Once a mainstay of the Chinese alumina industry, and alumina production in the former Soviet block countries, the sinter route takes a pyro metallurgical approach to ‘lock-up’ the silica and thus prevent it from contaminating the product alumina. The process is significantly more costly, both capital and operating costs, thus most sinter plants have closed. It has the advantage of being able to treat very low ratio alumina to silica ratio bauxites (low A/S bauxites) and in its ability to produce a higher purity product (thus making it better for lower volume applications demanding higher purity or higher whiteness).
Another drawback to the sintering technology has been that in its traditional form the alumina produced is very fin grained, making it more difficult to handle in modern smelting equipment.
- Comminution: bauxite is ground with limestone and sodium carbonate or caustic.
- Sintering: The ground meal is charged to a large rotary calciner, typically fueled by coal or oil and heated to ~1200°C. The silicate minerals react with the calcium in the limestone to form dicalcium silicate (Ca2SiO4) rendering far less soluble in the later hydrometallurgical stages. Alumina forms the highly soluble sodium aluminates.
- Clinker cooling and grinding: the clinker from the sinter operation is cooled and ground.
- Leaching: The ground clinker is leached using sodium carbonate solution.
- Clarification: The pregnant liquor is separated from the clinker grinds
- De-silication: The pregnant liquor is heated and held to precipitate any silica picked up during the leaching stage.
- Clarification: The liquor is again clarified to remove any solids that would give silica carryover to the final product.
- Precipitation: The pregnant liquor is forced to precipitate tri-hydrate by injecting carbon dioxide (captured from the off gas of the sintering operation). The tri-hydrate is very fine grained. The material is also washed to ensure minimal liquor carryover to the hydrate material.
- Calcination: The alumina tri-hydrate (Al2O3.3H2O) is calcined at ~1000°C to drive off all chemically bound water, to leave alumina typically >99% Al2O3.