SINTEC building embraces circularity

Materials World magazine
,
29 Aug 2019

A Swansea University associate professor discusses the modern challenges in researching materials while adhering to strict operational guidelines.

In the modern world, cost and performance must be offset against recyclability and carbon footprint. Life extension, production impact, and regeneration via alternative end-of-life uses provide many opportunities to reduce environmental impact, but present significant challenges to make them practicable and economically viable. Small-scale testing provides a cost-effective route to commercial implementation of alternative use technologies prior to either pilot scale demonstrators and full-scale implementation.

Simulation and integrity testing under extreme conditions (Sintec) is a project led by the Steel and Metals Institute at Swansea University, UK, which provides unique facilities for materials testing and process simulation. Although the £1mln building is designed for testing, it was also put through its own approvals process to ensure safety and best practice. The building had to demonstrate it could safely store a range of gases including inerts, toxics and flammables with temperature capability of up to 1,800oC in combination with commercially sourced, in-house designed and built bespoke chamber/containments vessels. This capability helps the building embody the engineering version of reduce, reuse, recycle – build it clean, make it last, tidy up, and provide a place where it can be delivered.

Build it clean

Sustainable production requires transitioning from scarce raw materials and reducing polluting waste products, an example of the former being the move from tropical hardwoods to composite wood materials. Industrially, resins form a crucial part of these materials. While curing resins are often chemically or thermally driven, scaled curing will always be complicated by mass and energy transfer. An area of research in this field is working out how to control the resin cure at a cost and scale that is commercially viable while maintaining the desired product form.

Wood composites also need joining together with adhesives and metal fixings. The fixings and the material itself must have commercially acceptable operational lifetimes. Studying those materials’ lifespans and recyclability will also be addressed.

Working in collaboration with Lignia Wood Company Ltd, the project aims are to investigate thermal behaviour of the resin-wood composites to study the resin curing in composite substrates. In situ metrology will be studied against a programme of manufacturing optimisation. This data will be correlated with process variables to understand how these affect curing and resin-wood interactions and, in turn, the resulting product parameters, for example strength, hardness and colour. Product lifetime is also tested under accelerated weathering, and coatings will be developed to enhance product operational lifespan. Material recycling, for instance depolymerisation and material valorisation, will also be studied using chemical and thermal treatments.

Coatings are a common way of enhancing lifetimes by protecting components from particular environments. Though effective, sometimes these coatings have their own environmental issues, such as chromated cadmium coatings used to protect structural landing gear components from environmental degradation. Registration, Evaluation, Authorisation and Restriction of Chemicals REACH regulations dictate that use of chromates is strictly regulated. General corrosion is the primary cause for costly scrapping of components at overhaul. Thus robust corrosion performance of new high-strength corrosion-resistant steels when subject to typical in-service environments becomes a critical aspect in their ability to deliver the required extension in time between overhaul, the reduction in lifecycle cost and scrapped parts.

The current standard corrosion test procedure defined in ASTM B-117 does not provide a good correlation with outdoor exposure testing. Nor does it provide confidence in the applicability of materials for landing gear parts in service due to its lack of other environmental corrosion accelerators such as sulphur dioxide and nitrogen oxide. While outdoor exposure testing is seen as a gold standard, these tests often take years before results are available. Working in collaboration with Airbus, an alternative highly accelerated lifetime test was developed to establish the most representative conditions seen by in-service landing gear, providing accelerated lifetime testing to reflect a 10–14 year service life.

Many processes in iron and steelmaking are closed-loop, where byproducts are reused and recycled. However, in its current form, blast furnace ironmaking remains a carbon-intensive process. Sintec is researching a range of different reductants and fuel sources to decarbonise ironmaking while maintaining process efficiency. Working with the UK steel businesses, the aim is to transition the industry from using coal as its primary energy source to a mix of renewable energy, hydrogen and waste materials. Plastics are of particular interest. While technologies are available to recycle hard plastics, flexible plastics, such as those for plastic bags, are often considered as non-recyclable waste. This situation is further compounded because many countries have banned the export of hard plastic waste for recycling. However, Sintec’s material processing allied to new instrumentation for thermal and chemical analysis mean that we are exploring new ways to recycle all types of plastic.

Make it last

Objects should have the longest operating lifetime possible. Key to the successful prediction of how a material will perform in service is the ability to generate property data about it in service representative environments. This represents a challenge for a number of reasons, including not always knowing what that service environment is. However, it is this service environment, combined with any other service load interactions that will dictate a material’s useful life.

Numerous industrial machines are approaching the end of their working lives. Yet, with a better understanding of how material degradation occurs, the useful life remaining can be understood and plant life extended accordingly. Also, a better understanding of material behaviour under representative environments allows for new materials development, process changes, and efficiency savings. For instance, work on nickel alloys has highlighted the importance of getting this service environment right. Temperature, gas environments, level of other contaminant and stress all have a key role to play, and true service degradation and lifetime impact can only be understood when this situation is reproduced accurately and mechanistic understanding is gained.

For instance, traditional Type II hot corrosion produces a broad front attack with dual oxide scales. The Sintec team previously showed that introducing cyclic stress changes the attack to a deeper V-shaped pitting with sulphide particles precipitated on the grain boundaries, which is more significant when considering fatigue and crack initiation. Thermal cycle also plays a key role and Sintec is exploring the temperature range over which hot corrosion may be generated as in-service data suggests that hot corrosion is present outside of the expected temperature range. Mitigation strategies and new materials development can then follow to maximise component life and minimise the use of rare earth elements in new alloy design.

Asset integrity is a multi-faceted approach involving both remnant component life studies relating to life extension, but also includes aspects that are associated with licence to operate in accordance with Control of Major Accident Hazards site requirements. The latter are essentially safety cases to demonstrate to the health and safety executive that components or plants are operating within a safe design intent from the point of view of mechanical performance, such as fracture, fatigue, creep, environmental or stress interactions and stress corrosion cracking. The latter could include hot blast stoves, thermal hot spots in blast furnace shells and blast furnace downcomers.

Work covered in the new facility includes development of testing methods to predict issues surrounding these hot spots in blast furnace shells and involves time and temperature cycling, residual stress relaxation and development, and material degradation. Other recent examples include fatigue evaluation of the basic oxygen steelmaking (BOS) charger crane and an exercise to re-evaluate material choice for the shell of the new BOS vessels.

Make it safe

On occasion, environmental interactions can be so catastrophic that a completely new class of materials needs to be used. Since the Grenfell Tower fire in London, UK, in June 2017, reviews of building regulations have resulted in all combustible materials being banned for high-rise residential buildings. Further reviews for all building types are ongoing. With building designers not allowed to use existing highly efficient thermally insulating products, in particular polyisocyanurate PIR foams for tower blocks, demand is rising for non-combustible products. At approximately half the weight and thermal performance of the combustible alternatives, the result is lower thermal efficiency in new and refurbished buildings and greater structural costs.

In order to meet government targets on CO2 and gain efficiencies within building design, working in collaboration with TATA Steel UK, the facility is investigating novel non-combustible insulating materials that perform as well as polyisocyanurate foams, in particular a density less than 50kg/m3, thermal conductivity below 0.035W/mK and lend themselves to adhering to steel face skins.

Tidy up afterwards

Coatings are useful for environmental protection of components, but can generate contamination issues when the component finally needs recycling. For every tonne of steel produced through the blast furnace production route, about 20kg of iron-bearing dusts and sludges are expected to be generated through off-gas cleaning. These dusts can have iron in a concentration of more than 60% weight percentage, comparable to high-grade virgin ore, and are prime candidates for recycling.

But contamination with zinc can mean these materials are difficult to reintegrate into the ironmaking process, as high zinc levels have a deleterious effect on blast furnace production. This zinc is introduced into the iron material chain through galvanising, where a thin layer of zinc is adhered to the steel to improve its corrosion performance. This process has become critical in automotive steel production and most newer vehicles have a substantial galvanised steel content.

When galvanised products reach the end of their service life, the steel is returned to the plant for reprocessing of any zinc on the steel volatilises and is captured with the off-gas sludges and dusts. Current projections indicate that the loss of zinc from the galvanising material cycle in the form of steelmaking byproducts may put a strain on the global zinc supply chain as early as 2050.

A process for the re-introduction of zinc into the manufacturing cycle is required to make hot dip galvanised steel into a true circular economy material. Research centres around utilising another plant byproduct that contains high levels of carbon to reduce and volatilise the zinc from the high zinc byproducts of steel production. Zinc is then recovered as a fine oxide, which is suitable to return to zinc smelters and the iron dusts are recovered as direct reduced iron (DRI). DRI is an attractive resource for ironmaking as it is already mostly metallised, meaning less coke is needed to reduce it to molten iron. Displacing coke has economic and environmental benefits as it is expensive, production is CO2 intensive and it is non-renewable.

Moving towards a circular economy will require advances across a wide front. The ability to make realistic assessments of novel approaches quickly and cheaply using realistic small-scale simulations will be a significant asset across the whole range.