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Tanning Process

The process of converting raw hides and skins into leather is called tanning. Operations carried out in the beam house, tan yard, and post-tanning areas are often referred to as wet processes, as they are performed in processing vessels such as drums. After post-tanning, the leather is subjected to dry finishing operations. Processes employed in each of the above categories change depending on raw material used and the desired final product. Therefore, the environmental impacts vary from tannery to tannery. The processing of hides and skins in a tannery can be split into following four main categories:

Preservation of hides and skins
Beam house operations
Tanning operations
Post-tanning and finishing operations

Raw Hides/Skin - In order to prevent bacterial activity, the moisture content should be brought down to less than 30%. This dehydration is usually done by applying common salt (i.e., Sodium Chloride) to hides/ skins to the tune of 30-45% by weight.

Sorting - Hides and skins are sorted into several grades by size, weight, or quality.

Trimming - Trimming is generally carried out during the sorting process. Some of the edges (legs, tails and heads, etc.) of raw hides and skins can be cut off. Usually this is done in the abattoir, but it can also be carried out in tanneries.

Curing and storing

Curing is a process that prevents the decomposition of hides and skins from the time they are flayed in the abattoir until the processes in the beam house begins. Whenever a raw material cannot be processed immediately ("green"), it must be cured. Popular methods of long-term preservation are salting and drying. Methods for short-term preservation (2- 5 days) are cooling, using crushed ice or refrigerated storage, and biocides. Curing is done in the abattoir, at the hide market, or at the tannery. In certain cases it might be necessary to repeat the step in the tannery, (e.g) chilled hides can be salted for longer storage, if salting was not efficient enough. Hides and skins are generally stored on pallets in ventilated or air- conditioned and/or cooled areas, depending on the method of curing chosen. From storage, the hides and skins are taken to the beam house.

Soaking

The main purpose of this process is to remove salt used during curing, re-hydrating the material and to getting rid of unwanted materials such as dung, blood, soil, etc. The duration of soaking may range from several hours to a few days. Depending on the type of raw materials used, soaking additives such as surfactants, enzyme preparations and bactericides can be used. The process of soaking can be classified into three stages

Dirt Soaking

In dirt soaking, 300-400 % of water is used to remove the unwanted materials

Main Soaking

The purpose of main soaking is to re-hydrate the material. In this operation, water, non-anionic wetting agent (0.2% concentrated Soda ash (0.2% concentrated) and preservatives (0.0 5% concentration) are used.

Final Soaking

Only water is used for the washing purpose in this operation. Major part of salt associated with preservation of skin/ hides is removed during the soaking operation.

Liming

The purpose of this operation is to facilitate the removal of hair, flesh, fat (partially), inter-fibrillary protein and to open-up the fibrous structure for osmotic swelling. The process of liming can be broadly classified into two parts (i.e) dehairing and re-liming

Dehairing

Lime (8-10 %) along with Sodium Sulphide (3 %) is applied to the skin to remove hair.

Reliming

To open up fibrous structure, lime, soda ash, caustic soda, etc., are applied. The pH of the skin being processed will rise to 12- 12.5.

Fleshing

The excess fleshing is removed manually or by using fleshing machines. The quantity of wet fleshings is in the range of 10-15% of the weight of raw hides/skins.

Deliming

This is a process to adjust the pH between 8-8.5 in order to enhance the enzymatic activity, which converts some of the proteins into soluble forms. pH correction i.e., from 12-12.5 to 8-8.5 are done by using ammonium chloride in case of soft leather and ammonium sulphate in case of hard leather.

Pickling

Pickling is a process of correcting the pH suitable to the tanning operation and to prevent swelling of the leather i.e. dehydration of the leather. In this process, water (80%), salts (8-10%), formic acid (0.28- 0.3%), sulphuric acid (0.75 - 2% based on thickness) are applied.

Ph correction

for vegetable tanning, a pH between 4 and 4.5 is maintained whereas, pH between 2.5 and 7.3 is maintained in case of chrome tanning. Prevention of swelling: salts to the tune of 8-10% are used in this process to prevent swelling.

The tanning process is of two types i.e., chrome tanning and vegetable tanning. Of the total leather production in India, more than 80% is based on chrome tanning and the rest is based on vegetable tanning.

Chrome tanning

Basic chromium sulphate [Cr2(SO4)3] (7-10 %) containing 25% Cr2O3 and sodium sulphate (25- 30%) is used in chrome tanning. Part of the pickle bath is used for chrome tanning operation. The pH is increased to 3.8-4.0 at the end of chrome tanning process which is called basification. The semi-finished leather after chrome tanning is called wet blue.

Vegetable tanning

Plant extracts are used for the purpose of tanning in this process. The pH falls down from 4- 4.5 to 3-3.5. Though this process is free of any heavy metal use, the leather developed from this process has comparatively weaker capacity of heat resistance and dye-holding

Post-tanning operations comprise of re-chroming of semi-finished wet blue leather, neutralization, dyeing, fat liquoring and finishing. In case of post-tanning of vegetable tanned semi-finished leather, the operations involved are semi-chrome tanning, neutralization, dyeing, fat liquoring and finishing. However the operations vary depending on the final product.

Sammying

It is a mechanized process to remove excess moisture in the wet blue.

Sliting

After sammying, the material is split into required thickness using splitting machine.

Shaving & trimming

The semi-finished leather is leveled using the shaving machine. Re-chroming: Depending on the quality of wet blue, re-chroming is carried out to improve the chromium content in the leather.

Semi-chroming

In case of vegetable tanned semi-finished leather, chrome tanning is given depending on the final leather quality.

Neutralization

pH is adjusted to 4.5-6.5

Deying

The leather is coloured using dyes such as anionic dyes, acid dyes, direct, metal complex compounds and basic dyes.

Fat liquoring

Natural/synthetic oils are applied for fat liquoring, thereby imparting softness to the leather

Finishing

Phenolics, melamine, acrylics, polymers, naphthalene, etc., are used for finishing to impart fullness to the leather.