Effect of carbon on the performance of red mud-molten salt composites for thermal management and waste heat recovery applications

Abstract

• A novel composite material consisting of red mud, carbon and molten salt is investigated. • Carbon addition enhances the thermal conductivity by up to 13.45% (5 wt.%). • Energy density is only slightly reduced in a temperature range of 25-500 ℃. • However, the elastic modulus and coefficient of thermal expansion are affected. • These properties coupled with thermal cycling lead to progressive porosity increase. Red mud (RM) is a major waste of the aluminium industry with a present amount exceeding 4 billion tones, typically compacted in dams or dykes (stockpiles). This practice is highly problematic due to its high alkalinity and traces of heavy metals. Despite of many efforts, its present global utilization rate is only ∼15%. In this work, we attempt to enhance the thermal properties of previously reported novel composite phase change materials (CPCMs) based on RM, by adding graphite. The fabricated, carbon-containing, CPCMs demonstrate an overall good performance over the temperature range of 25 to 500 ℃. The latent heat ranges from 47.3 to 65.5 J/g, depending on salt and graphite contents. An optimal thermal conductivity is found to be up to 0.79 W/mK (25-500℃) on average. The specific heat capacity is on an average up to 1.23 J/(g ℃) when the PCM is in the solid phase (25-220 ℃) and 1.25 J/(g℃) when the PCM is in the liquid phase (220-500 ℃). No variations in the melting point or latent heat are observed after 96 cycles. Optimal energy storage density is calculated to be 1100 MJ/m 3 . Addition of graphite leads to a reduction in the coefficient of thermal expansion and the elastic modulus of the CPCM resulting in an increase in its porosity and thus a reduction in energy storage density. The working temperature of this novel CPCM makes it a cost-effective and compact solution for high-temperature heat storage/heat recovery applications.