Rare earth elements (REEs) play an irreplaceable role in supporting the advancement of the global economy and technology, but their limited supply drives researchers to devise effective strategies for the recovery of REEs from alternative sources, including coal fly ash (CFA). In this work, a new column leaching method using citric acid as the lixiviant was proposed to enhance the recovery of REEs from CFA at room temperature. The mechanism of column leaching was investigated based on sample properties, leaching results, and the adsorption characteristics and complexation of citric acid. REEs in the CFA were found to be hosted into the amorphous aluminosilicate matrix in the form of oxides and fluorides using SEM-EDS. Compared to the maximum recovery of total REEs (TREEs, < 30 %) in the conventional agitation leaching tests, the recovery of TREEs in the column leaching tests increased significantly to 64.8 %, while the leaching rate of impurities (Al, Ti, and Fe) was less than 13.7 %. Results of the Zeta potential measurements showed that the citric acid and rare earth cations were adsorbed on the surface of CFA particles. These adsorption phenomena may make REEs bind to the CFA surface, thereby resulting in low leaching recoveries of REEs. Rare earth-citrate complexes in the lixivium were isolated by antisolvent precipitation, demonstrating that rare earth cations can be complexed by citrate. During the column leaching process, the leached rare earth ions were complexed with citrate and then carried away with the flowing liquid phase, which improved the recovery of REEs.

Coal preparation technology is an important and widely useful way to achieve clean utilization of coal. Furthermore, it has a huge effect on the recovery of critical elements in coal, including lithium (Li) and rare earth elements (lanthanides and yttrium, REY). In this study, the migration of Li and REY during coal preparation was investigated from industrial practice and laboratory. After industrial separation, Li is significantly enriched in the coal gangue with high ash content, while REY mainly migrates to fine coal, clean coal, and coal slime. Results of screening and float-and-sink from the laboratory show that Li is enriched in the +3 mm particle size fraction and the +2.0 g/cm3 density fraction, while REY is greatly enriched in the -0.5 mm particle size fractions and the -1.8 g/cm3 density fractions. Modes of occurrence of Li and REY greatly affect their migration routes during separation. Results of sequential chemical extraction and oxidation decomposition reveal that more REY occurs in organic matter-bound form compared to Li. REY-bearing minerals (monazite and bastnaesite) in feed coal were found by SEM-EDS, but they are embedded in organic matter. In addition, oxidation decomposition experiments confirmed that REY can directly bind to organic matter in coal, especially heavy REY. These findings demonstrate that REY is closely associated with the organic fractions, while the majority of Li occurs in inorganic minerals for these coals. This research could give support to clarify the Li and REY migration during coal preparation and their extraction in the future.

Germanium (Ge) is a crucial strategic element, featuring extensive application domains and considerable value. The extraction of Ge from lignite and its combustion products constitutes a potentially significant source. This study investigated the migration and transformation of Ge during thermal conversion with diverse atmospheres and temperatures. Ge is retained and enriched in ash during thermal conversion in air atmosphere, but tends to volatilize in nitrogen atmosphere. The transformation of Ge during lignite thermal conversion was simulated by FactSage 8.2. The results demonstrated that Ge is first reduced to GeO in a reducing atmosphere. Meanwhile, S in the system mainly exists in the form of H2S, and GeO quickly reacts with it and transforms into GeS and volatilizes. In the oxidizing atmosphere, Ge transforms into SiO2-GeO2 solid solution which is unfavorable to the extraction of Ge. This study can offer beneficial references for the development of technologies for the concentration and extraction of Ge from lignite.

Critical metals in coal-based byproducts have been gradually regarded as one of the most important alternative resources due to the market parameters, strategic perspective, and advanced technology. In this study, the mechanism of calcination-acid leaching for rare earth elements (REY) recovery from coal gangue of Jungar coalfield was proposed based on the results of sample characteristics, extraction law, kinetic analysis, and typical mineral leaching. The micro-particles of bastnaesite and monazite were found and embedded in the kaolinite via micro-observation with SEM-EDS. Kaolinite in coal gangue calcined at 600 ℃ was transformed into highly positively reactive metakaolinite, enhancing the dissolution of aluminum (Al) and exposing the rare earth minerals encapsulated within the particles. Meanwhile, bastnaesite was thermally decomposed into rare earth oxides and rare earth fluorides. Rare earth oxides are more easily dissolved by acid, and the fluoride ions and the aluminum ions form a stable complex ion [AlF6]3-, which promotes the dissolution of rare earth fluoride. In addition, a high REY and Al recovery (78.8% of REY, and 88.5% of Al), came true from calcined coal gangue at optimal conditions. The leaching kinetics showed that REY leaching followed the hybrid control model of interfacial transfer and diffusion through solid layer, while Al leaching is controlled by chemical reaction. The leaching test of bastnaesite and monazite shows that the leaching efficiency of LREY is higher than that of HREY in coal gangue, which is controlled by monazite. This may be related to the crystal structure and the radius of trivalent rare earth ions. This study provides a useful reference for the extraction of REY from coal gangue.

Lithium (Li), an important strategic metal, has been paid attention to by researchers, entrepreneurs, and political circles, especially its recovery from coal-based solid waste. In this paper, the occurrence modes of Li in coal gangue (CG) have been studied through sequential chemical extraction, distribution correlation, and intercalation-leaching test. The CG was pretreated via grinding-thermal activation to selectively leach Li. The correlation between Li and magnesium (Mg) during the leaching process was also investigated. Finally, the migration mechanism of Li during the grinding-thermal activation process was analyzed using solid-state nuclear magnetic resonance for the first time. The results of the first three experiments demonstrated that Li primarily exists within the kaolinite. The extraction of Li from CG after grinding-thermal activation is 44.82 % using 0.2 M (NH4)2SO4 (pH = 4.0), while aluminum (Al) was almost insoluble, exhibiting the selective leaching of Li. 84.76 % of Li and 17.63 % of Al are obtained at pH = 1.0. The correlation analysis result suggests that lithium ions (Li+) exist in the octahedron of the lattice as charge compensation. The migration mechanism is explained by the chemical shift of Li+, which moves from -0.196 ppm (raw, in the octahedron) to -0.319 ppm (ground, octahedral destruction) and further to -0.564 ppm (thermal activation, out of the octahedron). This paper can provide a theoretical basis for the occurrence of Li and a new method for selective extraction of Li from CG.

H2TiO3 (HTO) is one promising type of lithium-ion sieves (LISs) due to its theoretical adsorption capacity, stability and selectivity, but its actual adsorption ability is restricted. Since the larger radius of Cu ions than that of Ti4+, Cu doping was performed in this work for the purpose to improve adsorption performance by enlarging Li+ transport channels. The doping content of Cu as well its effect on structures, properties and adsorption performance of LISs were examined by various techniques. After trace doping of Cu, the hydrophilicity and specific surface area of HTO-Cu-0.04 (0.04 is the doping content) were improved, which can increase adsorption sites and accelerate Li+ diffusion process. And thus, HTO-Cu-0.04 owned enhanced adsorption capacity than HTO (35.58 vs 22.41 mg center dot g(-1) at 298 K) and some other metal doped HTO in literature despite different adsorption conditions. The Li+ adsorption on HTO-Cu-0.04 is spontaneous and endothermic, following Langmuir isotherm model and pseudo-second-order kinetics model. The adsorption selectivity was examined in the simulated brine of Li+, Na+, K+, Ca2+, and Mg2+, which was analyzed by the distribution coefficient (K-d,K-Li) and separation factor (alpha(Li)(M)). After five adsorption-desorption cycles, the adsorption capacity of HTO-Cu-0.04 amounts to 95.8 % of the initial value, showing high stability. This work provides guidance for doping engineering to increase Li+ diffusion channels and hydrophilicity of LISs for improved Li+ recovery.

Solid-phase reaction is a promising method to synthesize lithium-ion sieves H2TiO3 (HTO) for simple production process. However, the uneven mixing of raw materials during the solid-phase reaction causes the agglomeration phenomenon. In this work, silane coupling agent (3-aminopropyl)triethoxysilane (KH550) was employed for surface modification of HTO to yield the HTO/KH550 composite by forming covalent bond. The crystallinity, composition, morphology, and porosity of HTO/KH550 were characterized by a series of techniques. The agglomeration phenomenon was well alleviated and HTO/KH550 showed increased surface area. The adsorption experiments were detailed carried out by varying the different factors. HTO/KH550 exhibited improved lithium adsorption capacity and adsorption rate constant (25.61 mg center dot g- 1 and 0.0037 mg center dot g- 1 center dot h-1 vs 22.41 mg center dot g- 1 and 0.0020 mg center dot g- 1 center dot h- 1 of HTO) derived from surface modification by KH550. Ion exchange mechanism in the adsorption process was revealed by X-ray photoelectron spectroscopy (XPS). Selective adsorption experiments were performed and HTO/KH550 exhibited the partition coefficient (Kd) of Li+ much higher than the competing ions (Na+, Mg2+, K+, and Ca2+). Furthermore, HTO/KH550 showed excellent cyclic stability with the adsorption capacity loss of only 3.3 % after five adsorption-desorption cycles. This work is of guiding significance to synthesize surface modified lithium-ion sieves for industrial production.

Lithium is an important strategic metallic element, which plays a vital role in the production of batteries. Coal gangue has been regarded as a promising resource of lithium. In this study, the coal gangue collected from Jungar coalfield contains 437 ppm of lithium, which is significantly higher than the average content of coal (31.8 ppm) in China. Lithium extraction from the coal gangue was systematically investigated via the integrated process of roasting and acid leaching with the mechanism analysis. The majority (94%) of lithium was extracted when raw samples are roasted at 400 degrees C for 20 min before 4 h leaching with 2 mol/L hydrochloric acid at 60 degrees C, which represented the optimal conditions. X-Ray Diffraction patterns and surface microscopic analysis of the roasting samples showed that thermal decomposition of kaolinite, as the carrier minerals of lithium in the gangue, happened during the roasting, resulting in the easy release of lithium by acid leaching. It was found that diffusion is the dominant control factor of lithium leaching based on the kinetic analysis. In this paper, an integrated process of roasting and leaching with moderate conditions was developed and performed for lithium extraction from coal gangue.

Coal fly ash (CFA) obtained from pulverized coal furnaces is a highly refractory waste that can be used for alumina and rare-earth elements (REEs) extraction. The REEs in this type of CFA are associated with a mullite and amorphous glassy mass that forms a core-shell structure. In this research, it was shown that complete dissolution of amorphous aluminosilicates from the mullite surface with the formation of the low-alkali mullite concentrate prior to sulfuric acid leaching with the addition of (NH4)2SO4 helps to accelerate the extraction of REEs. The extraction degree of Sc and other REEs reaches 70–80% after 5 h of leaching at 110 °C and acid concentration of 5 M versus less than 20% for the raw CFA at the same conditions. To study the leaching kinetics of the process, the effects of temperature (90–110 °C), liquid-to-solid ratio (5–10), and leaching time (15–120 min) on the degrees of Al and rare-earth elements (REEs) extraction were evaluated. After 120 min of leaching at 110 °C and L/S ratio = 10, the extraction of Al was found to be lower than 30%. At the same time, total REEs (TREE) and Fe extraction were greater than 60%, which indicates that a part of the TREE was transferred into the acid soluble phase. After leaching, the residues were studied by laser diffraction (LD), X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM-EDS) to evaluate the leaching mechanism and the solubility of Al- and Fe-containing minerals, such as mullite, hematite, and amorphous aluminosilicate.

A two-step method involving microwave-assisted hydrothermal synthesis and solid-phase calcination was employed to prepare the Li1.6Mn1.6O4 (LMO) precursor. Material characterization demonstrated the structural stability of LMO even after acid pickling, resulting in the creation of the ion-sieve H1.6Mn1.6O4 (HMO) with the most notable Li+ adsorption capacity of 29.45 mg.g(-1). The adsorption isotherm of HMO adhered to the Langmuir isothermal model (R-2 = 0.9929), indicating a smooth and monolayer adsorption process. Simultaneously, the adsorption kinetics of HMO was consistent with the pseudo-second-order kinetic model (R-2 = 0.9911), indicative of chemisorption. Upon undergoing five adsorption-desorption cycles, HMO's adsorption capacity remained above 22.32 mg.g(-1). Evaluation of the distribution coefficient (K-d) and the separation factor (alpha(Li)(Me)) for HMO in both coal gangue leaching solution and simulated salt lake brine highlighted its exceptional selectivity. Notably, HMO achieved a remarkable 99.9% adsorption rate in the coal gangue leaching solution. These outcomes underscore the efficacy of the synthesized HMO in highly selective Li+ recovery from both coal gangue and salt lake brine, additionally emphasizing the potential of coal gangue as a valuable lithium resource with significant prospects for future development and utilization.

Coal fly ash (CFA) as a potential resource for rare earth elements and yttrium (REY) has attracted extensive attention over the past few years. This paper focuses on studying microwave systems for the leaching of REY from CFA. Leaching parameters including hydrochloric acid (HCl) concentration, microwave heating time, microwave heating rate, and temperature were assessed. The optimal recovery of 74.91% REY, 86.77% light rare earth elements (LREY), and 34.79% heavy rare earth elements (HREY) are obtained at the conditions of 3 M HCl, T = 220 degrees C, heating rate 39 degrees C/min, and leaching time 30 min. The closed microwave system produces hydrogen chloride fluid or hydrogen chloride gas that is easier to react with CFA than hydrogen ions. It was revealed by the analysis of XRD, particle size, porosity, and SEM that fracture development benefits the entrance of leaching reagents into the inner layer of CFA. A positive linear correlation was found between leaching efficiency and REY3+ ionic radius. The stability of the REY-O bond also remains consistent with the ionic radius, leading to behavior differences during the leaching. This work was targeted to provide a theoretical and technological foundation for microwave-assisted HCl leaching for the extraction of REY in CFA.

Critical metallic elements in coal gangue have great utilization potential, especially due to the current shortage of these metals. This paper focused on examining the feasibility of physical separation (screening and float-sink tests) and calcination treatment for the enrichment of critical elements (Li, Ga, and rare earth elements plus yttrium (REY)) from coal gangue. The impacts of these enrichment methods on the acid leaching recovery of these elements were then studied. Screening tests indicated that Li and Ga were enriched in >0.125 mm size fraction and the content of REY was highest in <75 μm size fraction. Float-sink tests showed that high-density fractions were enriched in Li and Ga, and low-density fractions were enriched in REY. Physical separation cannot significantly improve the leaching rate of Li, Ga, and REY. Notably, Li, Ga, and REY were enriched significantly, and their acid leaching recoveries were increased by 54~68% after calcination under 400 °C. Sequential chemical extraction tests showed that the majority of insoluble Li, Ga, and REY was converted into soluble forms at the above temperature, which is attributed to the formation of amorphous metakaolinite and the decomposition of organic matter. Based on the results, a conceptually combined flowsheet was proposed for the extraction of Li and Ga from coal gangue.

This paper first studied the occurrence modes of critical metals (lithium: Li, niobium: Nb, and rare earth elements: REY) in coal gangue (CG) by sequential chemical extraction, and the distribution and enrichment degree of critical metals in different density and size fractions were also studied, and then CG was pretreated with intercalation and calcination to strengthen the extraction of critical metals. The results of sequential chemical extraction showed that the residual fraction was the dominant occurrence mode of critical metals. The enrichment ratios of different density fractions were 0.57-1.23 for Li, 0.03-1.34 for Nb, 0.69-1.36 for REY, and different size fractions were 0.89-1.23 for Li, 0.46-2.21 for Nb, 0.63-1.17 for REY. Density separation and particle size classification can be used to enrich critical metals. Direct leaching of 3 M hydrochloric acid resulted in 5.76% Li and 0.74% Nb recovery. Further, the leaching efficiency of Li was improved to 17.83% using pretreatment of intercalation, and the crystal structure of kaolinite was retained in the leaching residue. In comparison, calcination pretreatment led to 92.74% Li recovery, while the leaching efficiencies of Al and Fe were correspondingly improved. This paper can provide a theoretical foundation and technological support for the extraction of critical metals from CG.

Coal fly ash (CFA) as a secondary resource has received a great deal of attention over the past few decades. Research on rare earth elements (REEs) and unburned carbon removal by flotation from CFA is burgeoning, respectively. It will be beneficial if REEs could be rich in the flotation product with the carbon removal. In this study, the optimization of flotation for the unburned carbon removal was investigated and near 100% removal came true with loss on ignition of 56% and the yield of 7.7%. The optimal conditions contain rotation speed of 600 rpm and time of 30 min using the collector of 1200 g/t, the frother of 140 g/t, and the density of 130 g/t for the flotation. The majority of REEs occurred in the tailing after the flotation due to the low yield and REEs contents of the concentrates. The difference of REEs distribution in the tailing and concentrate is more significant in the milled sample. The morphology was also observed to explain the microscale behavior of REEs during the flotation.(c) 2021 Elsevier B.V. All rights reserved.

粉煤灰作为煤炭燃烧过程中主要的伴生产物,其资源化利用对煤炭工业可持续发展具有重要意义.文章主要介绍了粉煤灰基本理化性质及其资源化提取现状,包括提取未燃炭、空心微珠、磁性微珠、氧化铝、稀土元素、锂元素、镓元素和铀元素.探讨了粉煤灰资源化提取存在的问题,并给出了合理的建议.

The present work focuses on simultaneous recycling of Li and Co from crushed products of mixed electrode materials using mixed organic acids, in which benzenesulfonic acid and formic acid were cooperatively used as the leaching reagents. Results show that the optimal leaching efficiency of 97% Co and 99% Li were obtained under the conditions of 13 mol/L benzenesulfonic acid, 1.5 mol/L formic acid, a solid to liquid (S/L) ratio of 30 g/L, and 40 min reaction time at 50 degrees C. Meanwhile, the leaching of Li and Co fits well to logarithmic rate model with apparent activation energy of 32.7 and 47.0 kJ/mol in this given leaching system, respectively. Besides, cobalt was directly recovered from the leach liquor as pure cobalt benzene sulfonic with the recovery efficiency of 99%, and lithium can be entirely precipitated by adding phosphoric acid. Further, the reaction mechanism involves the leaching-hydrating-complexing model of LiCoO2 particles was proposed based on the dissolution behavior of metals and then verified by morphological and phase characterization (i.e. FT-IR, XRD and SEM-EDS) of the recycling product. The whole process is found to be effective and sustainable for recovery of Li, Co and graphite from mixed industrial crushing product of spent LIBs. (C) 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

以N-异丙基丙烯酰胺和亲水性单体丙烯酸钠为原料,采用水溶液自由基共聚法制备了一系列N-异丙基丙烯酰胺基温敏吸水聚合物.利用傅里叶变换红外光谱仪和热重分析技术对所制样品的物理结构和性质进行表征,研究了不同丙烯酸钠单体摩尔比对聚合物临界溶解温度(LCST)、溶胀性和退胀性的影响,并研究其溶胀动力学.结果表明:通过实验成功制备出高分子共聚物,共聚物的热分解温度随着丙烯酸钠摩尔比的增加而降低;聚合物的LCST随着丙烯酸钠摩尔比增加而逐渐提高,但响应速率呈现逐渐降低的趋势;溶胀过程符合non-Fickon动力学方程.

Magnetite nanoparticles were prepared in a weakly basic solution at the low reaction temperature by the co-precipitation method. As a comparison, the oxidative precipitation method was also applied in this study. The structure, morphology, and other properties of the obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), and thermogravimetric analysis (TGA). The above characterization data indicate that small size and narrow size distribution are found for magnetite nanoparticles prepared by the co-precipitation method. Further magnetic property and Zeta potential results illuminate that magnetite nanoparticles prepared by this method display a quasi-superparamagnetic property and a good dispersion in the aqueous solution. Based on the investigation results, the magnetite nanoparticles with a quasi-superparamagnetic property and a fine dispersion can be facilely prepared in a weakly basic solution at the low reaction temperature by the co-precipitation method.

A temperature-sensitive,magnetic,and water-absorbent resin was prepared in this work by copolymerizing N-isopropyl acrylamide and sodium acrylate to envelop the pre-synthesized Fe3O4 nanoparticles,in which the Fe3O4 magnetic nanoparticles were prepared by coprecipitation method,and the envelopment was conducted by free-radical copolymerization in aqueous system.The obtained sample was characterized by X-ray diffraction(XRD),Fourier transform infrared spectrometer(FTIR)and thermal gravimetry(TG).Based on temperature sensitivity,swelling property,and deswelling property,it can be found that the resin possesses good water absorption and temperature sensitivity,as the low critical solution temperature (LCST) and swelling ratio were 50 ℃ and 116.74 g/g,respectively,and water loss in the fine coal was 77.90％ at 70 ℃ within 30 min.When the sample was fully swelled by water and treated by ultrasonication for 30 min,only a small amount of Fe3O4 was removed,illuminating a relatively stable magnetic property.

One novel superabsorbent polymer with temperature-sensitive and magnetic properties was prepared by the designed route. As a comparison, the other superabsorbent polymer was prepared by changing the above route. The obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), superconducting quantum interference device magnetometer (SQUID), and gel permeation chromatography (GPC). The temperature-sensitive properties of the obtained polymers were investigated by measuring swelling ratios at given temperatures. The superabsorbent polymer exhibiting the preferable temperature-sensitive, magnetic properties and swelling ratio was employed in the dewatering process of the fine coal. The moisture mass content in the fine coal was decreased to less than 10% within the mixing time 60 min. In addition, it can be found that the polymer with relatively stable responsive properties and swelling ratio can be gained by this method based on the regeneration of the used polymer in the dewatering process.