Abstract: The hot-dip galvanising (HDG) process provides steel items with a protective zinc layer against corrosion. Its main advantages are the long-term durability of galvanised steel and the low energy demand of HDG compared to the steel production itself. However, an intensive use of primary zinc and natural gas, and the generation of spent pickling acids have environmental consequences. In this context, previous studies were focused on the environmental assessment of steel production, but the HDG process has not been analysed in detail yet.
This work evaluates the environmental impacts of two HDG plants located in Spain using the life cycle assessment (LCA) methodology and a cradle to grave scope. These plants have differences in the production size (1:10) and the manufacturing process both related to the final application of the galvanised steel pieces. HDG plant #1 is engaged in the production of steel profiles and sections for the building and construction sector, and HDG plant #2 deals with engineering steel used in the manufacture of tools, components for engines, etc. On this basis, this work provides an overview of the discontinuous galvanisation in Spain. The study takes into account the extraction of raw materials including the steel production, its transport to each HDG plant, the waste and wastewater treatment, and its transport to the waste treatment facilities. The functional unit is one metric tonne of galvanised steel. The life cycle inventory was collected by questionnaires completed by the HDG plants in 2016, 2017 and 2018, and for secondary data, Thinkstep and Ecoinvent databases were used. The system was modelled using the Gabi software, and CML 2001 as impact assessment method. Abiotic resources Depletion [kg Sb eq.], Abiotic-fossil resources Depletion [MJ] and Global Warming Potential [kg CO2 eq.] were some of the impact categories selected.
The results showed that the environmental impact of steel production is much higher than the contribution of hot dip galvanisation. The main differences between both plants are found in ADP-fossil, since the HDG plant #1 incorporates a cogeneration unit that makes the consumption of natural gas more efficient, producing thermal energy and electricity at the same time. Most of this thermal energy is used in the drying and molten zinc bath stages, and one part of the electricity is consumed, and the other is sold to the electricity grid, thus improving the environmental performance of the system.
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