Soluble silicates in leather production

Introduction A European Community funded project to reduce the volume of solid tannery wastes by transferring them into byproducts (figure 3 for byproduct uses) has led the Testing & Research Institute, Vienna, Austria, the LGR, Reutlingen, Germany, the SSIP, Naples, Italy, and tanneries from Austria, Germany and Italy to develop procedures using aqueous solutions of alkali silicates, so called 'wasserglass', in beamhouse and tanning processing. The work has been carried out due to concerns about the amount of solid waste produced in the leathermaking process. Much of the solid can only be sent to landfill because the protein is tanned. A new EU directive which will limit the amounts of solid waste that can be disposed of has forced many tanners into looking at alternatives. Wasserglass is a 50% aqueous solution of sodium silicate and sodium hydroxide and is a basic and inexpensive chemical that can be used in a number of applications. Two processes are highlighted here, one for liming, and one for pretanning. For an alternative liming trial, soaked and fleshed hides or calfskins were treated with 2.5-3% Na₂S and 5-7% Wasserglass in 100-200 % float. For adjustment of alkalinity, 2-2.5% NaOH was required. After lime splitting, the hides were delimed, bated and acidified with formic acid, in order to fix the silicates, and were then shaved. Only when heavy shaving was needed, eg for the production of automotive leather, was an additional amount of wasserglass added prior to the neutralisation. The various stages are set out in Figure 1. The procedure was tested on leathers destined for automotive and upholstery leathers, shoe upper leathers from heavy hides, and shoe upper leathers from calfskins. A comparison of the resulting leathers is shown in Table 1. The results suggest that it is easier to split hides processed with wasserglass than the controls. It was also noted that, although all the leathers were dyed in the same bath, those silicate-stabilised crust leathers were a deeper shade, while remaining a uniform colour. Effluent values are shown in Table 2 and, although the COD and SS values for the experimental liming processes tend to be higher, subsequent process parameters show better exhaustion leading to an overall reduction in the effluent loading. Further work is intended to confirm this. The second process involves pretanning hides with wasserglass. The alternative liming process can be continued (see Figure 2) and leather can be produced without the need to resort to metal wet-white tannages or aldehydes prior to shaving after the initial tannage. A control tannage was carried out, and the effluent results are also shown in Table 2. The leathers produced were of comparable quality to the control processed leathers. Some of the physical parameters are set out in Table 3. Conclusion The main advantage of using wasserglass seems to be that nearly all the shavings from the liming processing can be utilised in other ways. Even tanning with aldehydes restricts the possible usage of the shavings from the tannage, but using wasserglass opens up possibilities for recycling the shavings into, for instance, sausage casings. Another advantage from the tanner's point of view is that for the post-tanning processes, the leather seems to be more receptive to the reagents, leading to greater uptake of chemicals, and bolder dyeings.