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Extraction of Metals.

Extracts from this document...

Introduction

Occurrence of Metals in the Earth Crust Metals are found in the earth's crust. The least reactive metals are found native as elements e.g. silver, gold, but most are found in compounds, usually oxides or sulphides. The occurrence in nature of metals will affect the cost of extracting and engineering them. Most metals are found as ores usually either as an oxide or a sulphide. Sulphides are even rarer than oxides as these were formed when the earth's crust was solidifying and there was plenty of sulphur from volcanic activity. Oxides became present later in the earth's history when oxygen became more abundant. Redox reductions are used to obtain most of the metals that we all take for granted. Most metals are found occurring naturally in rocks called ores. They are in compounds, chemically bonded to other elements. However, there are metals with very low reactivity that can be found as the elements themselves. We say these metals are found native in the ground. Examples of metals found natively are copper, silver, gold and platinum. Copper and silver are also mined as ores. Gold is expensive as it is rare and difficult to separate from the waste rock. All metals above copper in the Reactivity series are found as ores. Rocks that contain a high enough percentage of a metal to be extracted commercially are known as ores (what percentage is required depends on the value of the metal). Aluminium and iron are the most abundant metals in the earth's crust, and they are widely distributed around the earth's crust, which explains why they are so widely used. ...read more.

Middle

in the heat. CaCO3(s) CaO(s) + CO2(g) The calcium oxide (an alkali) reacts with acidic impurities such as sand (silicon dioxide) producing a molten mixture of compounds, mainly calcium silicate, called slag which floats on the iron. Slag is used in construction (e.g. road making and cement manufacture). CaO(s) + SiO2(s) CaSiO3(s) Pollution problems arise from using carbon, giving CO2 (greenhouse gas) and CO (toxic). Also the roasting of sulphide ores to give oxides leads to SO2 pollution, resulting in acid rain. Steel Making For this process the iron from the blast furnace is used to make steel. The iron obtained contains many impurities, up to 3 to 4% carbon plus some other non-metals. The impure iron is very brittle - it cracks easily - and is called 'cast' or 'pig' iron (after the name given to the moulds). Most of this will be turned into steel, which is much tougher. Steel is mainly iron (over 98%) with a small amount of carbon left in it. Other metals can also be added to improve its properties for particular requirements. To do this we must remove the carbon and other impurities. This can be done by blowing oxygen onto the molten pig iron using a Basic Oxygen Converter. The impurities in pig iron: carbon 3% to 5% silicon 1% to 2% phosphorous (0.05% to 1.5%) sulphur (0.05% to 1%) The carbon burns in the oxygen and escapes as carbon dioxide: C(s) + O2(g) CO2(g) The sulphur can also be oxidised and escape from the pig iron in the form of sulphur dioxide gas: S(l) ...read more.

Conclusion

Roasting Process: It is the process of beating the concentrated ore in the presence of excess air. Volatile impurities will be expelled. Zinc sulphide in its ore, zinc blende (ZnS) is roasted to obtain zinc oxide (ZnO) 2ZnS(s) + 3O2(g) 2ZnO(s) +2So2(g) from air Reduction: The conversion of metal oxide into metal (by removal of oxygen) is called Reduction. Generally, the 3 methods used are: * Reduction by heating the oxide * Chemical reduction * Electrolytic reduction Reduction by Heating the Oxide (Heating Process): The oxides of metals that are low in the Reactivity series can be reduced to obtain the metals by heating their ore. For example, mercuric oxide (HgO), obtained from its ore mercuric sulphide (HgS), when heated to about 300 degrees C, forms mercury metal. Heat 2HgO 2Hg + O2 (Reduction) mercury Roasting and reduction processes are carried out simultaneously. Chemical Reduction Process: Under this process the oxide of metals that are in the middle of the Reactivity Series are reduced to free metals using chemical reducing agents such as carbon, aluminium, sodium or calcium. Electrolytic Reduction Process: The oxides (or chlorides) of highly reactive metals (such as sodium, magnesium, aluminium and calcium) cannot be reduced by using carbon or aluminium. Electrolytic reduction is used for the above metals. The cathode acts as a powerful reducing agent by supplying electrons to reduce the metal ions into metal. Fused alumina (molten aluminium oxide) is electrolysed in a carbon lined iron box. The box itself is the cathode. The aluminium ions are reduced by the cathode. At the cathode: electrolysis Al 3+ + 3 e- Al Aluminium ion aluminium atom This concludes my investigation on the extraction of metals. Filename: Extraction of Metals Page: 1 of 15 ...read more.

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