excessive floating dross, methods to reduce and correct this issue are available through the AGA Troubleshooting Guideline Dross Pimples on Final Product.
Some zinc will be lost permanently and is not considered recoverable. This can include zinc splatter and the zinc that ends up on chains, fixtures and wire. However, you can minimize loss in these areas with careful handling procedures to minimize submersion in zinc, and through careful material selection to minimize reactivity with zinc (and therefore total zinc pickup).
Zinc can also be lost when items are required to be stripped and re-galvanized. The zinc dissolved into a strip tank is not recoverable for production when the acid is shipped for disposal, or when the ferrous/zinc sulfate crystals from acid recycling are sent for reuse or to landfill. To minimize the potential for large surface defects that require stripping and regalvanizing, the AGA has a Troubleshooting Guideline available on Bare Spots and a Dr. Galv article on Improving Rinsing Operations.
© 2019 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.