The central concept of CEReS is a co-processing approach for the treatment of coal and electronic wastes. The process targets acid mine drainage (AMD) generating coal wastes and uses these to generate a leaching solution (lixiviant) to solubilise metals from printed circuit boards (PCBs) taken from electronic wastes (e-wastes). Around this, a novel flow-sheet is envisaged, handling the identification and assimilation of waste materials and the downstream recovery of metals and other value products. This brings together two waste streams from opposite ends of the supply chain; turning each into a novel resource in a single, coherent ‘grave-to-cradle’ process.
The proposed co-processing flow-sheet is summarised in Figure 2, and can be considered in four parts:
- AMD-generating coal mine wastes are recovered from existing mine waste dumps or during ongoing production. Low-grade PCBs (shredded) are bled off from existing waste electronic and electrical equipment (WEEE) or e-waste handling streams. Therefore, there are no mining or comminution costs associated with the supply of raw materials for the CEReS process.
- A catalytic cracking circuit is used for pre-treatment of the PCBs resulting in a metal-rich char. Catalytic cracking partially converts the organic fraction into a liquid fuel (which can be used for electricity generation for downstream processes) and a carbon-rich solid residue (with potential applications as a reducing agent in metallurgy or steelmaking). A quenching process removes halogens (mostly bromine) as a saleable brine. The low temperatures and reducing conditions prevent the formation of dioxins while maximising product recovery.
- A bioreactor system is used to oxidise sulfide minerals in the coal wastes (effectively accelerating AMD production), resulting in the production of a ferric iron-sulfuric acid lixiviant. This is used to leach base and other soluble metals from the PCB char. The stabilised coal wastes (depleted of their AMD-generating potential) can be used, for example, as backfill in rehabilitation of the mine site or in civil engineering (aggregates).
- Valuable metals (Cu and potentially REEs, Ga and Ta) are recovered from the pregnant leach solution (PLS) by appropriate downstream process and the raffinate recycled to the coal waste bioleaching reactor (to maximise efficient process water use). This circuit will include a system for managing the iron content, as significant amounts will be encountered throughout the process as a whole. Precious metals, lead and tin will report to, or remain in, the solid leach residue and will be physically separated to produce concentrates suitable for established refining metallurgy.
The fundamental idea behind CEReS lies in the observation that there is an excess of acidic effluents from coal wastes released in the environment (in the form of AMD) and a need for cheap, effective lixiviants in hydrometallurgical treatment of PCBs. Coupling these both fluxes in an intelligent manner will result from one side in a reduction in the environmental impacts of these wastes through the generation of benign residues, and from other side in the recovery of strategically important metals . The process is small-scale and flexible with high-potential for replication.
The exchange of wastes, by-products, and energy among closely situated companies is one of the distinctive aspects of industrial ecology. By organising the co-development of remediation and recycling technologies in very specific local context (proximity of post mining and urban waste collection/treatment sites) and by including a LCA approach in the project development, CEReS will demonstrate the applicability of an industrial ecology approach to both coal mining and recycling businesses. CEReS seeks to deliver a practical, flexible, sustainable and profitable alternative to classical waste management systems for both coal and post-consumer wastes. CEReS will mainly target a waste flow (low grade PCBs) that is currently not treated by pyrometallurgy. Therefore, CEReS is more a complement to, rather than in competition with, existing technical solutions (Figure 3).
The overall aims of CEReS are the development and integration of innovative solutions for the co-processing of coal mining wastes and e-waste and the technological, socio-economic and environmental validation of this concept. This involves major technology improvements leading to the design of a full scale e-waste/mine waste co-processing plant in order to improve both treatment of AMD sources and recycling of e-waste (based on collected volumes). The validation of the integrated process will be strongly based on industrial ecology considerations, including a life cycle thinking approach at global and local scales.
The general objectives of the project can be summarised as follows:
- The use of a case study to demonstrate that the CEReS concept allows the profitable and sustainable co-processing of historic and current coal production and electronic wastes; and
- On a more global perspective, the development of a new business model that will integrate sustainability criteria in the co-development of remediation and recycling technologies.
The scientific and technological development of this project will use Poland as a case study. This country is the largest coal producer in the EU, currently producing over 70 Mt hard coal per year. This leads to the production of 25-20 Mt sour mining waste per year, adding to the >600 Mt coal waste currently laying in dumps. Tauron Wydobycie SA is part of the Tauron Polska Energia group, and owns two mines: Sobieski and Janina. Janina has the largest coal reserves in Poland, but, at current production rates, Sobieski will continue production for a further 20 years. The Polish coal mining sector has been suffering from well-publicised economic problems for some time. Government-led restructuring of the industry means Tauron Wydobycie SA will likely soon acquire the ailing Brzeszcze coal mine. Therefore, Tauron, and the entire Polish coal mining sector has an urgent need for technologies to improve coal mine performance.
By providing an alternative treatment route, well adapted to local contexts the project should decrease the environmental impact of coal mine wastes and enhance e-waste recycling. Through the integration of the flow-sheet into current operations, a new business model is foreseen that creates new economic opportunities for the treatment and valorisation of these wastes. This increases regional economic output through the production of high value products and so improves regional and European economic competitiveness.
Some components of the flow-sheet will be based on the transfer of existing technologies. However, the RTD tasks are significant, involving optimising individual processes for maximum performance and their integration into a single, functional flowsheet. More holistic approaches will be required to examine the wider uptake and deployment. The flow-sheet must be small-scale and flexible with a large potential for replication in various locations. There is no such thing as ‘average’ coal wastes or PCBs, so the flow-sheet will have to account for significant fluctuations in feed material(s). Therefore, fundamental research is necessary to understand the effects of various factors on individual processes. This will provide data on process tolerances and help make the process robust and able to handle variations in feed materials into the flow-sheet, and between the individual sub-processes.