Industrial waste silica-alumina gel recycling: Low-temperature synthesis of mullite whiskers for mass production

Analysis of Influencing Factors in the Preparation of Mullite Whiskers from Recovering Silicon-Rich Waste under Low-Temperature Conditions

“…On the contrary, at a lower temperature (700 °C), although no crystal diffraction peaks of mullite were detected, diffraction peaks of topaz phase were presented (Al 2 [SiO 4 ](F, OH) 2 , orthorhombic, (PDF#12-0765)). However, the diffraction peak intensity of the mullite phase of the sintered sample at 750 °C is lower than that at 720 °C, which may be due to the secondary growth of columnar mullite from the top at 750 °C, and the secondary growth occurs in the interior, forming many fine mullite whiskers with poor crystallinity, resulting in the decrease of the diffraction peak intensity, which is similar to results described in previous work [ 38 , 39 ]. The intensity of the diffraction peak of the mullite phase increases rapidly when the sintering temperature of the sample is increased from 800 to 875 °C.…”

“…Na 2 SO 4 is currently the most commonly used flux in the preparation of mullite whiskers, and its addition can effectively reduce the sintering temperature of the reaction [ 38 ]. The addition of Na 2 SO 4 during the preparation process, which reacts with aluminum compounds to form Al 2 (SO 4 ) 3 , results in the following chemical reactions [ 38 , 39 ]: …”

“…As Al 2 (SO 4 ) 3 decomposes, γ − Al 2 O 3 and silicon-rich waste gradually form mullite crystals by the V–L–S reaction mechanism in the liquid-phase reaction system provided by the fluorine atmosphere of aluminum fluoride and sodium sulfate. Meanwhile, the liquid phase matrix growth atmosphere of sodium sulfate accelerates the diffusion rate of the medium and atoms, and the low viscosity of the liquid phase accelerates the reaction rate at the liquid/solid phase boundary, which in turn has a significant fluxing effect on the Al 2 O 3 -SiO 2 system, thus melting at a lower temperature and promoting the growth of mullite crystals and the nucleation growth of mullite crystals [ 39 ].…”

“…The results show that the length of mullite whiskers increased significantly, while the diameter of whiskers increased slowly. It has been reported that aluminum fluoride trihydrate determines the development and growth of mullite, which grows linearly with increasing addition [ 38 , 39 ]. Compared with previous studies on the preparation of mullite whiskers from recycled waste [ 24 , 25 ], the present technique enables the preparation of a large number of mullite whiskers with uniform morphology and a certain interlocking structure and aspect ratio at a lower temperature, which expands the application prospects of whiskers.…”

Analysis of Factors Affecting the Preparation of Mullite Whiskers from Silica-Rich Slag and Application Studies

“…The molten salt method [37,38] promotes whisker growth due to its unique reaction atmosphere, but the reaction temperature is above 1200 • C. Many scholars have devoted themselves to the preparation of mullite whiskers using the low-temperature [39,40] molten salt method. Rashad et al [41] prepared mullite whiskers by utilizing the catalytic effects of AlF 3 in a reaction environment at 1100 • C. Wang et al [42] grew mullite whiskers using Al 2 (SO 4 ) 3 -Na 2 SO 4 as a molten salt system at 1023 • C. Chen et al [43] and Abdullayev et al [44] further reduced the preparation temperature to below 900 • C by adding aluminum fluoride. At the same time, there are also many scholars [45] focusing on the influencing factors of mullite whisker growth, but they do not take into account the most important factors of the molten salt system, the rate of temperature rise, or the heating time as regards their effects on the structure of mullite, and studies have yet to be undertaken on the effects of mullite whiskers on ceramic toughness.…”