Many EU policies aim at waste prevention and reduction. In order to succeed in reduction of waste sent to landfills, the policies encourage material reuse as secondary raw materials or as energy source. Incinerating waste is a well-known waste-to-energy technique. However, innovative thermal treatments are arising as an alternative to traditional processes: in particular, pyrolysing or
high temperature gasifying processes. When using biomass or waste, these methods can increase energy efficiency, reduce gaseous emissions to be depurated, and/or enhance solid-residue inerting.
Furthermore, the properties of the residue (char) from these new processes can be used as solid fuel or as secondary raw material. The properties of char strongly depend upon the nature of the starting materials and the process conditions. For instance, when high carbon content is present, the process can be tuned to favour the material recovery rather than the energy production. The residue can then be used as substrate for active carbon production. Another promising application is carbothermal synthesis in order to achieve high added value products, such as ceramics (Si3N4, SiC, TiC, NbC, etc.).
Take for instance silicon nitride (Si3N4), an advanced ceramic material that is used for the manufacturing of structural parts of gas turbines, engines and high temperature systems. The most common method for its production is direct nitridation of silica powders at temperatures ranging from 1200 to 1500°C:
3Si(s) + 2N2(g) → Si3N4(s)
However, it can also be produced through the carbothermal reduction of silica in N2 gas. At temperatures ranging from 1200 to 1700°C, it is two-step reaction that brings the formation of gaseous SiO as an intermediate. The final product adopts the most reactive crystalline form of the material
3SiO2(s) + 6C(s) + 2N2(g) → Si3N4(s) + 6CO(g)
Some residual products have been tested for obtaining Si3N4, for example rice husks, a by-product of the rice milling process. The initial material mostly contains organic matter and silica. It is therefore a suitable raw material for the production of silicon nitride through carbothermal reduction at high temperature. The synthesis of ceramic materials from waste residues showed various morphologies according to the process conditions, depending on temperature and milling grade of the starting materials.
This test proves that it is possible to transform waste into high added-value materials such as silicon nitride with excellent wear, corrosion resistance, and remarkable high temperature mechanical properties. Research on other waste types is currently on-going, for instance project TyGRE deals with tyres. All this knowledge is required in order to design appropriate industrial processes that will enable more high-volume waste reutilization and would be more competitive than combusting techniques.
Portofino, S.; Galvagno, S.; De Girolamo Del Mauro, A.; Calò, E.; Synthesis of ceramic materials from waste residues