Aluminium is industrially prepared by using a process called the Hall-Héroult process, which involves the electrolysis of alumina mixed with molten cryolite. Let's break down the process step by step:

1. Raw materials:
- Alumina: Alumina is extracted from bauxite ore through a chemical process called the Bayer process. Bauxite ore is rich in aluminium oxide, which is converted into alumina through various purification steps.
- Cryolite: Cryolite (Na3AlF6) is a fluoride mineral that is used as a solvent in the electrolytic reduction of alumina. It helps to lower the melting point of alumina, making the process more energy-efficient.

2. Preparation of the electrolyte:
- Alumina is mixed with molten cryolite in a ratio of about 2:1. This mixture forms the electrolyte, which is used in the electrolytic cell.
- The addition of cryolite helps to lower the melting point of alumina from 2072°C to around 950°C, reducing the energy required for the electrolysis process.

3. Electrolysis process:
- The electrolytic cell used for aluminium production is called a reduction cell or a Hall-Héroult cell.
- The cell consists of a carbon-lined steel shell with a layer of carbon blocks as the cathode (negative electrode) and a carbon anode (positive electrode) suspended in the molten electrolyte.
- When an electric current is passed through the cell, electrolysis occurs.
- The alumina in the electrolyte dissociates into aluminium ions (Al3+) and oxygen ions (O2-).
- The oxygen ions react with the carbon anode and form carbon dioxide gas (CO2), while the aluminium ions are reduced at the cathode and deposited as molten aluminium metal.

4. Collection of molten aluminium:
- The molten aluminium metal collects at the bottom of the reduction cell and is periodically tapped off.
- The collected aluminium is further processed and purified to remove impurities before being used for various industrial applications.

In conclusion, aluminium is industrially prepared by the electrolysis of alumina mixed with molten cryolite. This process helps to lower the melting point of alumina and reduce the energy consumption required for aluminium production.