Fluidized-bed homogeneous crystallization of α-Al(OH)3 for continuous aluminum removal from aqueous solution: Parameter optimization and crystallization mechanism

Abstract

Fluidized-bed homogeneous crystallization (FBHC) is utilized to recover dissolved aluminum from the solution. Aluminum is crystallized homogeneously using NaOH as a precipitant. Under the optimum pH of 9.7; cross-sectional surface loading (L) of 0.5 kg/m2.h; up-flow velocity (U) of 62 m/h, the total aluminum removal (TR%) in the FBHC reactor reached about 98.5% that is corresponding to the residual aluminum concentration of 10 mg/L. The crystallization ratio of aluminum pellets reached about 93%. The aging process of FBHC effluent solution in the sedimentation tank (SED) lowered the residual dissolved aluminum concentration to below 0.2 mg/L due to the polymorph change from amorphous Al(OH)3 to crystalline Al(OH)3. SEM image shows the round-shape pellet product of FBHC that consists of two different structures of the inner and outer parts due to the different operating supersaturation index. Meanwhile, XRD and FTIR patterns show that during 24 h of aging time, the precipitated compound was changed from amorphous Al(OH)3 to pseudo-AlOOH, and then α-Al(OH)3 crystal. Bayerite (α-Al(OH)3) was predominating the compound of the final pellet product. The production of low-moisture bayerite leads to a disposal cost reduction of up to 80%. FBHC proposed an alternative method to treat aluminum ions in the solution by reclaiming it as high separability and low moisture Al(OH)3 granulated crystal.