Free of salt high-pressure deliming of animal hides
The wastewater pollution of tanneries is of high concern. The investigation of technologies to minimize the consumption of chemicals in the leather production process can reduce the environmental burden. We focus on the reduction of ammonium salts in the leather production process. Salt-free deliming of animal hides with compressed carbon dioxide as deliming agent is performed for the first time in a technical scale 20-L drum. As a result, CO2-deliming at 30 bar and 30 °C is two times faster than conventional deliming. In addition, the deliming efficiency is slightly improved. The initial calcium (Ca) content of the hides of 8 g/kg reaches the lowest value of 2 g/kg after a process time of 3 h. However, a process time of 60 min is sufficient to reach an elimination of 50 wt% of the initial lime. The resulting Ca-content of 4 g/kg after 60 min CO2-deliming at 30 bar is comparable with the Ca-content of conventional delimed hide. We clarify that the ampholytic character of the collagen itself enables a buffering of the pH-value at pH-7. The stable pH-value supports the selection of specific bating enzymes that decompose non-collagen proteins. No buffering salts contaminate the wastewater. The high-pressure CO2-deliming process has high potential to reduce wastewater emissions, save costs for chemicals, and process time in industrial beamhouse applications.