Mineral and mineral processing
Mineral and mineral processing
Losses to residues
Losses to tailings and not-mined material
Figure 10.5 The product life cycle. Products that link minor and major metals are depicted with combined arrows; phases where both are separate are shown using separate arrows. * In product manufacture, metal demand is governed by product design, and thus the coupling between minor and major metal has disappeared.
Losses to residues
Ore metal extraction through recycling. Separate minor and major metal streams (recyclates) are created for secondary raw materials production. To enable efficient recovery, recyclates with the proper quality are required; this depends on the quality of the EoL treatment as well as on the material combinations in the (consumer) product. Unfavorable combinations increase losses in the EoL phase and recycling. Choices in product design can thus have a lasting effect on the sustainability of material life cycles. Dedicated, high-tech metallurgical processes are usually needed to recover precious and special metals effectively; however, some loss to slag, dust, and other residues will inevitably occur. Incomplete liberation of the metal combinations during EoL treatment leads to additional losses. Mixed material particles will be treated in the recovery process for the major component(s) in the particle. If no recovery technology for the minor components is installed, these will be lost. Thus, metal production from natural resources remains necessary to fulfi ll the demand caused by life cycle losses and market growth. However, the difference of material combinations that occur in primary and secondary resources generates specific challenges for the minor metals recovery.
Let us now turn to a discussion on raw materials production and the manufacturing phase for minor metals. For further discussion on life cycles, see MacLean et al. (this volume).
Opportunities and Limits in Extraction and Manufacturing
Metal production from natural resources is fundamentally different from production from EoL materials, because the ratio between major and minor metals is fixed by the mineralogy of the ore. Minor and major metals are mined together, but the loss of each during production can differ significantly. Primary production focuses on the major metal production and demand, which leads to complex mechanisms of primary minor metal supply. The demand of minor metals is determined (a) by the materials selection during product design and (b) by the efficiency of the manufacturing processes. These factors are discussed in turn to determine the boundaries in these phases of the life cycle.
Complex Supply Mechanism for Minor Metals
Technology metals are often found together with a major metal, usually one of the base metals. The mineral assemblies or combinations and the amounts are determined by the geological circumstances under which the ore has been formed. For example, gallium is found in bauxite (aluminium ores), germanium and indium typically with zinc, and PGMs with copper and nickel (Figure 10.6).
(Ta iD ARhJ
Figure 10.6 Coupling of major and minor metal production. The figure indicates which minor metals are produced as by-products of major metals.
Typical "by-products metals" (Ge, Ga, Se, Te, In) are found in the ores of major (carrier) metals at the ppm level (Wellmer, pers. comm.). The economic driver for mining here is clearly the major metal, determined by its share in the total intrinsic value (concentration times price). By-product metals can generate additional revenue, if they can be extracted economically; in some cases, however, they are also regarded as impurities that drive up production costs. Rhenium is special as it is produced as a by-product from molybdenum, which in itself is a by-product of copper (Figure 10.6). Some elements are by-product elements in certain cases, while they are also mined as target metals on their own (e.g., cobalt, bismuth, molybdenum, gold, silver, PGMs, and tantalum). Lithium is the exception, as it is produced from brines and found as mineral (spodumene) together with tin minerals (cassiterite).
Other minor metals occur as "coupled elements" without a real carrier metal. They are often found in the same group and have to be mined and processed together (Wellmer, pers. comm.). PGM, REE,4 and tantalum-niobium can be mentioned in this context. The ratio between the coupled metals in the ore and hence the specific metal productions vary per location. This does not always correspond to the market demand, so usually one element in the group becomes the driver for production.5
Rare Earth Elements (REE): La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y.
In the case of PGMs today, Pt is the production driver, but this can change, e.g., to Pd when its demand (price) increases significantly. The concentration of Rh and Ru is a factor 10 lower than Pt and Pd, making it unlikely that they will become "production-driving metals" even with high prices, since the adverse price effect from a large overproduction of Pt and Pd would overcompensate for gains that would result from increased Rh/Ru production. With a concentration 50 times lower than Pt/Pd, Ir is a by-product in a coupled metal production.
The distribution of minor metal mines and smelters throughout the world coincides partially with the regional concentration of base metal producers, and is governed by the location of the facilities able to recover the minor metals from the major metals' process residues. Table 10.4 gives the three major-producing countries for each minor metal based on the information found in the mineral summaries of the USGS. For most special metals, the combined output from the three main producers supplies over 70% of the market. This dependency on a few countries and/or producers makes the minor metal supply sensitive to disruptions due to, for example, process problems, political instability, or environmental hazards.
Certain losses during the life cycle of metals (and products) are inevitable. If we look in more detail at the raw materials and manufacturing phases (Figure 10.5), we see that losses occur during mining and mineral processing. Mining is selective; only the ore with sufficient metal content to be economically removed is mined. Minor metals will thus be left behind. When the presence of the minor metals leads to penalties or difficulties during later processing, the incentive to mine ores with a high minor metal content could be even lower. For example, the penalties that were levied for many years on selenium in concentrates by many Cu smelters could have made it unfavorable to mine Se-rich ores. However, high demand increased the price of Se in 2003, and this decreased or even eliminated penalties. As a result, mining of ores with higher Se content became more attractive (Yukon Zinc 2005).
Further losses occur during mineral processing when the gangue material is separated from the valuable minerals. To illustrate: flotation of Co oxide ores has a Co recovery between 50-70%, hand sorting of Li minerals from gangue has a yield of 60-80% (Ullmann 2002), and the recovery of In during concentration can be about 96% (Schwarz-Schampera 2002). The exact recovery for each depends on the deposit and methods used during mineral processing. Even in a perfect process, 100% recovery will not be obtained as a trade-off is made between the grade of the material (the purity) and the recovery (the amount) for both the major and minor (by-product) metals (Wills 2006). In addition to technological factors, the economics of the process also plays a role. Usually the losses of minor metals are much higher than the losses for the major metal; this occurs, in particular, when the minor metal is not incorporated in the major metal minerals, but is present in separate grains/particles for which the main recovery process is not optimized.
The separation between minor and major metals is usually achieved through smelting and refining. These processes aim to recover most major metals and the valuable minor metals. During smelting or recovery, minor metals can be:
Table 10.4 Major producing countries (primary production) and the percentages of minor metal production in 2007. Note: for Ga and Ge, the main producers are shown, as data on the actual market share is unavailable. For Si, data is from 2005 (Flynn and Bradford 2006).
Bi Co Ga Ge In Li RE Re Se Si Ta Te Ag Au Ir Pd Pt Rh Ru
53 61 10
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