As mentioned previously, to understand the full environmental impact of producing hot-dip galvanized steel, you must first determine the impact of steel making, zinc refining, and the galvanizing process with a life-cycle inventory (LCI) for each.
To fully understand and appreciate the environmental impact of producing hot-dip galvanized steel, it is necessary to start with the production of steel. The LCI of steel includes the mining of virgin material as well as the reuse of recycled scrap. Steel is the most recycled material in the world, with 70% of all steel produced made from recycled material. In addition to the raw material, the LCI also examines the energy consumed and emissions created by melting the material, casting the pieces into plates, beams, etc., and the impact of fabricating the steel for its end use. After fabrication, the steel is sent to the galvanizer’s facility to be coated. As Figure 6 shows, very little solid waste is created throughout the process.
Utilizing industry information from the GaBi database (a collection of environmental product declarations (EPDs)), an LCI was conducted to determine the amount of energy and emissions required to produce 1 kg of steel. The steel’s mass will also comprise the majority of the weight of 1 kg of hot-dip galvanized steel, but the resulting numbers take into consideration the weight of the zinc as well. Therefore, the values presented here represent the primary energy demand (PED), global warming potential (GWP), acidification potential (AP), and photochemical ozone creation potential (POCP) for the amount of steel contained in 1 kg of hot-dip galvanized product (Figure 7).
Other than the steel, zinc is the primary component of hot-dip galvanizing. Similar to steel, the production of zinc comes from both mined zinc ore and recycled sources. In fact, 30% of zinc produced annually comes from recycled material. The LCI for zinc analyzes the energy consumed and emissions created from mining, concentration, and refining. The refined zinc is sent to the galvanizer’s facility in large blocks or ingots to be melted in the kettle. Figure 8 shows a simplified look at zinc production. The zinc refining process also creates little waste; in fact, during the process byproducts such as copper, cadmium, and lead are separated from the zinc to be used for their own purposes.
Utilizing worldwide data collected from the zinc industry, an LCI was conducted to determine the environmental impact of producing 1 kg of special high-grade zinc. As was explained before, the majority of the hot-dip galvanized steel product’s mass is comprised of the steel. Therefore, the values represented here for zinc are based only on the amount of zinc present in 1 kg of hot-dip galvanized product (Figure 9).
Hot-Dip Galvanizing LCI
The final step to determining the LCA production phase for hot-dip galvanizing is to evaluate the energy demands and emissions generated by the hot-dip galvanizing process itself. The gate-to-gate study considers the additional energy and emissions of the process beyond the inputs of the steel and zinc. Figure 10 below depicts the additional products and energy required to coat the steel with zinc.
Utilizing world wide data collected from the galvanizing industry, an LCI was conducted to determine the environmental impact of coating steel with zinc during the hot-dip galvanizing process. The energy demand and emissions data for galvanizing had slight variations mainly due to the differences in the energy mix (electricity vs. natural gas), efficiencies within the process, and the differences in power grid mixes at various locations. Again, these average values represent only the gate-to-gate impact of the galvanizing process, excluding the impacts of the production of steel and zinc (Figure 11).
LCA Production Phase
The LCA production phase for hot-dip galvanizing (cradle-to-gate) combines the PED, GWP, AP, and POCP from all three LCI’s above: steel, zinc, and galvanizing. Therefore, at the point when the product leaves the galvanizer’s facility, the environmental impact of 1 kg of hot-dip galvanized steel is shown in Figure 12.