Metal Stocks in Society â Scientific Synthesis - UNEP

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Metal Stocks in Society – Scientific SynthesisFigure 1.The genericlife cycle ofa metal, withstock locationsindicated:1,metal in virginore bodies;2,metal in tailings;3,metal inprocessorstockpiles;2. The Concept ofStocksOne aspect of the availability of metals concernsthe natural (or virgin) stocks of metals:those deposited by geological processes inconcentrations suitable for being extractedand processed, now and in the future. Thetotal amounts of metals in such depositsare difficult to quantify accurately, but globalestimates are publicly reported (e. g., USGS,2008). This information is important to evaluationsof resource sustainability, but is notthe subject of this report and so is not treatedfurther herein.Import / Exportitself, fabricated into products, and thenput into use. During processing, waste rockand impurities containing small amounts ofthe target metals are deposited in “tailingsponds”. Some metal is held in stockpilesby processors, fabricators, and sometimesby governments. Eventually, the product isdiscarded, perhaps to recycling, perhaps lostto the environment. The metal in one form oranother moves rapidly through most of thesestages, but may stay in use for long periods– years, decades, perhaps a century or more.In some cases, such as metal in obsoleteundersea cables, the metal is no longer inuse, but not (yet?) recovered and recycled(Hashimoto et al., 2007). These “hibernating”stocks are potentially reusable, but their4,metal ingovernmentstockpiles;5,metal inmanufacturerstockpiles;Processing34Fabrication5Use6DiscardMgt.76,metal in-usestock;7,metal in recyclerstockpiles;Ore12Environment88,metal in landfillstockpiles.Emissions fromthe Use stage aresmall, and notindicated here.12A group of less well studied stocks aretermed “anthropogenic”; these are themetal stocks in society, already extracted,processed, put into use, currently providingservice, or discarded or dissipated over time.These types of metal stocks in society canbe appreciated with the help of the life-cyclediagram of Figure 1. Metal ores are minedfrom the ground, processed into the metalrecovery may well not be economicallyfeasible. All of these stocks are, in principle,the subject of the present report.Material flow analysis characterizes andquantifies flows of materials into, out of, andthrough a system of interest, equating flowsat each reservoir within the system by conservationof mass. In this analysis, the choice
Metal Stocks in Society – Scientific Synthesisof scale and level is critical (scale is “a spatial,temporal, quantitative, or analytic dimensionused to measure or study a phenomenon”[Gibson et al., 2000], as with a ruler,level is a position along the scale.) The quantityof mass of a chosen material that existswithin the system boundary of choice at aspecific time is considered stock within thesystem. In terms of units of measurement,stock is a level variable (i. e., kg), while flowis a rate variable (i. e., kg per unit of time). Ingeneral, the metal stock in society is highestby far when material is in use (rather thanin processing, fabrication, manufacturing, orwaste management).The metal portion of in-use stock can be definedin two ways. If an individual element isspecified, in-use stock of metal refers to thetotal mass of that element, regardless of itschemical form. If a metal alloy is specified,in-use stock of metal refers to the total massof that alloy (including all its constituent elements).13
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Metal Stocks in Society â Scientific Synthesis - UNEP