The process of refining the metallic ore in the form of alumina is known as Bayer process. Key steps involved in this process include (RocksandMinerals.com, 2010);
Crushing the bauxite rocks and grinding the particles into smaller granules of uniform size. The granules are then mixed with caustic soda solution at relatively higher temperature. The grinding mill converts the mixture into â€˜red mudâ€™ by blending the finer granules with the caustic soda solution. This red mud contains a solution of sodium aluminate and un-dissolved bauxite residues with particles of iron, and silica.
The red mud subsequently undergoes a process called digestion in which the alumina is dissolved with the help of a chemical reaction. The digester helps in dissolving alumina and produces a sodium aluminate solution while other impurities or un-dissolved particles are fished out with the help of â€˜flesh tanksâ€™ and â€˜settling tanksâ€™.
Gravitational properties of matter are used to settle the metallic particles of the solution. The liquid solution is taken out of the container with the help of series of fine filters. The gravy like red mud is subsequently stored into large storage ponds and the remaining liquid is evaporated, thus leaving concentrated alumina solution in the ponds.
Alumina crystals are prepared with the help of series of huge precipitators. Sodium aluminate taken out from the previous steps is pumped into the precipitators and then â€˜seed crystalsâ€™ are added for starting of the precipitation process. Alumina crystals grow around these crystals and their size grows bigger, these crystals settle at the bottom of the tank. These crystals are subsequently transferred to calcination kilns for removal of the remaining water content.
The process of calcination is used for removing the water content and thus producing anhydrous alumina i.e. alumina without water. Calcination is primarily a healing process.
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