3. Natural Resources

There are four main regions or "spheres" from which raw materials are taken (Figure 3.1). 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.

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 3.1 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 3.2. 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 3.2. 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.

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/S^2-), AlO(OH) (Al⁺³/O^2-,OH^-), and PbCO₃ (Pb⁺²/CO₃^2-). But in some cases the metal of interest resides in the formally anionic part as complex ions, e.g. CaWO₄ (Ca⁺²/WO₄^2-). 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 3.3. 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 3.4.

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 3.3). Gangue minerals may cause solid-liquid separation problems, e.g. clay minerals.