S

elements from ores usually involves two steps (Kesler 1994; Norgate, this volume). In the first step, known as beneficiation, the ore is broken physically into small fragments (crushing and grinding) to release the ore mineral, which is collected into a "concentrate." Waste from this process, known as tailings, contains small amounts of the desired ore mineral that could not be recovered economically; they might be reprocessed as demand and prices rise. Despite best efforts, concentrates are also impure. For instance, although copper makes up about 35% by weight of the common ore mineral chalcopyrite (CuFeS2), most chalcopyrite concentrates formed by beneficiation contain less than 30% copper with the rest consisting of waste minerals, usually attached to the chalcopyrite. Some ores are so fine-grained that they cannot be processed to make a pure concentrate. Exploitation of the large Pb-Zn-Ag HYC deposit at McArthur River, Australia, was delayed for decades because the lead and zinc minerals were so finely intergrown that they could not be separated into two different lead and zinc concentrates.

The second step, which involves extractive metallurgy and is commonly known as extraction, breaks the chemical bonds in the ore mineral to release one or more of its constituent elements. This step requires much more energy and is usually done by smelting, which uses heat to drive off sulfur as SO2 from sulfide ore minerals such as chalcopyrite (CuFeS2) or oxygen (as CO2, by combining with carbon in coke that was added during the process) from oxide ore minerals such as hematite (Fe2O3). Elements can also be liberated by dissolving the ore mineral. If the solvent is sufficiently selective (dissolves only the ore mineral and not the waste minerals), it can be used directly on the ore. For example, dilute solutions of acid and cyanide can be used to recover copper and gold, respectively, directly from ores. Use of selective-leach processing directly on ore (rather than concentrate) lowers the minimum grade that can be treated, thus expanding the resource.

Almost all ores contain one or more major elements, such as iron, copper, lead, zinc, or nickel, which are present in relatively high concentrations and usually form their own minerals (the ore minerals). These ores also contain other minor or trace elements, such as cadmium, cobalt, hafnium, indium, and scandium, which are present in relatively low concentrations and commonly do not form ore minerals and, instead, substitute for major elements in ore minerals. Elements with intermediate abundances such as gold, silver, and platinum can be mined from deposits of their own but are also recovered from deposits of copper, lead-zinc, and nickel, respectively, where they constitute important trace elements. The degree to which trace and intermediate-abundance elements are recoverable from major element ores varies greatly and depends on their mineralogical residence in the ore and the response of this mineral to beneficiation and extraction (Reuter and van Schaik 2008b; Hageluken and Meskers, this volume).

Flows of Mineral Resources

Insight into the movement of mineral-sourced commodities in society can be derived from substance (or material) flow analysis. Studies of this type identify major reservoirs and attempt to determine the flow (flux) of material among various reservoirs. In terms of understanding long-term demand, the most important stocks are the in-use material and the waste material, and the most important flow is the amount of material that is recycled. Minimizing the size and residence time of material in the waste management stock, usually through better recycling, is the most effective way to increase sustainability. All parts of the waste management stock have some limitations on recycling; end-of-life automobiles are a good source of steel but a poor source of trace elements, and tailings from beneficiation of many ores are too low grade to be re-treated. Even under optimal circumstances, however, recycling from end-of-life stocks is highly inefficient (Reuter, this volume; Reuter and van Schaik 2008b). For individual mineral commodities, substitution can play a role similar to that of recycling. Wellmer (2008) has pointed out that our actual need for mineral resources is for the services that they provide and not specifically for the minerals. Thus, if sufficient substitutes can be found, shortages can be delayed signifi cantly. In making substitutes, it is important not to select mineral resources that are less abundant (Graedel 2002). Two commodities with very different resource configurations, cement and copper, provide examples of the current situation.

Cement is produced by heating limestone, silica and other raw materials, which are mined with little or no waste except for small amounts of overburden that cover limestone in some areas. Kapur et al. (2009) have shown that 82-87% of the cement produced in the United States, almost all of which was produced since 1900, is still in use and that this amounts to a per-capita, in-use stock of almost 15 Mg. The growth rate of per-capita, in-use stocks of cement has slowed consistently through time, but is still greater than the rate of population increase. Recycling is limited and consists largely of downcycling of broken cement for use as aggregate. Remaining supplies of cement raw materials are enormous and can be considered essentially unlimited, although processing them to make cement requires enough energy to encourage efforts to develop products with a longer useable life (Kapur et al. 2009). Many other mineral resources that are used in rock or mineral form, such as aggregate, gypsum, and salt, are similar, with large to almost unlimited supplies, small to no recycling, and continued increases in use.

At the other end of the spectrum are most of the mineral resources that are used in elemental form, particularly metals such as copper. Production of copper involves mining, much of which is carried out in open pit mines that remove large volumes of waste rock, and extensive beneficiation and extraction that produce additional large volumes of wastes. For porphyry copper deposits, which are the most common source of copper, the volume of copper metal that is produced is considerably less than 1% of the volume of waste rock, tailings, and smelter slag. Only 43% of the copper that has been mined since 1900 in North America remains in use, whereas 18% was lost during extraction (mine tailings and production wastes) and another 34% was lost to postconsumer waste (landfills) (Spatari et al. 2005; Gordon et al. 2006). Approximately 40% of the copper that has been extracted has been recycled, largely into similar uses rather than downcycling. The per-capita, in-use stock of copper, currently about 0.17 tons per capita, has grown continuously since 1900; although growth slowed somewhat between the late 1940s and 1999, it appears to have increased again. Remaining supplies of copper ore are small relative to anticipated demand (Gordon et al. 2006), and similar stocks and flows characterize most other metals.

Recycling of mineral commodities varies tremendously from commodity to commodity (Table 7.1) for a wide range of reasons. Many of the commodities used in rock or mineral form (including phosphate, potash and nitrate fertilizers, road salt, graphite in pencils and lubricants, and feldspar in abrasives) are dispersed into the environment so completely in their present mode of consumption that they essentially return to the lithosphere as trace constituents. Others (e.g., clays in construction materials and ceramics, and gypsum in plaster and wallboard) undergo mineralogical changes during processing that render them unsuitable for reuse in their original markets. Most other rock and mineral commodities are simply too inexpensive to justify an effort to recycle them in today's economy, even where it might be feasible, although industrial diamonds are an obvious exception. Recycling of mineral commodities used in elemental form is considerably greater, but still surprisingly variable (Table 7.1). There are three main constraints on recycling of mineral resources that are used as elements. First, even where they are used in pure form, many of the commodities make up such a small part of their host unit that costs of retrieval are too high. Second, many other uses involve alloys and composite materials that effectively degrade the product making it useful only for downcycling applications. Finally, recovery of trace elements during recycling depends in part on the path or element that they will follow through the processing (Reuter, this volume; Reuter and van Schaik 2008b).

Estimates of long-term demand are also influenced by suggestions that countries will gradually dematerialize their economies as they evolve from manufacturing to services (Cleveland and Ruth 1999). In its simplest formulation, this means that use of mineral resources increases early in the history of a country but levels off and decreases later as material is used and recycled more effectively. Resulting plots of per-capita (annual) metal consumption versus per-capita GDP should have the shape of an inverted U, which has been called the environmental Kuznet curve. Evaluations of the validity of this relationship have produced conflicting results. Guzman et al. (2005) found that trends in copper consumption in Japan between 1960 and 2000 showed a decline typical of the down-going limb of a Kuznet curve. In a wider review of the relation in numerous countries, Cleveland and Ruth (1999) and Bringezu et al. (2004) found only a few examples that supported the relationship. Regardless of their relation to GDP, mineral resources will be critically important to society for the foreseeable future, and therefore estimates of remaining stocks are of wide interest.

Stocks of Mineral Deposits

The Reserve Base

Earth's stock of conventional mineral deposits includes material ranging from currently operating mines to deposits that have not yet been discovered and from conventional deposits to deposits of types that are not yet recognized. The most widely used system for classifying these is provided by the USGS (2007). In this classification, mineral resources are considered to be any concentration of material that can be extracted economically now or in the future (Figure 7.1). The key word here is "concentration"; in ores, concentration is often referred to as grade or ore grade. The term "economically" is not a key word because of the additional phrase "in the future," which allows for changes in cost structure that would permit exploitation of even very low grade material. Any body of rock that contains the commodity of interest in a concentration that is greater than its background concentration (average concentration in average rock) is part of the global resource. To stretch the point a bit, shoshonitic volcanic rocks in Puerto Rico and British Columbia, which contain about 200 ppm Cu (almost ten times the average crustal abundance for igneous rock; Kesler 1997), are mineral resources even though they are not likely to be exploitable economically at any time in the near future. In special situations, even enriched parts of the hydrosphere, such as saline lakes enriched in lithium or boron, might be

Cumulative production

Identified resources

Undiscovered resources

Demonstrated

Inferred

Probability range

Measured | Indicated

Hypothetical (°,r) Speculative

Economic

| Reserves e s

Inferred reserves

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