AI Bingchen, ZHAO Junxiang, ZHU Hu, LI Min, YI Guoqiang

(School of Materials and Chemical Engineering, Hubei University of Technology, Wuhan 430068, Hubei, China)

Extended Abstract:[Background and purpose] Secondary aluminum dross (SAD) is a harmful solid waste generated during the production and recycling of aluminum. The composition of SAD is too complicated to be dealt with. SAD contains a significant number of valuable elements, including aluminum, magnesium, silicon, etc. The comprehensive utilization of SAD is of great significance in environmental protection and resource utilization and it represents an important aspect of circular economy. To solve the problem of environmental pollution and waste of resources caused by the disposal of secondary aluminium dross, a novel waste-to-waste treatment method was proposed for fabricating magnesium-aluminium spinel refractory materials by utilizing secondary aluminum dross as the raw material and incorporating industrial refired magnesia.[Methods] The treated secondary aluminium dross was mixed with reburned magnesium sand, through ball milling with QM-3SP2 planetary mill for 10 min (ball to material ratio of 3:1 and speed of 400 r·min⁻¹). Samples with dimensions of 40 mm× 20 mm×8 mm were made from the ground powder using an LJ-24T-ST press at 15 MPa. The powder mixtures were heated at temperatures of 1000–1500 ℃ for 4 h, at a rate of 5 ℃·min⁻¹. A STA6000 TG-DTA was used to study the mixture roasting process at 35–1400 ℃ in air. Phase composition of the sintered samples was characterized by using an Empyrean X-ray powder diffractometer, at step size of 0.02° and scanning speed of 0.2 s/step. Microstructure of the sintered sample was observed by using a field emission SU8010 SEM. Samples with the size of 40 mm×20 mm×8 mm and span of 40 mm, were used to measure flexural strength, according to the three-point bending method, at crosshead speed of 1 mm·min⁻¹.[Results] The XRD peak intensity of MgAl₂O₃ was gradually increased, reaching the maximum at the theoretical level for the sample with 28.33% magnesia. Meanwhile, the crystallinity of MgAl₂O₃ initially was increased and then decreased, as the content of magnesia was increased, being consistent with the results of XRD. When the content of secondary aluminium dross and magnesia reached the theoretical point of 2.53 (magnesia content of 28.33%), the octahedral crystal structure was the most pronounced. Therefore, 28.33% is the optimal content of magnesia. With increasing sintering temperature, the intensity of the MgAl₂O₃ diffraction peaks gradually rises, reaching maximum at temperature of 1400 ℃. With increasing content of MgAl₂O₃, the crystallinity was initially enhanced and then declined, which was in agreement with the XRD results. After sintering at 1400 ℃, magnesium-aluminium spinel crystals were clearly visible with sharp edges, while large and small grains were combined forming nested structure and the size of the particles was uniform. Therefore, 1400 ℃ was the optimal sintering temperature. Accordingly, the material exhibited the highest flexural strength of 142.81 MPa.[Conclusions] Magnesia-aluminium spinel refractories were successfully prepared by mixing secondary aluminium dross with industrial refired magnesia. The following conclusions are arrived.

(1) With increasing content of magnesium sand, both the grain size and homogeneity were increased. When the content of secondary aluminium dross and magnesia reach the theoretical value of 2.53 (28.33% magnesia), the crystals exhibited octahedral structure with well-defined crystal faces.(2) With increasing sintering temperature, both the purity and crystallinity of refractory materials were significantly increased. The crystal growth rate was accelerated, while the grain size was increased. Additionally, the flexural strength was gradually increased as the calcination temperature was raised. However, when the temperature exceeded 1400 ℃, intensified strain mismatches between adjacent grains were induced, resulting in a reduction in flexural strength.(3) The sample with 28.33% magnesium sand after sintering at 1400 ℃ for 4 h possessed flexural strength of 142.81 MPa.

Key words: secondary aluminum dross; refired magnesia; sintering; MA spinel; flexural strength