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R&D / Materials 14. February 2024

Understanding the process of de-coating contaminated aluminium

Mechatherm International Ltd and Aston University have formed a research collaboration on the aluminium de-coating process to establish a new design of de-coating and heat recovery processes in the aluminium recycling industry.

In order to enhance recyclability of metallic material, paint, ink, paper, plastic and oil are being removed from the surfaces. This process is called „de-coating.
In order to enhance recyclability of metallic material, paint, ink, paper, plastic and oil are being removed from the surfaces. This process is called „de-coating.

By Ainul Nadirah Izaharuddin, Tim Hordley, Stuart Allen, Ahmed Rezk, Muhammad Imran

A new design of multi-chamber will be introduced in high efficiency of the de-coating process, reducing dross formation, improving the efficiency of aluminium recycling, reducing emissions and minimizing the energy consumption in aluminium recycling process.

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De-coating process involves pyrolysis and thermolysis to produce off-gas at temperatures 400-550 °C. In de-coating, the parameters involved include scrap types, scrap chamber temperature, oxygen level and scrap charge rate. The challenge of de-coating is to find the optimum condition of these parameters to gain the highest efficiency of the process and to find the lowest energy consumption in operating the scrap chamber. There are several technologies that have been used for de-coating, which include rotary drum and multi-chamber.

De-coating is the process to remove paint, ink, paper, plastic and oil in the surface of a metallic material to enhance recyclability. Most common products in aluminium are cans, extrusions, lithographic material, painted sidings, laminates, and chips and turning. The coatings comprise of organic and inorganic compounds. Through thermal degradation or oxidation, the chemical complex compounds are reduced to basic forms; for example polypropylene is reduced to carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), and water vapour (H2O).

In the industry, there are a number of companies that have designed the multi-chamber furnace for aluminium de-coating; this includes Mechatherm, Hertwich, Tenova and Sistem Teknik. The Aluminium Federation (Alfed) gives the opportunity to get involved in collaborations with business and stakeholders to foster innovation, promote best practice, develop skills and champion member interest, especially in the aluminium industry. Schmitz presented a handbook of aluminium recycling which included the whole process of aluminium recycling [1]. Moreover, the new technology of aluminium recycling needed to reduce the energy consumption and production of high quality recycled aluminium, which will be presented in this research.

Fig. 1 shows the summarisation of de-coating mechanisms based on previous research including by Schmitz [1], Steglich et al. [2], Dittrich et al. [3], Vallejo-Olivares et al.[4], and Engh et al. [5]. This is the summary of aluminium de-coating, which started with devolatilisation at temperatures 100-200 °C to reduce the moisture content of organic compounds.

Fig. 1: Summarisation of the de-coating mechanism.
Fig. 1: Summarisation of the de-coating mechanism.

Pyrolysis is a decomposition process in absence of oxygen to produce gases and char at temperature in the range of 400-550 °C, while thermolysis, also known as gasification, is a partial oxidation process to produce gas at temperature in the range of 400-550°C. Fig. 2 shows how the de-coating process happens in a double chamber, starting with the charging of scrap into the scrap chamber, followed by the aluminium de-coating process occurring by thermolysis at temperatures of up to 550 °C with a certain level of O2, which produces the off-gas (H2, CO, CO2 and CH4). The aluminium is heated in the scrap chamber, then pushed into the molten bath, once the de-coating process is complete, then melted continuously in the melting chamber at temperatures of 1050-1100 °C. This is the general view of aluminium de-coating process that gives effect to the aluminium yield and dross formation.

Fig. 2: Double chamber schematic view on aluminium de-coating process
Fig. 2: Double chamber schematic view on aluminium de-coating process

Fig. 3 shows the syngas production of window scrap for three different types, it shows that white scrap produced more CO2 because of high oxygen content in the volatile organic compounds (VOC’s). CO2 is one of the contributors of having more dross formation. The research that is implemented here defines how much heat can be recovered in the off-gasses produced after de-coating, These findings will determine the optimum parameters involved and minimise the energy consumption in aluminium de-coating process.

Fig. 3 : Syngas production of window scraps and UBC.
Fig. 3 : Syngas production of window scraps and UBC.

Acknowledgement

This partnership received financial support from the Knowledge Transfer Partnerships (KTP) programme. KTP aims to help businesses to improve their competitiveness and productivity through the better use of knowledge, technology and skills that reside within the UK knowledge base. This successful Knowledge Transfer Partnership project, funded by UK Research and Innovation (UKRI) through Innovate UK, is part of the government’s Industrial Strategy.

References

[1] Schmitz, C., Handbook of Aluminium Recycling (2nd Edition) - Mechanical Preparation - Metallurgical Processing - Heat Treatment. 2014, Vulkan Verlag. p. 69.

[2] Steglich, J., R. Dittrich, G. Rombach, M. Rosefort, B. Friedrich, and A. Pichat. Dross formation mechanisms of thermally pre-treated used beverage can scrap bales with different density, in Light Metals 2017. 2017. Springer.

[3] Dittrich, R., B. Friedrich, G. Rombach, J. Steglich, and A. Pichat. Understanding of Interactions Between Pyrolysis Gases and Liquid Aluminum and Their Impact on Dross Formation, in Light Metals 2017. 2017. Springer.

[4] Vallejo-Olivares, A., S. Høgåsen, A. Kvithyld, and G. Tranell, Effect of compaction and thermal de-coating pre-treatments on the recyclability of coated and uncoated aluminium, in Light metals 2022. 2022, Springer. p. 1029-1037.

[5] Engh, T.A., G.K. Sigworth, and A. Kvithyld, Principles of Metal Refining and Recycling. 2021: Oxford University Press.

Author

Ainul Nadirah Izaharuddin is a KTP Research Associate in Thermal and Fluid Modelling and Design, Master’s in Chemical Engineering with Entrepreneurship, PhD in Mechanical Engineering, with expertise of designing and modelling by using engineering software include Matlab, Aspen Plus, and Ansys. Having experience in Waste-to-Energy research with the universities in Malaysia, UK and Japan. She is also an Associate Member with the IMechE (Institute of Mechanical Engineers).

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