Emission reduction of aluminium anodising industry by production of Mg2+–Al3+–SO42--hydrotalcite-type compound

The synthesis of Mg2+–Al3+–SO42--hydrotalcite-type compound from the acid wastewaters of the aluminium anodising industry has been studied as a possible way of reducing the emissions to the environment, recovering simultaneously resource materials as a valuable mineral. The process of synthesis was carried out using rinse wastewater solutions generated from the anodising treatment when a cascade rinsing system is employed. The method of co-precipitation at constant pH was employed for such a process, using MgO as a source of magnesium. The synthesis was studied as a function of precipitation pH (8–10) and flow rate of reagent mixture (5–30 ml min−1). High pH of precipitation and low flow rate of reagent mixture (5–15 ml min−1) were found optimal to improve the crystallinity of the synthesised product. The mineral characterisation was performed using X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis and differential thermal analysis, all of which indicated characteristics typical of the desired compound. Almost 100% of the aluminium initially present in the acid wastewater solutions was recovered in the form of Mg2+–Al3+–SO42--hydrotalcite-type compound.