Enzymes
in Beam House Processing Leather
production consists of three main processes. These are:
- Beam
house process in which salt, dirt and hair are removed. The process involves
the following:
Desalting
and soaking the hides to remove salt (which is used to preserve skins). The process
uses a large amount of water (up to 20 cubic meter water per ton of hide. The
most significant the pollutants produced by the soaking process include salt,
hide surface impurities, dirt and globular protein substances dissolved in water.
Unhairing
and liming. Conventionally, unhairing is done by treating soaked hides in a bath
containing sodium sulphide/hydrosulphide and lime. The effluent from this process
is the most polluted effluent of the tanning process. The pollutants include suspended
solids, sulphides and nitrogenous material.
Deliming
and Baiting. In this pelt is processed in a bath of ammonium salt and proteolytic
enzymes. The pollutants from the process include calcium salts, sulphide residues,
degraded proteins and residual proteolytic enzymatic agents.
- Tanning
under which the hide is treated with chemicals to produce leather. Chrome is the
most common tanning agent used in the world. Conventionally, chrome tanning consists
of pickling, tanning and basifying. The main pollutants of the tanning process
are: chrome, chlorides and sulphates.
- Post
tanning (wet finishing), which includes neutralization, retanning, dying and
fat liquoring. The pollutants from the process include chrome, salt, dyestuff
residues, fat liquoring agents and vegetable tannins.
- Finishing
in which the leather is given desired properties. The main pollutants produced
during finishing are suspended solids and chrome.
In
addition to the above mentioned pollutants, which are discharged in the effluent,
leather production also produces emissions. These include: ammonia during deliming
and unhairing; sulphide during liming; chrome during chromate reduction and from
the buffing process. Also, alkaline sulphide may be converted to hydrogen sulphide
if the pH is less than 8.0.
Furthermore, particulate emission may occur
during shaving, drying and buffing. - Environmentally
friendly production methods and technologies.
The quantity of pollution load generated by the leather industry can be reduced
by:
Process modification to reduce the generation of waste and pollutants
in the beam house;
Reuse of chemicals (mainly sulphides and chrome) and
spent liqours.
Economical use and reuse of water;
The
modifications required to make various processes in leather making environmentally
friendly are described below.
- Desalting
and soaking. The salt load in the effluent can be reduced by:
- Decreasing
the amount of salt used to preserve hides by adding environmentally acceptable
anti-septics such as boric acid and sodium sulphide. It must, however, be mentioned
that the use of these preservatives reduces shelf life.
- Use
of improved methods of desalting by using Dodeca frames and desalting machines.
- Processing
fresh (green) hides, which have been preserved by chilling.
- Unhairing
and liming. The pollutants from these processes can be reduced by using the following
technologies:
- Recycling
spent float. This also leads to a reduction in the amount of water consumption.
- Enzymatic
unhairing. This can lead to a reduction in the use of sulphide, leading to a reduction
of COD by 30-40%.
- Deliming
and Bating. The environmentally friendly alternatives include: ammonia free deliming
and bating and carbon dioxide deliming.
- Chrome
tanning. Cleaner technologies to reduce chrome content in the effluent are:
- High
exhaustion process in which short floats at higher temperature and pH are used.
The process increases the extent of chrome exhaustion and reduces the chrome content
in the effluent.
- Recovery/recycling
of chrome. In this process, chrome in the effluent is recovered and reused in
tanning process.
- Low
or no chrome tanning.
- Post
Tanning. The methods to reduce the load of pollutants generated by these processes
are:
- High
Exhaustion
- Chrome
fixing in neutralization
- Chrome
precipitation.
- Replacing
nitrogenous compounds with other filling agents;
- Phasing
out environmentally hazardous chemicals with high COD and BOD values, and limited
biodegradability.
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