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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 \96 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


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