Monday, January 22, 2018

Caustic Embrittlement in Alumina Refinery

Hi Friends,

We all are always willing to achieve high chemical productivity, liquor productivity and inventory productivity in Alumina refinery aiming to substantial reduction in specific energy consumption thereby profitability of the plant. Also, we adopt chemical cleaning procedures particularly for descaling the tanks and pipelines of white areas of the plant at faster rate. It is always better to attain high productivity but not at the cost of safety and environment under any circumstance. In present post, we will discuss on the technical reasons for the failure of tankages in Alumina refinery and the possible control measures to avoid such failures in future.

In a few Alumina refineries of the World, failure of caustic storage tanks have been seen because of the circumferential cracking of vessels mainly at the welding zones caused by stress corrosion. The analytical results of the samples collected from the failure areas of the vessels clearly reveal that the Caustic soda at high concentration and high temperature enters the minute weld joints by capillary action and dissolves iron of the vessel progressively due to formation of Sodium Ferrate which develops cracks ultimately causing sudden failure of the storage vessel.

The abnormal behavior of concentrated Caustic soda at high temperature causing destructive cracking is termed as 'Caustic stress corrosion' or 'Caustic Embrittlement'. Caustic embrittlement generally occurs at weld joints, rivets and bends which are under stress. Detailed investigations such as visual, microscopic and metallographic examinations have clearly revealed that circumferential cracks are initiated at high temperature zone and propagated through grain boundaries.

Findings of various test work of fracture morphology confirm that such cracking vis-a-vis failures are mainly due to Caustic embrittlement owing to erratic behavior of Caustic soda at high concentration and high temperature.

In order to avoid / minimize corrosion and stress fracture risks in unlined steel tankages storing concentrated NaOH must not exceed the temperature of 45oC. 
The Caustic solution used for chemical cleaning of White area tanks, equipment and pipelines should be prepared by mixing fresh Caustic lye in Spent liquor to attain the resulting concentration of NaOH at around 400 gpl (max.). It is also recommended to ensure the temperature of this resulting solution below 82.2 degree C (180 degree F) as precautionary measure as the cracking in metal develops at accelerated pace in fresh caustic environment at temperature above 90 degree centigrade.

The test work confirms that Carbon steel is less susceptible to caustic compared to stainless steel because the cracking in carbon steel is generally inter granular. Carbon steel is considered to be most favorable material for Alumina refinery.

We would like to have your comments / remarks.

Rajendra Kunwar

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