EES 119/219

Lecture 4

Discussion of ore deposits

• Abundant metals

– Fe à Banded Iron Formation

– Al à Bauxite Deposits

Fe Characteristics

• Crustal abundance of Fe: 5.6 %; second most abundant metal in the crust (most abundant element in the earth)

à Major element, present in many rock forming minerals (e.g. Spinel)

• Fe is a major or minor constituent in all mineral classes

Chemical behavior of Fe

• Chemical behavior: occurs in two main valence states: Fe⁺² and Fe⁺³

– Fe⁺² ferrous iron à FeO

– Fe⁺³ ferric iron à Fe₂O₃

• The two types indicate different ways to react with oxygen

Iron in the crustal environment

• Ferric oxide may oxidize organic matter and thus become reduced to ferrous Fe

• Ferrous solutions may in turn be re-oxidized by atmospheric oxygen

• à Iron acts as catalyst in the cycles of carbon and oxygen in crustal compartments

Iron in the biosphere

• Fe acts to transfer oxygen from air or water to body tissues.

• In green plants, Fe is necessary for the formation of chlorophyll.

• Some bacteria possess an enzyme that transfers electrons from ferrous Fe to O₂, freeing them from the dependence on organic matter as an energy source

Iron ore deposits

• Because of the high abundance of Fe in the crust, Fe deposits are formed by many different processes

• Important for ore formation: ferric Fe has low solubility in water, ferrous Fe has high solubility

Fe Ore Deposits

• Grades: 30 à 60% Fe

– Fe in Fe₃O₄ 72% (Fe = 56; O = 16)

– Fe in Fe₂O₃ 70%

à Ore mineralization between 40 and 85 %

• Grades are determined for the extracted ore, typically accounting for the presence of gangue minerals, but not host rock

Total production of steel in 2006: ~ 1.2x10⁹ tons/yr (Fig. 1)

Fe as metal

• Iron is the most important metal for current civilization

• Fe is not present as native metal in the crust

• Ore Mineralogy of Fe

– Hematite Fe₂O₃ ferric iron

– Magnetite Fe₃O₄ (FeO ^. Fe₂O₃) ferrous + ferric

– Limonite Fe₂O₃xnH₂O (‘rust’)

– Siderite FeCO₃

• Pyrite FeS₂ and similar Fe occurrences are not considered ore minerals

Types of Fe deposits

– Magmatic deposits: late segregation of magma à Kiruna type deposit

– Exposure to weathering à oxidation to ferric Fe à other, more soluble materials are removed à lateritic deposits (similar to bauxite deposits)

– Banded Iron Formation (BIF): Most important type of Fe deposit – BIF contain between 10¹² and 10¹⁵ tons of Fe (Production 10⁹ tons/yr)

Example: Mesabi Range (Fig. 2; Fig. 3)

Characteristics of BIF

• Banded structure: alternation between Fe (Fe₂O₃) and Si (SiO₂) rich layers

• Small, uniform grains (absence of large pebbles etc.)

• Large deposits

• Age distribution

– Precambrian (1.8 Ga or older)

– Peak deposition ~ 2.0 Ga

Systematics of weathering

· Chemical Weathering (H2O, pH)à transport in solution

àfine grain deposits; continuous layers: limestone

· Physical Weathering (H2O, wind, freezing)à mechanical transport

àVariable grain size: conglomerates; sandstone

· Temperature and pressure are within narrow limits:

-30^oC < T< +60^oC p~1 atm

Increase in O₂ concentrations in the atmosphere: Fig. 4

Summary for BIF

• Formation reflect change in mobility between ferrous and ferric Fe

• Change in oxygen content of atmosphere

• Seasonal upwelling in coastal basins

• Large scale release of Fe from MOR and from erosion; transport possible only under low-oxygen conditions

• Formation of BIF reflects significant changes in the oxygen content of the atmosphere due to the occurrence of land plants

From Iron ore to steel

• Mining: large scale, typically surface mining

• Separation of ore and transport to smelter

• Production of pig iron

– Blast Furnace

• Production of Steel

– Steelmaking is the second step in producing steel from iron ore. In this stage, impurities such as sulfur, phosphorus, and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce the exact steel required

– basic oxygen furnace, also known as an LD converter

– An electric arc furnace is a system that heats charged material by means of an electric arc

Definitions:

• Pig Iron: Fe compound after the initial separation from oxygen

• à contains relatively high amounts of C, not used directly, but starting material for the production of steel (cast iron is form of pig iron)

• Steel: Fe metal containing between 0.5 and 1.5 % of C; often fortified with other Fe-allow metals (Cr; Mo; W etc.) to give it specific characteristics (toughness; hardness; elasticity etc.)