Green chemistry has been defined as the design of chemical products and processes to reduce or eliminate the use and generation of hazardous substances . This definition was formulated in the early 1990s and has since gained acceptance throughout the world, especially in the chemical and pharmaceutical industries [1,2]. The green chemistry approach strives to achieve sustainability at the molecular level. Anastas and Eghbali  consider the most important aspect of green chemistry to be the concept of design. Twelve principles of green chemistry have been set forth as design rules to aid practitioners in achieving the goal of sustainability . These principles apply to a broad range of chemical manufacture and their application has been successful in improving sustainability in the use of chemical substances in a variety of industries . Several of these principles that are particularly relevant to achieving metal sustainability in the mining and ore beneficiation industries are given in Table 1.
Table 1 Some principles of green chemistry applied to metals 
|Prevention||It is better to prevent waste than to treat or clean up waste after it is formed|
|Safer Solvents and Auxiliaries||Use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary, whenever possible and, when used, innocuous|
|Design for Energy Efficiency||Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized|
|Use of Renewable Feed Stocks||A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable|
Green Chemistry Principles and the Mining Industry
Despite wide and increasing use in chemical and pharmaceutical industry processes, green chemistry principles have been applied sparingly in the mining and ore beneficiation industries. This is unfortunate, since there is great need for improvement of metal separation and recovery procedures in these industries. In an analysis of the platinum group metal mining industry in South Africa, Mudd  has pointed out the large amount of waste generated and the enormous amounts of energy and water required to mine and process ore to recover desired metals. As ore grades decrease and greater depths are required to locate minable deposits, the amount of energy and water required increases dramatically. It would be desirable to reduce the necessity of mining virgin ore by better management of our existing metal supply through reduction of waste generation and achievement of improved recovery rates of metals from waste products.
Throughout human history, mining has been synonymous with waste generation . Remnants of this legacy are found in every corner of the Earth. In the United States alone, it has been estimated that there are as many as 250,000 acid mine drainage sites . A notable example is Berkeley Pit in Montana [7,8]. This pit is a remnant of underground and open pit copper mining in the area from about 1900 to 1983 when pumps were turned off and the pit began to fill with drainage water contaminated with a ‘witches brew’ of metals. It is, today, one of the largest Super Fund sites in the United States and has little hope of proper remediation. It is significant that this situation results from governmental inaction, not the lack of technology. In the 1990s IBC demonstrated a viable MRT process for metal removal from Berkeley Pit . China’s rare earth metal mining and processing industries are noted for their widespread pollution of the environment  and artisanal gold mining wreaks havoc on the environment through uncontrolled mercury emissions in parts of South America, China, and central Africa .
Solvent Extraction (SX) facilities, even for a small to mid sized mining operation, can occupy a footprint equal to several football fields of circuitry
Contrary to the foregoing, there are also notable examples of efforts by large mining corporations to turn the tide of waste generation in large scale mining. Companies such as Asarco, Impala, and Tanaka are striving to increase metal sustainability by installing clean chemistry processes in mining operations, ore beneficiation, and metal recovery procedures [3,11,12]. Such efforts are to be applauded and imitated, where ever possible. The consequences of continuing to use dirty processes in mining, ore beneficiation, and recovery of metals from end-of-life products have been presented and discussed . Beyond these giants of the industry, the work of start-up enterprises, such as Ucore Rare Metals and its work to reduce waste generation in rare earth processing via clean chemistry, are gaining increased recognition .
The latter half of the 20th Century saw great efforts in developed countries, such as the United States, Canada, and those in Western Europe to enforce by legislation stricter standards for metal pollution by industries. This action, prompted by many notable disasters from metal pollution of the environment in mid- to late-20th Century , was a recognition that we all share the same environment on Earth and that we have a stewardship responsibility. Unfortunately, not all nations share this view [13,15] . This stewardship concept was expressed in 1966 by Hardin  who discussed the dilemma of depletion of a resource shared in common by individuals, acting independently and rationally according to the self-interest of each, despite their understanding that depleting the common resource is contrary to the long-term best interests of the group. This phenomenon is referred to as the “tragedy of the commons” and may be applied to many human activities including environmental consequences of actions related to metal cycles.
Appropriate use of the commons is compatible with metal sustainability. However, improper use is antithetical to sustainability, because as a resource is depleted without replacing it, access to it by present or future generations becomes more limited. Once taken from the Earth, metals continue to exist in some form in the environment. The choice is whether the fate of metals should be managed as much as possible or whether metals should be discarded and allowed to move essentially unhindered through the commons. The first case is compatible with green chemistry principles. The second case is irresponsible in that severe environmental, economic, and health problems beyond the control of the polluting entity may result, as seen, for example, in Berkeley Pit, Montana [7,8] and in Chinese rare earth metal mining and processing .
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Once waste metals or end-of-life products containing them enter the commons, it is usually beyond the reach of present technology to recover the metals effectively, since their concentration is markedly reduced . However, the environmental and health effects of discarded metals persist into the indefinite future and can result in damage to the immediate surroundings, and, in some cases, such as mercury in artisanal gold mining , to the extended global community. It has been estimated that during the past 500 years approximately one million tons of mercury have been produced from mining operations . The presence of mercury in the commons is particularly insidious because mercury can be transformed by bacteria into methylmercury species, which can cross the blood-brain barrier in humans . Thus, recovery of metals before they enter the commons is desirable. However, this is a formidable task that is being tackled with mixed results by the global community .
Use of Solvent Extraction technology (alternatively referred to as “SX”) has become widespread in mining and ore beneficiation industries during the past half century, notably in the rare earth sector and across a wide range of industrial metals Notwithstanding this, the elimination of solvents is a increasingly a focal point for proponents of green chemistry for a variety of reasons. Anastas and Eghbali  have summarized these concerns. The elimination of solvents represents an important objective for green chemistry because they often account for the vast majority of mass wasted in chemical syntheses and processes. Moreover, many conventional solvents are toxic, flammable, and/or corrosive. Their volatility and solubility have contributed to air, water and land pollution, have increased the risk of workers’ exposure, and have led to serious accidents. Recovery and reuse, when possible, are often associated with energy-intensive distillation and sometimes cross contamination.
In an effort to address all of these shortcomings of SX methodologies, chemists have searched for safer solutions for use in the chemical and pharmaceutical industries. Solvent-less systems, water, supercritical fluids, and, more recently, ionic liquids are some examples of these new ‘‘green’’ answers. Where possible, the ideal situation would be to not use any solvent at all, because the decision to include an auxiliary always implies efforts and energy to remove it from a designated system. Efforts have therefore been devoted to developing solvent-less systems. This idea was reinforced by the finding that solvents account for most industrial waste. These observations by Anastas and Eghbali  may be difficult to implement in the mining and ore beneficiation industries where use of SX is deeply embedded. However, it is desirable to use creative thought to develop alternatives to solvents in these industries as is being done in the chemical and pharmaceutical industries.
In the case of mining, processing, and recovery of individual rare earth elements (REE – singular or plural), SX technology, which is used to process essentially all REE commercially used today, has produced many of the negative effects noted by Anastas and Eghbali . Use of solvent extraction has resulted in severe environmental and health problems in many nations, especially in China, where the very large majority of REE are processed . Although solvent extraction, when properly implemented and regulated, may have some utility as a separation process, its inherent drawbacks severely limit its desirability as a separation technique for the 21st century.
Solvent extraction is the antithesis of green chemistry. SX utilizes solvents that are typically toxic and corrosive, as well as being inherently volatile, flammable, and disposable only as a regulated waste. In addition, solvent extraction systems generally have low metal recovery rates and low metal selectivity, requiring many stages for effective separations. These inefficiencies require not only more expenditures of reagents, time, space, and labor, but produce large in-process metal inventories, significantly elongating the time in which final products are produced. Damage to the environment, resulting from such inefficiencies, are not accounted for in the cost of producing rare earth products. This situation is an example of negative externality, where the cost of producing the rare earth metals is borne, to a large degree, by society at large.
The only commercial non-solvent extraction process proven for production of the individual REE is Molecular Recognition Technology (MRT), which avoids the use of solvents and the attendant problems discussed above.
Molecular Recognition Technology: A Green Chemistry Process
MRT processes are compatible with the green chemistry principles given in Table 1. These processes have been described and examples given of their use during the past two decades in industrial metal separations and recovery [3,7,15]. The separation part of the MRT procedure makes use of a highly metal-selective, pre-designed ligand which is attached by a chemical bond to a tether which, in turn, is attached by a chemical bond to a solid support, such as silica gel to form a SuperLig® product.
In practice, this SuperLig® product is packed into a column. A feed solution containing a metal matrix including the target metal to be separated is then passed through the column. The target metal is selectively bound to the ligand and the remaining feed solution passes to raffinate. Metals contained in the raffinate can also be recovered, if desired, by a similar process using other SuperLig® products. Following washing of the column to remove residual feed solution, the target metal is eluted with a small amount of eluent. The resulting eluate solution contains the pure target metal, concentrated many fold over its concentration in the feed solution. Referring to the principles in Table 1, no solvents are used in the MRT separation process and minimal waste is generated in the procedure.
MRT based separation facilities are characterized by their small footprint, elimination of caustic solvents, low cost and ease of use.
In the case of rare earth processing such as the circuit designed for Ucore, molecular recognition design principles are used to design the ligands needed for the highly selective interactions with individual rare earth metals. In operation, these ligands remain attached by chemical bonds to the silica gel and can be used repeatedly without degradation. The MRT process is simple in design and operation resulting in minimal energy and water use. Space requirements are small since high metal selectivity reduces the number of stages required to achieve a desired metal purity.
Ucore’s MRT system for rare earth separation is now entering a phase known as “pilot plant”, in which the separations achieved at bench scale are now being transposed to bulk scale. The Ucore pilot plant will be designed to be fully automated and to produce the target metal(s) either on site of close to the point of origin. This is in contrast, for example, to Molycorp, which sends concentrates of their rare earth metals to China for processing. Rapid processing in the MRT process greatly reduces the pipeline or ‘lock-up’ of metal in the process, which is an important economic feature. The MRT process is much faster than solvent extraction or other separation processes. The high metal selectivity of the MRT process enables the recovery of essentially 100 % of the target metal.
As a result, in the Ucore MRT circuit, metals will not be discarded in tailings or to other parts of the commons. This is an important feature because valuable metal resources are conserved rather than depleted into the commons where they usually are unrecoverable and may impose environmental and human health risks. Loss of metals to the commons requires additional mining of those metals to replace the loss. Since mining has inherent large requirements for energy and water and generates large quantities of waste, it is desirable to conserve our already mined metal supply whenever possible.
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