2013년 9월 11일 수요일

Extraction Of Aluminium Metal From Aluminium Ore (Bauxite)- Done by Jo


Assignment 1: Extraction Of Aluminium Metal From Aluminium Ore (Bauxite)



  • Information about aluminium ore (Bauxite)
Bauxite, also known as aluminium ore, is one of the metals of greatest abundance in the world, with ample supply suffice to provide for the world-wide demand for aluminium for centuries. It is mainly composed of one or more aluminum hydroxide minerals, plus various mixtures of silica, iron oxide, titania, aluminosilicate, and other impurities in minor quantities.

The Aluminium metal is extracted from bauxite from two main processes; The Bayer process that involves conversion of iron into pure aluminium oxide, which then go through electrolysis for the aluminium to be seperated

Bauxite
(http://geology.com/minerals/photos/bauxite-14.jpg)











  • Extraction process (1) : The Bayer Process
The Bayer process simply refers to the reaction of the bauxite with hydroxide solution. The crushed aluminium ore is treated with sodium hydroxide solution of moderate concentration.Temperatures are typically from 140°C to 240°C; pressures can be up to about 35 atmospheres. High pressure is essential to the solution in order to keep the water in liquid state despite the high temperature of the solution. 

Through the reaction, aluminium oxide reacts with aluminium ore to give a solution of sodium tetrahydroxoaluminate. The equation is stated below.



Through this primary reaction, The impurities, which DO NOT react with sodium hydroxide, can now be separated from sodium tetrahydroxaluminate through the process of filtration. The impurities will be stored in huge lagoons after this process. This red mud have been a significant environmental issue due to its toxic nature.

Impact of the "red mud" on the water bodies
(data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/)

















The sodium tetrahydroxaluminate is then cooled for precipitation of aluminium hydroxide; The sodium tetrahydroxaluminate is "seeded" with some previously produced aluminium hydroxide, hence providing something for the new aluminium hydroxide to precipitate around.
The equation for this process goes like...



Now as aluminium hydroxide has been yielded, it is heated to get aluminium oxide. The heating takes places at around 1100 - 1200°C due to the strong resistance of aluminium against heat. Through the heating, the aluminium hydroxide is segregated into aluminium oxide and water.


And these processes are the main components of The Bayer Process. The aluminium oxide yielded would then go through electrolysis for complete extraction of the aluminium metal.


  • Extraction process (2) : Electrolysis
The extraction is done by electrolysis. But first the aluminium oxide must be made molten so that electricity can pass through it. As Aluminium oxide has a very high melting point (over 2,000°C), melting the metal would not be an economic solution. Hence, aluminium oxide is dissolved in molten cryolite, another aluminium ore, which used to be costly, but now widely available due to availability of lab production.

Cryolite (Na3AlF6)(http://upload.wikimedia.org/wikipedia/commons/thumb/7/7e/
816-_Ivigtut_-_cryolite.jpg/240px-816-_Ivigtut_-_cryolite.jpg)













Along with the molten crytolite, the aluminium oxide goes through electrolysis in an electrolysis cell

Simplified Electrolysis Cell
(http://www.bbc.co.uk/schools/gcsebitesize/science/images/gcsechem_35.gif)


















From the diagram, you can see an aluminium oxide electrolysis tank. Both the negative electrode (cathode) and positive electrode (anode) made of graphite are present in the tank, for the separation of aluminium metal from aluminium oxide.

 Aluminium is released at the cathode (negative electrode) and sinks to the bottom of the tank, where it is tapped off.. Aluminium ions are reduced by gaining 3 electrons.



 Oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away. 




Through this process of electrolysis, pure aluminium metal could be extracted from bauxite.

Aluminium Metal
(http://upload.wikimedia.org/wikipedia/commons/5/5d/Aluminium-4.jpg)



















  • Environmental Issues With Extraction Method.

  • Industrial waste products
During the extraction metal, many harmful by-products are created such as the "red mud"; It is is a mixture of various chemical compounds,such as a watery slurry of fine rock particles and salts, containing toxic heavy metals. It can even be slightly radioactive if the original bauxite contained radioactive minerals. Furthermore, The mud also has a high pH because of the sodium hydroxide solution used in the refining process, hence, destroying the plants and animal habitats if disposed into the environment. This industrial waste has been a significant subject of concern for the environmentalists over the years.

The flooding of the toxic red mud have affected the
residents in Devescer, Hungary
(http://www.cbc.ca/gfx/images/news/topstories/2010/10/06/tp-hungary-sludge-cp-953461.jpg)














  • Waste of lands and other resources
A large space of land is required to carry out different processes for the extraction of the aluminium metal as it is usually produced in large amounts due to the heavy demand for the metal. Also, since vast amount of electricity is needed for electrolysis, (to produce 1 mole of aluminium which only weighs 27 g you need 3 moles of electrons), there will be wastage of fossil fuels which is needed for production of electricity. Hence, not only this extraction process involve unnecessary usage of space, it also contributes to the rapid depletion of the finite natural resources.



  • Solutions
The solution for this problem is definitely recycling of the metal; not only does it help saving of raw materials, particularly electrical energy by not having to extract the aluminium from the bauxite, (Recycling aluminium uses only about 5% of the energy used to extract it from bauxite.), the environmental issues regarding the disposal of the industrial wastes can be greatly reduced as the amount of disposed waste materials would significantly drop. 
Hence, recycling eventually seem to be the most efficacious method for solving the environmental issue regarding extraction of metals. 



-THE END-  :D

Reference:


*If you are unable to read the equations given above, you can open the images in another tab to see it.












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