Precious Metals: A Resource Worth Recycling
Precious metals are among the most valuable of Earth’s resources. As long as humans have been on the planet they’ve coveted these metals. The pleasing appearance, tarnish resistance, and permanence of gold and silver have made them ideal choices throughout recorded history for jewelry, ornamentation of persons and objects, and investment in the form of coins, bars, and exchange-traded items. Platinum group metals (PGM) did not come on the historical scene until about 1800, but the high melting points, high corrosion resistance, and excellent catalytic properties of platinum, palladium, rhodium, ruthenium, iridium, and osmium make them indispensable for many industrial applications [1,2].
Commercial uses of precious metals are increasing rapidly both in number and quantity as high technology applications proliferate. Their electronic and chemical properties are exactly those needed to develop high technology products, which have fueled global economic growth in the 21st century. Without precious metals, many of the products and processes that we associate with our developing society would either not be possible or would have inferior performance.
Mining of precious metals has increased dramatically in the past few decades to meet demand resulting, often, in severe environmental damage and human health consequences as well as depletion of valuable mineral resources . Metals differ from other resources in that they remain with us forever in some form. Recycling metals following their use provides an important means to reduce the environmental burden resulting from mining primary ore, ensures the availability of a valuable secondary source of the metal, and conserves an irreplaceable resource that otherwise would be discarded. Challenges and benefits associated with recycling precious metals are presented here. Emphasis is placed on the need for greater use of green chemistry recycling processes for effective recovery of these precious resources to prevent their extensive loss to the commons. Molecular Recognition Technology (MRT) is presented as an effective green chemistry process in commercial metal recycling together with selected examples of its use.
Uses of Precious Metals
The automotive industry is the principal customer of PGM. Autocatalytic converters in vehicles require palladium, rhodium, and platinum to convert exhaust emissions to water and carbon dioxide [1,2]. PGM are essential components of catalyst systems for the production of petroleum and petrochemicals. In the electronics industry, storage capacities in computer hard disk drives are increased by certain PGM, which are also used extensively in electronic devices, multilayer ceramic capacitors, and hybridized integrated circuits. PGM are valuable in the glass manufacturing industry to produce fiberglass, liquid-crystal, and flat-panel displays. Exceptionally hard and durable properties of PGM alloys make them ideal as a coating for industrial crucibles used to manufacture chemicals and synthetic materials, including high-purity sapphire crystals used to make light-emitting diodes. Iridium is the most corrosion-resistant metal known and is used for many applications where this property is required, such as turbine fan blades for jet engines and spark plugs. Platinum does not corrode inside the human body and allergic reactions to this metal are rare, which makes Pt useful in medical implants such as pacemakers. Platinum is a constituent of cisplatin, a drug used in the treatment of testicular cancer . Cisplatin is one of the most effective cancer drugs known. Radioactive Pd-103 is used in brachytherapy as an effective treatment for prostate cancer . It has been suggested that PGM could play a crucial role in fuel cell technology, which has promise to revolutionize clean energy production for cars, homes, and businesses .
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The ability of Au to efficiently transfer heat and electricity is exceeded only by Cu and Ag, making it indispensable in electronics for semi-conductors and connectors in computer technology, especially because of its high corrosion resistance . The ability of Au to withstand oxidation has led to its widespread use as thin layers electroplated on the surface of electrical connectors to ensure good connections. Gold-plated connectors are an integral part of plugs and sockets for cable terminations, integrated circuit sockets and printed circuit boards. The more sophisticated the equipment and the greater the need for reliability, the greater the requirement to exploit the advantages of Au as a material. Thus, in telecommunications, computers, automotive electronics, spacecraft, jet aircraft engines, military applications, defense systems, and a host of other uses where safety is critical, Au is indispensable.
Silver has the whitest color, highest optical reflectivity, and highest thermal and electrical conductivity of any of the elements. In addition, silver halides are photosensitive. Silver has many industrial applications such as in mirrors, electrical and electronic products, and photography, which is its largest single end use . Catalytic properties of Ag make it ideal for use as a catalyst in oxidation reactions; for example, the production of formaldehyde from methanol and air by means of Ag screens or crystallites containing a minimum 99.95 weight-percent Ag.
Production of Precious Metals
Increased annual production of precious metals in recent decades reflects their value to a society that recognizes their intrinsic worth leading to storage of them in large quantities and to their essential use in products needed in rapidly developing new technologies . The increasing number of uses for precious metals has resulted in a significant upsurge in mining of virgin ore to meet demand, often with negative global environmental and health consequences . The result is the removal annually of a large amount of precious metals from the Earth that are either placed in storage or incorporated into products that eventually will come to the end of their useful life. Thus, the stage is set for loss of a large fraction of mined precious metals unless effective recycling of them takes place. Since metals are indestructible and can be reused indefinitely without deterioration of function, recycling is a highly desirable alternative to discarding them into the commons . However, the increasing number and variety of end-of-life (EOL) products present challenges to the development of effective recycling processes . This is especially true for high technology products that are manufactured in very large quantities with each item carrying small amounts of precious metals.
Earth’s upper crust contains about 0.0005 parts per million (ppm) of Pt . The occurrence of Au is ten-fold larger. Concentration of precious metals in Earth’s crust by natural processes makes them available to us at a much lower energy cost than would otherwise be the case. For example, nuggets of Ag, Au, and Pt are well known and have long been used as sources of these metals without need for mining and refining ore. Mining of precious metals requires expenditure of large amounts of energy and water, uses hazardous chemicals in large amounts, and generates large quantities of waste . Conventional refining and separation processes used to obtain pure metals require additional use of hazardous chemicals and generation of appreciable waste. Mining and processing of precious metals are good examples of negative externality factors at work, especially in non-Organization for Economic Co-operation and Development (OECD) nations where legislative constraints are often lacking or are not enforced. These factors argue for effective recycling programs to reduce the impact of mining and ore processing.
PGM are rarer than Ag and Au in Earth’s crust and are found, predominantly, in only a few regions, usually concentrated in magmatic ore deposits . Prior to 1920, nearly all global PGM production came from placer deposits in Russia and Columbia. Today, most PGM produced are from mineral deposits discovered in Siberia in 1919 and in southern Africa in the 1920s . The latter deposits are enriched in Pt and provide about 80% of the world’s supply of PGM. The former are enriched in Pd and provide a majority of the world’s supply of that metal. Significant development of these deposits did not begin until the 1960s, when industrial demands for PGM began to increase at a rapid rate. Hagelüken  estimates that 80% of all PGM ever produced has been mined since 1980.
Prior to about 1839, a large proportion of the world’s stock of Au was derived from ancient and South American civilizations, demonstrating that recycling is not a new idea. The annual output of Au at this time was about 12 tonnes per annum (pa). The remarkable series of ‘gold rushes’ in the mid- to late nineteenth century in California, Australia, South Africa, northwest Canada, and Alaska resulted in a world production of 150 metric tonnes (mt) pa by 1890. However, this amount pales by comparison with Au production in 2014, 2,860 mt . The 2014 number is approximately double that in 1980, 1,250 mt. Artisanal mining of Au has become an important source of the metal [3,10], particularly in China, Peru, and central Africa. It is estimated that approximately 20 % of the world’s annual total Au supply comes from artisanal Au mining . Extensive use of Hg in artisanal Au mining results in the release of immense amounts of Hg into the environment creating a major health hazard for miners and increasing the risk of Hg pollution for all of Earth’s inhabitants . It has been calculated that if all the Au ever mined, estimated at 170,000 mt, were assembled together it would fit in a cube approximately 68 feet on a side. Total Ag production from pre-history till 2001 is estimated by the U.S. Geological Survey (USGS) to have been about 1.26 million mt, half of which was mined in the last half century .
Importance of Recycling to Achievement of Metal Sustainability
Reasons for the need to develop effective recycling procedures to increase metal sustainability have been summarized [8,9]:
- reduction of environmental burden required to mine additional virgin ore to replace precious metals lost when EOL products are discarded;
- mitigation of environmental and health-related impacts of mining by reducing energy demand, CO₂ emissions, land and water use, and impacts on the biosphere caused by additional mining;
- extension of lifetime of, and preservation of, a valuable primary geological resource;
- reduction of geopolitical dependence resulting when metal resources are found, primarily, in only a few countries, as is the case with PGM;
- contribution to supply security for nations by partial decoupling production of precious metals from mine production of these metals;
- dampening of metal price fluctuations by improving demand-supply balances;
- creation of significant employment potential including high-technology jobs and infrastructure creation.
Effect of Closed and Open Metal Cycles on Recycling
Effectiveness of recovery of precious metals from EOL products depends on the degree to which it is possible to control movement of the metal throughout its life cycle . A closed metal cycle typically takes place in a business-to-business environment with no private consumers involved in its different steps. In such systems, the user of the metal-containing product (e.g., the chemical plant) returns the spent product directly to a refiner who recovers the metals and returns them to the owner for a new product cycle. Examples of closed cycles are found with chemical and oil refining process catalysts, where 80-90% of Rh, Pd, and Pt contained therein are recovered . The key is control of the precious metal throughout the process leading to its eventual recovery from the EOL product. Typically, the metals remain the property of the user for the entire cycle and the metal-refiner conducts recycling as a service (i.e., toll refining). The entire cycle flow becomes transparent in these cases and is professionally managed by industrial stakeholders, resulting in very small metal losses. When the PGM content is low, the economics of metal recovery are challenging. Molecular Recognition Technology (MRT) is a highly selective green chemistry process  that has been successfully used to recover individual PGM from spent petrochemical catalysts, even at ppm concentration levels . Essentially 100% of the PGM in the spent catalyst material is recovered using the MRT procedure.
Overall recycling rates are usually much lower in open cycles taking place in business-to-consumer environments . The effectiveness of recycling in open cycles varies over a wide range, depending on the degree to which the flow of the precious metal can be controlled. Two examples, automotive catalytic converters and e-waste, are discussed below.
Automotive Catalytic Converters. Owing to their high intrinsic value and ease of retrieval from the scrapped automobile, spent automotive catalytic converters are widely collected and the contained Pt, Pd, and Rh are recovered at high rates. However, despite the efficiencies of collection and recovery it is estimated that only 50-55% of Pt, Pd, and Rh in auto catalysts are ultimately recycled on a global basis .
E-Waste. Ritter  estimated that of the 2.4 million tons of e-waste discarded in the U.S. in 2010, 27% was collected for recycling, the remainder was landfilled. Of the 27%, it was estimated that 80% was shipped, usually illegally, to developing countries either for reuse or informal recycling. The amount of unrecoverable PGM and Au lost to the commons in this process is large. Considering that these data are for the U.S. in a single year, the cumulative amount of precious metals lost worldwide since the upsurge in mining these metals began a few decades ago is huge. Global recycling rates from spent electronic products for Pt, Pd, Rh, Ru, Ir, Au, and Ag range from 0 to 15% .
It is significant that there is a ready market for collected e-waste that is shipped from OECD to non-OECD nations for informal recycling. Some of the e-waste items are sold for reuse upon arrival in non-OECD nations, but most are subject to informal recycling by a large population using manual sorting and dismantling techniques . Recovery of precious metals, primarily Au, Pt, and Pd, from electronic scrap and electronic waste is accomplished in the informal sector in many thousands of backyard facilities [3,8], located mainly in non-OECD nations, such as China, India, Indonesia, Nigeria, etc. Pre-processing of the EOL products includes open sky incineration to remove plastics, ‘cooking’ of circuit boards over a torch for de-soldering, cyanide leaching, and Hg amalgamation. Over and above their disastrous effects on health and environment, the efficiency of such activities is very low. An investigation in Bangalore, India revealed that only 25% of the Au contained in circuit boards was recovered, compared to over 95% in integrated smelters . The Au not recovered in the process is lost to the commons. A UNEP report cited by Hagelüken and Grehl  provides a comprehensive overview on the situation in developing countries.
Concentrations of precious metals in many EOL products are much higher than those found in ore deposits. Furthermore, the product represents a different, usually much simpler, matrix for metal separation, less waste should be generated, and much less energy is required to recover the target metal(s), since there is no need to remove large quantities of gangue material.
An automotive catalytic converter contains approximately 2,000 g/mt of PGM in the ceramic block, compared to average PGM concentrations of <10 g/mt in most PGM mines . Considering the high environmental impact of primary production of precious metals arising from low ore concentrations, difficult mining conditions, high energy and water use, high chemical consumption, large waste generation, and other factors, recovery of metals from EOL products is appealing.
A typical primary gold mine will yield five grams of Au per mt . In electronic scrap, this figure, 10 years ago, was as high as 200-250 g/t for computer circuit boards and 300-350 g/t for mobile phone handsets. High metal prices together with progress in material developments and product design about that time triggered both a significant miniaturization of devices and components as well as a reduction of precious metal content. Since then, Au and Ag in PC motherboards declined by 40%, and Pd by 60%. Similar trends can be recognized for mobile phones. Still, even on today’s lower level, grades this high are very uncommon in natural Au deposits.
Mobile phones contain over 40 different chemical elements including base metals, such as Cu, Ni, and Sn; specialty metals including Co, In, and Sb; and the precious metals Ag, Au, and Pd. Metals, mostly Cu, make up about one quarter of the metal content in each phone . One mt of scrap mobile phones (equivalent to about 13,000 units without batteries), contained in 2010 an average of 3.5 kg Ag, 340 g Au, 130 g Pd, and as much as 130 kg Cu. In 2016, these amounts have dropped to 1.3 kg Ag, 300 g Au, 40 g Pd and 125 kg Cu, on average. The value of these metals can approximate up to $10,000/mt (although less at current low metal prices), with 80% or more of the total being due to precious metals present. By contrast, a single unit contains mg quantities of precious metals and ~9 g of Cu. Thus, the net value of one unit is below $1U.S., which does not provide an economic incentive for recycling. It is the sheer number of mobile phones in use that attracts attention for possible metal recovery. About 1.6 billion of these phones were sold worldwide in 2010, alone. In 2014, this number rose to 1.9 billion, including 1.2 billion smart phones. The number of phones produced is increasing yearly to meet demand as global population and affluence increase. The active lifetime of each phone is 2-3 years after which it is out of use in drawers, landfills, or sent to non-OECD nations for reuse or informal recycling [3,8,14]. The estimated cumulative global total of ten billion units, produced by 2010, would contain a total of 2,400 mt of Ag, 230 mt of Au, and 90 mt of Pd. The Au and Ag contents of the sales volumes of mobile phones and computers in 2010 alone are appreciable, being equivalent to ~4% of global mine production for Ag and Au and ~20% of that of Pd and Co .
As discussed above, recycling of precious metals can be put in two broad classes. First, closed cycles in which the metal is controlled from one business to another. Recycling of PGM from spent petroleum/petrochemical catalysts is an example of such a cycle, wherein the overall recovery rate of PGM is 80-90% . Second, open cycles in which the metal is partially controlled in business-customer interactions. Examples include recycling of automotive catalytic converters and recycling of e-waste. The key to high overall recycling rates is collection. Automotive catalytic converters are collected at relatively high rates due to their ease of retrieval and intrinsic value. Once collected, the spent catalyst has value and it can be sold, eventually winding up at a PGM refinery. MRT is used widely to refine spent automotive catalytic converters at both secondary and primary PGM refineries [17-21]. For those catalytic converters that are collected and subsequently processed, essentially all of the PGM can be recovered. As mentioned earlier, global recycling rates for Pt, Pd, and Rh from automotive catalytic converters range from 50 to 55% , indicating that only about half of the catalytic converters produced are collected for recycling.
E-waste is an example of an open cycle in which there is minimal control over the metal. In these cases, the owner of the spent product (e.g., a smart phone, or a personal computer) might be number x in line after a number of preceding (second hand) product owners. The last owner of a device must make a conscious decision to take it to a recycling facility, if such a facility is available in their region. It is not feasible, usually, for any of these owners to return the product directly to a metals refiner. Many high-technology products fall into this category including the increasing number of devices that control many of the functions in automobiles, as well as smart phones, computers, and other products. Metals used in these products are, generally, used in very small amounts, but the number of products manufactured is very large and, usually, the products have a short life time, being rapidly displaced by newer models. The result is that large quantities of precious metals are involved, but only small amounts are found in each item. Overall recycling rates of precious metals contained in these items are low, 0-15% , compared to those in closed or open cycles where the metal movement is partially controlled. Significant metal losses result.
Three conditions must exist for an urban mine to be productive. First, there must be value in the contained metals. This requirement is met in the case of precious metals. Second, enough of the component products or product parts must be collected to provide a critical mass for metal recovery to be profitable on a continuing basis. Third, efficient, cost-effective technology capable of extracting the target metal(s) must be available. The first and second conditions are met in the case of closed metal cycles; partially in the case of certain open metal cycles, such as automotive catalytic converters; and poorly in the case of small units containing precious metals, such as smart phones, computers, etc. The third condition is very challenging for traditional metal recovery technologies due to the low concentrations of precious metals and high concentrations of impurity metals.
Separation, Recovery and Purification of Precious Metals Using Molecular Recognition Technology
MRT SuperLig® products have been used for over two decades in the commercial separation, recovery and purification of precious and other metals [12,13,17-28]. Examples for individual precious metals are given in Table 1. High selectivity for target metals, even when they are present at ppm levels in the presence of much higher concentrations of impurity metals, is the key to the success of the MRT procedure. This feature makes possible, in column mode, high capacity loading of the target metal on the SuperLig® resin resulting in complete separation of this metal
Table 1. Examples of Separation, Recovery and Purification of Individual Precious Metals from Various Matrices using MRT SuperLig® Products
from impurity metals, which pass on to raffinate. Elution of the bound metal from the resin with a small volume of eluent results in concentration of the metal in the eluate solution, from which it is easily recovered in pure form. MRT systems are simple in design, occupy small space, have short metal inventory times, use relatively benign wash and elution chemicals, and employ no solvents. Binding and release of target metals by SuperLig® resins are rapid and the entire procedure can by incorporated on-line into existing processes. The ability of MRT systems to separate and recover metals at ppm concentration levels is important, since many metal cycles involve target metals at these low metal concentrations. MRT provides a viable, tested green chemistry procedure for selectively separating, recovering and purifying precious metals from a variety of spent products, including spent automotive catalysts, scrap, spent petrochemical catalysts, spent plating solutions, and leach solutions generated from e-wastes.
The power of the MRT approach is especially well illustrated in the separation, recovery and purification of individual PGM from spent automotive and petrochemical catalysts, where these steps are efficiently accomplished even when the PGM are present at ppm concentration levels in complex matrices. The ability to efficiently separate, recover, and purify PGM at these low concentration levels, in the presence of much higher concentrations of impurity metals, greatly reduces refining costs that are otherwise incurred through use of traditional refining techniques that, in order to be efficient, can require, depending on the process utilized, use of solvents and/or toxic chemicals combined with heat and pressure to increase the concentrations of PGM in solution.
Achievement of global metal sustainability requires increased use of highly selective and efficient green chemistry processes, such as MRT, for separation, recovery and purification of precious and other critical metals from a variety of spent materials, wherein these metals are present at very low levels making their recovery difficult using traditional processes . The degree to which metal sustainability can be accomplished means preservation of valuable resources that otherwise would be discarded to the commons and become unrecoverable.
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27. Gutekunst, G., Izatt, S.R., Izatt, N.E., Dale, J.B., Bruening, R.L. 2008. The Commercial Application of SuperLig® Molecular Recognition Technology (MRT) Products to Recycling of Potassium Gold Cyanide from Spent Gold Plating Solutions, International Precious Metals Institute, 32nd Annual Conference, Phoenix, Arizona, June 7-10.
28. Izatt, S.R., McKenzie, J.S., Bruening, R.L., Izatt, R.M., Izatt, N.E., Krakowiak, K.E. 2016. Selective Recovery of Platinum Group Metals and Rare Earth Metals from Complex Matrices using a Green Chemistry-Molecular Recognition Technology Approach. In R.M. Izatt (Ed.), Metal Sustainability: GlobalChallenges, Consequences, and Prospects, Wiley, Oxford, U.K.: In Press, publication September 2016
Reed M. Izatt received a BS degree in Chemistry from Utah State University (1951) and a PhD degree in Chemistry with an Earth Sciences minor ... <Read more about Reed M. Izatt, PhD>