The deliming operation in
leather
Leather is a strong, flexible and durable material obtained from the tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffalo, pigs and hog ...
processing is a drum/paddle or pit based operation where two main objectives are met:
* Removal of
alkali from the pelt and the consequent deswelling of the
fibres.
* Lowering of the liquor pH to the values used in the bating process.
Deliming operations of cattle hides usually last two hours and are generally associated with the alkaline phase of
beamhouse operations. The progress of deliming in pelts is monitored by the checking of
pH values of process liquors and in the cutting of a pelt cross-section.
Phenolphthalein
Phenolphthalein ( ) is a chemical compound with the formula C20 H14 O4 and is often written as "HIn", "HPh", "phph" or simply "Ph" in shorthand notation. Phenolphthalein is often used as an indicator in acid–base titrations. For this applica ...
is used to monitor deliming pelt cross-sectional progress.
Removal of pelt alkali and deswelling
* As an acid is added it lowers the pH value of the internal pelt solution. This
neutralizes the solution alkalinity. The lowering of the pH results in rapid
protonation of the collagen basic groups.
Lowering of liquor pH
* The acid present in the deliming agents neutralizes the process liquor pH. Typical values after effective, thorough deliming will range from 6 to 9. Ammonium salt deliming pH values should be pH 8 to 9.
Deliming types
The deliming process can be done with acids that can be rapid in their pH adjustment. Buffering salts like ammonium salts predominated the 20th century.
Ammonium sulfate
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...
and
ammonium chloride can be used as deliming agents and they follow the following chemistry:
:(NH
4)SO
4 → (
ammonium) + (
sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
)
The ammonium ion is then free to penetrate the pelt cross-section and further ionise to act as an acid:
: (
ammonium) + H
2O → NH
3 (
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...
) + H
3O
+ (
hydronium)
The protons can then serve two functions, namely to
protonate basic groups of the collagen and
neutralize solution alkali chemicals.
Other weak acids can be used such as
boric acid
Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolve ...
,
acetic acid,
formic acid,
lactic acid
Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as nat ...
,
phosphoric acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
and
carbonic acid. Carbonic acid is used in a deliming process called
carbon dioxide deliming. Strong acids such as
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
and
sulfuric acid can also be used but their use is limited.
References
{{leather
Leathermaking