Alumina

The bauxite ore is initially crushed so as to reduce the ore size. Diaspore bauxites exhibit greater hardness in respect to gibbsite ones and therefore are subjected to a two-stage crushing procedure.

Crushed bauxite is transferred to mills where bauxite particles are grinded with lime and caustic soda. After grinding, bauxite particles display suitable size and surface area for the chemical reaction.

Digestion is the heart of the alumina production procedure. Here the chemical reaction between the grinded bauxite slurry and caustic soda takes place under high pressure and temperature in high pressure vessels known as digesters. The energy required to increase the temperature of the bauxite slurry in the digestion vessels is provided by high pressure steam, which is produced in MYTILINEOS Cogeneration Station located next to the industrial complex of Aluminium of Greece. After the digestion process alumina is extracted to the caustic soda solution forming an alumina-soda solution (sodium aluminate).

The components of the bauxite ore that remain insoluble during the digestion process (iron oxides, titanium etc.) are separated from the alumina-soda solution through sedimentation in large tanks known as settlers. In the settlers the insoluble bauxite components sink at the bottom of the tank forming a red mud, while the alumina-soda solution remains in the surface. The alumina-soda solution known as pregnant liquor is further processed to recover the dissolved alumina as solid product.

Over the years, the company has invested in advanced technological equipment that allow the treatment of the bauxite tailings, while it participates in several European projects aiming at finding future applications for the bauxite residue.

ALUMINIUM OF GREECE was the first company in the world to adopt the high-pressure filtration technology. The company’s investment decision to install filter presses reflected at the time its vision to minimize the company’s environmental footprint in the most safe and efficient way via the adoption of the Best Available Techniques in bauxite residue handling (BAT). The filter presses allow ALUMINIUM OF GREECE to lower the moisture of the produced bauxite residue and hence to safely deposit it as dry filtercake on land, on the residue storage site that is located next to the refinery, meeting this way the EC waste directives. The deposition of Bauxite Residue the residue storage site takes place in line with environmental permits and geotechnical studies.

Precipitation is the reverse phenomenon of the digestion process. In precipitation the alumina that is dissolved in the alumina-soda solution (pregnant liquor) is recovered as a solid via crystallization. In precipitation the alumina-soda solution is progressively cooled, a process that results initially in the formation of small alumina crystals (hydrate alumina) which then grow and agglomerate to form larger crystals.

Solid hydrate alumina can be sold as a product or is fed into calciners for the production of its anhydrous form.

Anhydrous alumina also known as metallurgical alumina or calcined alumina constitutes the final product of the Bayer Process and the raw material for aluminium production in aluminium smelters. For the production of anhydrous alumina, the alumina solids that are formed in the precipitation stage are transferred to calciners, where they are processed at temperatures of up to 1100°C to remove moisture and chemically-connect water.