Material Flow Analysis of Scarce Metals: Sources ... - ACS Publications

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Material Flow Analysis of Scarce Metals: Sources, Functions, EndUses and Aspects for Future Supply Laura Talens Peiró,*,† Gara Villalba Méndez,‡ and Robert U. Ayres† †

INSEAD - Campus Europe, Boulevard de Constance, 77305 Fontainebleau, France Department of Chemical Engineering, Edifici Q, Universitat Autònoma de Barcelona (UAB), ES-08193 Bellaterra, Barcelona, Spain

‡

S Supporting Information *

ABSTRACT: A number of metals that are now important to the electronic industry (and others) will become much more important in the future if current trends in technology continue. Most of these metals are byproducts (or hitch-hikers) of a small number of important industrial metals (attractors). By definition, the metals in the hitch-hiker group are not mined by themselves, and thus their production is limited by the demand for the major attractors. This article presents a material flow analysis (MFA) of the complex inter-relationships between these groups of metals. First, it surveys the main sources of geologically scarce (byproduct) metals currently considered critical by one or other of several recent studies. This is followed by a detailed survey of their major functions and the quantities contained in intermediate and end-products. The purpose is to identify the sectors and products where those metals are used and stocked and thus potentially available for future recycling. It concludes with a discussion of the limitations of possible substitution and barriers to recycling.

1. INTRODUCTION Scientific progress has made it possible to discover and make use of more and more elements in the periodic table. A number of metals that were formerly scientific curiosities have now become essential for new applications. One example is the use of phosphors based on scarce metals in LEDs now competing with older types of light for many applications such as automobile headlights and streetlights as well as interior lighting. A different example of new applications of scarce metals is the ultrapowerful permanent magnet alloys now used in small electric motors (e.g., for disk drives and electric cars), generators for wind turbines and magnetic resonance imaging (MRI) scanners, magnetic cooling, and so on. A third example is new quaternary alloys for thin film photovoltaic. Concern about the future need for a number of byproduct metals has resulted in a number of reports discussing sources of supply and their criticality.1−4 In these reports, the criteria for defining criticality vary but the metals assessed as critical are mostly agreed upon.5 The UNEP and EU reports identify cobalt, gallium, germanium, indium, and tantalum, and two groups of metals, namely the platinum group metals (PGMs) and rare earth elements (REE) as critical.1,2 Lithium is included as critical only by the UNEP. In general, we can say that there is a list of 5−11 metals plus 2 groups of metals that are regarded as critical by some criteria (part 1 of the Supporting Information). As the interest in REEs keeps growing, the Ö ko-Institut published a second report for UNEP specifically focused on that group.6 The report suggests that future shortages are likely for dysprosium, europium, lanthanum, neodymium, praseodymium, terbium, and yttrium. Such shortages will affect applications such as LCDs, permanent magnets, nickel metal © 2013 American Chemical Society

hydride batteries, efficient lighting devices, automotive catalysts, and future new technologies such as magnetic cooling and hightemperature superconductors. This situation is also particularly critical in regard to the future potential for thin film photovoltaic modules (e.g., copper-indium-diselenide, cadmium-telluride, gallium-arsenide or thin film silicon) virtually all of which require one or more of the critical metals.7 As result, the future supply of critical metals has become a serious concern, especially for those that are not mined as such but are now obtained exclusively, or mostly, as byproducts of major industrial metals: aluminum, copper, iron, nickel, tin, and zinc. This list includes gallium (from bauxite); molybdenum, rhenium, selenium, tellurium (from copper ores), germanium, and indium (from zinc ore); cobalt (from copper ore and nickel ore) and rhodium and ruthenium (from PGM ore). The socalled rare earth elements (REEs) are included even though they are not geologically rare because almost 50% of the world’s production is currently obtained as byproduct of iron ores in China.8 The underlying problem of increasing production in this case is that the REEs are very difficult to refine, and refinery wastes constitute a major disposal problem. This article constitutes a simple material flow analysis (MFA) to illustrate the industrial metabolism of a specific set of geologically scarce metals from sources to products. The scarce metals included in the study (based in part on suggestions from reviewers) are cobalt, gallium, germanium, indium, niobium, molybdenum, rhenium, selenium, tantalum, and tellurium plus Received: Revised: Accepted: Published: 2939

December 19, 2011 January 22, 2013 February 13, 2013 February 13, 2013 dx.doi.org/10.1021/es301519c | Environ. Sci. Technol. 2013, 47, 2939−2947

Environmental Science & Technology

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even less than the crustal average. This situation seems to apply to copper, lead, mercury, platinum group metals (PGMs), silver, tin, and zinc, in particular.24,25 These metals (as well as gold) are typically mined for themselves. A number of other metals that are geologically less scarce (including but not limited to the REEs) are not found anywhere in high concentrations but are distributed essentially as contaminants, or trace elements, in the concentrations of minerals of other attractor metals, to which they are chemically similar. The attractors include iron, though most of the rare critical metals are associated with copper, lead, nickel, or zinc ores. Graedel has called the attractor element and its entourage parent and daughters, which wrongly implies that the attractor actually created the others in some abiotic process.26 Verhoef et al. use the terms carrier and co-elements in the metal wheel to explain the metal interconnections and interdependencies.27 Hereafter, to avoid inappropriate analogies, we call the chemically similar but scarcer metals hitch-hikers because they accompany attractors. Figure 1 displays the worldwide production of attractor and hitch-hiker metals in 2010 (in tonnes). This figure shows the mineral sources for the production of attractors, and the hitchhiker metals produced as byproducts from them. Most of the information presented is from USGS commodity reports and minerals yearbook.13,28 For certain metals, additional informa-

the PGMs and REE groups. We note that cobalt, molybdenum, and niobium are exceptions to our general characterization of the group as byproducts (or hitch-hikers) insofar as they are currently coproducts of some copper and nickel ores and are mined for themselves. REEs are coproducts of iron ores in China, whereas PGMs are mined, as a group, in Siberia and South Africa.

2. MATERIAL FLOW ANALYSIS Material flow analysis (MFA) applies the conservation of mass to quantify stocks and flows of natural resources through transformation processes leading to intermediate or final products.9 MFA has been used to study the industrial metabolism of major base metals (iron and steel, aluminum, copper, zinc, lead, and nickel) to quantify waste residuals generated by processing.10,11 In economics, this is sometimes characterized as the biophysical perspective.12 The ultimate purpose of studying flows of substances such as metals through an economy is to identify inter-relationships and possible hot spots such as potential supply bottlenecks, opportunities for materials substitution, or unexpected sources of environmental damage from process wastes. The MFA in this article consists of two parts. The first part quantifies the global flows of these metals from different sources in 2010 focusing on their mineral ores and on primary metallurgy. In this part of the study, we include cobalt, molybdenum, and niobium, all obtained partially as byproducts and likely to become critical in future years. The second part of the MFA focuses on flows of the various metals identified above in relation to functions. It also includes estimates of the consumption of scarce critical metals in intermediate and endproducts. Cobalt, molybdenum, and niobium are excluded from this part of the MFA because their uses are comparatively wellknown and their annual production levels exceed 50 thousand tonnes. The results of the MFA contribute to understanding the potential and limitations for future supplies. Data sources for the MFA are from the U.S. Geological Survey commodity reports and minerals yearbooks and the British geological survey.13,14 For most scarce metals, the functional uses are derived from market share information.8,15−21 For the platinum group metals and rare earth elements (REE), data are scattered in scientific publications, specialized reports, technical descriptions and, in some cases, restricted due to confidentiality. For platinum group metals, we use the reports published by Butler.19 For uses of rare earth elements, we made educated guesses based on REE output, the use pattern published in Roskill reports, and by Morgan.21−23 The data reported in this paper are entirely based on secondary sources. 3. SOURCES OF HITCH-HIKER METALS The term hitch-hiker conveys a geological relationship that helps explain the underlying reasons for potential scarcity problems discussed in the MFA. The point is that mineral resources are not randomly distributed in Earth’s crust but by complex geo-chemical processes. This leads to quantity-grade distributions that differ from the usual assumption of lognormality. In consequence, some elements that are geologically scarce in terms of parts per million in Earth’s crust, are nevertheless found in easily recoverable mineral deposits with high concentrations, whereas the bulk of the atoms are distributed randomly in ordinary rock with concentration

Figure 1. Worldwide production of mineral ores, attractor and hitchhiker metals in 2010. 2940

dx.doi.org/10.1021/es301519c | Environ. Sci. Technol. 2013, 47, 2939−2947


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Figure 2. Scarce metals by function, intermediate, and end-products.

tion was collected from other sources: for aluminum,11,14 for copper,29,30 for iron ores,31 for germanium,17,18 for nickel,32−36 for platinum group metals,19 and for rare earth metals8,15,16 (part 2 of the Supporting Information for a more exhaustive explanation). As illustrated in Figure 1, some metals are partially obtained from minerals in which they are predominant or significant as such. This group includes cobalt, molybdenum, niobium, and tantalum. Cobalt is mostly (85%) obtained as a byproduct of nickel and copper, but 15% is mined for itself from cobalt

arsenides.14 Molybdenum is partly (40%) obtained from molybdenum sulfides (mainly molybdenite) and 60% as a byproduct of copper ores.37 Niobium is mainly produced from niobium oxide minerals (pyrochlore) plus a marginal amount (less than 1%) as byproduct of tin. Tantalum is mostly (87%) produced from mineral oxides (tantalite), whereas 13% is a byproduct of tin. Although the production of niobium and tantalum from attractor ores is currently minimal, we include both metals in Figure 1 as hitch-hikers because the hitch-hiker share is likely to grow in the future. 2941



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Supporting Information, which shows the quantities of each scarce metal used per function and intermediate product. 4.1. Dopants for Semiconductors. Dopants are trace elements added to semiconductors to improve the electrical conductivity, absorption or emissions in various wavelengths, and so forth. They are used in intermediate products such as printed circuit boards, phosphors, thin films, detectors, and optical fibers. There are several critical hitch-hikers used in printed circuit boards: indium, gallium, and germanium. In 2010, 6 tonnes of indium were used in semiconductor chips, about 106 tonnes of gallium (mostly as gallium arsenide) converts electrical signals into optical signals, and 9 tonnes of germanium were used for a variety of compounds.38−40 Phosphors exhibit phosphorescence (emit light) and fluorescence (emit light previously absorbed) materials. Phosphors consist of a host material with an added activator or dopant. The composition of phosphors varies depending on the color of light desired. For red light, the RE oxides used includes yttrium, europium, and gadolinium. For green light, the hosts used are lanthanum, cerium, and yttrium, whereas terbium and gadolinium are used as activators. For blue light, the dopant commonly used is europium oxide. Phosphors are largely used for lighting (84%), followed by LCDs (12%) and plasma displays (4%).21 Indium and gallium are used in virtually all light-emitting diodes (LEDs) and laser diodes to transmit data and to improve image stability in certain equipment.35 In 2010, about 425 tonnes of indium as indium tin oxide (ITO) were utilized in thin film for flat panel displays (including touch screens) and 11 tonnes were used for LEDs. Germanium combined with magnesium as fluoro-germanates (Mg28Ge10O48 and Mg56Ge15O66F20) are used in lamps as a fluorescent coating.35 There are four thin-film PV technologies (all using critical metals): cadmium-telluride (CdTe), copper indium gallium selenide (CIGS) and triple-junction cells made of gallium arsenide (GaAs), germanium and indium gallium phosphide (GaInP2). Triple-junction cells are the most efficient solar cells to date.41 In 2010, 3 tonnes of gallium, 6 tonnes of germanium, 57 tonnes of indium, 325 tonnes of selenium, and 124 tonnes of tellurium were used for this purpose. Detectors are devices that can detect a change in temperature, pressure, light intensity, humidity, γ radiation, and so forth. Indium is mainly employed for temperature detection in alloys with bismuth in combination with lead, cadmium, and tin, to hold alarms, water valves, and door opening mechanisms where other methods of measurement are impracticable.35 Optical fibers are flexible, thin, and transparent fibers made of pure silica and germanium that allows the transmission of high volume data over long distances with low loss and immunity to electromagnetic interference as compared to metal wires. In 2010, about 66 tonnes of germanium was used for this purpose. 4.1. Catalysts. Catalysts are substances generally used to speed up the rate of chemical reactions. They are not consumed and can be reused. There are several combinations of PGMs and REE oxides used as wash-coating in the honeycomb of automobile catalytic converter.42,43 In 2010, we estimate that this use consumed 6 126 tonnes of cerium, 340 tonnes of lanthanum, 204 tonnes of neodymium, 204 tonnes of palladium, 99 tonnes of platinum, 136 tonnes of praseodymium, and 30 tonnes of rhodium. Although most of this use can be regarded as dissipative, used converters are to some extent collected and recycled at present to recover palladium, platinum, and rhodium.19 Catalysts are used during fluid catalytic cracking (FCC) in oil refineries to improve the yield of

Metals with similar chemical properties like the platinum group metals (PGM) and rare earth elements (REE) are generally assessed as groups. In each of these groups, we can differentiate subgroups based on production quantities. As shown in Figure 1 for PGM, there is a first group of metals produced in multiples of 100 tonnes per year (platinum and palladium), a second group produced in multiples of 10 tonnes per year (rhodium and ruthenium), and a third group produced in very small quantities (iridium and osmium). Thus rhodium, ruthenium, iridium, and osmium can be regarded as byproducts of platinum and palladium. In the case of REEs, there is a first group produced in annual amounts greater than 10 000 tonnes (cerium, lanthanum, neodymium, and yttrium), a second group that includes praseodymium and dysprosium, both produced in multiples of 1 000 tonnes. Then, a last group of REEs formed by samarium, europium, gadolinium, and terbium, all produced in quantities of the order of 100 tonnes. As result, most of the REEs can be regarded as hitch-hikers or byproducts of lanthanum, cerium, neodymium, and yttrium. The rest of the metals included as hitch-hikers in Figure 1 are all currently produced as byproducts of attractor metals. Copper processing is a major source of rhenium, tellurium and selenium. Germanium and indium are byproducts of zinc, and gallium is a byproduct of aluminum. Overall, almost all major industrial base metals have hitch-hiker metals associated with them. Apart from REEs, copper minerals have the most diversity of byproduct metals. Copper ores, worldwide, are associated with 9 byproduct elements (arsenic, cobalt, germanium, gold, molybdenum, rhenium, selenium, silver, and tellurium) plus some PGMs. At present, the two most critical hitch-hiker metals produced partly or mainly from copper ores are cobalt and tellurium.

4. TECHNOLOGICAL FUNCTIONS, INTERMEDIATE, AND END-PRODUCTS OF SCARCE METALS The second part of the MFA involves quantifying the use of each hitch-hiker metal in relation to their main functions in intermediate and end-products. Quantifying the use of scarce critical metals in products helps identify the dissipative versus the nondissipative flows. The latter may be candidates for recycling in the future. By dissipative flows we mean the quantities in process wastes and the quantities that are nonrecoverable in the productś use phase such as fuel additives or polishing agents. We also include the quantities embodied in end-use products that appear to be unrecoverable because the concentrations are so tiny, such as the elements in miniature chips that are embodied in credit cards, keys, and so on. The amount of scarce metals in intermediate and/or final products has been estimated based on use patterns and material composition of products. In some cases, we find it more appropriate to end our estimate at the intermediate product to simplify the figures. For example, we quantify the amount of indium contained in printed circuit boards but not the amounts embodied in other electrical and electronic equipment. The next paragraphs summarize the major functions performed by, intermediate and end-products that use scarce metals. Figure 2 summarizes this information for the year 2010. The metals included are indium, gallium, germanium, rhenium, selenium, tantalum, tellurium, platinum group (PGM) and rare earth elements (REE). Amounts for REE are included as elements although they can be used also as oxides and mischmetal. Part 3 of the Supporting Information provides more detailed explanation and includes Table S2 of the 2942



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superalloys, as well as in rock drilling and other applications (also abrasives below). Tantalum is used together with tungsten for high-temperature resistance and strength. In 2010, about 161 tonnes of tantalum were used for this purpose.1,7 The scarce critical metals cerium, indium, iridium, lanthanum, neodymium, praseodymium, rhenium, selenium, tantalum, and tellurium are all used in alloys with steel, copper, lead, cast iron, and zinc. 4.4. Additives: Glass and Ceramic Industries. Additives − like alloying elements − are substances added to augment natural properties of elements. Major uses of additives involving scarce critical metals take place in the glass and ceramic industry to preserve the quality and appearance of coatings and for coloring purposes. In the glass industry, the scarce metals used as coloring additives are: indium, selenium, rhodium and the REEs cerium, lanthanum, neodymium, praseodymium, and yttrium.23 These metals are generally added to neutralize the green color displayed by iron compounds in impure glass and to improve the UV and IR absorption. Indium is used in infrared optics and rhodium is used during the melting of liquid crystal display glass to reduce defects.19 In the ceramic industry, cerium, lanthanum, neodymium, and praseodymium and yttrium are added to stabilize zirconia in order to increase material strength.48 Selenium is also used in ceramics to produce red ruby color. 4.5. Abrasives. REEs are used in abrasives for polishing in the manufacture of LCD, optical glass, mirrors, photomasks, plate glass, lenses, and cut glass. They provide high mechanical abrasion and react with the glass surface to provide a high quality finish. 51,52 Morgan estimates that the average composition of polishing powder is: 32% lanthanum, 65% cerium, and 4% of praseodymium.21 In 2010, usage was 4 331 tonnes of lanthanum, 8 938 tonnes of cerium, and 481 tonnes of praseodymium. About 40% of rare earth polishing powder was consumed in the LCD industry.53 Although rare earth polishing powder is regarded as dissipative use, there is ongoing research to recover and use it as adsorbent for removing inorganic arsenic from water.54

light hydrocarbons such as gasoline and liquefied petroleum gas (LPG). The catalysts most widely used are zeolites containing cerium and lanthanum, which improve catalytic stability at high temperature, increase catalyst activity, and gasoline selectivity.35,44 Scarce metals are also used as catalysts in the production of plastics, fibers, nitrogen-based compounds, and other processes. We estimate that 30 tonnes of germanium, 2 tonnes of iridium, 3 tonnes of rhodium, 7 tonnes of ruthenium, and 100 tonnes of tellurium were used for the production of industrial chemicals based on the commodity description for each metal in 2010. These catalysts are mostly recycled. Platinum is currently used as a catalyst for fuel cells producing electricity by combining hydrogen (the fuel) and oxygen (from air). As fuel cell technology develops and matures, the increased demand for platinum can in part be compensated by improvements in efficiency which presently show a reduction in platinum required from 1.2 g to 0.2g per kW produced.45 4.2. Electrical Storage: Batteries and Capacitors. Electricity is difficult to store for any length of time, except in another form of energy, for example, gravity (pumped storage), heat, kinetic (e.g., flywheels) or chemical (batteries). Currently, electricity can be stored as such (in superconductors) or as electric charges (in capacitors). Electrochemical devices (batteries) are the most important for new (especially mobile) technologies. Scarce metals are mainly used in nickel metal hydride (NiMH) as mischmetal composed of lanthanum (50%), cerium (33%), neodymium (10%), praseodymium (3%), and samarium (3%).21 In 2010, the amount of REEs used in NiMH batteries was 6 331 tonnes of lanthanum, 4 229 tonnes of cerium, 1 266 tonnes of neodymium, 418 tonnes of praseodymium, and 418 tonnes of samarium. Pillot estimated 65% of NiMH batteries were used for HEV vehicles, 18% for retail (toys and household tools), 9% for cordless phones, and 8% in other electrical and electronic devices.46 Capacitors can store and regulate the release of an electrical charge and are essential components of all kinds of electrical and electronic circuits. In 2010, capacitors used 605 tonnes of tantalum, about 60% of its global production.47 In the future, fuel cells will most likely substitute batteries in power generation for buildings, in generators for portable equipment and internal combustion engines of vehicles. 4.3. Alloying Elements. Alloys, a mixture of two or more metals, are used to augment the properties of individual metals. The characteristics of alloys vary depending on their composition. Scarce metals are used as alloys in permanent magnets, cemented carbides, and other alloys. The first magnets containing REE, samarium cobalt, are being replaced by neodymium iron boron magnets (NIB), which are both more economical and twice as strong for the same weight.48 On average, NIB magnets contain 30% rare earths which are made up of 70% neodymium, 24% praseodymium, 5% dysprosium, and 1% terbium.21 The amount of REE in NIB magnets in each end product is estimated based on the electric vehicles registered, wind turbine installation, and magnetic resonance imaging units in 2010.6,49,50 We assume that the remaining amount of REE in NIB ends up in electrical and electronic devices. The amount of REE in NIB magnets for 2010 were 4 804 tonnes for electric vehicles, 1 300 tonnes for wind turbine, 644 tonnes for magnetic resonance imaging, and 17 315 tonnes for electrical and electronic devices (part 3 of the Supporting Information for more details). Cemented carbides are ultrahard materials used in machining carbon steel, stainless steel or

5. LIMITATIONS AND POTENTIALS OF FUTURE SUPPLY OF SCARCE METALS Sections 3 and 4 above summarize the main flows and transformations of scarce critical metals from sources to endproducts. The results show the dependence of scarce hitchhiker metals on mineral ores and attractors and thus provide insight on the limitations and potential of the future supply from primary sources (attractor metals) and end-products that contain these metals. Figure 1 shows that the production of many scarce metals regarded as critical by different reports depends on the production of attractor metals. Examples of these materials are gallium from aluminum, germanium, and indium from zinc, and rhenium, selenium, and tellurium from copper. Cobalt production depends mainly on two attractors: nickel and copper. Molybdenum and rare earth element (REE) are partially affected by the production of copper and iron ores, respectively. The production of tantalum and niobium can be marginally distressed by tin and platinum group metals (PGM) by nickel. Figure 2 shows the use of the scarce metals in intermediate and end-products in relation to their functions. Supply restrictions can affect the sustainability of new technologies that depend on phosphors, fiber optics, and thin films such as light-emitting diodes (LEDs), flat panel displays, photovoltaic 2943


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2% from nickel

47% from iron ore

REE

13% from tin 100% from copper

100% from zinc 100% from zinc 60% from copper 50% Ir > 25−50% Ru > 10−25% Os < 1%

Material Flow Analysis of Scarce Metals: Sources ... - ACS Publications

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