Fast high-pressure tanning of animal skins by accelerated chromium sulphate complexation

Abstract

In the conventional leather production process, basic chromium sulphate (BCS) penetrates fully inside the collagen matrix during tanning and forms oligo-nuclear complexes that stabilize the skin structure during basification. Accelerated chromium sulphate complexation by basifying the BCS before full penetration leads to surface tanning and low leather qualities. Therefore, long processing times of 16 h for tanning and basification are common in modern tanneries. We used sodium hydrogen carbonate to basify BCS before penetration. However, instead of conventional tanning we performed CO 2 tanning at 30 bar. The influence of accelerated chromium sulphate complexation on the leather quality was investigated. To estimate the structures of the formed chromium complexes, we considered the pH trend of the tanning solution during basification depending on the amount of basifying agent. The distribution of chromium oxide (Cr 2 O 3) and the shrinkage temperature (T s) in three different layers of the cross section were analysed. The analytics allowed the assessment whether the tanning agent penetrates through the whole cross section or adheres to the surface during CO 2 tanning. We show that CO 2-intensified tanning allows accelerated chromium sulphate complexation and a reduction in the tanning time from 16 to 4 h. The Cr 2 O 3 is equally distributed, and T s above 100 °C meets the standards of high-quality wet blue. However, the pH value of the tanning solution must not rise above 4.5 during basification. Otherwise, the T s decrease dramatically. The feasibility of the process is demonstrated in a 1700-L drum, with a batch size of about 500 kg. The CO 2-intensified tanning process has high potential to save time and chemicals and reduce emissions in industrial applications and is commercially competitive.

Removal of Chromium (III) and Reduction in Toxicity in a Primary Tannery Effluent Using Two Floating Macrophytes

“…Regarding chromium, most removal studies focus on Cr(VI) despite the fact that Cr(III) is currently widely used in industrial processes. The conventional leather tanning process uses basic chromium sulfate (Cr(OH)SO 4 ) [ 46 ]. However, few studies have comparatively examined the ability of these species to remove toxicity and genotoxicity in tannery effluents [ 47 ].…”

Hexavalent chromium release over time from a pyrolyzed Cr-bearing tannery sludge

“…Despite the development of novel and cleaner technologies for leather production 5 , 6 , presently more than 80% of the global leather production involves tanning using chromium salts 7 . Basic Cr(III) sulfate (Cr(OH)SO 4 ∙ n H 2 O) is the primary chromium salt used in the leather industry, because of its high penetration capability into the collagen matrix 8 , which results in a stable fibrous network resistant to bacterial attack and endows skin with excellent hydrothermal stability, elasticity and a comfortable feel 9 . The excess of chromium remains in the tanning liquor and is usually precipitated as trivalent chromium hydroxide using coagulants, which have high removal efficiency 10 .…”