Our production processes
Today, more than 95% of alumina is produced from bauxite using the Bayer process. Bauxite is the ideal raw material because it contains alumina in a non-combined form and in great quantities (30% to 65%).
The Bayer process is the industrial route to produce alumina, named after its creator Karl Josef Bayer in 1887. This was one year after the simultaneous invention of the electrolytic process by Hall and Héroult to manufacture alumiium from alumina.
The Bayer process was implemented for the first time in 1893, by the Pure Alumina Company, in Gardanne (France), chosen for its setting close to both bauxite and the coal necessary for thermal energy.
The mineral bauxite is firstly crushed to form grains with a diameter of less than 30 mm in hammer crushers. It is then mixed
with liquor recycled from the down- stream process and then ground further to obtain fine grains with a diameter of less than 315µm.
This grinding is necessary to increase the contact surface between the liquor and the bauxite and to improve the yield of the digestion. The recycled liquor comes from the filtration stage after hydrate precipitation.
This liquor is enriched in soda (NaOH) and lime (CaO) before the grinding stage to aid digestion by making the li-quor more “aggressive” towards the bauxite. The permanent recycling of liquor and more generally of water, is the source of the synonym for the Bayer process: the“Bayer Cycle”.
The bauxite-liquor mixture is a red suspension (or slurry) that is sent into the digesters.
The bauxite-liquor slurry that results from grinding is preheated then sent into digesters for several hours.
The temperature and the pressure at which the slurry is digested depends on the bauxite and on the type of process. A gibbsitic bauxite can be digested
at atmospheric pressure whereas tens of bars and more than 200°C are necessary to make the alumina contained in diasporic bauxite soluble. The alumina is dissolved in the liquor in the form of sodium aluminate while the other compounds are deposited in the form of insoluble scale. The digesters need regular maintenance to clean this scale. The slurry is diluted after digestion in order to make settling easier.
During the digestion stage of the Bayer process, two phenomena occur:
- Dissolution of the alumina contained in the bauxite
- Formation of solid residues: so called, "red mud" or bauxite residue.
Here is the main reaction of the Bayer process:
This reaction is reversible depending on the pressure and temperature. The digestion conditions displace the balance to the right and so the alumina contained in the bauxite can be dissolved. Other compounds are also dissolved and these are the source of the impurities in aluminas produced by the Bayer process.
The mud is made up of the insoluble bodies coming from the bauxite and from the solid desilication products that are formed during digestion.
The objective of this stage is to separate the two slurry phases :
- The liquor containing sodium aluminate
- The residue from which the alumina has been extracted
The residue and the liquor that make up the slurry are separated by settling. The solid particles fall to the bottom of the settling tank (which has a very large diameter) and are extracted by pumping towards the mud wash. The floating liquor is filtered then sent to be precipitated.
There are two objectives in washing the residue after its extraction from the settler:
- To recuperate the sodium aluminate, which will be used again in the Bayer cycle.
- Cleanse the residue of soda so that it can be stored in a natural environment.
After being washed, the soda content in the residue is very low. It is simply bauxite without its alumina content. At Gardanne, 100% of the residue are now stored on land. After appropriate treatment, the residue is transformed into Bauxaline® which can be used for example as sublayers for road construction.
This stage marks the beginning of the “white side” of the process. The sodium aluminate liquor, cleansed of the mud, is no longer red. The precipitation or the crystallization of the hydrate is known as “decomposition”.
The liquor is cooled, diluted with the water from the red mud wash then sent into huge thickening tanks (several thousands of cubic metres).
The alumina hydrate slowly precipitates from tank to tank as the temperature decreases. The floating suspension is recuperated in the last thickening tank.
The liquor is then filtered to separate the wet hydrate from the liquor cleansed of alumina. This liquid is then sent to the bauxite digestion where it will be enriched in soda and in lime.
The precipitation kinetics are very slow, which explains the need for very large volume tanks. In fact, for a given liquid flow, a large volume corresponds to a long residence time in the thickening tanks.
In order to accelerate the nucleation of the hydrate, 90% of the wet hydrate recuperated after filtration is recycled and used as a crystallization seed.
The alumina hydrate, Al(OH)3 , that is obtained is then filtered, washed and dried before storage. It can be sold as such - wet or dry - or it can be calcined into alumina, Al2O3.
The crystallization of the alumina hydrate (in the form of gibbsite) is a complex stage of the process. The mechanisms of crystallization involve nucleation, growth and agglomeration, which makes the operation complicated.
The precipitation conditions will determine the particle size distribution, the morphology and the level of impurities in the gibbsite and therefore in the calcined alumina. Precipitation occurs by diluting and cooling the liquid. These conditions help displace the reaction, below, to the left:
Several factors affect the balance and the speed of the precipitation:
- The temperature
- The soda concentration
- The alumina hydrate concentration
- The gibbsite seed
Two major types of processes can be distinguished:
- The “European” technique in which conditions lead essentially to a growth mechanism.
- The “American” technique which promotes strong agglomeration, , rather than growth,
The crystalline structures that are obtained in the two cases are different. Further information about alumina hydrate can be found in the section Hydroxides in World of Alumina.
Wet hydrate is calcined in rotary kilns (long rotating furnaces that are slightly inclined). These are the most suitable type for the preparation of calcined aluminas. Static kilns with a fluidized bed are uniquely reserved for transition aluminas that are used to produce aluminium metal.
All of the characteristics of calcined alumina are extremely variable and depend on the conditions of calcination. Soda is the main impurity in alumina produced by the Bayer process and this can be a hindrance for certain technical applications. Several methods for soda extraction exist, such as hydrate washing or the use of silica to manufacture an alumina with a very low soda content.
Calcination of hydrate :
The transformation of gibbsite into alpha alumina successively gives rise to the following phenomena while the temperature is rising:
- A massive water vapor release between 250 and 400°C fluidises the alumina so that it begins to flow like liquid in the inclined kiln.
- At around 1000-1250°C the exothermic transformation into alpha alumina occurs which makes the temperature rise by several degrees. The appearance of alpha alumina crystallites modifies the morphology of the grains that become rough and friable.
- For the gibbsite to be completely transformed into alpha alumina it must remain at the transformation temperature for about 1 hour.
- Halogenated compounds, also called “mineralizers” are used to catalyse the transformation and orient the morphology of the alpha alumina crystallites.
The mineralizers can also form volatile components with soda therefore reducing the level of this impurity . Soda can also be reduced by reaction with silica. These reactions compete with the combination of soda and alumina to form beta alumina.
Calcined alumina is produced as particles of white powder. These particles comprise of alpha alumina crystallite agglomerates, which typically vary from 0.5 to 10 µm in size. The higher the degree of calcination, the larger the crystallite size.
Calcined aluminas can be produced with a wide range of surface areas and crystallite sizes which can then be sold as-is or ground to produce various particle size distributions. Common grinding methods include continuous ball milling and air-jet milling. A more intense grinding method is batch ball-milling, or so called supergrinding, which can seperate the alumina crystallites.
Further information can be found in the section Calcined Alumina in World of Alumina.