Hydrated alumina Al2O3.3H2O was produced from spent alkaline baths resulting from aluminium transformation industry. Batch and continue laboratory and pilot tests were performed in order to assess technical viability of producing reactive alumina directly from the baths by a precipitation/crystallization process and at the same time of recovering and recycling the associated sodium hydroxide to the extrusion process. The influence of three parameters: time, seed quantity and dilution were evaluated using a statistical method. It was found that time and dilution are the factors which have more influence on aluminium precipitation yield, allowing to reach values greater than 45%. The hydrated aluminas obtained were characterized by granulometric analysis, scanning electron microscopy and x-ray diffraction. Therefore, apart from minimizing the environmental impact of the aluminium transformation industry by reducing a waste quantity, it is obtained a marketable sub-product and recovered a raw material (NaOH), which can be recycled within the aluminium transformation process.