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Aluminium Purification Methods: Baeyer’s, Hall’s, Serpeck's and Goldschmidt Aluminothermic Process

Most of you must have visited your villages during your summer vacations to meet your grandparents right?

There are huge trees and green fields in villages. Have you noticed a bee's hives hanging over the branches of tree? Many of you are probably aware of this.

Tell me, how many of you like eating honey? What type of question is this you may say!! Everyone enjoys eating such pure honey that directly comes from the honeycomb. Children love eating honey in bread, chappatis and many more. Honey is a lovely product that is regarded as a top health food all over the world. Honey is one of the most extensively used sweeteners, and it has numerous health benefits. It is used as a foundation for many traditional remedies, especially in Ayurveda, by various civilizations throughout the world.

Wait! Why am I telling you the benefits of honey? We don’t have to focus on honey but we need to focus on the process. Yes! How we are extracting honey from bee hives and honeycombs and making it feasible to use. Bringing it to the People and Making it Profitable is all that we need.

Similarly, like honey, we also extract metals and make them profitable to use. Raw metals contain many impurities which can’t be used in such a way, so there are different processes to extract metals. Today we will study the extraction of aluminium in detail. So let’s begin.

Table of content:

What is Aluminium?

Aluminum is a silvery-white metal and the 13th element in the periodic table. According to an astounding statistic, aluminium makes up more than 8% of the earth's core mass, making it the most common metal in the world. After oxygen and silicon, it is the third most prevalent chemical element on the globe. With 8.3% of the earth's crust made of aluminium, it is the third most abundant element in the universe. Slate, clay, and various silicate rocks all contain it. Only the combined state contains it.

Occurrence:

  • Aluminium is the third most plentiful element on the planet, accounting for 8.3% of the earth's crust. 
  • It's found in clay, slate, and a variety of silicate rocks. It can only be found in the situation in which everything is blended.
  • The following minerals are essential although pure aluminium oxide is colourless, impurities can give it a variety of colours. Some impure forms of alumina, Al2O3, occur naturally and are attractive, uncommon, and highly valued.
  • Sapphire, alumina with Ti4+ impurities (typically blue), Ruby, alumina with Cr3+ impurities (usually red), and Topaz, alumina with Fe3+ impurities (usually red) are gem-quality corundum.

Purification method-Baeyer’s process, Hall’s process, Serpeck's process, Calcination of aluminium hydroxide:

Bauxite ore, which is commonly contaminated with ferric oxide and silica, is used to extract aluminium. Because it is difficult to remove iron and silicon from aluminium metal, the removal of ferric oxide and silica from bauxite ore before electrolysis is required. The presence of these elements renders the aluminium metal brittle and prone to corrosion. The extraction of aluminium from bauxite ore involves the following three processes:
i)Bauxite ore purification, i.e. elimination of ferric oxide and silica.
ii) Al2O3 electrolytic reduction
iii) Aluminum purification via electrolysis.

We will study here some important processes used in the purification of bauxite.

Bauxite ore purification: The following processes are utilised to purify bauxite ore.

  • Baeyer’s process: This method is mostly used on bauxite ore with ferric oxide as the major contaminant. Because the ore is usually red in colour, it is known as red bauxite.

To convert any ferrous oxide present in the powdered ore into ferric oxide, it is first roasted at a low temperature. It is subsequently digested in an autoclave at 150C for many hours with a concentrated sodium hydroxide solution (density 1.45 g mL-1). While aluminium oxide dissolves in caustic soda to generate soluble sodium meta aluminate (NaAlO2), ferric oxide and silica remain insoluble and settle to the bottom. Filtration is used to eliminate them.
Al2O3.2H2O(s) + 2NaOH(s) --> 2NaAlO2(aq) + 3H2O(aq)
When sodium meta- aluminate undergoes hydrolysis with the generation of Al(OH)3 precipitate, the solution is stirred with freshly precipitated Al(OH)3 for 36 hours.
NaAlO2(aq) + 2H2O(aq) --> Al(OH)3(s) + NaOH(aq)
The precipitate is washed and dried before being used. The NaOH solution can be concentrated and used once more.

  • Hall's Process: Sodium carbonate is fused with bauxite. Sodium meta-aluminate is formed when aluminium oxide, Al2O3, reacts with sodium carbonate to generate sodium meta-aluminate. Water is employed to remove the fused bulk, although Fe2O3 and SiO2 remains insoluble in the residue.
    Al2O3(s) + Na2CO3(s) --> 2NaAlO2(s) + CO2(g)
    Warm the sodium meta aluminate solution to 50-60 ℃ and pump carbon dioxide through it. As a precipitate, Al(OH)3 separates.
    2NaAlO2(aq) + CO2(g) + 3H2O(aq) --> 2Al(OH)3(s) + Na2CO3(s)

The filtrate is then washed, dried, and filtered. The Na2CO3 solution is concentrated and reused.

  • Serpeck's Process: When silica is present in significant proportions in bauxite ore, this technique is used. The ore is combined with coke and heated to 1800°C in the presence of nitrogen, resulting in the formation of AlN (aluminium nitride).
    Al2O3(s) + 3C(s) + N2(g) --> 2AlN(s) + 3CO(g)
    At this temperature, silica is converted to silicon, which volatilizes.
    SiO2(s)+2C(s)-->Si(s)+2CO(g)

Aluminium hydroxide is formed by hydrolyzing AlN with water.
AlN(s) + 3H2O(aq) --> Al(OH)3(s) + NH3(g)

The ammonia collected as a valuable by-product is a distinguishing advantage of this method.

  • Calcination of aluminium hydroxide: The aluminium hydroxide precipitate obtained in the preceding step is calcined in a rotating kiln at 1500°C to obtain pure aluminium oxide, Al2O3 (commercially referred to as alumina).
    2Al(OH)3(s) --> Al2O3(s) + 3H2O(aq)

Gold Schmidt's alumino-thermic process:

Because it has a strong attraction for oxygen at high temperatures, aluminium has the ability to displace elements that are less electropositive than itself from their oxides. Because the reaction is so exothermic, the metal that is liberated is obtained in a molten state and is protected from oxidation by a layer of fluid slag made of Al2O3.
Oxide + Al(s) --> Al2O3 + metal + heat energy
The alumino-thermic method is based on this reaction. Goldschmidt was the one who discovered it. 

There are two main applications for the procedure:

1. Extraction of metals and non-metals: Metals like Cr, Mo, and Mn, as well as non-metals like boron and silicon, can be extracted from their oxides.
Cr2O3(s) + 2Al(s) --> 2Cr(s) + Al2O3(s) + heat energy
3Mn3O4(s) + 8Al(s) --> 9Mn(s) + 4Al2O3(s) + heat energy
B2O3(s) + 2Al(s) --> 2B(s) + Al2O3(s) + heat energy
SiO2(s) + 2Al(s) --> 2Cr(s) + Al O3(s) + heat energy
Aluminum powder is added to the oxide. With the use of a cartridge, the mixture is ignited in a sand-encased fire clay crucible (containing magnesium powder and barium peroxide). The reaction generates a considerable quantity of heat, which fuses both the alumina and the free element. There are two distinct layers that can be easily separated.

2. Thermite welding of metals:

  • Metal welding, particularly steel welding, is another application.
  • A sand and clay mould surrounds the fractured part to be welded, which is heated to redness with a gasoline torch or a blast lamp. In a crucible lined with magnesite and with a plug hole, a 1: 3 mixture (known as thermite) of aluminium powder and Fe2O3 is is placed. 
  • This is covered with a magnesium ribbon and put into a combination of magnesium powder and barium peroxide. A magnesium ribbon is used to light the thermite.
  •  The temperature rises to over 2500°C as iron oxide is converted to iron. The molten iron is then tapped into the weldable sections. The shattered iron's hot surface melts and mingles with the molten iron, resulting in a flawless weld.

Practice Problems:

Q1. The fluoride ore of aluminium is referred to as ___________

A. turquoise,
B. bauxite
C. cryolite
D. feldspar,

Answer: C

Solution: Cryolite, or Na3AlF6, is an aluminium fluoride ore. Bauxite (Al2O3.2H2O) is an aluminium oxide ore. Feldspar (KAlSi3O8) is an aluminium silicate ore. Turquoise, (AlPO4Al(OH)3.H2O, is an aluminium phosphate ore.

Q2.__________ is a poor conductor of electricity in its purest form.

A. Alumina
B. Silica
C. Aluminium
D. Cryolite

Answer: A

Solution: Alumina is a poor conductor of electricity in its purest form.The challenges are overcome by combining alumina, cryolite (Na3AlF6), and fluorspar (CaF2) in a 20: 40: 20 ratio. This mixture has a fusion temperature of 900°C and is a good conductor of electricity.

Q3._____________ process is used purification of bauxite ore

A. Froth flotation
B. Blast furnace
C. Baeyer's Process
D.Thermite

Answer: C

Solution: Baeyer's Process is mostly used on bauxite ore with ferric oxide as the major contaminant. Because the ore is usually red in colour, it is known as red bauxite.

Q4. Aluminium being more electropositive in nature displaces ____________ from their salt solutions.

A. Zn metal
B. Cu metal
C. Pb metal
D. All of the above

Answer: D

Solution: Because Al is more electropositive, it displaces copper, zinc, and lead from their salt solutions.
Here are the reactions of these metals with Al.
3ZnSO4(s) + 2Al(s) --> Al2(SO4)3(s) + 3Zn(s)
3CuSO4(s) + 2Al(s) --> Al2(SO4)3(s) + 3Cu(s)
3PbSO4(s) + 2Al(s) --> Al2(SO4)3(s) + 3Pb(s)

Frequently asked questions-FAQ

Q1. What are the procedures for obtaining aluminium from bauxite?

Solution: These are the few steps which need to be followed while obtaining aluminium from bauxite

  • It's done entirely by hand–picking, grinding, and so on.
  • Impurities in bauxite ore often include ferric oxide and silica.
  • Crushing and pulverising are used to dress Bauxite ore.
  • A magnetic separation process is used to remove ferric oxide contaminants from Bauxite.

Q2.What is the benefit of performing extraction many times?

Solution: To ensure that the maximum amount of the target molecule is separated, the entire extraction procedure is usually performed numerous times. As a result, the phase holding the original combination must also be saved.

Q3.What is the impact of aluminium extraction on the environment?

Solution: The strip-mining procedure used to extract bauxite from the earth eliminates all native vegetation in the mining region, resulting in a loss of habitat and food for local species, as well as severe soil erosion.

Q4.Explain the process of solvent extraction.

Solution: The difference in solubility or distribution coefficient between these two immiscible (or slightly soluble) solvents causes a component to transfer from one to the other during solvent extraction. When compared to other separation procedures, it has a better separation effect than chemical precipitation, as well as a higher degree of selectivity and faster mass transfer than the ion exchange approach.

 

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