Formation of solid RbCl from aqueous solutions through membrane crystallization

Rubidium (Rb) is a very precious alkali metal with a high economic value. Of the rubidium compounds, RbCl is probably the most widely used. In the present work, RbCl crystals were obtained by vacuum membrane crystallization (V-MCr). Two different membranes were utilized: (i) one in polypropylene (PP) and (ii) one in poly(vinylidene fluoride)(PVDF). The membranes differ in material but have equal pore and porosity, and the experiments with the two membranes were conducted at the same temperature, vacuum pressure and flow rate values. While RbCl crystals were not obtained with the PVDF membrane due to wetting, good crystals were achieved from experiments with the PP membrane, with dispersion of crystal size distribution around of the average size of just 33.6 %. Another important result is that, in addition to the operating conditions, the membrane material is decisive for the success or failure of the membrane crystallization operation.