Alumina Processing

Alumina, or aluminum oxide, has the chemical formula Al₂O₃ and is produced from the refining of bauxite. The primary use of alumina is for the production of aluminum metal, but it is also used in refractories, ceramics, polishing and abrasive applications, as well as a fire retardant/smoke suppressant. Bauxite is refined to alumina primarily using a process known as the Bayer Process as well as, to a lesser degree, the sinter process.

The Bayer Process was developed and patented by Karl Joseph Bayer in 1888. The process is based on the fact that hydrated aluminum oxides are soluble in caustic at elevated temperatures and pressures. The solubility of aluminum oxide varies widely, according to the form in which it is present. Alumina occurs in bauxite in the trihydrate form (gibbsite) and as the monohydrate (boehmite and diaspore). The trihydrate is more soluble than the monohydrate. The process may briefly be described, as follows^1-3. Bauxite is digested in a caustic soda solution at elevated temperatures and usually under pressure. After digestion, the solution containing the dissolved aluminum oxide, in the form of sodium aluminate, has suspended in it the residue from the bauxite. This insoluble residue, called 'red mud,' consists predominantly of iron oxide, titanate and silica based compounds. The red mud is separated from the sodium aluminate rich solution, with the aid of synthetic flocculants in vessels referred to as Thickeners, Decanters or Settlers. The terminology used is dependent on the operating company. The clarified liquor is further polished (mud particles removed) via filtration. Upon cooling, alumina trihydrate is then precipitated from the liquor, filtered and washed before it is calcined at extremely high temperatures. The product derived is anhydrous alumina, Al₂O₃.

The underflow (mud) from the Thickeners, in addition to the mud removed at filtration, still has entrained in it a significant amount of liquor containing caustic and alumina. Most of this is recovered by washing the mud in a Counter Current Decantation Circuit (CCD circuit). Synthetic flocculants are also used here to aid in the mud/liquor separation process.

The entire process may be represented by the equations:

Extraction
Al₂O₃.3H₂O + 2NaOH → 2NaAl(OH)  (1)

Precipitation
2NaAl(OH)₄ → Al2O₃.3H₂O + 2NaOH (2)
 

Calcination
Al₂O₃.3H₂O → Al₂O₃ + 3H₂O (3)

The dissolution and mud separation stages are generally referred to as the "Red Side" of the circuit, while the precipitation, alumina filtration, and calcination are referred to as the "White Side."

An alternative process, used primarily with diasporic and high silica containing bauxites, is the sinter process.⁴ There are different variations of this process, but the common principle is that the bauxite (or in some cases red mud) is heated to a high temperature (~1200°C) with a calcium-containing chemical in the solid phase to convert the alumina containing minerals into a form that is more easily extracted and to convert the silica and titanate minerals into a calcium form that is not soluble. The variations include the lime sinter process (which generally uses calcium carbonate or calcite), the lime-soda process (which uses a combination of calcium carbonate and sodium carbonate), and the Bayer-sinter process (which involves the use of the Bayer process first, and the red mud is then treated in the sinter process to extract the alumina lost to sodalite by-product ). The sinter process is used extensively in China and, to a lesser extent, in eastern Europe (Russia, Ukraine, and Turkey).

In the lime sinter process, the mixture ratios and conditions are designed to optimize the production of various calcium aluminum silicates, which are easily extracted with an alkaline solution (either caustic or more commonly sodium carbonate) at atmospheric pressure and temperatures of about 95-100°C. The gibbsite is precipitated by either seeding or carbonation. The equations following describe the primary reactions in this process:

2CaCO₃ + SiO₂ → 2CaO.SiO₂ + 2CO₂
12CaCO₃ + 7Al₂O₃ → 12CaO.7Al₂O₃ + 12CO₂

In the lime-soda process, a source of soda (usually sodium carbonate) is added to promote the formation of sodium aluminate, while the calcium carbonate forms calcium silicates and calcium titanates. The equations below describe the primary reactions in this process:

2Al(OH)₃ → Al₂O₃ + 3H₂O
Al₂O₃ + Na₂CO₃ → 2NaAlO₂ + CO₂