takes place. Tanning is a process in which skin and hides of animals are processed
through many stages to make leather (Dargo and Ayalew 2014). This process uses
variety of chemicals, such as sodium sulfide, lime, ammonium sulfate, vegetable
tannins, sodium chloride, chrome salts, and bactericides, and further high quantity of
water is used for its processing (Cooman et al. 2003). Conventional tanning technology is adversely affecting the environment due to large amount of chemical and
organic pollutant generation. These generated pollutants mostly present in effluent
discharge and emerge as a serious threat to the environment.
16.2 Tannery Process
Manufacturing of leather during tanning process is a complex method which is
performed into two parts: beam house operations and tanning process. Figure 16.1
shows various processes that are involved in the manufacturing of leather. In first
step, i.e. beam house operations, blood and dirt from hides of animals are removed
by washing which is followed by softening and salt removal from hides by soaking
in water (Cassano et al. 2001). Fatty acids are also removed by fleshing. In order to
provide better penetration of tanning agents and hair removal, swelling of hides is
required which is done by liming. We use alkaline medium of sulphide and lime for
chemical dissolution of the hair and epidermis. To facilitate de-hairing, sodium
sulphide is added (O’Flaherty et al. 1965). In the process of liming, a high concentration of sodium sulphide, organic matter, and lime is produced which joins the
effluent. To remove hair remnants and to degrade proteins, hides are neutralized by
ammonium salts followed by treatment with enzymes. This leads to generation of
high ammonium waste in the effluent.
After these stages, hides are prepared for tanning by a process known as pickling.
Acids, mainly sulphuric acid, are added in order to adjust the pH value of hides
(Cassano et al. 2001). In order to prevent hides from swelling, salts are added. In
tanning process, reaction of collagen fibers present in hides takes place with chromium, tannins, alum, or other chemical agents. There are a number of compounds
which are used as tanning agents; some of them are alum, formaldehyde, syntans,
heavy oils, and glutaraldehyde. Nearly 300 kg of chemicals are used per ton of hides
during the tanning process. Table 16.1 shows list of chemicals that are consumed for
leather processing.
On the basis of tanning agent, tanning operations are further divided in chrome
tanning and vegetable tanning. In vegetable tanning, natural organic substances are
used which are derived from plants, and it is done in series of vats (Zywicki et al.
2002). In the case of chrome tanning, chromium salts are used, and it is performed at
higher pH. Out of the total leather produced nearly 90% uses chromium salt for
tanning (Stein and Schwedt 1994). When tanning is completed, tanned leather is
piled down, wrung, and graded for required thickness and quality. Retaining, fat
liquoring, and dyeing are the additional steps which are used in chrome tanning
unlike vegetable tanning. During fat liquoring, oil gets introduced into the skin
580
N. Patel et al.
through many stages to make leather (Dargo and Ayalew 2014). This process uses
variety of chemicals, such as sodium sulfide, lime, ammonium sulfate, vegetable
tannins, sodium chloride, chrome salts, and bactericides, and further high quantity of
water is used for its processing (Cooman et al. 2003). Conventional tanning technology is adversely affecting the environment due to large amount of chemical and
organic pollutant generation. These generated pollutants mostly present in effluent
discharge and emerge as a serious threat to the environment.
16.2 Tannery Process
Manufacturing of leather during tanning process is a complex method which is
performed into two parts: beam house operations and tanning process. Figure 16.1
shows various processes that are involved in the manufacturing of leather. In first
step, i.e. beam house operations, blood and dirt from hides of animals are removed
by washing which is followed by softening and salt removal from hides by soaking
in water (Cassano et al. 2001). Fatty acids are also removed by fleshing. In order to
provide better penetration of tanning agents and hair removal, swelling of hides is
required which is done by liming. We use alkaline medium of sulphide and lime for
chemical dissolution of the hair and epidermis. To facilitate de-hairing, sodium
sulphide is added (O’Flaherty et al. 1965). In the process of liming, a high concentration of sodium sulphide, organic matter, and lime is produced which joins the
effluent. To remove hair remnants and to degrade proteins, hides are neutralized by
ammonium salts followed by treatment with enzymes. This leads to generation of
high ammonium waste in the effluent.
After these stages, hides are prepared for tanning by a process known as pickling.
Acids, mainly sulphuric acid, are added in order to adjust the pH value of hides
(Cassano et al. 2001). In order to prevent hides from swelling, salts are added. In
tanning process, reaction of collagen fibers present in hides takes place with chromium, tannins, alum, or other chemical agents. There are a number of compounds
which are used as tanning agents; some of them are alum, formaldehyde, syntans,
heavy oils, and glutaraldehyde. Nearly 300 kg of chemicals are used per ton of hides
during the tanning process. Table 16.1 shows list of chemicals that are consumed for
leather processing.
On the basis of tanning agent, tanning operations are further divided in chrome
tanning and vegetable tanning. In vegetable tanning, natural organic substances are
used which are derived from plants, and it is done in series of vats (Zywicki et al.
2002). In the case of chrome tanning, chromium salts are used, and it is performed at
higher pH. Out of the total leather produced nearly 90% uses chromium salt for
tanning (Stein and Schwedt 1994). When tanning is completed, tanned leather is
piled down, wrung, and graded for required thickness and quality. Retaining, fat
liquoring, and dyeing are the additional steps which are used in chrome tanning
unlike vegetable tanning. During fat liquoring, oil gets introduced into the skin
580
N. Patel et al.