C.Muralidharan, H.Purushotham,
K.Parthasarathy, PG Rao and KV Raghavan
Central Leather Research Institute, Adyar,
Chennai
S.Suryanarayanan, N.Sudhakar
and S.Ravichandran,
M/s.Tejoomals Tannery, Pernambut
In place of Ammonium Sulphate,
Co2 is an environmentally friendly alternative
in leather processing.
1.0
Process modifications and control strategies
adopted
To meet the objectives mentioned
above, following process modifications &
control strategies have been adopted.
1.1 Ammonia free (Carbon dioxide)
deliming
1.2 High exhaustion chrome tannage
1.3 Rechroming with high performance syntans
1.4 RH control and recycling system
1.5 Chemical preparation and dosing system
1.6 Hot water generations and additions
1.7 Improved drainage system
1.1
Ammonia Free (Carbon Dioxide) Deliming
Ammonia salts are ideally suited
for the technological requirements of deliming
as they can act as buffers and readily react
with lime. However, use of ammonium salts
adversely affects the effluent characteristics
and creates an unpleasant working condition.
Conventional ammonium deliming contributes
nearly 75-80% of ammonia in the effluent.
Use of organic acids for deliming is often
associated with the problem of drawn grain
(or) acid shock resulting in poor grain
qualities and area yield. Carbon dioxide
does not pose these problems when used for
deliming as it has an ability to act as
buffer in the float. It has high affinity
for lime and is comparatively inexpensive
to use.
The other advantages associated
with Carbon dioxide deliming are:
- Reduction of nitrogenous discharges in
the effluent
- Reduction in BOD & COD to an extent
of 50%
- Reduction in ammonia odour
- Improved grain characteristics and
- Easy handling and automation ability
The process adopted for deliming
is given below. Carbon dioxide is let into
the drum through the axle hold by a rubber
tubing attached to a manifold system connecting
a bundle of 3 or 4 carbon dioxide cylinders.
The carbon dioxide flow can be controlled
through a ball valve and pressure regulator.
The rate of flow can be closely monitored
through a flow meter.
The critical operating parameters,
which have decisive influence on the efficiency
of the system and quality of the leather,
are:
- hide – float ration
- Carbon dioxide gas addition rate and
- Time of bate addition
Hide- float ration is an important
factor for Carbon dioxide deliming. Higher
float levels allow better dissolution of
Carbon dioxide resulting in promotion of
faster deliming. A float level of 150% on
the limed pelt weight has been recommended
for successful deliming. The time taken
for Carbon dioxide deliming is more or less
same as that of conventional ammonium deliming.
The rate of addition of Carbon
dioxide is another important factor for
proper deliming. To speed up the process
and to overcome the lime blast problem,
high flow rates of Carbon dioxide is suggested
initially until the buffering pH of 6.5-7.0
is reached. Then the flow rate of Carbon
dioxide is brought down till completion
of deliming as inferred from the disappearance
of pink colour at the cross section of pelt
when tested with phenolphthalein.
Since more of the bate formulations
available in the market have maximum activity
at a pH of 8.3, it is desirable to add the
bate immediately after commencement of Carbon
dioxide injection as the pH drops down to
a level around 7.0 during most of the deliming
period.
Courtesy : Printed
with permission from Dr. C.Muralidharan,
Central Leather Research Institute, Adyar,
Chennai
