Sintering kinetics and alumina yield in lime-soda sinter process for alumina from coal wastes

Utilization of coal fly ash in China: a mini-review on challenges and future directions

The rapid economic development in China places a large demand for energy, and as a result, thermal power plants in China are producing an enormous amount of coal fly ash (CFA) which causes severe environmental pollution. This paper briefly describes the current production and utilization status of CFA in China and identifies the challenges confronting sustainable CFA utilization as the Chinese economy is being transformed. These issues include a regional imbalance in supply and demand, reducing demand in the real estate industry as well as stricter laws for environmental protection. Viable directions for future CFA utilization are proposed, for example, production of CFA-based ceramic tiles, recovery of elemental resources, agricultural melioration, treatment of wastewater and flue gas, and production of high-volume CFA concretes. This paper has some guiding significance for sustainable and cleaner utilization of CFA in China and even worldwide. Graphical abstract.

Aluminum extraction technologies from high aluminum fly ash

Coal fly ash (CFA), an industrial by-product of high-temperature combustion of coal in coal-fired power plants, is one of the most complex and largest amounts of industrial solid wastes generated in China. It is widely recognized that CFA should be considered as a potential alumina resource to substitute bauxite. In this review, the features of high-alumina fly ash and aluminum recovery technologies are first described. Later, the merits and drawbacks of alumina extraction technologies in recovering more valuable materials are compared in terms of extraction mechanisms and equipment requirements. It is shown that “predesilicating-sodium carbonate (Na2CO3) activation-acid leaching” is currently a promising method in achieving multimetal synergistic extraction. Finally, the hydrochloric acid and sulfuric acid combination process is proposed as a sustainable development of the predesilicating-Na2CO3 activation-acid leaching process. The findings of this review provide theoretical guidance for novel developments and applications of aluminum extraction technologies.

Alumina Extraction from Coal Fly Ash via Low-Temperature Potassium Bisulfate Calcination

Owing to the depletion of bauxite and increasing demand for alumina, calcination methods for extracting alumina from coal fly ash (CFA) were developed. However, these methods have disadvantages such as the need for high temperatures and the emission of toxic gases. Hence, in this study, Al2O3 was extracted from CFA via low-temperature potassium bisulfate calcination technology. Effects of the potassium bisulfate amount, calcination temperature, and calcination time on the alumina extraction efficiency were investigated using X-ray diffraction, thermal gravimetry, scanning electron microscopy, differential scanning calorimetry, and energy-dispersive spectroscopy. It was found that this technique could recover alumina efficiently, and potassium bisulfate significantly contributed to the degradation of mullite and corundum phases. Al2O3 in CFA was converted into soluble K3Al(SO4)3. With a KHSO4/Al2O3 molar ratio of 7:1, calcining temperature of 230 °C, and calcining time of 3 h, the alumina extraction efficiency reached a maximum of 92.8%. The Avrami–Erofeev equation showed the best fit with the kinetic data for the low-temperature calcination of CFA with KHSO4. The activation energy was 28.36 kJ/mol.

Green synthesis of mesoporous γ-Al2O3 from coal fly ash with simultaneous on-site utilization of CO2

Mesoporous Al₂O₃ with crystalline framework walls has expanded all over the world due to the various potential applications especially in catalysis. Here, we develop a green and facile approach for the conversion of coal fly ash (CFA) into ordered mesoporous γ-Al₂O₃. The practical and promising lime-sinter method was comprehensively studied for the extraction of aluminum from CFA as a first step. The extraction efficiency of aluminum could reach up to 87.42%, through calcining with CaCO₃ at 1390°C for 1 h and then dissolving in Na₂CO₃ solution at 70°C for 0.5 h. Combined with the urgent demand for CO₂ emission reduction, simulated purified flue gas was introduced to precipitate the Al(OH)₃ precursors without structure-directing agents for just 1 h, followed by calcining at only 400°C or 550°C. A series of characterizations were conducted to discuss the effect of precipitation temperature and calcination temperature, resulting the superior product (Al₂O₃-65/550) with high surface area (230.3 m² g^-1), crystalline γ-Al₂O₃ phase and ordered mesostructure. This proposed strategy, integrating the on-site recycling of CFA and utilization of CO₂, appears to be promising for scalable production of mesoporous γ-Al₂O₃.

Research and industrialization progress of recovering alumina from fly ash: A concise review

Fly ash, a by-product of high temperature combustion of coal in coal-fired power plants, is one of the most complex and largest amount of industrial solid wastes generated in China. Its improper disposal has become an environmental problem. Now it is widely realized that fly ash should be considered as a useful and potential mineral resource. Fly ash is rich in alumina, making it a potential substitute for bauxite. With the diminishing reserves of bauxite resources, as well as the increasing demand for alumina, recovery of alumina from fly ash has attracted extensive attention world-wide. The present review describes, firstly, the generation and physicochemical properties of high alumina fly ash found in northern China and then focuses on the various alumina recovery technologies, the advantages and disadvantages of these processes, and in particular, the latest industrial developments. Finally, the directions for future research are also considered.