Removal and Control of Impurities in Alumina Refinery

Hi Friends,

Generally there are two types of impurities in plant liquor of Alumina refinery classified as inorganic and organic impurities. We know that bauxite is the basic input raw material used for production of alumina. The majority of bauxite deposits in the World has moderate to high level of organic carbon. These organic carbons goes into solution on dissolution of bauxite with caustic soda. The organic carbon keeps on increasing with circulating plant liquor and hence build up in organic impurities. Metallic carbonates and oxides of precious metals present in liquor are the inorganic impurities. Inorganic impurities cause scaling and thus removed either as byproducts or by causticization where as organic impurities directly affect the process efficiencies.  

The organic impurities have detrimental effects in process because of the adverse impact of impurities on plant performance particularly due to excessive foaming, lower productivity, colour of product and quality of product alumina.

Excessive foaming affects the volume utilization of plant and facilities where as lower liquor productivity causes high energy consumption, thus it becomes essential to minimize the impurities level in plant liquor by adopting techno-economically viable process.

As such, a few of the following techniques are used in the plant to minimize and control the overall  impurities level in plant liquor-

• Physical sorting of stems of trees from ROM Bauxite,
• Beneficiation of ROM Bauxite,
• Seed washing
• Salting out and causticization,
• Liquor burning and
• Adsorption.

Though liquor burning is one of the latest developments for removal of organic impurities but it is a capital intensive proposition and it also generates excess carbon dioxide.

Therefore, process expert should take final decision about adoption of most appropriate, environment friendly and economically viable process before implementation in the plant.

Trust, I have broadly described the topic to the extent possible. We seek your views / suggestions / remarks / comments, if any.

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Thanks and regards.

Kunwar Rajendra