Abstract

The commercial chrome tanning of hides and skins complies with conventional understanding and is carried out at pH 2.5 to 4.0. The efficiency of chrome utilisation in many standard systems is between 75—85%. There are proprietary systems that can be used to increase chrome utilisation but even these options leave chrome in the tradewaste discharge and problems of meeting stringent limits remain. The research reported in this paper seeks to minimise the environmental impact of chrome tanning by changing the basis of present chrome tanning technology.

A novel chrome tanning technique for tanning lamb pelts and lime split hides has been developed whereby pickling is completely eliminated and tanning conducted immediately after deliming at pH 7 to 8.8. This novel technique also does not require basification or masking agents for the completion of chrome tanning. This new approach gives a high exhaustion of chrome to the extent of 98% for lamb and over 99% for hides with improved yield of leather at reduced cost. Substantial amounts of salt are eliminated from the process and reduced acid usage encourages lower levels of plant and equipment corrosion.

The study has investigated the reactivity of a standard chrome tanning solution by examining the individual reactivities of the constituent complexes in relation to their charge characteristics.

Development of the novel no-pickle chrome tanning system has demonstrated that the differently charged ionic complexes can all provide a contribution to the tannage, as measured by chrome uptake from solution, and maintain commercially acceptable properties.

A mechanism suggested for the effectiveness of the no-pickle tannage is based on providing a wider pH range for the complex—substrate interaction and reversing the conventional pH trend by starting at elevated pH and allowing the intrinsic acidity of the chrome tanning reagent to reduce the pH to conventional levels.

The no-pickle process has been evaluated in bulk production trials and its advantages confirmed. In a further extension of this research, where pretannage for aqueous degreasing is concluded at elevated pHs that conventionally have required re-acidification for chrome tannage, it has been shown that pH adjust ment is unnecessary both in terms of re-acidification and basification.