An Eco-Friendly Acid Leaching Strategy for Dealkalization of Red Mud by Controlling Phase Transformation

The alkaline components in red mud represent one of the crucial factors restricting its application, especially for the construction and building industry. The phase state of alkaline components has a significant influence on the dealkalization of red mud. In this work, an environmentally friendly acid leaching strategy is proposed by controlling the phase transformation of red mud during active roasting pretreatment. With a moderate roasting temperature, the alkaline component is prevented from converting into insoluble phases. After acid leaching with a low concentration of 0.1 M, a high dealkalization rate of 92.8% is obtained. Besides, the leachate is neutral (pH = 7) and the valuable metals in red mud are well preserved, manifesting a high selectivity and efficiency of diluted acid leaching. The calcination experiment further confirms the practicability of the strategy in the construction field, where the cementitious minerals can be formed in large quantities. Compared with the traditional acid leaching routes, the diluted acid leaching strategy in this work is acid saving with low valuable element consumption. Meanwhile, the secondary pollution issue can be alleviated. Hence, the findings in this work provide a feasible approach for the separation and recovery of alkali and resource utilization of red mud.

Preparation of a Novel Cement from Red Mud and Limestone

Red mud (RM) is an industrial waste obtained from the Bayer process which is usually discharged into marine or disposed into a landfill causing pollution for the surrounding water, air, and soils. Thus, disposal of RM is an environmental concern, and it should be recycled effectively. Because RM consists of iron- and aluminum-rich phases, it is possible to be processed into cementitious material and utilized for construction purposes. This research fabricated a type of cement from the mixture of RM and limestone. The mixture was sintered at temperature of 1180 °C to obtain the clinker of the novel hydraulic cement with C2S, C3A, and C4AF minerals. In which, C2S, C3A, and C4AF are respectively belite, alite, and tetra-calcium aluminoferrite compounds that are characteristic hydraulic minerals of Portland cement clinker. The specifications of this cement were tested and evaluated in this study such as chemical and mineralogical compositions, fineness, specific surface area, mechanical strength after 3, 7, and 28 days.

Solidification performances of contaminants by red mud-based cementitious paste filling material and leaching behavior of contaminants in different pH and redox potential environments

Considering the urgent need for disposal of red mud and the comprehensive treatment of coal mined-out areas, this paper presented red mud-based cementitious paste filling material (RMFM) to achieve the purpose of green filling treatment. However, the solidification performance of alkaline RMFM for contaminants can be affected when in contact with acid goaf water in practice, which may in turn causes secondary pollution to the surroundings. The leaching tests of RMFM under different pH and redox potential (Eh) conditions were designed to investigate the effects of environmental elements on the solidification performance of RMFM, and primarily investigated the treatment effectiveness of RMFM on goaf water. The test results manifest that the acidic and oxidizing environments could damage the hydration products generated by alkali and sulfate activation, thus affecting the solidification performance, while the alkaline and reducing environments could effectively prevent the release of the contaminants by enhancing the degree of alkali activation and inhibiting oxidation acid forming process. In the possible exposure environment, RMFM could effectively stabilize its own pollutants without secondary pollution. In addition, the powder RMFM samples had significant removal effects on heavy metals, the values of Cu, Pb, and As removal efficiency all reached more than 96.15%.

The impregnation of ZnO onto ZSM-5 derived from red mud for photocatalytic degradation of methylene blue

Photocatalytic degradation of Methylene Blue (MB) by zinc oxide/zeolite socony mobile-5 (ZnO/ZSM-5) composites was investigated. The ZSM-5 material was synthesized from red mud by a two-step hydrothermal method to which ZnO loadings at different mass ratios were subsequently performed. Characterizations using X-ray diffraction (XRD), Fourier transform infrared spectroscopy, and scanning electron microscopy were carried out to identify the formation of ZSM-5 and ZnO/ZSM-5. ZSM-5 and ZnO/ZSM-5 have cubic microcrystallite morphologies. ZnO loading in the ZnO/ZSM-5 composites was successfully performed and confirmed by the appearance of wurtzite peaks in the XRD spectra that matched the Joint Committee on Powder Diffraction Standards data. The presence of ZnO in ZSM-5 leading resulted in a decrease in the surface area and pore size as confirming by nitrogen adsorption-desorption isotherm experiments. The band gap of the samples was measured using UV-Vis diffuse reflectance spectroscopy. The optimum photocatalytic degradation of MB was observed at a ZnO loading of 34% w/w dubbed 34-ZnO/ZSM-5. The influence of the initial concentration of MB was also investigated at 80, 90, and 100 mg L− 1 using 34-ZnO/ZSM-5 and ZSM-5. Liquid chromatography–mass spectrometry characterization was performed to analyze the degradation products.