Unit 6 - The Abundant Metals

Question 1

What properties are unique to metals?

Answer 1

Metallic properties include opacity, toughness, ductility, malleability, fusibility, and high thermal and electrical conductivities.

Question 2

What critical percentage (by weight of occurrence in the Earth’s crust) classifies a metal as “abundant”?

Answer 2

The critical percentage separating abundant from scarce metals is 0.1 per cent by weight concentration in the Earth’s crust.

Question 3

Which are the two most important factors in determining whether a rock is an ore? Explain.

Answer 3

The two most important factors in determining whether a rock is an ore are the mineralogy, or form of the mineral, and the grade or proportion of metal content. These factors determine which process and how much energy will be needed to extract the metal.

Question 4

All of the ore minerals for the abundant metals are oxides, hydroxides, or carbonates, rather than silicates. Why?

Answer 4

Silicate minerals are rarely used as ores for metals because they are difficult to handle and very expensive to smelt.

Question 5

Of all the metals, why does iron dominate?

Answer 5

Iron is the dominant metal because of its abundance and accessibility. Furthermore, iron is easily smelted, and its special properties allow it and its alloys to be tempered, shaped, sharpened, and welded to make a strong and durable product.

Question 6

Differentiate between the ferrous and ferric states of naturally occurring iron.

Answer 6

Ferrous iron (Fe+2) occurs under the influence of reducing conditions. It is relatively soluble. Siderite and chamosite are examples of ferrous iron minerals. Ferric iron (Fe+3) is extremely insoluble. It occurs under oxidizing conditions. Ferric minerals include hematite or goethite.

Question 7

What accounts for the three types of iron ore produced by igneous activity?

Answer 7

The first type of iron ore is produced by magmatic segregation, in which magnetite minerals crystallize and settle out onto the floor of a mafic magma chamber. The second type is produced through contact metamorphic deposition, in which hot fluids from an igneous intrusion react with (and sometimes replace) the wall rock, leaving a mixture of coarse-grained iron oxides (among other things). The third type of iron ore occurs wherever sea floor volcanism takes place. Instantaneous cooling of magmatic vapours and fluids by contact with sea water results in large deposits of iron oxides.

Question 8

Describe the formation of a laterite, and discuss its importance as a source of iron.

Answer 8

Laterites are formed in tropical regions where abundant rainfall has leached the soils, leaving large areas of hard red iron oxide deposits. In this reaction, iron hydroxide Fe (OH)3 converts irreversibly to FeO·OH through a dehydration process. Although such leaching results in extremely poor soil, some laterites contain thirty per cent iron, which may represent large future sources of iron.

Question 9

Compare the method of formation and location of bog iron deposits with ironstone deposits.

Answer 9

Both bog iron and ironstone are types of sedimentary iron ore deposits. Bog iron is the smaller of these deposits. Although bog iron was important in the seventeenth and eighteenth centuries, it is not mined today. Bog iron deposits occur in glaciated areas and coastal plain sediments. Reducing conditions caused by decaying vegetation puts iron into solution, where it oxidizes and precipitates as lenses and sedimentary cement.
Ironstones deposits are much larger than bog iron deposits. Ironstone deposits are continuous sedimentary beds that vary from tens to hundreds of kilometres wide, and several metres to tens of metres thick. They are thought to have been formed when soluble ferrous iron was transported by ground water or deep basin fluids to shallow, near-shore marine sediments. These oxidizing conditions allowed the transformation to ferric iron both through direct sedimentation and through diagenetic replacement.

Question 10

Describe the origin of banded iron formations.

Answer 10

The formation of banded iron deposits is still a great mystery. We know that these deposits were made during Precambrian times, when the Earth’s atmosphere and surface waters probably contained little free oxygen. Under these reducing conditions, the iron could have been in the ferrous state and could have been dissolved in water and carried to the oceans. The banding, however, implies that there was periodic precipitation and accumulation of iron; that is, oxygen was periodically available to enable precipitation and the alteration of iron to the ferric state. There is disagreement about what caused this.`

Question 11

What is the predominant form of mining for iron ore today? Why is this method preferred?

Answer 11

Open pit mines account for about eighty-five per cent of the world’s iron mining today, because the iron bodies lie close to the Earth’s surface and have wide dimensions. Also, compared to an underground mine, an open pit mine allows for a larger production capability, is less expensive to operate per tonne of ore, and is easier and safer to operate.

Question 12

Differentiate between direct shipping ores and taconites.

Answer 12

Direct shipping ores are those which contain more than fifty per cent iron by weight, and which can be processed effectively at the smelter after only being crushed. Taconites are lower-grade ores composed of a mixture of iron oxides (commonly forty to fifty per cent) and silicates.

Question 13

Describe the process of beneficiation.

Answer 13

Beneficiation is an upgrading process by which taconites are changed into pellets ranging between sixty-three and sixty-five per cent iron that are used for smelting. The taconites are simply ground to a powder from which impurities are removed. This powder, consisting of iron oxide, is bound into pellets with clay or fine volcanic ash.

Question 14

Discus the roles of coke, iron ore, and limestone in the smelting of iron.

Answer 14

In the smelting of iron, coke is added to produce carbon monoxide on combustion. The CO then reacts with the iron ore (Fe2O3) to produce free iron (2Fe) and carbon dioxide. Limestone is used to form a slag which absorbs undesirable elements from the charge.

Question 15

What elements are added to iron to create stainless steel?

Answer 15

Chromium, copper, and nickel are added to iron to add corrosion resistance (Table 7.4, p. 245). A corrosion-resistant, iron-based alloy is commonly called stainless steel.

Question 16

What country is the greatest supplier of iron to the United States?—Of steel?

Answer 16

Canada is the greatest supplier of iron ore to the United States, while the European Union and Japan have been its greatest suppliers of steel.

Question 17

What is the main use for manganese?

Answer 17

Manganese is invaluable as a scavenger of minor detrimental impurities, such as sulphur and oxygen, during the smelting of iron. Up to seven kilograms of manganese are needed to produce each metric ton of iron or steel.

Question 18

Manganese nodules are actually composed of which metallic minerals?

Answer 18

Manganese nodules contain manganese as well as iron, nickel, copper, cobalt, and zinc.

Question 19

Why, according to Kent, are manganese nodules found only on the surface of the ocean bottom and not in the sediments below?

Answer 19

Manganese nodules are found only on the surface of the ocean bottom because the bottom ocean waters are oxidizing and so promote the growth of the nodules. Within the sediments, the micro-environment is a reducing one, which causes the minerals to dissolve.

Question 20

What is thought to be the source of metal salts responsible for growth of the nodules?

Answer 20

There is a correlation between abundant surface micro-organisms and rich ocean beds of nodules. This correlation implies that the metals are somehow secreted by the plankton, pass into solution, and are subsequently precipitated on the ocean floor.

Question 21

Identify possible environmental problems associated with dredging for manganese nodules.

Answer 21

Dredging could disturb benthic organisms with clouds of fine sediment.

Question 22

What are the useful qualities of aluminum?

Answer 22

Aluminum is light (weighing only about one-third as much as iron or copper), malleable, ductile, and easily machined and cast; it resists corrosion and is an excellent conductor of electricity.

Question 23

Describe the formation of the aluminum ore, bauxite.

Answer 23

Lateritic weathering takes place in tropical and subtropical regions where rainfall is abundant, where groundwater is of the right pH, where there are aluminous parent rocks, and where there is subsurface drainage but low relief. In lateritic weathering, chemical leaching occurs faster than mechanical erosion. During this intense chemical weathering process, the more soluble elements, such as sodium, potassium, magnesium, and calcium, are removed in solution; the residue is mainly iron hydroxide and certain types of clays, such as kaolinite (Al2Si2O5(OH)4). Decaying vegetation can make percolating waters slightly acidic, so that the clays are dissolved, leaving behind lateritic deposits of aluminum and iron hydroxide. If the aluminum hydroxides form a high enough ratio with the iron hydroxides, the resulting deposit is an aluminum-rich laterite called bauxite.

Question 24

Why do bauxite-producing countries export the ore, rather than process it?

Answer 24

Although mining bauxite is simple and inexpensive, smelting and producing aluminum is costly and energy-intensive. Bauxite resources tend to be located in less developed parts of the world where electricity is very expensive. Hence, the ore is crushed, washed to remove impurities, dried, and shipped to importing countries.

Question 25

Why is the import of scrap aluminum equated with the import of energy?

Answer 25

Re-melting an aluminum can to produce another can requires only five per cent of the energy that was needed to smelt the original aluminum, so importing scrap aluminum creates an energy saving of ninety-five per cent.

Question 26

What five countries have the greatest bauxite reserves?

Answer 26

The top five countries in bauxite reserves, in descending order, are Guinea, Australia, Brazil, Jamaica, and India.

Question 27

What are the two major uses of titanium?

Answer 27

Titanium is used as an alloy to give a high strength-to-weight ratio, a high melting point, and a great resistance to corrosion. However, ninety-five per cent of titanium is used in the preparation of white titanium oxide pigment for paint, paper, plastic, rubber, and many other materials.

Question 28

What are the two oxide minerals of titanium? Why is titanium oxide more easily obtained from mafic rocks when associated with magnetite rather than hematite?

Answer 28

The two oxide minerals of titanium are rutile (TiO2) and ilmenite (FeTiO3). Titanium minerals are quite easily obtained from magnetite in mafic rocks, because magnetite is both coarse-grained and magnetic; conversely, with hematite, titanium occurs as very fine-grained intergrowths.

Question 29

What do titanium oxide minerals and the gold panner’s “black sand” have in common?

Answer 29

Titanium oxide and black sand are often the same thing. Titanium oxide minerals are very resistant to weathering and erosion, and have higher specific gravities than quartz sand and other weathering residues; therefore, they act as placers and are often found with gold.

Question 30

What environmental hazards are associated with the processing of titanium?

Answer 30

Processing ilmenite to produce pigment generates up to 3.5 metric tonnes of toxic sulphate and sulphuric acid waste per tonne of product.

Question 31

What are the important uses of magnesium?

Answer 31

Like the oxide magnesia (MgO) and the silicate mineral forsterite (Mg2SiO4), magnesium is used as a refractory material in steel and some base metal industries. As a metal, magnesium is most often mixed with aluminum to produce a light-weight, corrosion-resistant alloy, or in compounds to produce a variety of products.

Question 32

From what sources is magnesium obtained?

Answer 32

Magnesium may be obtained from forsterite, brucite, magnesite, dolomite, brines, and evaporite minerals.

Question 33

Identify the three countries that produce the most magnesite.

Answer 33

China, Russia, and North Korea produce the most magnesite.

Question 34

Silicon does not normally occur in a free form in nature. Why not?

Answer 34

Silicon, the second most abundant element in the Earth’s crust, does not normally occur in a free form in nature, because silicon combines very easily with oxygen (the most abundant element) to form quartz (SiO2), which is the most abundant mineral in the Earth’s crust.

Question 35

Why is silicon valued in metallurgy?

Answer 35

In metallurgy, silicon is valuable as a deoxidizing agent, because of its affinity for oxygen, with which it forms quartz. As an alloying agent, silicon adds strength and reduces scaling and corrosion at high temperatures.

Question 36

Which country is the greatest exporter of silicon? With what industry is silicon use most closely associated?

Answer 36

Norway is the world’s largest exporter of silicon because it has abundant hydroelectrical power to produce it and only a small steel industry (the greatest user of silicon) to consume it.