Municipal solid waste incineration (MSWI) bottom ash contains economically significant levels of silver and gold. Bottom ashes from incinerators at Amsterdam and Ludwigshafen were sampled, processed, and analyzed to determine the composition, size, and mass distribution of the precious metals. In order to establish accurate statistics of the gold particles, a sample of heavy non-ferrous metals produced from 15 tons of wet processed Amsterdam ash was analyzed by a new technology called magnetic density separation (MDS). Amsterdam’s bottom ash contains approximately 10 ppm of silver and 0.4 ppm of gold, which was found in particulate form in all size fractions below 20 mm. The sample from Ludwigshafen was too small to give accurate values on the gold content, but the silver content was found to be identical to the value measured for the Amsterdam ash. Precious metal value in particles smaller than 2 mm seems to derive mainly from waste of electrical and electronic equipment (WEEE), whereas larger precious metal particles are from jewelry and constitute the major part of the economic value. Economical analysis shows that separation of precious metals from the ash may be viable with the presently high prices of non-ferrous metals. In order to recover the precious metals, bottom ash must first be classified into different size fractions. Then, the heavy non-ferrous (HNF) metals should be concentrated by physical separation (eddy current separation, density separation, etc.). Finally, MDS can separate gold from the other HNF metals (copper, zinc). Gold-enriched concentrates can be sold to the precious metal smelter and the copper-zinc fraction to a brass or copper smelter.

This paper aims to summarize, propose, and discuss existing or emerging strategies to shift towards a circular economy of materials. To clarify the landscape of existing circular practices, a new spectrum is proposed, from product-based strategies, where entire products go through several life cycles without being reprocessed, to material-based approaches, extracting, recycling, and reprocessing materials from the waste flow. As refillable packaging does not lose any functionality or value, when re-used through many life cycles, product-based strategies are globally extremely efficient and must be promoted. It appears however that their implementation is only possible at the scale of individual products such as packaging containers, relying on the cooperation of involved companies and consumers. It appears more and more urgent to focus as well on a more systematic and flexible material-oriented scheme. The example of circular glass recycling is a success in many countries, and technologies become nowadays available to extend such practices to many other materials, such as rigid plastics. An ideal would be to aim at an economy of materials that would imitate the continuous material cycle of the biosphere. Technological and business strategies are presented and discussed, aiming at a relevant impact on circularity.

An innovative route for plastics recycling is proposed, based on a combination of a logarithmic sorting process and colour plus high-resolution near-infrared (NIR) sensors. Although counterintuitive, it is shown that such a technology could sort clean flakes from rigid packaging waste into a very large number of different plastic grades with modest sorter capacity, provided that the chosen sensor is able to differentiate correctly between any two grades of plastics in the waste. Tests with high-resolution NIR on single pixels of transparent flakes from different types and brands of packaging show that this is indeed the case for a selection of 20 different packaging items bought from shops. Moreover, the results seem to indicate, in line with previous research, that high-resolution NIR data can be linked to important physical plastic properties like the melt flow viscosity and tensile strength. The attraction of deep sorting of waste plastics with relatively cheap sensors and modest sorter capacity is that the present industrial practice of tuning plastic grades to specific applications could coexist with commercial high-grade recycling at high levels of circularity and low carbon footprint. Therefore, advanced recycling technology is likely to be a societal alternative to phasing out plastics for rigid applications.

Solid waste sorting is an important pre-treatment in recycling to improve the efficiency of material recovery and reduce costs. Motivated by the PEACOC project on metal recovery from solid wastes, an innovative magnetic density separation (MDS) process has been developed for solid waste sorting. It has intrinsic advantages over conventional gravity separation technologies and the previously industrialized MDS process. The new MDS process applies an inclined planar magnet and a horizontal basin containing a static magnetic fluid as the separation medium. A particle sliding phenomenon is identified as a feature that could help the separation. Experiments have been carried out to demonstrate the role of the MDS in concentrating valuable metals in shredded PCBAs and reducing metallic contaminants in plastic fractions of shredded wires. A pilot scale facility is introduced to show the design to achieve continuous production and to reduce the consumption of ferrofluid.

Motivated by the PEACOC project on metal recovery from solid wastes, an innovative magnetic density separation (MDS) process has been developed for granular material sorting. It has intrinsic advantages over the existing industrialized MDS by eliminating fluid turbulence and particle jamming problems. The new MDS applies an inclined planar magnet and a horizontal basin containing a static magnetic fluid as the separation medium. A particle sliding phenomenon is identified as a feature of the process which could help the separation. The MDS successfully sorted shredded PCBAs to concentrate valuable metals and sorted shredded wires to reduce metallic contaminants in the plastic fractions. A pilot scale facility is introduced to show the design to achieve continuous production and to reduce the consumption of ferrofluid.