Valorization of Industrial Wastes to Low-Carbon Minerals

This study concentrates on developing methods for utilizing industrial waste materials such as red mud and phosphate slag. In this study, an electrochemical process was developed for recovering iron metal powder from red mud. Red mud is a waste material from aluminum refineries. Although red mud can be utilized as a construction and building material, its use is becoming controversial due to its high metal content, which contributes to its toxicity. However, the high iron oxide content in red mud makes it a good feedstock for producing iron. In this project, we employed an electrochemical technique for extracting iron powder from red mud. A thermochemical pretreatment method was developed for red mud to enhance the efficiency of iron extraction. The impurity content in iron powder produced from the thermo-chemically treated red mud sample was significantly low as compared to iron produced from untreated red mud. In the next chapter, the feasibility of blending phosphate slag with Type IL cement in concrete making was studied. A comparative analysis between concrete samples containing a mixture of phosphate slag and Type IL cement and those with 100% Type IL cement was conducted to understand the cementing capabilities of phosphate slag and its impact on the compressive strength of the concrete. Results indicated that the addition of slag decreased the compressive strength of the concrete. However, concrete samples exhibited better mechanical performance under weak acidic conditions or carbonation. This trend reversed with increasing alkalinity, such as with NaOH addition. An attempt was made to balance the pH effect and the amount of HCO3- ions in the mix. Both water-curing and temperature-curing conditions were tested, revealing that temperature-cured samples had lower compressive strength compared to water-cured ones. Thermogravimetric analysis and X-ray diffraction showed that structural water in the form of CCSH phase was present in water-cured concrete, while aragonite and calcite were present in both curing conditions. The ratio between carbonate and CCSH phases was correlated with mechanical performance, suggesting that these reaction conditions can enable low-carbon cement formulations. The results indicated that the introduction of phosphate slag into the Type IL mix decreased the compressive strength of the concrete. However, the mechanical behavior improved under high carbonation or weak acidic conditions. An attempt was made to balance the pH and number of carbonates in the mix; however, the strength decreased with the increase in alkalinity. Temperature-curing was carried out apart from water-curing, to observe the influence of temperature on the mix. Results revealed that the introduction of temperature adversely affected the compressive strength of the concrete. Thermogravimetric analysis and X-ray diffraction results showed that the structural water content was there in the form of CCSH phase in the water-cured sample, while aragonite and calcite were present in both curing conditions. The ratio between carbonate and CCSH phases were correlated with mechanical performance, suggesting that these reaction conditions can enable low-carbon cement formulations.