The process of tanning leather involves five main steps: pre-tanning, tanning, selecting, dressing, and finishing. During tanning, chemicals are used to change the structure of leather fibers and make them more durable and resistant to bacteria and fungi. Common tanning methods include mineral tanning using chromium salts, vegetable tanning using plant materials, and oil tanning using fats. The tanning process generates significant solid and liquid waste, as up to 80% of the raw hide material is released as solid waste and processing one ton of hides produces 15,000-50,000 liters of wastewater.

1. Tanning changes the chemistry inside the leather fibers to make it more
difficult for the enzymes from bacteria and fungi to break them down –
this makes the leather durable. Leather tanning is the process to prepare
skins and hides for leather production.

2. The process of tanning involves five distinct stages:
STEP 1: PRE-TANNING
Soaking – When the leather arrives at the tannery it may be soaked to
extrude salts used in preserving leather. This is done in revolving
drums which can hold up to 200 hides.
Liming – Hair and epidermis are removed and a solution of lime
(calcium hydroxide) and sodium sulfide is applied to soften and enhance
the hide for softness and flexibility needed for upholstery leather.
Splitting – The hide is split into layers. The top, or grain, layer will
produce a fine, smooth grain leather. The bottom is used for suede or
split leather for other uses.

3. STEP 2: TANNING
This is the process which converts pre-tanned hide into leather.
Mineral tanning is normally done with alkaline chrome-3 salts. It penetrates
the hide fairly quickly (24-48 hours). This results in a pale duck-egg blue,
which, after processing, yields a fine, soft, modern finish. When there is
absence of chromium tanning, other methods combine vegetable with
polymers and syntans as an alternative.
Other tanning methods are:
Pure Vegetable Tanning (10-12%) (oak, spruce barkquebracho, tara
pods, olive leaves, rhubarb roots or mimosa)
Oil Tanning (yak butter, brain, fish oil, sebum or marrow )
Combination Tanning

4. STEP 3: SELECTING
After tanning, excess water is removed from the hide. Hides are then graded
according to the quantity and locations of natural features and flaws. Aniline
and Nubuck leathers. Heavily coated or embossed leather can utilize a lesser
quality of hides.
STEP 4: DRESSING
Shaving – Hides are given a uniform thickness.
Dyeing – Dyes are added to color leather.
Re-tanning – Additional tanning substances are sometimes added to modify the
physical characteristics of the leather to suit its final use.
Setting – A process which mechanically removes creases and excess water.
Drying – The hides are stretched dried on large frames or vacuum dried.
Trimming- The rough and ragged edges are removed.

5. STEP 5: FINISHING
The purposes of finishing are:
 To minimize the appearance of grain blemishes without losing the natural
beauty of the leather product.
 To give the required degree of gloss.
 To ensure the leather is soft, malleable and moldable.
 To give a more protective surface.
 To provide a surface that can be easily cleaned.
 To give special effect such as an antique look.

8. Waste generation
 Solid waste
 80% of raw material is released as solid wastes.
Out of 1000 kg of raw hide, nearly 850 kg is generated as solid wastes in
leather processing. Only 150 kg of the raw materials is converted into
leather
 Fleshing: 56-60%
 Chrome shaving, Chrome splits and buffing dust: 35-40%
 Skin trimmings: 5-7%
 Hair: 2-5%
 Conventional pre tanning and tanning processes account for nearly 90%
of the total pollution from a tannery
 About 15,000–50,000 L of water is required for processing 1 metric ton
of raw animal hides which generates 15–50 m3 of wastewater
 Volatile Organic Compounds Pollutants such as ammonia, hydrogen
sulfide, volatile hydrocarbons, amines, and aldehydes are emitted to the
atmosphere from tannery plants as effluents