Alumina Refining Process Flows

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Alumina Refining Process Flows

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Alumina Refining Process Flows

Alumina Refining Process Flows

Alumina Refining Process Flows

Virtually all alumina commercially produced from bauxite is obtained by a process patented by Karl Josef Bayer (Austria) in 1888. The Bayer process involves the following steps:

Digestion—bauxite is ground and slurried into a caustic soda (NaOH), which is then pumped into large pressure tanks called digesters. The sodium hydroxide reacts with the alumina minerals to form soluble sodium aluminate (NaAlOH).

Clarification—the solution from the digestion step is depressurized and processed through cyclones to remove coarse sand. The remaining fluid is processed in thickeners where flocculants are added to agglomerate solids, which are removed by cloth filters. These residues (red mud) are washed, combined, and discarded, and the clarified solution (containing the NaAlOH) is passed to the next step.

Precipitation—the solution from the clarification step is seeded with alumina seed (very small) crystals to aid precipitation of larger agglomerated alumina crystals. The product-sized crystals are separated from the small crystals (recycled as seed) and are washed to remove entrained caustic residue. The agglomerates are moved to the next step.

Calcination—The agglomerates of NaAlOH are placed in rotary kilns or stationary fluidized-bed calciners at temperatures that can exceed 960ºC (1,750 ºF), which drives off the chemically combined water leaving a residue of commercial-grade alumina (Plunkert, 2006, Red Mud Project 2010a).

Materials flows within the Bayer process are dependent on the grade of the bauxite being processed and the amount and character of the non-alumina minerals contained in the bauxite. Figure 5 shows selected materials flows for a model ―European‖ and ―World‖ alumina refinery, in 2005。

Figure 5 presents the inputs to and outputs from alumina production per metric ton alumina as kilograms (gross), metric tons (gross), and cubic meters of the various materials. In 2005, the amount of bauxite required, for a model ―Europe‖ and the ―World‖ alumina refinery, to produce one t of alumina was 2.2 t and 2.7 t respectively, which indicates that on average European smelters were processing higher grade bauxite than the rest of the world. Red mud production from these models followed this trend reciprocally— that is, 706 kg/t for the European model, and 1,142 kg/t for the worldwide model (European Aluminium Association, 2008). Reduction of red mud waste by processing higher grade bauxite is not likely to represent a strategy for long-term sustainable production of aluminum because average grades of metals produced typically decline with time as higher grade deposits are produced before lower grade ones.

The most important output from the Bayer process after alumina is red mud. The composition of red mud worldwide varies as follows: Fe2O3, 30 to 60 percent; Al2O3, 10 to 20 percent; SiO2, 3 to 50 percent; Na2O, 2 to 10 percent; CaO, 2 to 8 percent; and TiO2, trace to 25 percent. Red mud is a highly complex material, and its ultimate chemistry depends on the nature of the original bauxite ore. It is highly alkaline and contains a variety of elements and mineral species in small sizes and contains as much as 50% water (Red Mud Project, 2010b).

In the past, red mud has been disposed of at sea, or contained in lined lake-size containment compounds (Red Mud Project, 2010c). While these practices still are used, research is ongoing to find better ways to recycle and reuse red mud—for example, as building materials (bricks, roofing and flooring tiles), catalysts, ceramics, fillers, fertilizers, light-weight aggregates, metallurgical fluxes, and recovery of other metals (Red Mud Project, 2010d–e). In 2008, alumina refining worldwide produced about 93.2 Mt of red mud (roughly 40% water), of which 83.9 Mt was attributable to aluminum production. The reuse of red mud offers an opportunity to develop new industries based upon the wastes from alumina production.

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