Alumina refinery plant

The Bayer Process

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The Bayer Process Wheel

The Bayer Process

The Bayer Process is the principal industrial means of producing aluminum oxide (alumina). The process was discovered by the Austrian chemist Karl Bayer. The process was patented in 1889. Today, it produces nearly all the world’s alumina supply as an intermediate in aluminum production. The process dissolves the aluminum component of bauxite ore in hot sodium hydroxide (caustic soda), removes impurities from the solution, precipitates alumina trihydrate, which is then calcined to aluminum oxide. A Bayer Process plant is principally a facility for heating and cooling a large recirculating stream of caustic soda solution. Bauxite is added at the high temperature point, red mud is separated at an intermediate temperature, and alumina is precipitated at the low temperature point in the cycle. Bauxite usually consists of two forms of alumina—a trihydrate form gibbsite (Al2O3·3H2O) and a monohydrate form Boehmite (Al2O3·H2O).

Raw Materials

The bauxite supply for Gramercy Alumina is mined in Jamaica. This bauxite is transported to Gramercy designated ships. After clearing U.S. Customs, the ships are discharged at the Gramercy dock using two gantry cranes. The bauxite is conveyed to a covered storage building and to a rotating trommel screen, which removes oversized materials. The bauxite that is processed through the trommel screen is conveyed to bauxite bins. Dust collectors throughout the area reduce dust emissions in the transportation of bauxite.

Digestion

Bauxite and caustic soda solution are mixed and pumped as a slurry to storage tanks. Lime slurry is added to remove impurities as the Alumina is extracted in a two-stage digestion process. The material from this digest is cooled by passage through flash tanks. After dilution with mud washer overflow, the mixture passes to sand cyclones in Clarification for further processing.

Clarification

The primary function of this area is the separation of mud residue from the alumina-rich liquor leaving the Digestion area. The separation of mud from the Digestion liquor is performed in two steps: sand cyclones, and a mud settler. The mud, once separated, is washed to recover liquor containing caustic and alumina. Starch and synthetic flocculant are used as settling aids. The washed mud residue is pumped to impoundment areas on the plant site.

Filtration

The primary objective of the Filtration Department (Press Floor) is to provide final ‘polishing’ of the alumina-saturated liquor prior to entering Precipitation. The mud solids, if not removed, represent contamination in the final products. Filtration is the removal of solid particles from a fluid by pumping the liquor through a lime filtering media on which the solids are deposited.

Heat Interchange & Evaporation

The primary function of the Heat Interchange (HI) units is to cool the saturated caustic solution fed to Precipitation through vacuum flashing. The secondary function is the transfer of the energy to the cool caustic solution leaving Precipitation. In Evaporation, water is removed by flashing the liquor to lower pressure to increase the caustic concentration.

Precipitation and Classification

Alumina extracted from the bauxite in Digestion is recovered from the liquor in Precipitation by adding seed hydrate, allowing the alumina in solution to precipitate and attach to the seed particles. As the alumina precipitates on the seed, the particles grow to product size. The precipitators containing the product hydrate are pumped to begin the Classification process, where the product-size hydrate and smaller sized seed-hydrate are separated in gravity settlers.

Calcination

The Calcination area receives hydrate slurry from Precipitation. The hydrate is filtered and washed to remove caustic soda to meet customer specifications. The washed hydrate is sold as

  • Calcined (smelter grade) alumina (Al2O3); washed hydrate processed at high temperature in rotary kilns;
  • Dry hydrate (Al2O3·H2O); washed hydrate processed in a hydrate dryer;
  • Wet cake; washed hydrate loaded out without drying free moisture.

In the load out area, product is stored in bins, then transferred into railcars, barges, or trucks.

Powerhouse

The Powerhouse provides steam, electricity, compressed air, and water to the Gramercy Alumina production areas. Power boilers generate 900 psig steam. Steam turbine generators and gas turbines produce electricity.

Red Mud Impoundments

The red mud from Clarification is pumped into sealed diked impoundment areas, where the mud is decanted by gravity and allowed to dry. The supernatant liquor is either brought back to the plant via the washer train, or is neutralized and disposed.

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