The extraction of aluminium involves two steps that is, purification of bauxite by Baeyer's process and electrolysis of alumina.
1. Purification of bauxite by Baeyer's process
In the Baeyer's process, the bauxite ore is heated with concentrated NaOH solution under pressure (Aluminum is purified by leaching method). The alumina dissolves as sodium meta aluminate. The other materials present in the ore are left as insoluble part. This solution is filtered in ore are left as insoluble part. This solution is filtered off. From the solution Al(OH)3 is precipitated by adding freshly prepared Al(OH)3 to the cold dilute solution and agitating.
Al2O3 + 2NaOH -------> 2NaAlO2 + H2O
NaAlO2 + 2h2o -------> NaOH + Al(OH)3
The precipitated Al(OH)3 is dried and ignited to get pure alumina.
2Al(OH)3 --------> Al2O3 + 3H2O
2. Electrolysis of alumina
The alumina is dissolved in a mixture of molten cryolite and fluorspar which lowers the melting point. It is then electrolysed in a rectangular steel tank with carbon lining, which serves as cathode. Anode is set of thick carbon rods suspended from the top into the fused Al2O3. The temperature is maintained between 1200 and 1310k. Oxygen is evolved at the anode.
Al2O3 -------> 2Al3+ + 3O2-
At cathode: 2Al3+ + 6 electron -------> 2Al
At cathode: 3O2- --------> 1.5 O2 + 6 electron
Aluminium formed at the cathode gets collected at the bottom of the electrolytic cell from where it is removed periodically.
The metal obtained by this method is about 99% pure. Further purification is carried out by Hoop's electrolytic method. The electrolytic refining of aluminum is carried out in Hoop's cell. The cell consist of iron tank having a lining of carbon. It has three layers of molten liquids with different densities. Molten impure aluminum in the bottom layer which along with the carbon lining acts as the cathode. The middle layer is a mixture of molten fluorides of sodium, barium and aluminum in the molten fluorides of sodium, barium and aluminum in the molten state. This act as the electrolyte. Top layer consist of molten pure aluminum in which a number of carbon electrodes are suspended. These carbon electrodes act as the cathode.
When electric current is passed, the aluminium ions from the middle layer move to the top layer and are discharged at the cathode as pure aluminium. At the same time, an equivalent amount of aluminum from the bottom layer migrates to the middle layer leaving behind the impurities. The pure aluminum is removed from the tapping hole from time to time.
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1. Purification of bauxite by Baeyer's process
In the Baeyer's process, the bauxite ore is heated with concentrated NaOH solution under pressure (Aluminum is purified by leaching method). The alumina dissolves as sodium meta aluminate. The other materials present in the ore are left as insoluble part. This solution is filtered in ore are left as insoluble part. This solution is filtered off. From the solution Al(OH)3 is precipitated by adding freshly prepared Al(OH)3 to the cold dilute solution and agitating.
Al2O3 + 2NaOH -------> 2NaAlO2 + H2O
NaAlO2 + 2h2o -------> NaOH + Al(OH)3
The precipitated Al(OH)3 is dried and ignited to get pure alumina.
2Al(OH)3 --------> Al2O3 + 3H2O
2. Electrolysis of alumina
The alumina is dissolved in a mixture of molten cryolite and fluorspar which lowers the melting point. It is then electrolysed in a rectangular steel tank with carbon lining, which serves as cathode. Anode is set of thick carbon rods suspended from the top into the fused Al2O3. The temperature is maintained between 1200 and 1310k. Oxygen is evolved at the anode.
Al2O3 -------> 2Al3+ + 3O2-
At cathode: 2Al3+ + 6 electron -------> 2Al
At cathode: 3O2- --------> 1.5 O2 + 6 electron
Aluminium formed at the cathode gets collected at the bottom of the electrolytic cell from where it is removed periodically.
The metal obtained by this method is about 99% pure. Further purification is carried out by Hoop's electrolytic method. The electrolytic refining of aluminum is carried out in Hoop's cell. The cell consist of iron tank having a lining of carbon. It has three layers of molten liquids with different densities. Molten impure aluminum in the bottom layer which along with the carbon lining acts as the cathode. The middle layer is a mixture of molten fluorides of sodium, barium and aluminum in the molten fluorides of sodium, barium and aluminum in the molten state. This act as the electrolyte. Top layer consist of molten pure aluminum in which a number of carbon electrodes are suspended. These carbon electrodes act as the cathode.
When electric current is passed, the aluminium ions from the middle layer move to the top layer and are discharged at the cathode as pure aluminium. At the same time, an equivalent amount of aluminum from the bottom layer migrates to the middle layer leaving behind the impurities. The pure aluminum is removed from the tapping hole from time to time.
For more visit Extraction of aluminium (new)
Related article
Extraction of sulphur
Extraction of copper
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Who is the author of this article?
ReplyDeletehari
ReplyDeleteHow can i help u?
What is the electrolyte used in hoope's cell for aluminium extraction ?
ReplyDeleteHoope's cell is used for refining of aluminum.
ReplyDeleteAluminum obtained from the aluminium extraction process contains impurity. So refining is done to remove the impurity.
The electrolyte is middle layer containing ,molten mixture of AlF3, BaF2 and NaF.
Extraction of Aluminium from bauxite
ReplyDeleteSir,
We are interested in extracting aluminium from bauxite. I have gone through ‘ extraction of aluminium from bauxite mastering chemistry help. We would like the detail study for extraction of aluminium from bauxite. I have read through your process of extraction.
Step One:- amount of NaOH required to dissolve bauxite in one litre of NaOH. At what temperature should the mixture be heated and what is the pressure required
Step Two:- The solution is filtered and insoluble is removed
Step Three:- What temperature for cold solution should be maintained and how much should it be diluted for one litre of Al2O3
The precipitate is removed and dried and then ignited( at what temperature)
We have inquiry for export of aluminium for china.Please furnish further detail.
Thank You
Suhas
Hai Suhas,
ReplyDeleteI am glad to know that you are interested in aluminum extraction. There may be some difference in theory and practical procedures. So it is better to seek advice from someone who has practical knowledge about Extraction of aluminum.
How ever Iam glad to help you.
In step 1 the purification of bauxite is known as Leaching. Temperature is at 500k and Pressure at 35 atm.
I have added a new post for you please read
http://entrancechemistry.blogspot.com/2012/07/leaching-metallurgy.html
I think you are from maharashtra, aren't you?
Dear Hari,
ReplyDeleteI have some questions I would like clarified as to the completion of hoop's method. I have looked up as much as I could find about it and am still searching for more answers because the resources behind it seem to be limited. The lack of information leaves me with questions about the specifics of what is being done and I hope you could clarify.
I have built a furnace and have melted aluminum. However I would now like to purify this aluminum so I can create some of my own aluminium alloys. If I do not have a more pure batch of aluminum then my works will be less productive.
If I had the box already created to extract the impurities via hoop's method. Would I need to add in other chemicals to the molten aluminum I already have in order to cause the process to work. In other words. What exactly am I going to need to put inside of the box besides the impure aluminium.
I remember doing a school project where we separated hydrogen and oxygen in water by using carbon sticks and electricity. I assume the process is very similar to that; I think during the separation of water the only thing required was the water itself inside of the container that is. However, in the Bayer process elements are mixed in to lower the melting temperature and other things like that with aluminum.
In one of the above comments you answered a similar question. But I would like some direct confirmation.
"The electrolyte is middle layer containing ,molten mixture of AlF3, BaF2 and NaF."
Are those elements a side effect of Hoop's Method given off by the impure aluminum or am I going to need to mix in some of or all of those elements.
What is the minimum amount of electricity required to make this process work.(This does not have to be exact. More of a general answer should do just fine). I read that they can use 6 volts but 100,000 amps in the Bayer process. Surely that is not necessary to make this happen.
I would really like a response to these questions. I have many more questions regarding aluminum. However I am restraining myself from asking more.
please respond. I will check this site again but I would love an Email from you. Magnes3d@live.com
Yes you need to add those, Cryolite (Na3AlF6), AlF3, BaF2, CaF2
DeleteMay be 1000 amps ... not sure
How can you increase the yield of aluminium?
ReplyDeleteNo practical knowledge on Aluminium extraction. This link may help you !!!!!!! http://my.alacd.com/tms/2003/PAPERS/03_531X_351.PDF
ReplyDeletehow is the silica(gangue) removed from the ore i.e from bauxite
ReplyDelete