## 7 Conclusions  
The rising interest in the valorizing of industrial by-products is one of the main reasons for exploring different separation methods of leather waste along the production-chain in the tanning industry. Recently, we showed that through a detailed and accurate separation of waste, during leather processing, an optimal zero-waste condition may be attained [25]. The main reason that led to this success is that the tanned leather waste raw material was made economically advantageous for other production cycles. The advantage is to avoid the expensive and complex pretreatment of waste, some of which must be disposed of in landfills. Consequently, the mechanical and chemical properties, linked to the structural complexity of the leather, have been preserved allowing for a wider spectrum of uses. Three types of waste, corresponding to three different stages of skin leaking, i.e., PTD, CS (chrome tanning) and OT (cod liver oil tanning), were used as fiber-fillers for vulcanizing NR. The study of the vulcanization process indicates such fibers produce an accelerating effect, especially noted for OT-waste. The mechanical tests performed on compounds show a reduction of the tensile strength of materials in working conditions and an increased hardness, especially in the compounds containing PTD and CS fibers. Therefore, the compounds can be usefully used for the production of flooring supports, soles and heels for shoes and anti-slip rubber grip for chairs. Mixtures obtained with OT fibers, exhibit also a significant increase in \(\sigma_{b}\). Generally, compounds can be used as materials with functions requiring greater tensile strength (e.g., hoses, conveyor belts). In order to further increase the characteristics of hardness and breaking strength, the mixes are reinforced with carbon black. Usually, the incorporation of carbon black into NR causes an increase in \(\sigma\) and \(\sigma_{b}\) with a corresponding decrease in \(\epsilon_{b}\). While for PTD and fibers, expected results were recorded, for OT fiber, on the contrary, there was a strong increase in both \(\sigma_{b}\) and \(\epsilon_{b}\), compared to the reference compounds. Thus, experimental results show that leather waste can be used, without any pretreatments, to manufacture products destined for different uses. Herein, as an example, formulations containing 10 wt% of PTD waste were used to manufacture car mats and floor laminate supports; while formulations with 10 or 20% of PTD were used to make soles and heels for shoes and anti-slip rubber grip for chairs. Leather waste was used without chemical treatments so that the release of chromium could be aroused. In this regard, both the formulations and the artifacts were monitored for the release of chromium (VI) traces [28; 31]. However, concerns about possible synergistic harmful effects between rubber and tanned waste (e.g., chromium release) [35] have prompted us to carry out chemical analyses for a whole year [33; 34]. Results showed that the amount of Cr(VI) was below the instrumental detection limits, indicating that this metal is either absent or according to the European Regulation less than 5 mg per kg of product [32]. The examples presented in this research seem to confirm the idea that efficient waste separation and their proper use lead to a twofold advantage. On the one hand, the environmental impact of tanneries is reduced, while, on the other hand, leather industries have, with the waste, an alternative source of gain. Figure 6: Products manufactured from RMPTD\({}_{10}\) formulation. (**a**) Laminate floor covering. The white surface is the top layer, the black band is the backing layer; (**b**) car floor mat; (**c**) soles and heels for shoes and anti-slip rubber grip for chairs. **Author Contributions**: Author Contributions: Conceptualization, L.A.; G.B., Methodology, L.A.; G.B., numerical analyses were carried out by B.M. All authors have read and agreed to the published version of the manuscript. **Funding**: This research received no external funding.