liming operation in Tanning process and Lime blast

Liming

Liming is the treatment of soaked hides and skins with milk of lime with or without the addition of sharpening agent such as sulphide (Na2S,NaSH), Cyanides, amides, markaptan etc.

Objectives of Liming

1. To remove hairs, nails, hooves and other keratinous proteins.
2. To remove some of the interfibrillary soluble proteins like mucines etc.
3. To remove the natural greases and fats.
4. Swelling and plumping of collagen fibres.
5. Swelling of the flesh and adipose tissue.
6. Splitting the fibre bundles into individual fibres.
7. Emulsify of the skin fats.
8. Opening of more reactive sites for tanning.
9. To bring the collagen to a proper condition for satisfactory tannage.

Why liming is the most important operation in Tanning process? 

Liming is the most important operation because the main objects of the liming are to remove hair, nails and natural fats, swelling and plumping of collagen fibers  and to split up the fiber bundle into individual fiber. The liming process must be balanced those so that the various actions are as far as possible carried out to the desired extent. For example, Goat skins are require more plumping than sheep skins and where the tight grained leather is produced. Thus it is cleared that the qualities of the finished leather are largely controlled in this liming process. Thus high capable of polishing by rubbing abrasive, resistance of sole leathers, high tensile strength of pickling band or belting leathers etc. are largely dependent on the liming process. For those reasons, The physical properties of finished leather largely depend on the liming process. And that is liming is the most important operation.

Test of Liming process 

Phenolphthalein test: By rolling the leather and removing water from it, we can test it in phenolphthalei. From the color (pink) we can be confirmed that, the liming process is completed. 

What is Lime blast

When the hides are hauled from lime liquor and kept exposed to air on the floor, Calcium carbonate is formed on the pelt 

surface and inside the hair - pits. This is called lime blast .
An important reaction associated with lime pelt is the damaging fault called " lime blast ". It's a fault of leather.

Why this fault

When hauled leather kept exposed, the calcium hydroxide reacts with carbon dioxide and make calcium carbonate precipitated. The reaction is given below,

Ca(OH)₂ (s) + CO₂ (g) = CaCO₃ + H₂O

It is not always possible to remove the lime blasts from the pelt during the next operation and may create trouble during tanning, dyeing, finishing. 

Remedy of Lime blast

The fault can be seen as dull areas on limed hides but becomes mere apparent when the resultant leather is dyed and dried. this problem can be solved by-

• The lime pelt should not be kept exposed on the air for a long time.
• If necessary, the top most limed hides should be kept on the pile with their flesh side out.
• Sulphopthalic acid is very satisfactory chemical for removing lime blast. It is strong enough to dissolve lime blast without grain swelling.