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Chapter 8: Problem 11

How is alumina melted in the electrolytic extraction process of molten alumina?

Short Answer

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Question: Briefly describe the steps involved in the electrolytic extraction process of molten alumina. Answer: The electrolytic extraction process of molten alumina involves preparing an electrolyte mixture of alumina and cryolite, setting up an electrolytic cell with graphite anodes and cathodes, heating the electrolyte to melt cryolite and dissolve alumina, applying electric current to initiate electrolysis, and collecting pure aluminum from the cathode while oxygen gas is released at the anode.

Step by step solution

01

Preparation of electrolyte

In the electrolytic extraction process of molten alumina, a solvent called electrolyte is needed for the melting. The electrolyte is a mixture of alumina (Al2O3) and cryolite (Na3AlF6). Cryolite is added to the mixture because it helps to decrease the melting point of alumina and increases the electrical conductivity of the solution.
02

Setting up the electrolytic cell

An electrolytic cell is set up consisting of a container, inert graphite anodes, and cathodes. The container is lined with graphite, which acts as the cathode, while the graphite anodes are suspended in the cell. The electrolyte mixture is added to the electrolytic cell.
03

Heating the electrolyte

The electrolyte mixture in the electrolytic cell is heated to a temperature between 950掳C and 980掳C. This temperature will melt the cryolite and allow alumina to dissolve in it, forming a homogeneous molten solution that is ready for the extraction process.
04

Applying electric current

An electric current is applied to the electrolytic cell. The electric current is passed through the anodes, the molten electrolyte, and the cathodes. Due to the electric current, the molten alumina undergoes electrolysis, and aluminum ions (Al^3+) and oxide ions (O^2-) are formed.
05

Formation of aluminum at cathodes

The aluminum ions (Al^3+) are attracted towards the cathode (graphite lining) and receive three electrons to form pure aluminum atoms. These aluminum atoms then deposit on the cathode, forming a liquid layer that can be collected. The reaction at the cathode can be described as: Al^3+ + 3e^- -> Al (s)
06

Formation of oxygen at anodes

The oxide ions (O^2-) move towards the anodes, where they lose their extra electrons, combine and form oxygen gas (O2) which is released at the anode. The reaction at the anode can be described as: 2O^2- -> O2 + 4e^- To summarize, alumina is melted in the electrolytic extraction process by dissolving it in molten cryolite at high temperatures in an electrolytic cell. An electric current is then applied, causing electrolysis and the formation of aluminum at the cathode and oxygen at the anode.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Alumina
Alumina, chemically known as aluminum oxide ( Al_2O_3 ), is a key material in the extraction of aluminum. It is extracted from the mineral bauxite through the Bayer process. In aluminum extraction, alumina is dissolved in a molten mixture for the electrolytic process to occur. This mixture primarily consists of alumina and cryolite. By itself, alumina has a high melting point which makes it challenging to manage during extraction. Hence, the importance of another substance that assists in the melting process.
Cryolite
Cryolite ( Na_3AlF_6 ) plays a crucial role in the electrolytic extraction of aluminum. This mineral is used as a solvent to reduce the melting point of alumina from over 2000掳C to between 950掳C and 980掳C. Reducing the melting temperature makes the process significantly less energy-intensive. Additionally, cryolite contributes to enhancing the conductivity of the solution, which is vital for the electrolysis process. By lowering the energy requirements and improving conductivity, cryolite makes the entire process more economical and efficient.
Electrolytic Cell
An electrolytic cell is the apparatus in which electrolysis occurs. For aluminum extraction, the cell is constructed to contain the molten electrolyte, with components that conduct electricity efficiently. The cell consists of a container lined with graphite that functions as the cathode. Graphite anodes are suspended overhead to immerse in the solution. This setup allows for the effective flow of electric current through the molten electrolyte, prompting the decomposition of alumina into its elements. It is where the controlled environment of high temperature and electrical flow comes together to facilitate extraction.
Electrolysis
Electrolysis is a chemical process that involves passing an electric current through a liquid or solution to trigger a chemical reaction. In aluminum extraction, electrolysis is used to separate aluminum ions ( Al^{3+} ) from oxide ions ( O^{2-} ) within the molten electrolyte. The electric current causes these ions to migrate toward the respective electrodes: aluminum ions to the cathode and oxide ions to the anode. This movement and subsequent reactions at the electrodes lead to the formation of aluminum metal and oxygen gas, respectively, promoting the overall objective of aluminum extraction.
Graphite Anodes and Cathodes
In the electrolytic cell, graphite is used for both anodes and cathodes. The inert nature of graphite makes it ideal for withstanding high temperatures and chemical exposure without reacting. At the cathode, aluminum ions receive electrons to form aluminum metal, while at the anode, oxide ions release electrons and form oxygen gas. The reactions at the electrodes are pivotal for converting ionic compounds into elemental forms. The graphite lining of the cell also helps in collecting the liquid aluminum that forms, making it a practical choice for repeated cycles of the process.

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