Hydrometallurgy: Principles And Applications
The past 10 years have been blessed with a much greater awareness of environmental issues in the extractive-metallurgy sector. This period has seen the launch of the Journal of Sustainable Metallurgy in 2015  and an acknowledgement that the total environmental impact of a metallurgical process can only be properly evaluated via a lifecycle assessment (LCA) or, better still, via a multicriteria assessment (MCA). Unfortunately, such studies do not provide the metallurgist with practical guidelines as to how to improve the sustainability of a metallurgical flowsheet. In the area of environmentally friendly chemistry, the Twelve Principles of Green Chemistry have proven their worth in making chemical syntheses safer and greener (Table S1) [7, 8]. However, most of these principles are more closely associated with the synthesis of organic compounds and are not relevant to extractive metallurgy. The Twelve Principles of Green Chemistry were reformulated for engineering practice as the Twelve Principles of Green Engineering (Table S2) , and the Nine Principles of Green Engineering of the Sandestin Declaration (Table S3) . Although some of these principles can be applied to extractive metallurgy, many others are less relevant because they were devised for manufacturing. At present, it is fair to say that such guidelines for extractive metallurgy are lacking. For this reason, we decided to formulate a set of design principles adapted to the field of hydrometallurgy that we hope will spur the development of more sustainable hydrometallurgical processes [11, 12]. These guidelines contribute to the targets of Goal 12 of the United Nations Sustainable Development Goals: Ensure sustainable consumption and production patterns .
Hydrometallurgy: Principles and Applications
As guidelines for the design of circular hydrometallurgical flowsheets, we are proposing an interrelated set of principles,: i.e., the 12 Principles of Circular Hydrometallurgy (Table 1). These principles are to help metallurgical engineers achieve the goal of circularity in hydrometallurgy. As such, they are practical guidelines, presented in the form of imperatives, with each principle elaborated in more detail below. The Principles of Circular Hydrometallurgy have been numbered from 1 to 12. Although one could argue that some principles are more important than others, their order does not reflect a strict hierarchy. Some principles are more general (e.g., Principle 1: Regenerate reagents), whereas others are more specific (e.g., Principle 8: Electrify processes wherever possible). The principles are not independent and can often be combined to even more powerful principles, as explained in a separate section.
Leaching residues could find applications as raw materials to produce construction materials such as bricks, but extensive testing is required to establish the immobilization and non-leachability of traces of heavy metals. These leaching studies must conform to regulations, but these regulations are very country specific. Leachable heavy metals are a showstopper for the valorization of industrial process residues, including metallurgical leaching residues. Even if the material conforms to the regulations, there will be concerns over the potential release of immobilized heavy metals if these materials are to be recycled in the future (cf. second life) .
Hydrometallurgical processes should be designed to maximize mass, energy, space, and time efficiency. This is also one of the principles of green engineering . This Principle is not only important for the design of sustainable processes; it also matters to the economics of a hydrometallurgical plant, since it maximizes the recovery of valuable elements or products at minimum cost.
The most useful and powerful method for real-time measurements of metal content is without doubt X-ray fluorescence (XRF), especially in the form of energy-dispersive X-ray fluorescence (EDXRF) . XRF has been widely been used in the mining and mineral-processing industries for real-time on-line measurements [135, 136, 137]. The technique is also routinely used by the metallurgical industry to measure metal concentrations in solutions, but these are typically not real-time on-line measurements, but rather off-line analyses. On-line XRF would be a very interesting technique for monitoring hydrometallurgical processes, but on-line XRF analyzers for solutions are not routinely available yet. In hydrometallurgy, inductively coupled plasma optical emission spectrometry and mass spectrometry (ICP-OES and ICP-MS) are often the analytical methods of choice for the off-line analysis of complex liquid process streams, but it is not straightforward to adapt these techniques to on-line applications.
In these days of rapidly developing artificial intelligence (AI), it is not surprising that the different branches of AI, such as expert systems, fuzzy logic, neural networks, and machine learning, could find applications in efficient process control for hydrometallurgy. However, the implementation of AI methods in hydrometallurgy is slower than in related industrial domains, such as minerals processing . Still, AI methods have been applied to leaching processes , solvent extraction [141, 142], and electrowinning [143, 144]. Decision and optimization methods can be used to decide between alternative hydrometallurgical unit processes [145, 146], while the digitalization of a hydrometallurgical process can be seen as part of a broader narrative, i.e., the digitalization of the circular economy [147, 148].
Although there is no strict hierarchy among the principles and their numbering is somewhat arbitrary, Principle 1 (Regenerate reagents) is by far the most important principle because no circular hydrometallurgical flowsheets can be designed without regeneration of reagents. The 12 Principles are not independent, and they can be combined to even more powerful overarching principles. Hence, there can be synergies between the principles. Principle 4 (Maximize mass, energy, space, and time efficiency) is related to several other principles. Integration of materials and energy flows (Principle 5) leads to higher efficiencies (Principle 4). More efficient processes with a lower reagent consumption (Principle 4) result in less need for regeneration of reagents (Principle 1). Real-time analysis and digital process control (Principle 11) induce more efficient processes (Principle 4), while combination of circular hydrometallurgy with zero-waste mining (Principle 12) is a form of process intensification (Principle 4) as it allows to largely omit mineral-processing operations, linking mining directly to hydrometallurgy. Less minerals processing also translates into a substantial energy saving since size reduction by milling and grinding is highly energy intensive (energy efficiency is addressed in Principle 4). By decreasing the activation energy of hydrometallurgical processes (Principle 7), mass, energy, space, and time efficiency can be maximized (Principle 4). Concentration of potentially hazardous elements and their encapsulation in an inorganic host matrix (Principle 6) represent a form of waste prevention (Principle 3). Reduction of the chemical diversity (Principle 10) facilitates the regeneration of reagents (Principle 1). Many methods for regeneration of reagents (Principle 1) constitute electrochemical methods, thereby boosting Principle 6 (Electrify processes wherever possible).
To provide a compass that can guide the metallurgical engineer in developing circular flowsheets in hydrometallurgy, a set of design rules has been provided, i.e., the 12 Principles of Circular Hydrometallurgy. Although we realize that the choice of these principles is somewhat arbitrary and that other principles could be imagined, we are nevertheless convinced that these principles make powerful tools to show the direction of future research and innovation in hydrometallurgy, also in academia. This is especially the case if these qualitative rules are combined with quantitative assessments of the materials and energy balances for newly developed hydrometallurgical flowsheets. Because only by applying sustainability metrics (e.g., LCA and MCA) it is possible to objectively compare different options for new processes, for both primary mining and recycling flowsheets. A circular use of chemicals in a primary-mining-based process might beat a non-circular use in a concurrent recycling flowsheet. The question is what we should do if we develop a very elegant circular flowsheet that is not economical to build. Which of the principles can be sacrificed first? Is there a hierarchy to these principles? There is no easy answer. Only a system-level approach will work. What is key is that, instead of introducing more complexity, we should aim for minimalism and simplicity, so that the hydrometallurgy of the future evolves to a form of low-energy-input, circular hydrometallurgy.
This book is concerned with the theoretical principles of hydrometallurgical processes and engineering aspects. The hydrometallurgical processes of production of copper are discussed and leaching of chalcopyrite as the main sulphide mineral of copper processed in industry is used as an example. The book is suitable as a university textbook for students of metallurgy.
The course is delivered during the first quadrimesterThursdays pm; B 52, - 1/433This course is presenting the unit oprations which are utilized within the final stage of the industrial metal production chain. Thus, although a link exists, the material covered in this course differs from the one already seen in the Mineral Processing and Soild Waste and By-product Processing courses. The theoretical principles of extractive metallurgy are extensively presented and computer and laboratory exercises are used to facilitate the comprehension of the subject. 041b061a72