Preparation of calcium aluminate and spinel by hydrolysis and calcination from secondary aluminum dross

The direct extraction of alumina from secondary aluminum dross (SAD), which is a dangerous solid waste, is difficult. Moreover, this process easily produces a large amount of solid waste residue, which is not easily utilized. In this paper, a new green process was developed to prepare calcium aluminate and Mg-Al spinel from SAD by hydrolysis-calcification roasting. The effects of calcium oxide (CaO) content, sintering temperature, and holding time on the properties of calcium aluminate were investigated by single-factor experiments. The phase transformation mechanism of calcium aluminate was studied by thermodynamic analysis, X-ray diffraction analysis, X-ray fluorescence spectroscopy, and scanning electron microscopy. Under the optimal conditions (Ca/Al molar ratio of 0.8, sintering temperature of 1300 degrees C, and holding time of 2 h), the main calcium aluminate phases are CaAl2O4 and Ca2Al2SiO7, the soluble alumina content of the calcium aluminate sample is 49.71 wt.%, and the main phases of the acid-insoluble residue are Mg-Al spinel and a very small amount of CaTiO3. The Ca/Al ratio is the key factor affecting the calcium aluminate phase-with increasing Ca/Al ratio, the calcium aluminate phase is transformed from CaAl4O7 to CaAl2O4 and eventually to Ca12Al14O33, and the Si-containing phase changes from Ca2Al2SiO7 to CaSiO4.