The formation mechanism of Al13 and its purification with an ethanol–acetone fractional precipitation method

The formation mechanism of Al13 is discussed via the investigation of the distribution and transformation of Al13 in the precipitate and in the solution during the preparation of polyaluminum chloride (PACl). After the NaOH is injected into the AlCl3 solution, the monomeric Ala species reacts with OH− immediately to form the high-molecular-weight Alc species, which may exist either in the solution or in the precipitate, depending on the system temperature and the rate of base injection. Subsequently, the Alc species reacts with Ala to generate the medium-molecular-weight Alb species, i.e., Al13. The Al13 species can be separated and purified from PACl solutions with an ethanol–acetone fractional precipitation method. The results from ferron assay and liquid- and solid-state 27Al NMR show that the purity and the extraction yield of the extracted Al13 depend on the initial Al13 concentration. A higher Al13 content in the initial PACl solution leads to higher Al13 purity and to a higher extraction yield. A maximum purity of 99.5% for Al13 can be achieved by the described ethanol–acetone precipitation method.

► Ala reacts with OH− to form Alc, and Alc reacts with Ala to generate Al13.
► An ethanol–acetone fractional precipitation method is proposed for Al13 purification.
► The purity and the extraction yield of Al13 depend on the initial Al13 concentration.
► A maximum purity of 99.5% for Al13 can be achieved by the proposed method.