What is mining? "Mining is the process of excavating minerals of economic value from the earth's crust for benefit of mankind". Here we can assume that the earth's crust, the outer surface of the earth, including the oceans, lakes and rivers, extends to depths of 30 to 50 km or so. For mining operations one should have a working knowledge of geology.
The word Geology means science of the earth and deals with the nature and origin of the rocks that constitute the earth. A person interested in the extraction of minerals from the earth is, however, concerned with the thin surface of the rocks which make up the earth's crust for a depth of a maximum of 5 km so that geology enables him to locate and to decide the sites most economic for mining or quarrying.
In geology the terms mineral and rock have precise but different meanings. A mineral is a homogeneous and naturally occurring substance having definite physical properties and a composition that may be expressed by a chemical formula. The chemical composition of a mineral, as found in the earth, may be the same as that of an artificially prepared chemical compound in the laboratory but the physical characteristics may differ; e.g. lead sulphide, PbS, is generally available in the laboratory as an amorphous powder. It has quite different physical properties from galena, Pbs, the name of the naturally occurring mineral, often in the crystalline form. Some few minerals occur as single elements, e.g. native gold., silver, graphite, but most minerals are composed of two or more elements in chemical composition, e.g. quartz (SiO2), hematite (Fe2O3), etc. Most rock-forming minerals are oxides, chlorides, sulphides, carbonates, sulphates or silicates. A rock may be composed of one mineral only, but is usually a mechanical mixture or aggregate of two or more minerals. For example, granite is a rock composed essentially of three separate minerals; quartz, felspar and mica. Whereas minerals can be considered as aggregates of chemical elements, so rocks are really aggregates of minerals. They contain minerals in varying proportions and they have no definite chemical composition.
Bed rock is any rock lying in the position in which it was formed; it is therefore not broken up.
Country rock of an ore body is that rock which is predominant in the area and which contains the ore body. The country rock forms the foot wall and the hanging wall.
A seam is a mineral deposit limited by two, more or less parallel planes, a shape which is typical of sedimentary rocks. The term is generally used for coal, e.g. a coal seam.
When excavating a useful mineral, the uneconomic rock or mineral associated with it which has to be excavated and discarded is called rejection, dirt or waste in coal mining practice and gangue in metal mining practice.
An ore is a rock which contains mineral and which can be used for economical extraction of metal after processing to separate mineral from gangue. Ores usually occur as veins or lodes.
If an ore, when subjected to metallurgical processes, yields only one metal, it is called a straight ore.
A mineral deposit is .a rock or mineral that is of economic value and repays its extraction from the earth.
A vein, (or lode) shown in fig. 1.1. is a crack in the earth's crust filled with mineral. This filling can occur by precipitation of the mineral from the mineral-rich water or by the cooling of the magma filling the crack or by the separation of the mineral from vapours and gases rising up the crack. Veins, like seams, have a strike, a dip and a thickness but for the same vein, alt these are usually quite variable.
A number of nearly parallel veins constitute compound (odes.
Orebody is the part of a vein that carries the ore. Generally all parts of a vein are not ore.
Note that:
the country rock contains the vein;
the vein contains the ore body;
the orebody contains the ore;
the ore contains the minerals;
the mineral contains a metal (or) metals.
Fig. 1.1
MINERALS
Minerals possess definite physical properties by virtue of which they can be distinguished from one another. The most important physical properties are as follows:
Colour: Some minerals possess a characteristic colour, e.g; galena, magnetite, olivine, etc; but in some others the clour is variable, e.g. quartz.
Specific gravity: Most rock-forming minerals have a specific gravity between 2 and 4.
Lustre: The lustre may be metallic (like galena or iron pyrites), pearly (like talc) or silky.
Taste and smell: Rock salt, alum and some other minerals can be recognised by their taste.
Streak: A few minerals, when drawn over paper or over an unglazed procelain plate, leave a coloured mark known as the streak; for example, graphite gives a black streak; hematite leaves a cherry red streak.
Crystalline form: A crystal is geometrical solid bounded by smooth plain surfaces called faces and capable of increasing jn size by the deposit of fresh material on the outside of these surfaces. The faces in a crystal show a definite geometrical pattern and the angles between th3 faces are constant; for example, quartz crystallises in the hexagonal system, while mica or muscovite crystallises in what is called the monoclinic system, and rock salt, in the cubic system. The crystallisation may take place by: (a) deposition from solution (b.) slow cooling from the molten state, or (c) direct change from; a vapour to a solid.
Cleavage: Many crystals have a tendency to split along one or more direction parallel to an actual or possible crystal face. This splitting gives plane surfaces known as cleavage planes. For example, mica cleaves in one direction only; galena (lead sulphide) cleaves in three planes at right angles, forming perfect cubes.
Fracture: When a crystal breaks independently of the cleavage plane, it is said to fracture. The property is prominent in minerals with poor cleavage.
Hardness: This term gives the relative ease with which minerals can be scratched. In practice hardness is measured by reference to a set of minerals given below so arranged that the first member can be scratched by all the others, the second by all except the first, and so on.
Moh's scale of hardness is as follows:
1. Talc
2. Gypsum
Scratched by finger nail
3. Calcite
4. Fluorspar Scratched by a knife
5. Apatite
6. Orthoclase Scracely scratched by a knife.
7. Quartz
8. Topaz
9. Corundum Not scratched by a knife. 10. Diamond
It may be observed that:
A finger nail will scratch up to about 2.5.
A pen knife will scratch up to 6.5.
When testing the hardness of a mineral window glass can be used as a substitute for apatite.
Protodyakonov strength number; Hardness of rocks is expressed by Protodyakonov strength number in Russia. The number indicates the relative ease with which a rock can be broken, e.g. strong lignites and weak clay shales have Protodyakonov strength number as 1.5 to 2; strong coals and anthracites have strength number as 2; exceedingly strong quartzites and gabbro-diorites have the number as 20-25, the highest number. Other rocks have the numbers inbetween.
Electrical and magnetic properties of minerals and the properties dependent on light are also made use of in distinguishing minerals which react in a distinctive manner to the tests.
Common minerals
There are about 107 elements that have been isolated and recognised in the laboratory. Of this number, however, there are only 8 that enter into the compdsition of the earth's outer portions in abundance. In fact these 8 elements make up some 98%of the earth's observable crust. These are (in order of abundance);
O2 - 47% Si - 28%
Al - 8% Fe - 5%
Ca - 3.5% Na - 2.5%
K - 2.5% Mg - 2.0%
Total 98.5%
The combinations of some of these eight common elements among themselves have produced the most common rock-forming minerals that constitute the bulk of the rocks. These most common rock-forming minerals are feldspars, quartz, mica, amphiboles, pyroxenes, and olivine.
The other 96 elements are relatively scarce, in that they represent only 1.5% by weight of the earth's crust e.g. Cu — 0.0045%, Pb - 0.00015%, Au - 0.0000007%,
There are about 2000 catalogued mineral specimens but the real economic targets of mining activity are 100 minerals including the native minerals, the hydro carbon minerals and a few types of economic rocks used as house construction materials.
Of the above rock-forming minerals some can be considered as essential minerals while some others are accessory minerals and secondary minerals.
(a) Essential Minerals: These make up the bulk of the rocks and are always silicates with the exception of quartz and the carbonates.
(b) Accessory Minerals: These are present only in small quantities in a rock.
(c) Secondary Minerals: These are derived from the break-down of the others.
Classification of economic minerals:
a) Metallic minerals: Minerals that yield metals:
Precious metals: gold, silver, platinum.
Base metals: Copper, lead, zinc, tin.
Steel industry metals: iron, nickel, chromium, manganese, cobalt, molybdenum, tungsten, vanadium, tantalum, etc.
Light metals: aluminium, magnesium, titanium, etc.
Electronic industry metals: Cadmium, bismuth, germanium, mercury, selenium.
Radioactive metals: Uranium, radium, caesium, zirconium, beryllium, rare earths, etc.
b) Non-metallic minerals:
Insulating materials: Mica, asbestos, silimanite.
Refractory materials: Silica, alumina, zircon, graphite, etc.
Abrasives: Corundum, emery.
Gems: garnet, diamond, topaz, emerald, saphire, etc.
General industrial minerals: Phosphate rock, lime stone, rock-salt, baryte, borates, felspars, magnesite, gypsum, potash, clays, sulphur.
c) Fuel minerals:
Solid fuels: Anthracite, coal, lignite, oilshale.
Liquid fuels: Petroleum oil, natural gas.
The common rocfc-forming minerals are described below in brief.
Silicates
The Felspars: These are all complex silicates of aluminium with potash, soda or lime and are most abundant in igneous rocks. Potash felspar, also known as orthoclase, is the most common type. It is white, grey or pink in colour with a glassy lustre. When weathered it leaves a hydrated silicate of aluminium known as kaolin or china clay. The soda and lime felspars are known as plagioclase. They also decompose and disintegrate in a similar way. Plagioclase felspars occur in most Igneous rocks especially the darker varieties rich in lime. (sp. gr = 2.6 to 2.7; H = 6). H means hardness.
Mica: This occurs as white variety known as muscovite (a silicate of aluminium and potassium) and as black variety known as black mica or biotite (a silicate of aluminium, iron and magnesium). The mica can be easily scratched by a finger nail and has a well-developed cleavage. It is a common constituent of igneous rocks and crystalline, schists (Sp. gr = 2.7 to 3.1; H = 2.5).
Hornblende and Augite: These minerals are complex silicates of clacium, magnesium and iron. They are both greenish black in colour. Hornblende is also called amphibole. It is rather a dull, black mineral, forms six-sided crystals, is found in most igneous rocks and has Sp. gr. = 3-3.5; H = 5 to 6. Augite (also called pyroxene) is black but more brilliant than hornblende, forms monoclinic crystals and is found in most of the basic igneous rocks. It often alters to chlorite. (Sp. gr. = 3 lo 3.5; H = 5 to 6),
Olivine (also called peridot): It is a silicate of magnesium and iron found in basic igneous rocks such as dolerite, basalt and peridote. It is greenish and looks like quartz (Sp. gr. = 3.2 to 4; H — 6. to 7).
The above minerals which contain iron and magnesium are known as ferro-magnesian minerals and are generally found in abundance in the more basic igneous rocks (basalt, dolerite, etc.).
Oxides
Quartz (Silica, SiO2): It is an important constituent of the granite and other acid igneous rocks, and a chief constituent of sandstones where it occurs in the broken form (Sp. gr. = 2.6; H = 7).
Magnetite (Magnetic oxide of iron, Fe3O4): It is black in colour and (eaves a black streak (Sp. gr. = 4.9 to 5.2; H = 6).
Hematite (Ferric oxide of iron, Fe2O3): It is bluish black in colour. Earthy varieties are known as red ochre. The streak is always cherry red (Sp. gr. = 4.5 to 5.3; H = 6).
Limonite (Hydrated ferric oxide, 2Fe3O3, 3H2O): It amorphous, brown or nearly black in colour and the streak yellowish brown (Sp. gr. = 3.6 to 4; H — 5).
Bauxite: It is essentially a hydrated aluminium oxide l-kO), dirty white in colour. It results from the decay and weathering of aluminium rocks, often igneous, under tropical conditions. (Sp. gr. = 3.5; H = 2.5).
Carbonates
The common carbonates are of calcium and iron. All carbonates effervesce when treated with dilute hydrochloric acid.
Calcite (CaCO3): It is an essential constituent of marble* chalk or* limestone; it is white or colourless and gives a white streak (Sp. gr. •• 2.9; H = 3).
Siderite (FeCO3): Col -mr is brown in various shades and the streak is white (Sp. gr. = J.7 to 3.9; H = 4).
Dolomite: This is a carbonate of calcium and magnesium, Ca, Mg (CO3)2. Calcite and dolomite constitute the hirgcr bulk of the limestones, but are sometimes found as secondary minerals in the igneous rocks. (Sp. gr. = 3; H = 4).
Sulphides
The common sulphides are the sulphides of iron, lead, zinc and copper.
Iron pyrite (FeS2): Brass yellow in colour, this mineral is sometimes found interspersed in a coal seam. e.g. in the Pench Valley Coalfield. The streak is greenish or brownish black. Coal containing iron pyrites is observed to be more liable to spontaneous heating than other coal (Sp. gr. = 4.8 - 5.1; H = 6 to 6.5).
Galena (PbS): It is grey in colour and has a grey streak. (Sp. gr. = 7.2 to 7.7; H = 2.5).
Sphalerite, Blende (ZnS): It is also known as black jack. It is usually brown or black, the streak being reddish brown (Sp. gr. = 3.9 - 4.2; H = 3.5 - 4).
Chalcopyrite, Copper Pyrite (Cu2S, Fe2 S3,): It is golden yellow in colour; streak is greenish black (Sp. gr. = 4.1 — 4.3; H = 3.4 — 4).
Sulphates
Gypsum (Hydrated calcium sulphate, CaSO4, 2H2O): It forms colourless crystals, but some varieties may be white or grey. Plaster of paris is made from gypsum by heating it to expel some of its water of crystallisation and then grinding it to a fine powder (Sp. gr. = 2.3; H = 2).
Chlorides
Rock Salt (known as common salt, NaCl): It is colourless or white when pure (Sp. gr. = 2.2; H = 2).
Other rock-forming minerals are:
Kaolin (also called china clay, Al2 03, 2SiO3, 2H2O): It may be described as hydrated silicate of aluminium. It is a soft, white, calyey material crumbling to powder when pressed between fingers. Felspars when decomposed yield china clay.
Fireclay: Any clay or shale that does not fuse below 1580°C is called fireclay. The predominant constituent of these clays is the mineral kaolinite with the formula AI2 O3, 2SiO2, 2H2O. Besides alumina and silica the fireclay also contains oxides of iron, calcium, magnesium, sodium and potassium.
fluorspar: (CaF2): It may De colourless, purple, green or yellow; lustre is vitreous but often transparent. It occurs in veins of metalliferous ores with galena (Sp. gr. = 3; H = 4).
Baryte (BaSO4): It may be white, yellow, red or blue with a vitreous lustre and is commonly found in mineral veins associated with the ores of lead and zinc. (Sp. gr. = 4.5; H == 3).
Uranium oxides: Uranium is the metal used for production of nuclear energy. Two uranium isotopes U-235 and U-233 are fissionable materials. Uranium ore contains 0.03 to 0.1%of uranium. The mined ore is processed to give a marketable concentrate containing 75%
Rare Earth Metals: This is a group of metals which includes metals like cerium (Ce), lanthanum, erbium, yttrium and others. Thorium is closely associated jn nature with, this group. Thorium is a metal related to titanium and has a sp. gr. of 11. Monazite is a phosphate of cerium metals, but is industrially important as a source of thorium compounds as it contains a small percentage of thorium oxide or thorium silicate. Monazite: (Sp. gr. = 5.27; H = 5.5).
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