Sustainable and environmentally friendly production of leather

Focus of the leather producing industry is the transformation of untreated skins that are brittle under dry conditions and may rot under wet conditions into durable, imperishable leather by processes which incorporate tanning agents into the hides. The traditional processes comprise a multitude of different steps (up to 70), are time-consuming and water intensive. An accrual of 20 to 40 t of wastewater per tonne of hides is typical. About three fourth of the wastewater contain less critical compounds like salts and surfactants, while one fourth of the wastewater may contain metal cations and/or organic tanning agents. In order to intensify the processes of leather formation a new method of high-pressureimpregnation was developed. The impregnation steps are carried out in aqueous systems in the presence of pressurized carbon dioxide, which may be either in its liquid, gaseous or supercritical state. The processes are considerably accelerated. Tanning for instance is achieved already after a few hours, if the process is operated at elevated pressure between 30 and 100 bar, while the traditional process may take some ten hours. Similar acceleration was found for other steps like washing, pickling or defattening of the treated hides. The water consumption can be reduced to about one fourth of the traditional method (Weidner and Geihsler, 1995). An overview of results achieved in lab and pilot scale will be given. Process principles will be described and the acceleration of the tanning process will be demonstrated. In lab scale traditionally tanned hides were produced. With a standardized experimental approach, the time dependency of the ion content in the tanning liquid was measured when tanning was performed in the presence of CO2. Moreover, the influence of CO2 pressure on the tanning time and the influence of the concentration of the tanning agent were investigated. Based on this lab-scale results experiments in pilot scale were performed. The upscale-factor based on the mass of treated hides was 400%. A quality control of the leathers produced showed high quality.