Preferred material of construction for tanks, plant & equipment

Hi Friends,

Selection of suitable material of construction (MOC) for plant and equipment plays vital role in enhancing their useful life and there by life of entire Alumina refinery. In present post, we will briefly discuss about the recommended material of construction for major and vital equipment of Alumina refinery.

As such, Alumina refinery adopting Bayer's process has high alkaline environment as caustic soda is the basic leaching agent used for dissolution of alumina from bauxite. Since mild steel (carbon steel) is the most favorable material for handling alkaline chemicals like caustic soda and aluminate liquor, hence most of the plant, equipment, tanks and structural components are made of carbon steel with corrosion allowance varying from 1.5 mm to 3 mm in Alumina refinery. As aluminium, copper and zinc are highly susceptible to caustic attack, hence use of components made of aluminium, copper, zinc and their alloys are totally avoided in Alumina refinery. 

Storage tanks for caustic soda, lime, fuel oil, aluminate liquor, hydrate slurry, bauxite slurry, residue slurry and concentrated sulfuric acid are fabricated with carbon steel confirming to IS 2062. The shell of digesters, shell & tube heat exchangers and flash tanks are fabricated in workshop using unfired boiler quality steel confirming to A516 grade 70. Cast iron and cast steel are respectively considered as favorable materials of construction for water and condensate pumps. For slurry and liquor pumps, hi-chrome steel having chromium content ranging from 27 to 29% Cr is the preferred material. 

Alloy-20 is used as MOC for dilute sulfuric acid pumps where as preferred MOC for storage tanks of dilute sulfuric acid is either rubber lined carbon steel or SS316L. 

Hope, the material of construction for tanks, plant and equipment has been described to the extent possible. Please process your views / suggestion / remarks / comments, if any.

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Thanks and regards.

Kunwar Rajendra