Bayer process - bauxite to alumina

The Bayer process is widely used because it is cost-effective and efficient, producing alumina with a high degree of purity suitable for further processing into aluminium metal. There are a number of other acid and thermal methods of refining bauxite, but those acid or electrothermal processes generally are either too expensive or do not produce alumina of sufficient purity for commercial use. 

Where is Bauxite?

Bauxite is 2-20m below ground. Easier to mine as opposed to other mining operations. Soft deposit layer, no drill and blast required.

What is Bauxite?

The mineral composition of bauxite can vary depending on the specific deposit, but the following are typical ranges for the primary minerals found in bauxite:

Gibbsite Al(OH)3: 60-70%      (aluminium hydroxide)

Boehmite γ-AlO(OH): 15-30%    (pronounced bay-mite, aluminium oxide hydroxide)

Diaspore α-AlO(OH): 5-15%     (compound is the same as Boehmite but in different structure)

The γ and α in Boehmite and diaspore indicate the specific crystal structure of the mineral.

Bauxite often contains iron oxide (Fe2O3), calcium oxide (CaO), titanium oxide (TiO2) and silica (SiO2)

Iron oxides : 10-30%

Silica (in the form of quartz or clay minerals): 5-20%

Titanium oxides : 1-10%

Bayer Process

1. Crushing and grinding of bauxite ore into fine power, for increasing the surface area for subsequent chemical reactions.

2. Desliming

Mixed with water to create slurry. Finer particles (i.e. slime) are separated from coarser particles. The slimes are typically rich in silica so are removed.

3. Digestion (probably the most important step)

The basic idea is putting bauxite into some sort of solution, allowing the aluminium parts  to dissolve while the impurities don't. Then easily separate the solution (liquid) and impurities (solid). After then the solution precipitate the aluminium parts back in a much purer form. Below is the process:

The deslimed slurry is mixed with hot (140-250 degree) and concentrated sodium hydroxide (NaOH) in high-pressure (2-3 MPa) digester. During this step, the aluminium-containing minerals in the bauxite react with and dissolve in the sodium hydroxide to form soluble sodium aluminate, while impurities remain as a residue, such as iron oxides and silica, which are not soluble in sodium hydroxide. The sodium hydroxide solution is also called caustic soda. Note, the iron oxides and silica do react with hot and concentrated sodium hydroxide under high pressure, so some pollution may happen if the conditions allow.

Possible reactions during digestion under different conditions:

* For gibbsite, 140°C to 150°C with moderate Sodium Hydroxide Concentration, it forms sodium tetrahydroxoaluminate

         Al(OH)3 + NaOH -> Na[Al(OH)4]

* For gibbsite, 200°C to 250°C with higher Sodium Hydroxide Concentration, it forms sodium aluminate

         Al(OH)3 + NaOH -> NaAlO2 + 2H2O   (preferred)

* For Boehmite and Diaspore, 200°C to 250°C with Moderate to high Sodium Hydroxide Concentration and Elevated pressure:

       AlO(OH) + NaOH + H2O -> Na[Al(OH)4] 

* For Boehmite and Diaspore, 250°C to 270°C with high Sodium Hydroxide Concentration and elevated pressure:

       AlO(OH) + NaOH -> NaAlO2 + H2O  (preferred)

Note, Boehmite and Diaspore have the gamma structure and alpha structure of AlO(OH), but reactions are the same.

NaAlO2 is the preferred outcome

For producing alumina (Al2O3) that is ready for remelting in the electrolytic production of aluminium (via Hall-Heroult process), sodium aluminate (NaAlO2) is preferred as the intermediate compound during the Bayer process. 

Why NaAlO2 is preferred? 

The formation of NaAlO2 in the Bayer process is optimized for high-temperature digestion, which efficiently dissolves aluminium from bauxite, particularly in the presence of boehmite and diaspore, alongside gibbsite. Secondly, NaAlO2 can be easily processed to precipitate itself back to aluminium hydroxide Al(OH3). 

Why dissolving it first and precipitating it back? 

Aluminium hydroxide (gibbsite, boehmite and diaspore) is dissolved in sodium hydroxide first, in order to leave the impurities (iron oxide, silica, etc.) behind. After that, when the solution is diluted and cooled, it precipitates back to aluminium hydroxide, which allows for the removal of residual sodium hydroxide and other impurities, resulting in a purer form of aluminium hydroxide for the following Hall-Heroult process. 

Precipitating Al(OH)3 from NaAlO2

After digestion, the sodium aluminate solution is typically diluted and cooled. This reduces the solubility of Al(OH)3 in the solution, encouraging precipitation. The primary chemical reaction that occurs during the precipitation is:

    NaAlO2 + 2H2O -> Al(OH)3 + NaOH

This is the reverse of the gibbsite reaction in digestion. If small crystals of aluminium hydroxide (seed crystals) are added into the sodium aluminate solution, it promotes / accelerates the precipitation.  So now a much purer form of  Al(OH)3 has been extracted from raw bauxite. Also the output NaOH may be reused for the next round of digestion?

Wait, Na[Al(OH)4] can also precipitate back to Al(OH)3 

Why NaAlO2  is still preferred? if they both precipitate back to Al(OH)3

High temperature Efficiency: The formation of NaAlO2 requires higher temperatures, which accelerate the digestion process, leading to faster reaction rates and improved efficiency in dissolving bauxite.

Energy optimization: while higher temperature requires more energy, the overall efficiency can offset the energy cost, particularly in processing boehmite & diaspore which already requires higher temperatures.

High concentration efficiency: The formation of NaAlO2 requires higher concentrations of sodium hydroxide, which again lead to more efficient digestion, same as the effect of high temperature

Better purity: The higher concentration of sodium hydroxide can improve the purity of the final aluminium hydroxide precipitate, as impurities are less likely to co-precipitate under higher concentrations.

Flexibility: The high temperature and high concentration conditions allow to process various bauxite types, especially those not rich in gibbsite, e.g. with a lot of boehmite and diaspore.

Impurities in Bauxite

Some of the impurities in bauxite, particularly silica, do react with sodium hydroxide during the digestion step of the Bayer process.

Silica present in the bauxite reacts with sodium hydroxide to form sodium silicate, which can remain in solution or form insoluble compounds.

SiO2 + 2NaOH -> Na2SiO3 + H2O

The formation of sodium silicate is problematic because it can react with alumina to form complex sodium aluminosilicates, and cause losses of both alumina and sodium hydroxide, making the process less efficient. This may be managed by selecting low-silica bauxite, desilication before digestion, or use lime CaO to react silica out.

Iron oxides (e.g., hematite, Fe₂O₃; goethite, FeO(OH)) typically do not react with sodium hydroxide under the conditions used in the Bayer process.  It would need higher temperature and pressure. Instead, they remain as solid residues, contributing to the formation of red mud.

Titanium Dioxide (TiO₂), like iron oxides, it also remains largely unreacted and becomes part of the red mud.

Calcium Compounds may react to form calcium aluminate or other compounds, depending on the conditions. This needs handling if calcium is present in significant quantities in the bauxite ore, or lime is used to control silica.

Outcome of digestion

After digestion, the solution contains dissolved sodium aluminate. The solid impurities (red mud) will be separated from the clear solution. The clear solution will be further processed to precipitate and recover alumina.

4. Clarification

The outcome from the digestion stage  is slurry that contains sodium aluminate and undissolved impurities (red mud). This mixture is allowed to settle in large tanks, where the red mud is separated from the clear sodium aluminate solution.

This process relies on both physical and chemical principles to achieve effective separation. It simply allows the slurry / mixture to sit undisturbed so that the solids can settle out due to gravity.  The equipment is clarification tank / settling tank / thickener. As the slurry remains in the tank, the red mud particles begin to settle at the bottom due to gravity. The clear liquid (overflow) is collected at the top of the tank. It may have rakes or scrapers to periodically remove settled solids from the bottom of the tank as well. Another equipment is flocculator. In some cases, flocculants (chemical additives) are used to aid in the settling process. These flocculants help to agglomerate fine particles into larger flocs, which settle more quickly.

5. Precipitation

The clarified sodium aluminate solution is cooled and seeded with aluminium hydroxide crystals. This induces the precipitation of aluminium hydroxide Al(OH)3 from the solution.

Without the seeds, it still precipitates but at a much slower pace.  The reaction as mentioned earlier above, is:

        NaAlO2 + 2H2O -> Al(OH)3 + NaOH

The aluminium hydroxide precipitates out of the solution as a solid, and the sodium hydroxide is recycled. The precipitated aluminum hydroxide, often referred to as hydrate, typically appears as a fine, white, powdery substance.

6. Calcination

The aluminium hydroxide Al(OH)3 is then washed, dried and subjected to high-temperature calcination (~1000°C-1100°C) to remove the water. This heating process, known as calcination, drives off water molecules and converts the aluminium hydroxide into alumina (anhydrous/waterless alumina Al2O3)

        2Al(OH)3 -> Al2O3 + 3H2O

Final Product and by-product

The final product is a white powder, alumina Al2O3, which is then used as the input for aluminium production via the Hall-Héroult process in electrolytic reduction cells.

Red Mud is a significant by-product of the Bayer process. It's a highly alkaline residue that must be managed carefully due to its environmental impact.