During past ten to fifteen years, the chemistry and metallurgy of pure and ultra-pure minor metals (MMs) and their compounds have been better understood. The innovative disclosures made MMs an indispensable material for production of efficient appliances required for energy, environment, defense, and social amenities.
The global demand of many MMs, especially rare earth elements (REE) alone has increased from 60 kt in the year 2000 to 180kt in 2011. As a result the gap between demand and supply of several MMs and their prices has increased. This phenomenon compelled the producers of advance appliances to re-think concerning the subject of alternative resources on one hand and conservation of metals & minerals on the other.
For conservation, it is necessary to recover/recycle the MMs used in the production of advanced commodities after end of life (EoL). In the recycling process the biggest problem is associated with the acquisition of dissipated spent-appliances from the end users.
The logical solution of the problem therefore lies in creation of awareness among the users of advanced appliances about MMs and also about the harmful impact of unsafe disposal of the spent appliances. Such awareness will not only improve the traceability & collection of dissipated spent materials for safe disposal for recovery/recycling but also create an environment for indigenous technological developments in the field of MMs.
Genesis of Minor metal Resources
Minerals are naturally occurring substances of almost definite chemical composition, physical properties & atomic structure and an ore is a type of rock which contains minerals/ metals that can be extracted by mining, providing the content of minerals or metals are economically exploitable.
From the geological point of view the genesis of MM resources could be explained on the basis of mineral sources, their transportation and traps, prevailing during mineral transformation (constitutional change). As per the above concept, the large, medium, small & minor deposits were formed when the sources, transportation and traps were adequate or inadequate.
The origin of MMs may be classified based on the source rocks orientation and subsequent deposits i.e. metals ( Pt, Pd, Cr, V, Ni etc.) are associated with igneous or mafic (SiO2-45 to 52%) and ultra-mafic (SiO2-<45%) rocks, predominant in South Africa. Similarly the metals (W, Mo, Ge, Nb, Ta, In, Sb, Bi, Te, U, REE etc.) are associated with reduced alkaline granitic rocks, predominant in China and some MMs (Li, Co, Ga) appeared from multiple sources.
In Chile Li is produced from brine and in Congo and Zambia cobalt from cobaltite (CoAsS), but the main source of the Co persists as a by-product of copper and nickel mining. The REE occurs in alkaline igneous rocks through hydrothermal ores and metamorphic skarns (silicates) & REE are mineralized in carbonatites (rich in CO3) & lateritic bauxites etc.
The placer deposits (accumulation of valuable minerals formed by gravity separation during sedimentary processes) of rivers & sea often contain ilmenite, monazite, rutile, zircon, chromite, wolframite, cassiterite etc. which contain various MMs. China is the only country which has maximum deposits of REE in the world.
According to the USGS-2008 report the global reserve of REE per country are shown in parentheses (China-57.72, CIS-13.62, US- 9.08, Australia-3.76, India-0.84, Brazil-0.05, and Malaysia-0.02 and others-14.91 % each). The important MMs are produced in the following countries shown in parentheses; [Australia-(Ta-60%, Ti-42%), Brazil-(Nb-90%), China-(REE-95%, Sb-87%, W-84%), Chile-(Li-60%), Russia-(Pd-45%), South Africa-(Cr-41%, Rh-79%, Pt-77%,V-45%), & USA - (Mo34%)].
Nomenclature of Minor Metals
The nomenclature system for various groups of metals has been in practice for so long that each group has been given a a unique name, for example light & heavy metals, precious metals, rare earth metals, ferrous & non-ferrous metals etc.. Today we often use the names based on their end uses. During the early seventies the MMs were known as “by-product metals” since many MMs are found along with the base metals like Pb, Zn & Cu ores and are co-extracted during their processing e.g. [(Se, Te are recovered from Cu & Pb and Mo-from Cu & Re from Mo), (Ge, In & Cd from Zn), (Ga & V from Al), (Ta, Nb from Sn), (Hf Zr from Rare Earth), & (Rh-from Pt)].
Several precious metals and MMs are recovered during Cu electrolysis, the metals more noble than Cu such as Ag, Au, Se, Te etc settle to the bottom of the cell as anode slime. During the primary Cu refining, slime with metals (Ag-5 to 10%, Au-0.5 to 1%, Te-2 to 4%, Se-20 to 30%, Cu-2%) is produced. During secondary Cu production from scrap, impurities like Pb, Cu, Sn and Ag are produced.
The significant cobalt bearing minerals are often associated with copper minerals and pyrite and pyrrhotite iron sulphides, and Co recovered from Cu-Co sulphide concentrates. Ga and V are extracted from Bayer liquor of alumina extraction. During hydrometallurgical extraction of Zn, the ZnO is leached with acid and the resultant liquid phase contains the zinc and the solid residue contains Pb & Ag.
Besides by-product, there are several MMs which are also recovered through the mining route as well e.g. the V minerals (vanadinite & carnotite) are mined in South Africa, China, and Russia. The Li minerals (spodumene (LiAl(SiO3)2 & lepidolite (KLi2Al(Al,Si)3O10(F,OH)2) are also mined for production of Li2CO3.
The Ti-ferrous ore (Fet 47–52%, FeO 8–9%, TiO2 14–15%, Al2O3 5–7%, SiO2 4–8%) are mined for Ti extraction. Nb & Ta ore (Tantalite) are mined for extraction of Nb and Ta. Similarly the W minerals (wolframite, scheelite) and Hg mineral (cinnabar), Sb mineral (stibnite) etc. are also recovered by mining. The REE minerals Monazite (RE PO4), Bastnaesite (fluoro-carbonate of La & Ce), Xenotime (YPO4), RE- ion-adsorption clays (heavy-REE and Y) are also recovered through mining.
To avoid the contradiction over the nomenclature and also to symbolize the host of MMs the traditional term “Byproduct metals” was gradually replaced by the term “Minor metals”, in most transactions but not absolutely.
Characterization of Minor Metals
In the periodic table the elements are arranged in various groups according to their respective physical & chemical properties and termed as: alkali, alkaline earth, transition metals, metalloids etc., but in trade & industries the terminologies like “ferrous / non-ferrous, base / precious, “major / minor” metals are used.
The higher quantitative availability of Fe & Al makes them the major metals in comparison to the remaining minor metals. The former are relatively abundant in the earth’s crust and economically produced in million tons, comparing against the minor metals which are less abundant and produced merely in tons and kilo tons per year. Literally the term minor is a denotation of the “state of being lesser,” in comparison to major and it is true for all MMs.
The eminent MM observer Jack Lifton, in his article “The age of the technology metals” (Resource investor.com, 2008/Sept & 2009/Jan) suggested that “there are few or no permanently minor metals, only metals the uses for which go from minor to major” and thus may be termed as: electronic, power, performance & structural metals. Subsequently he suggested “break them (MMs) into two groups: (as by - product metals and primary minor metals). As a result, in further investigations, the following definitions for MMs were found in various publications.
According to Mine Fund bulletin - 2011 (Alpha Fund, LLC) “minor metals are so described not because they are unimportant, but because their productions is relatively small and they are usually by-products of other metal. They are also defined by their comparatively specialized uses and difficulty in refining for manufacturing.” The World Finance report (April 2011) “steel and aluminium are two prime major metals and are found in almost every area of manufacturing and construction. By definition MMs have lower turnover value than their more weighty peers in the major sector, although the unit value of the MM may be high.”
As a result of Metal bulletin “MMs are non-ferrous metals of smaller volume of international trade and frequently rated higher value than base metals and often obtained as by-products.” To ascertain the quantitative limits of smaller volume of MM production an arbitrary limit thought to be appropriate, came into existence.
The other terminologies used for MMs because of their circumstantial expositions, are often called “strategic & critical”. The materials required for defense are “strategic”, while those, for which a threat to supply could be involved, are “critical”. Besides these, some more terminologies like “peak, exotic, technology, specialty and green minor metals” etc. are also used to symbolize their end uses. An example can be seen in Graedel et al, in “Recycling Rates of Metals” (A status report-UNEP-2011) used the term specialty metal for MMs particularly for recycling prospects, similarly UNEP DTIE SCP publication labeled the MMs like In, Ge, Ta, PGM, Co, Ga, RE metals etc. as “green MMs”.
In the absence of universal terminology for MMs, often various terminologies based on their specific applications are used, which are synonymous and overlapping each other and as a result, some terminological ambiguity persists. To provide a universal nomenclature, it would be fair to define MMs as; “a group of non-ferrous primary metals which are produced comparatively in fewer quantities and considered more valuable than major metals. Each element is selectively chosen from alkali & alkaline earth metals, transition metals and metalloids.” This definition indicates that the metals are selectively chosen from several primary non-ferrous metals by preference as essential elements for advanced technological applications and trading.