Bauxite Extraction and Processing
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Questions and Answers

What is the purpose of the coke powder layer in the electrorefining of aluminum?

  • To enhance the conductivity of the electrolyte
  • To act as a catalyst for the reaction
  • To increase the density of the molten aluminum
  • To prevent corrosion at the liquid-solid-air interface (correct)

    • Which component of the electrolyte mixture is essential for adjusting density in the electrorefining process?

  • CaF2
  • Cryolite
  • BaF2 (correct)
  • Al2O3

    • Why is copper deliberately added to impure aluminum during the electrorefining process?

  • To improve the flow of electricity
  • To increase the density for layer separation (correct)
  • To promote the oxidation reaction
  • To enhance thermal conductivity

    • What occurs at the cathode during the electrorefining process of aluminum?

    <p>Pure aluminum is deposited (C)</p> Signup and view all the answers

    What is the primary purpose of adding cryolite in the electrolytic reduction of Al2O3?

    <p>To decrease the melting point of the alumina (D)</p> Signup and view all the answers

    Which chemical species is formed in the Bayer process when aluminum hydroxide reacts with sodium hydroxide?

    <p>Al(OH)4- (D)</p> Signup and view all the answers

    In the electrolytic reduction process, what role does the carbon lining of the iron tank serve?

    <p>It acts as the cathode for the electrolysis (B)</p> Signup and view all the answers

    What is the result of the reaction between CO2 and hydroxide ions in the context of aluminum processing?

    <p>Production of water and carbonate ions (D)</p> Signup and view all the answers

    What is an adverse effect of impurities in bauxite during the extraction of alumina?

    <p>Reduced efficiency in chemical reactions (C)</p> Signup and view all the answers

    Which component is NOT part of the molten mixture used in the Hall process?

    <p>Sodium hydroxide (C)</p> Signup and view all the answers

    During the electrolytic reduction of alumina, what is produced at the anode?

    <p>Oxygen gas (B)</p> Signup and view all the answers

    In the Bayer process, which compound is primarily removed from bauxite to purify alumina?

    <p>Silica (B)</p> Signup and view all the answers

    What occurs at the anode during the Hall Process?

    <p>Al → Al3+ + 3e (B)</p> Signup and view all the answers

    Why is pure Al2O3 not used as the electrolyte in the Hall Process?

    <p>It volatilizes at high temperatures. (D)</p> Signup and view all the answers

    Which of the following statements accurately describes the behavior of impurities in the Hall Process?

    <p>They remain intact in the bottom layer. (D)</p> Signup and view all the answers

    What is the primary reason that aqueous solutions of Al salts cannot be used as electrolytes?

    <p>Hydrogen is discharged instead of aluminum. (C)</p> Signup and view all the answers

    What reaction occurs at the cathode during aluminum extraction from Al(NO3)3?

    <p>2H+ + 2e → H2 (B)</p> Signup and view all the answers

    Why is molten AlCl3 unsuitable as an electrolyte?

    <p>It sublimates easily and is a poor conductor. (A)</p> Signup and view all the answers

    In the Hall Process, what does the bottom layer typically represent?

    <p>The cathode where aluminum is deposited. (B)</p> Signup and view all the answers

    What is the role of the hydroxide ions at the anode during the electrolysis with aqueous aluminum nitrate?

    <p>To produce oxygen gas and water. (B)</p> Signup and view all the answers

    Flashcards

    Electrorefining of Aluminium

    A process to purify aluminium by using electrolysis. Impure aluminium is taken as anode, and the graphite rod is cathode.

    Electrolyte in Aluminium Electrorefining

    Molten mixture of cryolite and BaF2, saturated with Al2O3. BaF2 is used instead of CaF2 to adjust density

    Graphite rods in Electrorefining

    Graphite rods act as electrodes in the electrolytic cell, connecting the electrical circuit

    Impure Aluminium

    Aluminium containing other metals or impurities.

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    Copper melt purpose

    Copper melt is deliberately added to impure aluminium to increase its density and form a separate layer (bottom layer).

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    Electrolytic reduction of Al2O3

    A process using electricity to reduce aluminum oxide (Al2O3) into aluminum metal.

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    Molten alumina composition

    A mixture of 20% aluminum oxide, 60% cryolite, and 20% fluorspar.

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    Electrolytic reduction container

    An iron tank lined with carbon that acts as the cathode.

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    Anode material

    A graphite rod used as the anode in the electrolytic reduction process.

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    Cathode material

    The iron tank lining, which acts as the cathode.

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    Cryolite

    A mineral that lowers the melting point of alumina and makes the electrolytic reduction process more efficient.

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    Fluorspar

    A mineral that also helps lower the melting point and improve the electrolytic process.

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    Powdered coke layer

    A layer at the top of the container, which is maintained during the electrolytic process.

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    Electrolysis of Aluminum

    A process used to produce pure aluminum, using molten cryolite as an electrolyte, not pure aluminum oxide.

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    Aluminum Anode

    The top surface of the bottom layer in the electrolysis process, where aluminum metal is oxidized (loses electrons).

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    Aluminum Cathode

    The bottom surface of the top layer in the electrolysis of aluminum, where aluminum ions gain electrons to become aluminum metal.

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    Molten Cryolite Electrolyte

    Used in the electrolysis of aluminum to reduce aluminum ions (Al3+) by providing the ionic medium for the reaction.

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    Impurities in Aluminum

    Substances like iron (Fe), silicon (Si), and copper (Cu) that remain in the bottom layer of the electrolysis process, not part of the pure aluminum.

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    Why not use pure Al2O3?

    Pure aluminum oxide (Al2O3) has a high melting point and causes high energy consumption issues, aluminum vaporization issues, and potential oxidation concerns.

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    Aqueous Al Salt Electrolyte

    Aqueous solutions of aluminum salts are not suitable for aluminum electrolysis because hydrogen gas is produced at the cathode instead of aluminum.

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    Molten AlCl3 Electrolyte

    Molten aluminum chloride is a poor conductor of electricity because its nature is covalent.

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    Study Notes

    Bauxite Extraction

    • Bauxite is the most common ore used for aluminum extraction.
    • Two types of bauxite: red (impurities Fe₂O₃, SiO₂, TiO₂) and white (impurities SiO₂, Fe₂O₃, TiO₂).
    • Main steps of aluminum extraction from bauxite:
      • Beneficiation of bauxite to produce pure alumina
      • Electrolytic reduction of pure alumina
      • Electrorefining of aluminum

    Bayer's Process

    • Beneficiation process for red bauxite.
    • Process stages shown in Figure 6.24.
    • Finely powdered bauxite is digested with 45% NaOH at 150°C and 80 lb pressure for 8 hours.
    • Reaction in step one: Al₂O₃ + 2OH⁻ + 3H₂O → 2[Al(OH)₄]⁻
    • Reaction in step two: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O
    • Residue is Fe₂O₃, TiO₂ + other insoluble impurities
    • Filtrate is NaAlO₂ + Na₂SiO₃
    • Al(OH)₃ is precipitated (step 2) using water and a seeding agent, or CO₂.
    • Pure Al₂O₃ is obtained after ignition at approximately 1100°C.

    Hall's Process

    • Beneficiation process for low-grade red bauxite.
    • Various stages in Figure 6.25.
    • Finely divided bauxite ore mixed with Na₂CO₃ and CaCO₃, fused at 1100°C.
    • CO₂ is passed, and filtered to obtain Al₂O₃.
    • Key reactions:
      • Step 1: Al₂O₃ + Na₂CO₃ → 2NaAlO₂ + CO₂↑
      • Step 2: SiO₂ + Na₂CO₃ → Na₂SiO₃ + CO₂↑
      • Step 3: Fe₂O₃ + Na₂CO₃ → 2NaFeO₂ + CO₂↑
      • Step 3: CaO + SiO₂ → CaSiO₃
      • Step 3: 2NaAlO₂ + CO₂ + 3H₂O → 2Al(OH)₃↓ +Na₂CO₃
      • Step 3: 2Al(OH)₃ → Al₂O₃ + 3H₂O

    Serpeck's Process

    • Beneficiation process for white bauxite in Figure 6.26
    • Finely powdered bauxite + coke powder heated to 1800°C in N₂ gas.
    • Reactions:
      • Step 1: Al₂O₃ + 3C + N₂ → 2AIN(s) + 3CO↑
      • Step 2: SiO₂ + 2C → Si↑ + 2CO ↑
      • Step 3: AIN + NaOH + 3H₂O → Na[Al(OH)₄] + NH₃ + H₂O
      • Step 3: Na[Al(OH)₄] → Al(OH)₃ ↓ + NaOH

    Electrolytic Reduction of Al₂O₃

    • Molten alumina (20%), cryolite (60%), and fluorspar (20%) are used in an iron tank.
    • Carbon lining acts as the cathode.
    • Graphite rod acts as the anode.
    • Reactions: Al₂O₃ → 2Al³⁺ + 3O²⁻
    • At cathode: Al³⁺ + 3e⁻ → Al
    • At anode: 2O²⁻ → O₂ + 4e⁻
    • Cryolite helps reduce melting temperature to 900°C and improve conductivity.

    Electrorefining of Aluminium

    • Impure aluminium mixed with copper melt is used, with a graphite lining.
    • Pure Al layer acts as the cathode.
    • The electrolyte is a molten mixture of cryolite, and BaF₂ or CaF₂.
    • Impurities like Cu are removed by this process.

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    Description

    This quiz covers the extraction of aluminum from bauxite, including the key processes involved such as Bayer's and Hall's processes. Learn about the types of bauxite, the steps of aluminum extraction, and the chemical reactions that take place during the beneficiation processes. Test your knowledge on these essential topics in metallurgy.

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