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Mining                                                                            

Mining can be subdivided into 4 basic types:

1. Placer
2. Hydraulic
3. Hard rock
4. Open pit

After mining, metal ores are obtained. These ores are usually not pure and hence have to undergo a series of complex purification methods before being processed to form alloys.

Extraction of Metals

The separation of metals in a pure or relatively pure state from the minerals in which they occur naturally. Metals are found in the Earth either as compounds or, in the case of very unreactive metals, as the crust of the uncombined element. The rock that contains the metal is called an ore. Common metal ores include bauxite (aluminium oxide), haematite (iron oxide), zinc blende (zinc sulphide), galena (lead sulphide), and cinnabar (mercury sulphide).

A mineral can be a solid metallic or non-metallic element or a compound found naturally in the Earth’s crust.

A metal ore is a mineral or mixture of minerals from which economically viable amounts of metal can be extracted, i.e. its got to have enough of the metal, or one of its compounds, in it to be worth digging out! Ores are often oxides, carbonates or sulphides. They are all finite resources so we should use them wisely!

In order to extract a metal, the ore or compound of the metal must undergo a process called reduction to free the metal (i.e. the positive metal ion gains negative electrons to form the neutral metal atom, or the oxide loses oxygen, to form the free metallic atoms).

The chemical that removes the oxygen from an oxide is called the reducing agent i.e. carbon, carbon monoxide or sometimes hydrogen.

Generally speaking the method of extraction depends on the metals position in the reactivity series

The reactivity series of metals can be presented to include two non-metals, carbon and hydrogen, to help predict which method could be used to extract the metal.

lower Pt Au Ag Cu (H) Pb Sn Fe Zn (C) Al Mg Ca Na K higher in series

RULE: Any element higher in the series can displace any other lower element

Metals above zinc and carbon in the reactivity series cannot usually be extracted with carbon or carbon monoxide. They are usually extracted by electrolysis of the purified molten ore or other suitable compound

e.g. aluminum from molten aluminum oxide or sodium from molten sodium chloride.

The ore or compound must be molten or dissolved in a solution in an electrolysis cell to allow free movement of ions (electrical current). Theory given in the appropriate sections.

Metals below carbon can be extracted by heating the oxide with carbon or carbon monoxide. The non-metallic elements carbon will displace the less reactive metals in a smelter or  blast furnace e.g. iron or zinc and metals lower in the series.

Metals below hydrogen will not displace hydrogen from acids. Their oxides are easily reduced to the metal by heating in a stream of hydrogen, though this is an extraction method rarely used in industry. In fact most metal oxides below carbon can be reduced when heated in hydrogen, even if the metal reacts with acid.

Some metals are so unreactive that they do not readily combine with oxygen in the air or any other element present in the Earth’s crust, and so can be found as the metal itself. For example gold (and sometimes copper and silver) and no chemical separation or extraction is needed. In fact all the metals below hydrogen can be found as the ‘free’ or ‘native’ element.

Other methods are used in special cases using the displacement rule. A more reactive metal can be used to displace and extract a less reactive metal but these are costly processes since the more reactive metal also has to be produced in the first place! See Titanium or see at the end of the section on copper extraction.

Sometimes electrolysis is used to purify less reactive metals which have previously been extracted using carbon or hydrogen (e.g. copper and zinc). Electrolysis is also used to plate one metal with another.

The demand for raw materials does have social, economic and environmental implications e.g. conservation of mineral resources by recycling metals, minimising pollution etc.

Historically as technology and science have developed the methods of extraction have improved to the point were all metals can be produced. The reactivity is a measure of the ease of compound formation and stability (i.e. more reactive, more readily formed stable compound, more difficult to reduce to the metal).

The least reactive metals such as gold, silver and copper have been used for the past 10000 years because the pure metal was found naturally.

Moderately reactive metals like iron and tin have been extracted using carbon based smelting for the past 2000-3000 years.

BUT it is only in the last 200 years that very reactive metals like sodium or aluminium have been extracted by electrolysis

Iron oxide ore is mined in many parts of the world.

 The solid mixture of haematite ore, coke and limestone is continuously fed into the top of the blast furnace. This causes a reaction in burning that causes a high percentage of carbon monoxide, a harmful gas to be liberated to the environment.

We all know carbon gases are harmful and are greenhouse gases. Humans will be affected when they get warmer or even breathing in these gases.

• The coke is ignited at the base and hot air blown in to burn the coke (carbon) to form carbon dioxide in an oxidation reaction (C gains O).
• The heat energy is needed from this very exothermic reaction to raise the temperature of the blast furnace to over 1000^oC to effect the ore reduction.

Heat is given as part of energy conversion due to the thermal reactions the reactants formed ion the burning process.

• The molten slag forms a layer above the more dense molten iron and they can be both separately, and regularly, drained away. The iron is cooled and cast into pig iron ingots OR transferred directly to a steel producing furnace.
• The waste gases and dust from the blast furnace must be appropriately treated to avoid polluting the environment.

• The highly toxic carbon monoxide can actually be burnt to provide a source of heat energy, and in the exothermic reaction it is converted into relatively harmless carbon dioxide.

When carbon is converted to one another in the process and may not be as harmful as the raw carbon monoxide. But when greenhouse gases are concerned, carbon gases are harmful and bad for the humans lungs and breathing.

Iron from a blast furnace is ok for very hard cast iron objects BUT is too brittle for many applications due to too high a carbon content from the coke. So it is converted into steel alloys for a wide range of uses.

Raw iron is rarely used and is usually used as a stepping stone to be made in to alloys with other mixtures of metals.

Thus extraction of metals, iron, causes many complications to the human environment and may not be as harmless as it seems. The products being made on the way to become the industry iron, are harmful and toxic.

-A solution to the dust problems, would be that dust from mining-quarrying or processing can be reduced by air filter and precipitation systems and even hosing water on dusty areas or spoil heaps or carried away to somewhere else via tall chimneys.

- For noise pollution, it is very difficult to be controlled, or to find a solution to solve this problem. Sound-proofing the area is often not practical, but operations can be reduced for unsociable hours e.g. evening movement.

-Air pollution can be reduced by cleaning the ‘waste’ or ‘used’ air, water and waste gases etc. of toxic or acidic materials.

Some examples would be :

Toxic carbon monoxide from the blast furnace extraction of iron, it can be burnt as a fuel, but it must not be released into the air unless converted to biologically harmless carbon dioxide.

Sulphur dioxide gas from copper extraction of its sulphide ore is an irritating poisonous gas, which can also cause acid rain. However, it can be converted to the useful, therefore saleable, industrial chemical concentrated sulphuric acid, so you can remove a harmful pollutant and recover back some of the metal extraction costs, good green economics? Acidic gases like sulphur dioxide can be removed by bubbling through an alkali solution such as calcium hydroxide solution (‘limewater’) where it is neutralised and oxidised to harmless calcium sulphate. Cleaning a gas in this way is called ‘gas scrubbing‘.

Mining operations will disfigure the landscape BUT it can be re-claimed and ‘landscaped’ in an attempt to restore the original flora and fauna. However in the case of a limestone quarry, I’m afraid there is no way round the fact that huge chunks of beautiful hills get carted away if we want to use it as useful mineral.