Ferric alum and its production from Bauxite

Hi Friends,

In earlier posts, we have already discussed about specifications of various grades of Bauxite for non-metallurgical applications in industries. Bauxite used for manufacture of Ferric alum is popularly known as Chemical grade bauxite. High alumina content and low iron content are the basic requirements for such grade of bauxite. Here, we will discuss the specifications, uses, market potential and production process route for manufacture of Ferric alum in subsequent paragraphs-

Specifications of Ferric alum:

Aluminium sulfate having chemical formula of Al2(SO4)3.14H2O is known as alum. There are two types of alum - (i) Ferric alum and (ii) Non-Ferric alum. With bauxite, Non-Ferric alum can not be produced directly because of higher level of purity requirement. Broad specifications of Ferric alum conforming to IS 299 : 1989 are outlined below-

• Aluminium compound as Al2O3 : 15% min. (w/w),
• Iron as Fe : 0.70% max. (w/w).
• Insoluble matters : 0.50% max. (w/w),
• pH of 5% aqueous solution: 2.70 max.,
• Basicity: 0.50% max. (w/w),
• Free acidity : 0.50% max. (w/w),
• Lead as Pb : 30 ppm max.

Uses of Ferric alum:

Ferric alum is used as water treatment chemicals as it acts as coagulating agent for colloidal  impurities present in raw water drawn from reservoir / canal / river forming heavier particles resulting in faster settling, thus purifying raw water for drinking purposes after removal of other impurities by further treatment.

Market potential of Ferric alum:

Ferric alum has tremendous market potential for its huge requirement for water treatment, paper manufacture and waste water treatment including pH control. 

Production process route:

Ferric alum is produced by reaction of Bauxite with Sulfuric acid at controlled and optimized operating conditions in batches. The reaction vessel is made up of carbon steel properly lined with acid resistant bricks. Key parameters which affect the completeness of involved reaction between alumina content of bauxite and sulfuric acid are listed below-

• Particle size of bauxite,
• Concentration of sulfuric acid,
• Solid liquid ratio,
• Reaction temperature and
• Residence time.

Crushed bauxite with particle size ranging between 1.5 mm to 2 mm meets the process requirement. Measured volume of water is pumped to reaction vessel and then sulfuric acid procured at 98% purity is added slowly to the reaction vessel. Transfer of sulfuric acid is done through pump or compressed air system with necessary precautions. Agitator provided in the vessel makes the dilute sulfuric acid and then weighed quantity of solid crushed bauxite is added. Heating of reaction mixture is carried out to attain the temperature around 100 degree centigrade. The molar ratio of alumina and sulfuric acid is controlled at around 1:6 and solid liquid ratio of 1:5. The total residence time of about 6 to 8 hours ensure the completeness of reaction at 100 degree centigrade.
Now the product mixture is transferred and kept on floor outside for solidification by atmospheric air cooling. Each batch requires about 20 to 24 hours for solidification. To enhance the cooling rate fans are also used. Solidified lumps are stacked in product storage shed for transportation to desired destination.
The topic has been covered with broad details. Details of Non-ferric alum shall be published in the forthcoming post. Please put your views / suggestions / remarks / comments, if any.



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Kunwar Rajendra