Natural Resources

There are four main regions or “spheres” from which raw materials are taken (Figure 6). These are the lithosphere (crust), the biosphere (plants, animals, bacteria), the hydrosphere (oceans and freshwater) and the atmosphere. Metals extraction relies almost entirely on the lithosphere for metal bearing raw materials. However, the industry also relies heavily upon each of the other spheres: the hydrosphere for water; the atmosphere for air, or oxygen (concentrated from the air), and; the biosphere, e.g. animal proteins and carbohydrates as additives in metal processing and bacteria used in biological leaching of sulfide ores. It is intriguing to ponder the astounding differentiation between metal compounds in the earth’s crust and the oceans. Only three metal ions are found in the oceans at any appreciable concentration: Na+, 0.47%; Mg+2 0.053%, and; Ca+2, 0.010%. Some very valuable metals occur at very low concentrations in the oceans, e.g. gold at ~1 μg/L (~1 part per billion), but as yet there are no economic methods of extracting these. Note too that the lithosphere includes that roughly ¾ of the Earth’s surface which is covered by the oceans. This is largely unexplored. There is interest in this, but land-based mineral resources are still plentiful enough and economically much more viable. Oil and gas are notable exceptions.

Figure 6. Main terrestrial sources of raw materials.

How minerals are distributed in the earth’s crust is crucially important in the business of metal extraction. There are three aspects to this. One is the natural abundance of the element in the Earth’s crust. This is the average concentration in the lithosphere. A second is the occurrence of mineral deposits of the elements. Often these contain the pertinent minerals at considerably higher concentration than the average in the crust overall. It is fortunate that the elements are often not uniformly distributed. (Some elements are more evenly dispersed on average.) The third factor is that the mineral deposits must be able to be economically produced. The metals need to be present at suitable concentrations and in mineral forms that can be processed. Numerous factors go into the economic viability of a mineral deposit. To illustrate, copper has a natural abundance of 0.0068% by weight (68 g Cu per tonne of rock). But an economically producible copper mineral deposit would have contained around 1.8% Cu 100 years ago, whereas today a deposit with as little as 0.5% Cu could be economic to produce.

Table 1 shows the natural abundances of many commercially important metals and elements in the lithosphere. Most of the Earth’s crust is composed of oxygen (45.5%), silicon (27.2%), aluminum (8.3%), iron (6.2%), calcium, magnesium, sodium and potassium; these 8 elements comprise about 99% of the earth’s crust! Most of the transition metals and the other heavier metals are either rare or very rare, with a few exceptions (iron, titanium and manganese – among the earlier of the transition metals). Natural abundances are also illustrated in the form of a periodic table in Figure 7. Note that often the lighter elements are the more common, while the heaviest transition metals are relatively rare. Indeed the platinum group metals, gold and silver are among the very rarest elements. There are exceptions to the general rule. The lanthanides (also called the rare earths) are not actually all that rare, compared to some of the less abundant transition metals.

As mentioned previously, many elements are concentrated in mineral deposits. The major types of minerals are listed in Table 2. Where a mineral is a compound, it is commonly thought of as formally a combination of cations and anions. However, the nature of the bonds between the atoms is often a lot more complex than this. Components of ionic, covalent and metallic bonds, as well as weaker bonding interactions, like hydrogen bonding, may be involved. Hence one usually cannot regard minerals as actually being salts. PbS, for example, is formally comprised of Pb+2 and S2-, but in reality it is not composed of discrete cations and anions.

Table 1. Natural abundances in percent by weight of many commercially important metals and some non-metals in the Earth’s crust.

Li Alkali Metals Ni 0.099 Transition Metals
Na Pd 1.5x10-6
K Pt 1x10-6
Be Alkaline Earths Cu 0.0068
Mg Ag 8x10-6
Ca Au 4x10-7
Sc Transition Metals Zn 0.0076
Y Cd 1.6x10-5
La Hg 8x10-6
Ti Al 8.3 Groups III-VI
Zr Ga 0.0019
Hf In 2.4x10-5
V Tl 7x10-5
Nb Sn 0.00021
Ta Pb 0.0013
Cr Bi 8x10-7
Mo Lanthanides 8-660x10-5
W Th 0.00081 Actinides
Mn U 0.00023
Re Si 27.2 Semiconductors
Fe Ge 0.00015
Ru As 0.00018
Os Sb 2x10-5
Co Te 1x10-7
Rh Se 5x10-6
Ir S 0.034 Non-metals

Figure 7. Natural abundances of elements in the lithosphere.

Table 2. Classification of minerals according to their formally anionic components.

Mineral class Anion
Sulfides S2-, S22- (i.e. a polysulfide)
Selenides, tellurides Se2-, Te2- (rarer than sulfides)
Oxides/hydroxides O2-/OH-(common)
Halides Cl-, F-; rarely Br-, I-
Carbonates CO32-
Sulfates SO42-
Nitrates NO3-
Phosphates PO43-
Arsenates AsO43-
Silicates SiO44-, Si2O76-, many others (common)
Aluminosilicates [Alw(OH)xSiyOz]n- (numerous)
Chromates CrO42-
Molybdates MoO42-
Tungstates WO42-
Vanadates VO43-
Borates B(OH)4-, B4O5(OH)42-, etc. )others

The formalism is useful for classification purposes and for understanding the chemistry. Minerals are usually classified by the formally anionic components since there are many metals that form precipitates with the same anion, for instance there are many metal sulfides, such as Cu2S, CuFeS2, PbS, ZnS, etc. In many cases the metal values reside in the formally cationic parts of the minerals, e.g. CuS (Cu+2/S2-), AlO(OH) (Al+3/O2-,OH), and PbCO3 (Pb+2/CO32-). But in some cases the metal of interest resides in the formally anionic part as complex ions, e.g. CaWO4 (Ca+2/WO42-). Oxides/hydroxides, silicates/aluminosilicates and sulfides are the most commonly mined minerals. (There are also other classes of minerals which, in the context of metal production, are not commonly produced.)

A sample list of some of the more important minerals is shown below in
Table 3. Some rare and valuable metals may be disseminated in a variety of mineral matrices, for example gold may be hosted in quartz or pyrite or arsenopyrite etc. In some instances there are now few available ores of some metals (e.g. silver). In such cases the main means of production is as a by-product of another metal’s production. A partial list of applicable leaching systems for treating the mineral, or conditions under which it may at least partially be dissolved are also indicated in the table. Note the preponderance of acid, most often sulfuric acid (the cheapest of the “mineral acids”).

There are vastly more minerals than listed here, but this gives an indication of some of the more important ones. New minerals are still being discovered. Natural deposits may contain a diverse mixture of minerals, some at very low concentrations, which may have an impact on the extraction process. Thorough study of the mineralogy of an ore is crucial to a successful process. Many a plant has been doomed due to inattention to mineralogy.

In addition to minerals that contain metals of interest, ores inevitably contain gangue minerals. These have no commercial value in the context of the metal extraction process of interest. Very often the gangue constitutes by far the major fraction of an ore. This does not mean that some of the associated metals are not valuable. But, they cannot be economically processed from the given ore. For example, pyrite contains iron, the basis for steel, but there is not economically viable means to make iron from pyrite processed in the context of hydrometallurgy. Common gangue minerals and their reactivity are listed in Table 4.
Gangue minerals require careful attention. They may cause excessive consumption of costly reagents. They may host valuable minerals (e.g. fine crystals within gangue minerals, or chemical substituted within the gangue crystals, e.g. Ni in nickeliferrous limonite, Table 3). Gangue minerals may cause solid-liquid separation problems, e.g. clay minerals.
Table 3. Common and important minerals of various metals.

Metal Mineral Formula Principal Leaching Chemistry
Al Gibbsite
Boehmite
Diaspore
Bauxite
Corundum
Al(OH)3
AlO(OH)
AlO(OH)
Mixtures of above 3
Al2O3
Caustic (NaOH)
NaOH; less reactive than diaspore
NaOH; less reactive than gibbsite

NaOH; less reactive than boehmite

Cu Chalcopyrite

Chalcocite
Covellite
Bornite
Cuprite
Native Cu
Malachite
Azurite
Tenorite
Chrysocolla
Atacamite
Enargite
Tetrahedrite

CuFeS2 Main Cu mineral
Cu2S
CuS
Cu5FeS4
Cu2O
Cu metal (rare)
CuCO3·Cu(OH)2
2CuCO3·Cu(OH)2
CuO
CuSiO3·2H2O
3CuO·CuCl2·3H2O
Cu3AsS4
Cu3SbS3
HCl/oxidant, H2SO4/oxidant. Mainly treated by pyrometallurgy.
H2SO4/oxidant
H2SO4/oxidant
H2SO4/oxidant
H2SO4/oxidant
H2SO4/oxidant; NH3/O2
Dilute H2SO4
Dilute H2SO4
Dilute H2SO4
Dilute H2SO4
Dilute H2SO4
Difficult to leach or smelt
Difficult to leach or smelt
Zn Sphalerite
Marmatite
Zincite
ZnS Main
(Zn,Fe)S minerals
ZnO (uncommon mineral; roasting ZnS -> ZnO; common)
H2SO4/oxidant
H2SO4/oxidant
Dilute H2SO4
Fe Magnetite
Hematite
Limonite
Goethite
Siderite
Pyrite
Marcasite
Pyrrhotite
Arsenopyrite
Fe3O4
Fe2O3
FeO(OH)·nH2O
FeO(OH)
FeCO3
FeS2 (may bear Au)
FeS2
Fe1-xS, 1-x ~ 1
FeAsS (may bear Au)
Dilute acid
Unreactive
Acid (limonite may bear Ni)
Acid (goethite may bear Ni)
Dilute acid
H2SO4/oxidant
H2SO4/oxidant
Acid or acid/oxidant
Pb Galena

Litharge
Anglesite
Cerussite

PbS Main mineral; usually smelted
PbO
PbSO4
PbCO3
HCl/aq. Cl- salt/oxidant

HCl/aq. Cl- salt; HNO3
HCl/aq. Cl- salt
HCl/aq. Cl- salt

Ni Pentlandite
Garnierite
Millerite
Laterites, with
nickeliferrous
limonite
(Ni,Fe)9S8
(Ni,Mg)6Si4O10(OH)8
NiS
Complex oxides/silicates,
H2SO4 or HCl/oxidant
Strong H2SO4
H2SO4 or HCl/oxidant; NH3/O2
H2SO4; reductive roasting then leach with NH3/(NH4)2CO3
Co Smaltite
Cobaltite
Linnaeite
Laterites
CoAs2
CoAsS
Co3S4

CoS

H2SO4 or HCl/oxidant
As per Ni
"Concentrate" formed from laterite leach; leach with NH3/O2
Sn Cassiterite SnO2 Mainly by pyrometallurgy
Ti Ilmenite
Rutile
FeTiO3
TiO2
Very strong acids
Very strong acids
Mo Molybdenite
Molybdite
Wulfenite
Powellite
MoS2
MoO3
PbMoO4
Ca(Mo,W)O4
Mainly by pyrometallurgy
W Scheelite
Wolframite
CaWO4
(Fe,Mn)WO4
HCl, H2SO4
NaOH
Sb Stibnite
Jamesonite
Sb2S3
FePb4Sb6S14
Generally undesirable impurities. May leach in acid/oxidant
As Realgar
Orpiment
As4S4
As2S3
Generally undesirable impurities. May leach in H2SO4/oxidant, cyanide/O2 systems
Bi Bismite
Bismuthinite
Native Bi
Bi2O3
Bi2S3
Elemental Bi
Acid/chloride
Acid/oxidant
Acid/oxidant
Lanthanides (“Ln”) Monazite (La,Th,Ln)PO4 Hot strong acid; NaOH
U Pitchblende UO2.x;
x < 0.67
H2SO4/oxidant; Na2CO3/oxidant
Ag Argentite
Cerargyrite
Native Ag
Ag2S
AgCl
Ag metal
CN-/oxidant; S2O32-/oxidant
NH3; CN-
CN-/oxidant
Au Native Au
Calaverite
Sylvanite
Au metal
AuTe2
(Au,Ag)Te2
CN-/oxidant; S2O32-/oxidant
Hard to leach
Hard to leach
Hg Cinnabar HgS Usually by pyrometallurgy
PGM’s Various and complex

Table 4. Common gangue minerals. Superscripted Roman numerals, e.g. MII, refer to a formally +2 metal cation.

Mineral Composition Reactivity (Partial or Total Dissolution)
Quartz SiO2 (may host gold) Fluorides or strong NaOH
Feldspars M1-M2-Al-silicates
M1, M2 = Na, K, Ca
Relatively unreactive
Mica, Chlorite K2Al4(Si6Al2)O20(OH)4
Mg10Al2(Si6Al2)O20(OH)16
Dilute acids
Amphiboles MIIy-(Si4O11)(OH)x: many
forms; MII = e.g. Ca, Mg, or
both; includes asbestos
Strong acids
Clays Layered aluminosilicates Acids
Pyroxene,

Olivine

MIISiO3, (M1II,M2II)SiO3
MII, M1II, M2II = e.g. Ca, Mg
(Mg,Fe)2SiO4
Acids
Garnets,

Staurolite,
Kyanite

M3IIM2III(SiO4)3 MII = e.g. Ca, Mg, Fe; MIII = e.g. Al, Cr, Fe
(FeII,Mg,Zn)2Al9(Si,Al)4O22(OH)2
Al2SiO5
Relatively unreactive
Andalusite,
Sillimanite
Al2SiO5
Al2SiO5
Acids
Calcite CaCO3 Readily with dilute acids
Magnesite,
Dolomite
MgCO3
CaMg(CO3)2
Acids
Pyrrhotite Fe1-xS, 1-x ~ 1 Dilute acids; CN-/O2
Marcasite,
Pyrite
FeS2
FeS2
Acids/oxidants; CN-/O2
Magnetite
Limonite
Goethite

Hematite

Fe3O4
FeO(OH)·nH2O
FeO(OH)

Fe2O3

Acids (rates depend
Acids on particle
Acids size and
crystallinity.)
Practically unreactive

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