Unhairing and Liming of hides and skins

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General description

The aim of unhairing and liming is to remove the hair, epidermis, and to some degree, the interfibrillary proteins, and to prepare the hide or skin for the removal of adhering flesh and fat by the fleshing process.

Hair removal is performed by chemical and mechanical means. The keratinous material (hair, hair roots, epidermis) and fat are traditionally eliminated from the pelts mainly with sulphides (NaHS or Na2S) and lime. Alternatives to inorganic sulphides include organic sulphur compounds such as thioles or sodium thioglycolate in combination with strong alkali. Enzymatic preparations are sometimes added to improve the performance of the process. After liming in a tannery recognised as a 'technical plant' by the competent authority, the hides or skins are no longer subject to animal by-products controls.

Painting and liming of sheepskins

The aim of painting is to bring about the breakdown of the wool root within the skin, so that as much undamaged wool fibre as possible can be pulled easily from the pelt.

Paint, a solution of sodium sulphide at concentrations between 5 and 20 % thickened with an equal amount of hydrated lime, is applied to the flesh side of the skin and then left for several hours. The soluble chemicals in the paint penetrate the skin from the flesh side and dissolve the basal young epidermal cells of the epidermis and the wool or hair root, thus loosening the hair or wool which should be easily removed by wiping or light pulling. Paint can be applied either manually or by spraying machines. Several hours after the application, the wool can be 'pulled' from the skin, either manually or mechanically. After pulling, the skins are limed in process vessels, with the same purpose as the liming of bovine hides.

Fleshing

Fleshing is a mechanical scraping off of the excessive organic material from the hide (connective tissue, fat, etc.). The pelts are carried through rollers and across rotating spiral blades by the fleshing machine [33, BLC 1995].

Fleshing can be carried out prior to soaking, after soaking, after liming or after pickling. The process of fleshing is called green fleshing if the removal is done prior to liming and unhairing. If fleshing is performed after liming and unhairing, it is called lime fleshing. Sheepskins may be fleshed in the pickled state. Fleshing operations give rise to an effluent containing fatty and fleshy matter in suspension.

Splitting

The aim of the splitting operation is to produce hides or skins of a set thickness. They are split horizontally into a grain layer and, if the hide is thick enough, a flesh layer. Splitting is carried out on splitting machines, fitted with a band knife. Splitting can be done in the limed condition or in the tanned condition.

Deliming

After the liming process, the lime or other alkali in the skin is no longer required, and, in most cases, it has a detrimental effect on subsequent tannage. The deliming process involves a gradual lowering of the pH (by means of washing in fresh water or by weak acidic solutions or by salts such as ammonium chloride or sulphate or boric acid), an increase in temperature and the removal of residual chemicals and degraded skin components.

The extent of deliming to be achieved depends on the type of final leather; a thorough deliming results in a softer leather, whilst partial deliming gives a firmer leather. At this stage, the hides and skins are ready for vegetable tanning but, for chrome tanning, the delimed hides and skins have to be further processed by bating and pickling. Delimed skins must be taken to the next process immediately, as once the alkali has been removed, putrefying bacteria can thrive. The acidification of liquids which still contain sulphide may generate hydrogen sulphide gas. With prior treatment using hydrogen peroxide or sodium hydrogen sulphite to oxidise the sulphide, this problem can be avoided.

The use of CO2 instead of ammonium salts reduces the release of ammonia in the effluents.

Source: Joint Research Centre, Best Available Techniques (BAT) Reference Document for the Tanning of Hides and Skins, 2013

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