Use of sodium silicate to improve the environmental aspects of traditional chrome tanning: Development of a semi-industrial scaled process for high-quality bovine upper leather

As an alternative to the traditional process, a tanning process for light skins has been developed based on a two-step procedure in which a soluble sodium silicate ("water glass") is used as stabilizing agent and chromiumsalts are successively dosed to complete the tanning process. In a preliminary investigation on a laboratory scale, not only the stabilization effect of silicate has been confirmed (shrinkage temperature of the skins around 65°C) but also its capability of allowing a better penetration and fixation of chromium salts (half the dosage usually adopted in traditional chromium tanning was sufficient to obtain a final shrinkage temperature >100°C). Following the laboratory phase, in which the optimal dosages and procedures have been set up, the tanning procedure has been validated on a semi-industrial scale. Among four commercial sodium silicate solutions, one has been selected that has revealed the best performance. Following the optimal recipe (sodium silicate offer 1.5 % as SiO 2 pickled weight basis; chromium salt offer 1.5% as Cr 2 O 3 pickled weight basis) the final leather obtained, after a standard fatliquoring/dyeing treatment, showed very good organoleptic and physic-mechanical properties, quite similar to chrome tanned leather. The environmental improvements of the process consist in the preliminary stabilization of the leather so that it can be shaved by obtaining chromium free wastes; besides, the significant reduction of the chromium offer results in a drastic reduction of the chromium content (about one third of the amount typically present in the exhaust float of chromium tannage) of the exhaust effluent. This paper reports also the results of scanning electron microscopy (SEM) coupled with energy x-ray dispersion (EDX) used as investigations methods to follow the tanning process steps.