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The phase equilibrium of the ternary system K 2 SO 4 −KOH−H 2 O at 313.15, 333.15, 343.15, and 353.15 K was researched by the isothermal dissolution method. There are one invariant point, two univariant isothermal dissolution curves, and three crystallization regions. Neither double salt nor solid solution is found in the ternary system. At the invariant point of the ternary system, the composition of the solution is K 2 SO 4 0.03 and KOH 56.03 wt % at 313.15 K, K 2 SO 4 0.07 and KOH 57.21 wt % at 333.15 K, K 2 SO 4 0.08 and KOH 58.56 wt % at 343.15 K, and K 2 SO 4 0.09 and KOH 59.48 wt % at 353.15 K, respectively. According to the phase diagrams of the ternary system, it can be indicated that high KOH concentration and low temperature are beneficial to separating K 2 SO 4 from the alkali solution. Furthermore, the solubilities corresponding to the crystallization region of K 2 SO 4 in the ternary system at each temperature were calculated theoretically by using the Pitzer ion interaction model. The calculated K 2 SO 4 solubilities basically agree with experimental values at low KOH concentration. However, the high ionic strength was found to be a key factor that seriously influenced the calculation accuracy. All the data obtained in this work are significant to the design and optimization of the K 2 SO 4 crystallization process from the alkaline leaching solution in the alunite metallurgical industry.

Ultrafine particle classification can be realized using hydrocyclones with novel structures to overcome the limitations of conventional hydrocyclones with tangential inlets or cone structures. Herein, the hydrocyclones with different inlet structures and cone angles were investigated for classifying ultrafine particles. Computational fluid dynamics (CFD) simulations were performed using the Eulerian−Eulerian method, and ultrafine MnO 2 powder was used as a case study. The simulation results show a fine particle (≤5 μm) removal efficiency of 0.89 and coarse particle (>5 μm) recovery efficiency of 0.99 for a hydrocyclone design combining an arc inlet and a 30°cone angle under a solid concentration of 2.5 wt %. Dynamic analysis indicated that the novel arc inlet provided a preclassification effect to reduce the misplacement of fine/coarse particles, which cannot be provided by conventional tangential or involute inlets. Furthermore, the proposed design afforded comprehensive improvement in the flow field by regulating the residence time and radial acceleration. Subsequently, a novel hydrocyclone with an arc inlet and 30°cone angle was manufactured using the three-dimensional (3D) printing technology. Experiments were conducted for classifying ultrafine MnO 2 particles using the novel 3D-printed hydrocyclone and conventional hydrocyclone. The results demonstrate that the classification performance of the 3D-printed hydrocyclone was superior to that of the conventional one, in particular, the removal efficiency of fine particles from 0.719 to 0.930 using a 10 wt % feed slurry.

Phase equilibrium of ternary system K2O–Al2O3–H2O is the key in realizing seeded precipitation process of aluminum hydroxide product from alkaline leaching solution in the alunite tailings metallurgical industry. However, little work has been carried out about the solubility of the K2O–Al2O3–H2O system, especially for the concentrated alkali solution region. The corresponding phase equilibria at 323.15, 333.15, 343.15, and 353.15 K were researched by using the isothermal dissolution method and the Pitzer ion-interaction model. The calculated Al2O3·3H2O solubilities corresponding to the crystallization region of Al2O3·3H2O in the ternary system at each temperature basically agree with experimental values when K2O concentration is below 30 wt %. Furthermore, the ternary system at each temperature contains two invariant points, three univariant isothermal dissolution curves, and five crystallization fields corresponding to Al2O3·3H2O (gibbsite), K2O·Al2O3·3H2O, KOH·H2O, Al2O3·3H2O + K2O·Al2O3·3H2O, and K2O·Al2O3·3H2O + KOH·H2O. According to the phase diagrams of the ternary system, it can be indicated that the low alkali concentration and temperature are in favor of the crystallization of Al2O3·3H2O, while the high alkali concentration and low temperature are beneficial to the crystallization of K2O·Al2O3·3H2O.