The Prospects for Urban Mining

 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.

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