Aluminum production process-Bayer & Hall-Héroult

Mostly aluminum is produced in two ways.

1.         Primarily Production Process

1. By refining aluminum from grinding and extracting from bauxite ore - Bayer process

2. By using reduction process for crystalline alumina - Hall-Héroult process

2.         Secondary Production Process

1. By doing pre-treatment of scraps such as cleaning and sorting  

2. By process of smelting, refining or alloying of aluminum

Bayer process: Refining Bauxite

In this process, the bauxite ore is dried, crushed and ground into a powder. It is then mixed with preheated spent solution. Lime is added to increase the solubility of alumina and the resulting mixture is pumped to digesters along with a solution of caustic soda to extract the alumina. The operation proceeds at elevated temperatures (using high temperature steam) and pressures in
digesters. The range of temperature and pressure used depends on the type of bauxite: boehimite, gibbsite or diaspora. For gibbsite, the temperature range is the lowest (about 140^oC) while for boehmite, the highest temperature is used.(about 300^oC)  

A slurry containing dissolved sodium aluminate and a mixture of metal oxides called red mud is removed in clarifiers.  The red mud is washed to recover the chemicals and is disposed.

The aluminate solution is cooled and seeded with fine crystals of aluminatrihydrate to crystallize the hydrated alumina in precipitator tanks.  The alumina is precipitated as trihydrate as crystals which are then washed and then calcined in rotary kilns or fluid bed calciners to produce the aluminum oxide or alumina. 

The rotary kiln temperature is around 1200-1300^oC to remove water of hydration which then makes the alumina stable for use in the Hall-Heroult process. 

 Figure  Bayer process flow sheet

  

The Hall-Héroult Process: Reduction of Alumina to Aluminum

Aluminum is produced from alumina by electrolysis in a process known as the Hall-Héroult Process.  The alumina is dissolved in an electrolytic bath of molten cryolite (sodium aluminum fluoride) to reduce melting point and thus energy requirement of producing aluminum. An electric current is passed through the electrolyte and flows between the anode and cathode.  Molten aluminum is produced, deposited at the bottom of the electrolytic cell and periodically siphoned off and transferred to a reverberatory holding furnace.  There it is alloyed, fluxed and degassed to remove trace impurities.  Finally, the aluminum is cast or transported to the fabricating plants. The reaction is simplified as,

2Al₂O₃ + 3C  4Al +3CO₂ 

The equation shows that 2 tons of alumina and 0.5 ton of carbon produces 1 ton of aluminum. The heat of reaction is 290.4kcal/mol or 6.27kWh/kg of aluminum. This is however, the theoretical amount or minimum amount of energy required. 

Secondary production process

In today’s world where there is so much Aluminum just lying around everywhere, and can be used over and over indefinitely through recycling there is not much need for the production of Aluminum. 

Scrap recycling

Alternatively scrap materials with Al content of more than 20% can be recycled. To obtain this level of quality, all adherent materials must be removed and the scrap sorted according to alloy type and content. For example turnings are centrifuged and dried to remove the oil and water, iron is separated magnetically while used beverage cans are processed to remove the interior lacquer coating and the outside product display printing inks.

Aluminum hydroxide

Alternative Al can be produced from aluminum hydroxide Al(OH)₃. Aluminum hydroxide is a waste in aluminum production and can be assumed to be free of impurities as it has been produced. Thus, it can be calcinated directly requiring a theoretical amount of 3.38kWh/kg aluminum. Adding the energy required for carbon anode production and smelting, the total energy required is 17kWh/kg. This is only 43% of the energy required in primary Al production. 

Related posts

Aluminum sulphate production 

Aluminum hydroxide production