Inorganic foam structures by using residual materials
The residues material in this project is mostly decoupled with a temperature above 1000°C, cooled down, partly in a controlled manner and reused cold in further processes. Its thermal potential of about 1 kJ/kgK is not exhausted. In the last century the industry recognized the immense potential and developed various approaches for heat recovery. However, so far, none of these approaches has been technically implemented. Due to the low thermal conductivity of these mineral residues (max. 2 W/mK) a good heat transfer needs a large volumetric surface [Ren96; WAH05]. The results are small particles which preclude any further use. Caused by this contradiction the aim of this project is to develop a product, which can be produced by using the thermal potential of its source material. The main components of the inorganic material are silicon oxide (SiO2), calcium oxide (CaO2), alumina (Al2O3) and magnesium oxide (MgO). Foam glass, which is used as insulation material in civil engineering, is also made of these minerals. Its production process is divided into 6 steps [Deu08]: 1. Melting waste glass mixture at about 1250°C 2. Cooling the melt 3. Grinding the cooled melt with the addition of a foaming agent, such as e.g. coal 4. Foaming of the glass at 700-900 ° C 5. Cooling of the foam glass 6. And if necessary: cutting the foam glass plates. For the production of foam glass approximately 24.5 kJ/kg primary energy is required [Deu08]. Theoretically only 2.5 kJ/kg are required for melting and reheating of the material. The remaining 22 kJ/kg is assigned to the heat losses and the mechanical processes. This is where the development of the mineral foam starts: The high energy the residues are containing while leaving the process is to be used by a foaming agent such as calcite, magnesite or kaolinite. These minerals decompose at a critical temperature in a base component of the mineral, plus a gas like CO2 or H2O. The gas is forming pores and is therefore able to create inorganic foam structures. The expected result is a macro porous mineral foam with similar physical properties like foam glass. For the production of a mineral foam from molten residue material, a foaming agent and waste glass flour no additional primary energy is necessary. The scientific study focuses on uncovering the relationships between the starting mixture, the addition of waste glass powder, the thermal conditions and the material properties like porosity, pore size and distribution, as well as the thermal conductivity. Temperatures well above 1000°C are required for the analysis of mineral foam production. Thus, in addition to the initial mixture the method of mixing must be examined in the molten state. Parameter studies are required for both the powdered state (milled to <60 microns) analogous to foam glass manufacturing, as well as for the molten state.