Aluminium and its Extraction

EXTRACTION OF ALUMINIUM

Bauxite is an orange-red igneous rock, which occurs naturally in the lithosphere, from which Aluminium is extracted. It contains 30-54% alumina, Al2O3 and other impurities such as clay, Iron (III) Oxide (Fe2O3), Silica (SiO2), and Titania (TiO2). Australia was the top producer of bauxite in 2007, with almost one-third world share.

The Bauxite has to be purified by a process known as The Bayer Process. The Bayer process is the main industrial method of refining bauxite to produce alumina. The Bauxite rocks are crushed to smaller rock sizes. Then the bauxite is digested by mixing it with a hot solution of sodium hydroxide, NaOH at 175oC. This only dissolves the oxides of aluminium and silicon, but not the other impurities. The solution is further purified by filtering out the solid impurities. Carbon dioxide gas is then bubbled through the solution, which creates a weak carbonic acid, neutralising the solution and causing the aluminium oxide to precipitate, but leaving the other silicon impurities. The remaining solution is filtered once again and boiled to remove the water. The resultant product is purified aluminium oxide.

After the purified aluminium oxide has been produced, the aluminium can be separated by a process known as The Hall-Heroult method. The alumina is dissolved in molten cryolite (Na3AlF6). Then mixture is then heated to approximately 980oC, (aluminium oxide usually melts at a higher temperature, but due to the added aluminium fluoride, a considerable amount of energy is saved). The mixture is then placed in a carbon lined bath and a large electrical current is passed through it. This forms aluminium at the cathode and oxygen gas at the carbon anode. The oxygen gas reacts with the anode to give off carbon dioxide gas. The transformer generates a current from 220kA to 340kA, with a voltage of 1-2kV from 110kV; this shows how strong the bonds are between the aluminium and oxygen ions, as a large amount of electrical energy is used.

The solid aluminium that is formed is denser than the molten cryolite (at 1000oC) and therefore sinks to the bottom of the bath, which taken out. The liquid aluminium is removed by a vacuum tube called a siphon; this saves the use of high energy pumps.

IMPORTANCE OF PREDICTING YIELD IN THE IDENTIFICATION, MINING AND EXTRACTION OF COMMERCIAL ALUMINIUM ORE DEPOSITS

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IMPORTANCE OF PREDICTING YIELD IN THE IDENTIFICATION, MINING AND EXTRACTION OF COMMERCIAL ALUMINIUM ORE DEPOSITS

The yield of a metal is the amount of product (i.e. a metal) which is derived from a chemical extraction process, which is often expressed as a percentage of the theoretical maximum amount. It is very important for mining companies to predict the yield, to check if the extraction of aluminium from bauxite will be profitable or not.

To extract aluminium from bauxite it can be a very costly operation, the various costs that need to be considered before the mining actually starts; salaries of the miners and their housing, buildings, smelters, machinery, sites for waste disposal, transportation of aluminium etc. Then by predicating the yield, it can be evaluated, if the total costs can be covered by the amount of aluminium that can be extracted. The amount of aluminium that can be extracted depends on two things, the size of the bauxite ore body and the concentration of aluminium in the bauxite. The higher the concentration the greater the yield and the more economically viable the operation becomes.

DESCRIBE THE RELATIONSHIP BETWEEN COMMERICAL PRICES OF ALUMINIUM, ITS NATURAL ABUNDANCE AND THE RELATIVE COSTS OF PRODUCTION,

The commerical price of aluminium depends mainly on the cost of production. The cost of production is determined by the folloing factors:

The location and size of the ore body. In australia some ore bodes are located in procted natural reserves or aborignal lands, whihc means that the ore there cannot be mined, if it can be mined, there would be large costs both financially and environmetnally; (e.g. compensation to mine on aborigianl lands). Also if the ore is located very deep, it means more money is needed as the machines have to dig deeper. Another factor is the size of the ore body, it needs to be large enough to make sure that profitcs can be made, small ore-bodies may not even cover the start up costs.
The Concentration of metal in the ore body . A large high-grade ore body is economical to mine becuase it conctains a high concentration of the metal in the ore. Some low-grade (i.e. low concentration) deposits may still be economoical to mine if they are large enough and suitably located. Another way low-grade and uneconomical mines becomes efficient if new technologies are invented. The more lower concentration, the more expensive it is to mine.
Mining companies may need to build a railway line from the mining site to the cost, to be able to export the metal extracted. The more remotely located the mine, the greater cost in production.
The cost of producing alumiinium isalso based on how much money was intitially spent on initiaiting the mining operation. This includes the costs of exploration, the establishmnet of the extraction plant (purchasing of machinery and sites for disposal of waste). There are also ongoing costs such as miner’s saleries and housing for their families.

Therfore, as the cost of production rises, it means that automatically the final price of th e aluminium metal will also be generally higher. Also the more of an ore there is naturally in the lithosphere means that the price is lower. In conclusion, both the cost of production and the natural abundance of aluminium need to be factored in to determine the final cost of the aluminium.

ANALYSE INFORMATION TO COMPARE THE COST AND ENERGY EXPENDITURE INVOLVED IN TH EXTRACTION OF ALUMINIUM FROM ITS ORE

The refining of bauxite uses approximately 15000 MJ of energy per tonne of alumina. The smelting of alumina requires an extra 50000 MJ of energy per tonne of aluminium. Therefore the total energy expenditure in the total process of extracting aluminium from bauxite is 65000 MJ. During the Hall-Heroult method, only 5 V is constantly kept between the two electrodes. But an electrical current of, approximately 1 x 105 A, is needed to convert the aluminium ions to the metal, it is also responsible for keeping the electrolyte molten. In the USA, about 5% of all the electricity consumed is used to make aluminium. It costs approximately 100 times more to make a can by extracting aluminium from its ore, than to make it from the recycled aluminium.

For every 1kilogram of aluminium recycled:

14-21 kW/h less electricity is requires for electrolysis
5-8kg of bauxite does not have to be mines
20 kg of greenhouse gases are not emitted into the atmosphere

RECOUNT THE STEPS TAKEN TO RECYLE ALUMINIUM

The Aluminium is collected through local council initiatives and from organisations such as Planet Ark.
Alumium is not magnetic however steel does, so any steel can be removed by using a powerful magnests. The Aluminium is then sorted accoridng to alloy types manually
The aluminium is then compressed into bales; seprate bales is used for cans, these bales are then transported to plants for further recylcling.
The bales are then put into a rotary furnance, which has temperatures upto 780oC, this is sufficent for the aluminium to be melt. Thhe molten aluminium is then cast into ignots, which is then used to creat new drink cans, kitchenware and other various aluminium prodcuts.

JUSTIFY THE INCREASED RECYCLING OF ALUMINIUM IN OUR SOCIETY AND ACROSS THE WORLD

Landfill sites are being less common nowadays, because of recycling of the aluminium. The aluminium industry has also designed more efficient methods to create lighter cans, which means that about 40% less aluminium is needed to create these cans. If less aluminium is needed, this means that less aluminium needs to be mined, hence less energy is used.

Aluminium is a very important material that is used for the production of the bodies of automobiles. The reduction in car weight by using aluminium alloys means that there is lower fuel consumption. Hence less green house gases are released since the fuel is being used at a decreasing rate. Recycling aluminium crap to create cars means that approximately 95% of greenhouse gas emissions are saved.

In the recycling of aluminium, the smelting process needs about 65000 MJ of energy, unlike recycling which only requires 800 MJ. There is no need for combustion reactions, or electrolysis in the recycling processes, thus the major energy usages in the extraction of metals is saved.

This leads directly to the next important reason to recycle metal. Metals like aluminium are a non-renewable resource as they take many thousands even millions of years to form. Aluminium is widely used in modern society because of both its physical and chemical properties. The main physicals properties of aluminium are that it is cheap (mainly because of its abundance) and that is light and malleable. Aluminium also is relatively unreactive compared to other metals, because of the oxide layer that is formed on top.

The most common use of aluminium is in soft drink cans, because it is readily rolled, lightweight, odourless, and tasteless and it conducts heat away from the drink so it can be chilled. As more aluminium is recycles that means that less mines are needed, so that means less fluoride compounds are released during the production of aluminium.

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