Galvanizing is always carried out in an industrial works which contains all stages of the process. Steel comes in at one end and the finished galvanized product goes out at the other. There is a good spread of galvanizing plants across the UK and Ireland and steel does not have to travel great distances to a nearby galvanizing plant keeping transport costs and environmental impacts as low as possible. The main process consumable, zinc, is used very efficiently in the process. The dip operation ensures that any zinc that is not deposited on the steel is returned to the galvanizing bath. Zinc that oxidizes on the surface is removed as an ash and is readily recycled (sometimes on site). Dross formed at the bottom of the bath is removed periodically and has a high market value for recycling.
THE GALVANIZING PROCESS:
Inputs, emissions, wastes and recycling flows
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Process energy use
Energy is required to heat the hot dip galvanizing bath and this is usually supplied by natural gas. Although the galvanizing industry is not considered to be amongst the most energy-intensive sectors of industry, it has made great efforts to manage its energy use efficiently. The galvanizing industry has set targets for energy efficiency and encouraged improved energy management and new technology to achieve these targets.
Examples of these advances are:
- improved burner technology for greater energy efficiency
- more efficient bath lids (used during maintenance and/or down time)
- greater use of waste heat for heating of pre-treatment tanks
Emissions within the plant are carefully controlled to avoid disturbance or problems for the surrounding neighbourhood. Galvanizing plants are regulated under the EU Directive on Integrated Pollution, Prevention and Control. The industry has cooperated in the publication of a Best Practice Reference Note (BREF) for hot dip galvanizing.
The principal requirement of the BREF is to capture the non-hazardous particulates during dipping. These particulates are then filtered using either scrubbers or bag filters.
Regeneration and recycling
Pretreatment steps in the process are mainly aimed at cleaning the steel articles. Process consumables, such as hydrochloric acid and flux solutions all have important recycling and/or regeneration routes.
- spent hydrochloric acid solutions are used to produce iron chloride for use in treating municipal waste water. Many plants remove iron and zinc and recycle regenerated acid to the re-treatment tanks
- improved monitoring and maintenance of flux tanks means that these are rarely discarded to waste and only small volumes of sludge require periodic disposal. Closed-loop flux recycling is used in many plants
- ambient temperature acidic and biological degreasers have been developed
Galvanizing plants use relatively low volumes of water compared to other coating technologies. In fact, it is very rare for a galvanizing plant to discharge waste water. Any waste water that is generated can be treated and returned to the process, with only low volumes of stable solids sent for external disposal.
In some cases, it has been possible for galvanizing plants to eliminate the use of mains water by harvesting rainwater falling on the site. Rainwater can be collected through gutters and stored for later use.