Galvanised steel plays critical role in mining industry

Galvanised steel plays a critical role in infrastructure in the mining industry. Environmental conditions in the mining industry can often be aggressive and the service demands on steel extreme, subjecting it to maximum demands. According to the International Zinc Association (IZA) Africa, hot dip galvanised coatings are best to protect critical infrastructure in mining from corrosion.

Steel infrastructure in the mining industry ranges from: Steel shaft structures (buntons, station steelwork, headgear) to stope structures (roof supports, pipe hangers); and haulway structures (ore transfer stations, electrical stubbies) to surface structures (plant buildings, tank supports, pipe gantries, conveyor belts, stacks, stockpile structures, stairways, railings).

 The tough environmental conditions that mining steel is exposed to includes water immersion, impact and abrasive conditions, extended surface wetness, variable temperatures and corrosive underground atmospheres.

The primary mine environments are underground, at surface minerals beneficiation process plants, and on transportation equipment such as conveyors. Hot dip galvanised coatings have a good track record in protecting many structural steel parts in these environments.

South Africa’s deep gold and shallower coal mines have ever present corrosive water, which is where hot dip galvanised (duplex coated) steel really comes into its own. Thus, a cost-efficient, long-lasting, and maintenance-free protection system for steel should be specified, which means a duplex coated galvanised steel system, explains Simon Norton, executive director of the International Zinc Association (IZA) Africa.

Galvanising provides a tough and abrasion-resistant coating as well as long-term corrosion protection of steel in two ways – as a physical barrier to corrosive atmospheres, and by means of galvanic or sacrificial protection, which is unique to zinc – significantly extending the service life of steel in even the most challenging environments.

Underground

With increasing depth and distance from the shaft towards the working area, the corrosion severity and incidence of mechanical damage increases substantially. Underground workings are often wet. This water tends to be corrosive due to pick-up of blasting fumes and ingress of acidic waters into the working areas. Shaft areas become hostile environments for steelwork due to water, falling objects and dust, resulting in poultice formation that allows corrosion to continue underneath the poultice, thus attacking the steel.

Surface

Atmospheric environments are classified into different categories of severity by ISO 9223, which also provides a range of lifetimes for steel and zinc in these environments. There is a specific relationship between galvanised zinc coating thickness and expected coating life, defined as the time that can pass before coating maintenance is required to restore protection to the base metal.

Added corrosion protection: The duplex system

Organic (paint) coatings can be added for improved corrosion protection. If hot dip galvanised mine shaft steel is then overcoated with a tough organic paint system, the resultant duplex coating has an extended lifetime, saving the mine owner millions of dollars.

On the other hand, continuous galvanised sheeting is generally pre-painted in the same facility in which a thin zinc galvanised coating is applied. This allows for precise preparation of the galvanised surface prior to coil coating. Painted galvanised sheets can be roll-formed to give profiles as specified by the customer for roofing and cladding of a surface plant.

Combining paint coatings with galvanised coatings is referred to as a duplex system. Duplex systems benefit from a synergistic effect that enables the combined life of the painted galvanised steel to be longer than the sum of the individual lives of the paint and galvanised coatings in the same environment. The synergy multiplier ranges from 1.8 to 2.7, depending on the severity of the corrosion conditions.

Hot dip galvanising provides superior corrosion protection to steel. It is easily and swiftly applied and covers the entire surface of the steel article, even in inaccessible areas, provided the component is designed properly. Hot dip galvanised coatings offer a unique dual protection that prevents corrosion even if the coating is damaged.

Maintenance painting is often impossible in mining environments, meaning the only option with paint-only structures is complete replacement. Hot dip galvanised coatings, however, are hard and abrasion resistant. They provide long lasting and maintenance-free corrosion protection to steel even in challenging mine environments, and provide significant savings compared with other protection systems that require maintenance.

Reference:

https://www.zinc.org/wp-content/uploads/sites/4/2015/01/Hot-Dip-Galv-in-Mining-Brochure-36pp.pdf.

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