The formation mechanism of Al-13 is discussed via the investigation of the distribution and transformation of Al-13 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 Al-a species reacts with OH- immediately to form the high-molecular-weight Al-c 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 Al-c species reacts with Ala to generate the medium-molecular-weight Al-b species, i.e., Al-13. The Al-13 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 Al-27 NMR show that the purity and the extraction yield of the extracted Al-13 depend on the initial Al-13 concentration. A higher Al-13 content in the initial PACl solution leads to higher Al-13 purity and to a higher extraction yield. A maximum purity of 99.5% for Al-13 can be achieved by the described ethanol-acetone precipitation method. (C) 2011 Elsevier B.V. All rights reserved.