Hyperspectral scrap characterisation for scrap composition optimisation in steel recycling

Steel is an ideal recycling material as it can be recycled almost indefinitely, and steel recycling is a lot more energy-efficient than iron ore steel production. Steel making with usage of steel scrap in electric arc furnaces is heavily influenced by contaminants in the scrap, including non-ferrous metals, stones, or plastic. To produce high-quality steel, it is important to know what contaminants are part of the scrap to adapt the recycling process accordingly. This work presents a three-part processing pipeline to determine the scrap composition and optimise the recycling process parameters. The first part is hyperspectral imaging, recording images with 437 spectral bands in the short-wave infrared range. Next, deep learning-based image recognition, with multilayer perceptron, 2D and 3D convolutional neural networks (CNNs) have been compared. The 3D-CNN showed the best performance in detecting the 14 material classes in the scrap samples. Finally, a mixed integer optimisation is used to select the best scrap mix for the steel classes that are to be produced. The evaluation shows an accuracy of about 76% in detecting the 14 material classes correctly, with a higher accuracy for the steel class. The detected non-ferrous materials determine the scrap class which is input for the optimisation with its constraints that each scrap class has a minimum consumption to prevent storage overflows, and as little energy and additives as possible should be used.