In This Issue
Spring Issue of The Bridge on Urban Sustainability
March 18, 2011 Volume 41 Issue 1

The Prospects for Urban Mining

Wednesday, March 30, 2011

Author: T.E. Graedel

 Mining “urban ore” may provide an alternative to the continued extraction of virgin metals.

A half-century ago, the visionary urbanist Jane Jacobs (1961) proclaimed that “cities are the mines of the future.” This prediction was based on some hard facts that are more evident today than they were when Jacobs penned her words: (1) hard-rock mining is energy intensive and environmentally problematic; (2) as the metal content of rock being extracted continues to decline, the extraction rates continue to increase; (3) a large fraction of the metal being mined flows to cities, where it is used in structures, transportation, and the wide variety of products that are the hallmarks of modern life; and (4) this “urban ore” may provide an alternative to mining virgin metals.

The prospect of urban mining raises many questions. To what degree is its promise being fulfilled? What are the successes of urban mining? What are its challenges? Perhaps most important, does urban mining matter, or could it matter?

In this article, I review the current state of urban mining of metals in the periodic table and discuss how important Jacobs’ grand idea is turning out to be. A central message is that, as with more traditional mining, each step of the process—recovery, separation, sorting, and processing—must be evaluated as components of systems. Only by optimizing these systems can urban mining reach its potential.

Urban Mining Resources

Three significant questions relate to assessing the potential of urban mining resources: how much metal there is; when it will be available; and what form it is in. It is worth examining each of these questions in turn.

How Much Metal Is There?

From the standpoint of a specialist, the question becomes: What are the sizes of the in-use metal stocks in urban regions? There are two ways—“bottom up” and “top down”—to estimate these stocks.

For a bottom-up estimate, the analyst first identifies the major uses of a particular metal. Copper, for example, is used primarily for power distribution, transportation, plumbing, and consumer electronics. Next, the analyst must determine, based on field research and other information, approximately how often a metal is used for each type of use in the particular urban region (e.g., how many vehicles have been registered, how much copper plumbing is in a typical building, etc.) and second, the typical amount of metal in each unit of use. A straightforward calculation then provides an estimate of in-use stock.

The alternative, top-down method is more workable on the national and global levels. This method requires determining flows of the subject metal into each major use over a number of years and then applying product-lifetime determinations to estimate the amount of material that is no longer in use. The difference is the in-use stock.

Figure 1

It has been shown (Figure 1) that careful bottom-up and top-down studies yield similar results. Although the magnitudes of urban stocks are modest compared with the amount in existing ore deposits, they can be significant compared to annual demand: 538 teragrams (Tg) in-use stock and 29 gigagrams (Gg) annual demand for aluminum (IAI, 2006); 354 Tg in-use stock vs. 13 Tg annual demand for copper (Kapur and Graedel, 2006); and 18 petagrams in-use stock vs. 860 Tg annual demand for iron (Müller et al., 2011). (The data are for 2000 for copper and iron and 2004 for aluminum. Annual demand is based on studies by the U.S. Geological Survey for the appropriate year.) In each case, the in-use stock is about 20 times the annual demand, although much of that stock is still being used and is thus not available for immediate mining.

National and global results (i.e., top-down results) are not the same as urban results, of course. Binder et al. (2006) and Graedel and Cao (2010) have shown that both flow-into-use and stocks remaining in use follow per capita wealth. Because urban dwellers are, on average, significantly wealthier than rural dwellers, cities are magnets for materials, and the bulk of in-use stocks can be found in urban areas. Therefore, when bottom-up studies are not available (Kennedy et al. [2007] review the available information), national figures can be scaled to urban regions on the basis of per capita gross domestic product.

When Will Urban Stocks Become Available?

This question relates to how products are used and for how long. The baseline is set by the physical lives of products—a few years for electronics, a decade or so for vehicles, a few decades for industrial machinery. Other factors must also be considered, however, such as the improved utility of new products and social pressures (Allwood et al., 2010; van Nos and Cramer, 2006). In any case, it is clear that urban stocks of materials, especially large-magnitude stocks related to buildings and infrastructure, are released for possible reuse very gradually over a period of many years.

In What Forms Do Urban Stocks Exist?

This question is, perhaps, the most arresting. Unlike polyethylene in plastic bags or wooden flooring in buildings, which are used largely in pure form, metals may be present in a variety of forms, some simple, some complex. The form, which differs greatly for different metals, significantly influences recycling potential.

Table 1 lists the three predominant uses for each metal in the periodic table, accompanied by judgments as to whether, for each use, the metal is readily recoverable in (1) pure form, (2) multicomponent alloys (as in stainless steel), (3) complex assemblages (as in computer chips), or (4) dissipative forms (such as paint).

When this information is used to construct a “periodic table of recyclability” (Figure 2), it becomes immediately apparent that there are large differences. The only metals judged to be relatively easy to recycle in pure form in their principal uses are copper (used in pure form as a conductor of electricity and heat), lead (used in nearly pure form in batteries), and five precious metals (gold, silver, platinum, palladium, and rhodium).

Figure 2

Metals found predominantly in multicomponent alloys (e.g., 0.1 percent niobium in high-strength steel) are difficult, sometimes impossible, to recover. Recovering metals in complex assemblages (e.g., tantalum capacitors in electronics) is similarly challenging. Because metals in alloys and assemblages are commonly used in very small amounts, the separation process is extremely complicated. Metals in dissipative uses, of course, cannot be recovered.

The distribution of metals among the four groups is: only 12 percent in the easy to recycle group; 46 percent in the recyclable only as alloy group; 33 percent in the complex and unlikely to be recycled group; and 9 percent in the predominantly dissipative group. Thus materials scientists are facing a major challenge, which, so far, they have not addressed. The challenge is to invent and/or prescribe forms of materials that support a high level of product performance while simultaneously maintaining significant recycling potential. Until this challenge is met, the “urban mining” of almost all metals will remain problematic.

The Urban Mining Stock

Abandoned, Comatose, and Hibernating Stock

Not all metal in urban mines is destined to become available for reuse, or at least not for a very long time. For example, abandoned stock (e.g., port revetments, skyscraper pilings, etc.), is material used in a way that makes recovery difficult, expensive, and sometimes essentially impossible. Tanikawa and Hashimoto (2009) have shown that the amount of iron in abandoned stock in Japan is nearly equal to the amount providing above ground services.

A second example can be called comatose stock (e.g., obsolete copper conductors in buried cables). Like humans to whom the term comatose is applied, recovery of these materials is possible in theory, but for various reasons (e.g., difficulty of locating and retrieving it from buried cables, plus unfavorable economics) recovery is unlikely.

A third category is hibernating stock, material now asleep (not performing a useful service) that might someday wake up (e.g., an old cell phone in a drawer).

The Recycling Sequence

For the present discussion, I will not address problems raised by the three categories of material listed above. Instead, I will focus on the recycling sequence, using the example of a metal-containing product that has been discarded.

The common perception of recycling—if you drop an item into the right bin, good things will happen—does not take into account many of the social, political, and technological aspects of the recycling sequence. Collection is indeed vital to success, but collection is only the first step in recycling. Unless the discard is that rare item that consists entirely of a single material (e.g., a piece of copper pipe), the materials in the item must be separated from each other.

This happens with varying degrees of efficiency. For example, when an automobile goes through a shredder at a recycling yard, rivets, adhesives, and other “heroic joining” materials can make separation a challenge (van Schaik et al., 2004). Once separated, the materials must be sorted, a task performed (again with varying degrees of efficiency) using magnets, flotation, eddy-current separation, and a host of other technologies.

Only after the collection and separation steps have been completed can materials be moved to the final stage of metallurgical separation in which individual metals or alloys are reprocessed for reuse. This step may require a very high level of technology and capital investment. For example, state-of-the-art reprocessing of electronic circuit boards can be done in only a handful of facilities worldwide.

Figure 3

Figure 3 shows the recycling sequence and efficiencies typical of modern technology for electronics (some other product groups have higher efficiencies, some lower). The overall system efficiency is the product of the efficiencies of each step. In this typical scenario, only about one-quarter of the metal discarded in products actually ends up as recycled metal.

The sequence is further complicated because different players, many of whose roles are not well understood by outsiders, are responsible for each step in the process. It is easy to understand that collection is crucial, and if it is not carried out successfully, all else is of minimal benefit. Municipalities, the main actors at this stage, oversee regular pickups of discarded appliances, electronics, and other consumer products.

Next are demolition contractors, scrap yards, agglomerators, and metal marketers, who recover or trade many different kinds of discarded metal-containing products. Contractors perform with varying degrees of efficiency and are generally subject to little external oversight.

In some parts of the world, recycling is informal. For base metals (e.g., steel and copper) this approach probably works at least as well as the more structured approaches of highly developed countries. However, for metals in complex, highly integrated products, the informal approach is extremely inefficient.

Thoughts on the Potential of Urban Mining

In the introduction to this article, I posed three questions: What are the successes of urban mining? What are its challenges? Does urban mining matter, or could it? It should be clear at this point that the successes to date are not very significant or exciting, and that enormous challenges remain. Nevertheless, urban mining does matter. Every kilogram recovered and reused displaces a kilogram that must be mined and processed, with all the environmental, social, and economic implications those actions entail.

A conundrum is that contemporary product design is increasingly at odds with effective urban mining. The mixing of materials in products is increasing, and the improved performance that results is achieved at the expense of efficient urban mining (Dahmus and Gutowski, 2007). Clearly, product designers are crucial, if invisible and unrecognized, actors in the recycling chain. Good designs (and some certainly exist) can markedly improve recycling potential, and thoughtless designs from the end-of-life standpoint can effectively nullify it.

Urban mining today is rather successful when the economic incentives are high (e.g., gold in jewelry, rhenium in gas turbine blades, lead in batteries). However, absent economic incentives or other imperatives, our society seems likely to persist in mining and processing virgin metals, using them once or twice, and letting them dissipate back into the environment. This is not Jane Jacobs’ vision, nor is it ultimately sustainable.

We can do better as a society, but only if every participant in the chain—product designer, customer, collector, sorter, and processor—realizes that urban mining is about sustaining the resources on which all of technology depends, whether or not it is economical at the present time. Metals are gifts from the stars that were generated over billions of years; we should treat them with the awe and respect they deserve and devise ways to recycle them over and over. Only then will sustainability become a reality.

References

Allwood, J.M., M.F. Ashby, T.G. Gutowski, and E. Worrell. 2010. Material efficiency: a white paper. Resources, Conservation, and Recycling 55(2011): 362–381. DOI: 10.1016/j.resconrec.2010.11.002.

Binder, C.R., T.E. Graedel, and B.K. Reck. 2006. Explanatory variables for per capita stocks and flows of copper and zinc: a comparative statistical analysis. Journal of Industrial Ecology 10(1–2): 111–132.

Butler, J. 2010. Platinum 2010 Interim Review. London, U.K.: Johnson Matthey Public Limited Company.

Dahmus, J.B., and T.G. Gutowski. 2007. What gets recycled: an information theory based model for product recycling. Environmental Science & Technology 41: 7543–7550.

Du, X., and T.E. Graedel. 2011. Global in-use stocks of the rare earth elements: a first estimate. Environmental Science & Technology 45, in press.

Graedel, T.E., and J. Cao. 2010. Metal spectra as indicators of development. Proceedings of the National Academy of Sciences of the United States 107: 20905–20910.

Hagelüken, C. 2009. Recycling of Technology Metals—Opportunities and Limits. Presentation at R’09 Twin World Congress on Resource Sustainability, Davos, Switzerland, September 20, 2009.

IAI (International Aluminium Association). 2006, Global Aluminium Recycling: A Cornerstone of Sustainable Development. London: IAI.

IMOA (International Molybdenum Association). 2010. Molydenum Uses. Available online at http://www.imoa.info/moly_uses/molybdenum_uses.html.  Accessed Jan. 17, 2011.

Jacobs, J. 1961. The Death and Life of Great American Cities. New York: Random House.

Johnson, J., L. Schewel, and T.E. Graedel. 2006. The contemporary anthropogenic chromium cycle. Environmental Science & Technology 40: 7060–7069.

Kennedy, C., J. Cuddihy, and J. Engel-Yan. 2007. The changing metabolism of cities. Journal of Industrial Ecology 11(2): 43–59.

Kapur, A., and T.E. Graedel. 2006. Copper mines above and below the ground. Environmental Science & Technology 40: 3135–3141.

Kirk-Othmer. 2009. Selenium and Selenium Compounds, and Tellurium and Tellurium Compounds. Kirk-Othmer Encyclopedia of Chemical Technology. New York: Wiley-Interscience.

Mao, J.S., J. Dong, and T.E. Graedel. 2008. The multilevel cycle of anthropogenic lead. I. Methodology. Resources, Conservation, and Recycling 52: 1058–1064.

Matos, G.R., J.D. Jorgenson, and M.W. George. 2005. Historical statistics for mineral and material commodities in the United States. USGS Data Series 140, Reston, Va.

Müller, D.B., T. Wang, and B. Duval. 2011. Patterns of iron use in societal evolution. Environmental Science & Technology 45: 182–188.

Nakajima, K., K. Yokoyama, and T. Nagasaka. 2008. Substance flow analysis of manganese associated with iron and steel flow in Japan. ISIJ International 48: 549–553.

Nassar, N. 2010. Private communication, Yale University.

Perron, L. 2001. Vanadium. In Canadian Minerals Yearbook 2001, Natural Resources Mineral and Metal Commodity Reviews. Available online at http://www.nrcan.gc.ca/mms/cmy/content/2001/65.pdf.

Reck, B.K., D.B. Müller, K. Rostkowski, and T.E. Graedel. 2008. Anthropogenic nickel cycle: insights into use, trade, and recycling. Environmental Science & Technology 42: 3394–3400.

Silver Institute. 2011. Silver uses. Available online at www.silverinstitute.org/silver_uses.php. Accessed Jan. 17, 2011.

SQM (Sociedad Quimica y Minera de Chile, S.A.). 2007. SQM 2007 Annual Report. Available online at http://www.sqm.com/PDF/Investors/AnnualReport/SQM_Annual_ Report_227_EN.pdf. Accessed 4-12-2010.

Tanikawa, H., and S. Hashimoto. 2009. Urban stock over time: spatial material stock analysis using 4d-GIS. Building Research and Information 37: 483–502.

Tin Technology Limited. 2006. New ITRI study illustrates the reasons behind continued boom in tin use. St. Albans, U.K.: ITRI Innovation.

TZ Mineral International. 2007. Mineral Sands Annual Review. Perth, Australia: TZ Mineral International Pty Ltd.

USGS (U.S. Geological Survey). 2004. Minerals Yearbook, 2007. Reston, Va.: USGS.

USGS. 2005. Minerals Yearbook, 2007. Reston, Va.: USGS.

USGS. 2007. Minerals Yearbook, 2007. Reston, Va.: USGS.

USGS. 2009. Mineral Commodity Summaries, 2009. Reston, Va.: USGS.

USGS. 2010. Mineral Commodity Summaries, 2010. Reston, Va.: USGS.

Van Nos, N., and J. Cramer. 2006. Product lifetime optimization: a challenging strategy towards more sustainable consumption patterns. Journal of Cleaner Production 14: 1307–1318.

Van Schaik, A., M.A. Reuter, and K. Heiskanen. 2004. The influence of particle size reduction and liberation on the recycling rate of end-of-life vehicles. Minerals Engineering 17: 331–347.

Wang, T., D.B. Müller, and T.E. Graedel. 2007. Forging the anthropogenic iron cycle. Environmental Science & Technology 41: 5120–5129.

  

TABLES

 

TABLE 1   Principal Uses (by percentage of total use) and Recycling Potential of Metals in the Periodic Table

Metal

 

Principal Uses#

 

Reference

 

Li

 

Batteries (25,b), glasses (18,c), greases (12,d)

 

SQM, 2007

 

Be

 

Electronics (60,c), aerospace (20,c), appliances (10,c)

 

USGS, 2010

 

B

 

Ceramics (76,c), soaps (5,d), agriculture (4,d)

 

USGS, 2010

 

Mg

 

Refractories (40,d), chemicals (37,d), Al alloys (16,b)

 

USGS, 2010*

 

Al

 

Transport (28,b), buildings (25,b), electrical (18,a)

 

IAI, 2006

 

Sc

 

Al alloys (85?,b), lamps (?,c), electronics (?,c)

 

USGS, 2010

 

Ti

 

Pigments (94,d), aerospace (4,b)

 

USGS, 2010

 

V

 

Steels (85,b), Ti-Al alloys (10,b), chemicals (5,d)

 

Perron, 2001

 

Cr

 

Metal goods (30,b), buildings (25,b), indust. machinery (25,b)

 

Johnson et al., 2006

 

Mn

 

Steel (96,b), Al alloys (1,b)

 

Nakajima et al., 2008

 

Fe

 

Construction (45,b), transport (24,b), indust. machinery (20,b)

 

Wang et al., 2007

 

Co

 

Superalloys (49,b), chemicals (27,c), metallurgy (15,b)

 

USGS, 2010

 

Ni

 

Stainless steel (68,b), superalloys (11,b), plating (6,b)

 

Reck et al., 2008

 

Cu

 

Buildings (50,a), electronics (21,a), transport (11,a)

 

USGS, 2010

 

Zn

 

Galvanizing (55,b), Zn alloys (21,b), brass/bronze (16,b)

 

USGS, 2010

 

Ga

 

Electronics (67,c), optoelectronics (31,c)

 

USGS, 2010

 

Ge

 

Fiber optics (35,c), infrared optics (30,c), catalysts (15,c)

 

USGS, 2010

 

As

 

Wood preservatives (50,d), batteries (?,d), electronics (?,c)

 

USGS, 2007

 

Se

 

Glass (25,d), metallurgy (22,b), agriculture (19.d)

 

Kirk-Othmer, 2009

 

Sr

 

Pyrotechnics (30,d), magnets (30,b), alloys (10,b)

 

USGS, 2010

 

Y

 

Lighting (45,c), flat panel displays (33,c), glass additives (12,c)

 

Du and Graedel, 2011

 

Zr

 

Ceramics (54,d), refractories (13,d), metallurgy (13,b)

 

TZ Minerals Int’l, 2007

 

Nb

 

Steels (76,b), superalloys (24,b)

 

USGS, 2010

 

Mo

 

Steels (50,b), stainless steel (25,b), chemicals (14,d)

 

IMOA, 2010

 

Ru

 

Electrical (59,c), chemical (20,d), electrochemical (14,c)

 

Butler, 2010

 

Rh

 

Autocatalyst (86,a), chemical (8,c), glass (4,c)

 

Butler, 2010

 

Pd

 

Autocatalyst (54,a), electrical (17,c), jewelry (12,a)

 

Butler, 2010

 

Ag

 

Industrial (57,c), jewelry (20,b), photography (16,a)

 

Silver Institute, 2011

 

Cd

 

Batteries (83,b), pigments (8,d), platings (7,b)

 

USGS, 2009

 

In

 

Monitors (33,c), TV (24,c), computers (15,c)

 

Matos et al., 2005

 

Sn

 

Electrical (50,b), cans (18,b), chemicals (14,c)

 

Tin Technology Ltd., 2006

 

Sb

 

Flame retardant (40,d), transport (22,b), chemicals (14,c)

 

USGS, 2010

 

Te

 

Metallurgy (60,b), chemicals (25,d), electrical (8,c)

 

Kirk-Othmer, 2009

 

La

 

Catalysts (30,b), metallurgy (22,b), batteries (14,b)

 

Du and Graedel, 2011

 

Ce

 

Autocatalyst (35,b), metallurgy (31,b), glass additives (16,c)

 

Du and Graedel, 2011

 

Pr

 

Computers (27,b), audio systems (21,b), wind turbines (12,b)

 

Du and Graedel, 2011

 

Nd

 

Computers (29,b), audio systems (22,b), wind turbines (13,b)

 

Du and Graedel, 2011

 

Sm

 

Defense applications (70,b), batteries (30,b)

 

Du and Graedel, 2011

 

Eu

 

Lighting (50,c), flat panel displays (33,c), plasma (12,c)

 

Du and Graedel, 2011

 

Gd

 

Computers (32,b), audio systems (25,b), wind turbines (15,b)

 

Du and Graedel, 2011

 

Tb

 

Lighting (27,c), flat panel displays (20,c), computers (15,c)

 

Du and Graedel, 2011

 

Dy

 

Computers (33,b), audio systems (26,b), wind turbines (15,b)

 

Du and Graedel, 2011

 

Ho

 

Magnetics (100,b)

 

Du and Graedel, 2011

 

Er

 

Fiber optics (75?,c), lasers (20?,c), optical glass (5?,c)

 

Du and Graedel, 2011

 

Tm

 

X-ray (75?,c), lasers (20?,c), electronics (5?,c)

 

Du and Graedel, 2011

 

Yb

 

X-ray (75?,c), lasers (20?,c), electronics (5?,c)

 

Du and Graedel, 2011

 

Lu

 

Electronics (80?,c), medical (20?,c)

 

Du and Graedel, 2011

 

Hf

 

Superalloys (40?,b), nuclear power (30?,a), electronics (5?,c)

 

USGS, 2009*

 

Ta

 

Capacitors (68,c), other electronics (11,c), superalloys (8,b)

 

Nassar, 2010

 

W

 

Cutting tools (60), metallurgy (15?,b), superalloys (10?,b)

 

USGS, 2010

 

Re

 

Superalloys (77,b), catalysts (15,a), crucibles (8,a)

 

USGS, 2008

 

Ir

 

Electrochemicals (25,c), chemical (21,c), electrical (15,c)

 

Butler, 2010

 

Pt

 

Autocatalyst (46,a), jewelry (26,a), chemical (5,d)

 

Butler, 2010

 

Au

 

Jewelry (72,a), electronics (7,c), dental (2,b)

 

USGS, 2010

 

Hg

 

Mining (75?,d), medicine (10?,a), lighting (10?,c)

 

USGS, 2010*

 

Tl

 

Medicine (40?,d), radiation detection (30?,c)

 

USGS, 2010*

 

Pb

 

Batteries (75,a), pipe and sheet (5,a), cable sheathing (2,a)

 

Mao et al., 2008

 

Bi

 

Metallurgical (45,b), alloys (29,b), chemicals (25,d)

 

USGS, 2010

 

# (a) Largely existing or readily recoverable in pure form; (b) largely in multicomponent alloy form; (c) largely in complex assemblages; (d) largely dissipative uses.

* Use percentages are estimated in the present work, based on the cited text information.

 

 

About the Author:T.E. Graedel is Clifton R. Musser Professor of Industrial Ecology, Center for Industrial Ecology, Yale University, and an NAE member.