Preparation Of Sialon From Aluminum Dross

Recently, China has become one of the biggest electrolytic aluminum productioncountries. Aluminum dross is one of the main secondary wastes during aluminumrecycling procedure. It is necessary to utilize aluminum dross not only decreases thewaste of resources, but also reduces environment pollution. The reuse of aluminumdross is the last step of aluminum industrial production, and is thought as a worldwidedifficult problem.Sialon ceramics are considered as an attractive material for engineeringapplications because of their excellent mechanical, chemical, and thermal properties.The applications of Sialon ceramics have always been limited by their high cost forthe use of high-purity raw materials. Using aluminum dross to synthesize Sialon notonly protect environment but also decrease the cost of Sialon production. It is veryimportant in the theory and practice to achieve the high value utilization of aluminumdross.In this paper, aluminum dross was used as raw materials to synthesize Sialon. Theeffect of initial compositions, technology paramters and additives on phasecomposition and microstructure was systematically studied. Based on the study ofsynthesis, the effect of pressureless sintering and hot pressing sintering on themicrostructure and mechanical properties of aluminum dross-based Sialon wasstudied. To solve two main obstacles of glass and impurity in industry wastesutilization, the formation and distribution of glass phase and impurity in aluminumdross-based Sialon was also studied by TEM, HREM and SEM.In this paper, three systems （aluminum dross+slag+Si, aluminum dross+flyash+Si and aluminum dross+Si） were designed according to the compositions ofdifferent wastes. The aluminum dross and Si system was identified as the preferredsystem to synthesize Sialon materials. The high quality Sialon can be synthesized byusing the aluminum dross （high aluminum）. The phase compositions can be controlledby changing the ratios of Si to Al and synthesizing temperatures. Using the aluminumdross （low aluminum）, the densification of Sialon can be easily obtained, but a lot ofglass phases were formed in the Sialon.The addition of Y₂O₃（<3%） is helpful to the formation of AlN polytypoid.However, a lot of glass phases were formed with the excess addition of Y₂O₃. The addition of NH₄Cl （17-26%） is helpful to the nitridation reaction, but decrease theformation of AlN polytypoid. The content of elongated grains increased withincreasing the addition of NH₄Cl. The synthesis mechanism of aluminum dross andsilicon system was studied by DTA-TG and synthesis experiment in section. The mainreactions are the nitridation of Al at800℃and the nitridation of Si at1100℃. Thesynthesis of Sialon has been completed at1450℃.Using aluminum dross and Si system, dense β-Sialon-15R ceramics weresuccessfully fabricated. The relations of technology parameters, microstructure andproperties were studied. The study of pressureless sintering indicated β-Sialon formedequiaxed or short column grain, and AlN polytypoid showed fibre or long columngrain in liquid phase sintering mechanism. The in-situ reinforced microstructure canbe controlled by the synthesis parameters. When the ratio of Si/Al is1.5, the optimalmicrostructure is formed in β-Sialon-AlN polytypoid ceramics at1650℃for6h.Thus, the composition （Si/Al is1.5） is designed as the initial composition in the studyof hot pressing sintering. The results indicated that hot pressing pressure wasbeneficial to grains rearrangement. Increasing hot pressing temperature, more liquidphase was formed and the grain growth was promoted. The densification wasaccelerated and the in-situ reinforced microstructure was formed. A in-situ reinforcedmicrostructure was formed above1550℃. The in-situ toughening effect of fibre-likeAlN polytypoid was found, and was identified by crack deflection and crack bridging.The β-Sialon-15R ceramics obtained the highest density of3.2g/cm³at1750℃, theVickers hardness （Hv10）, bending strength and fracture toughness of the materialcould reach with value of12.3GPa,432MPa and4.31MPa m^1/2, respectively.The situation of glass phase was analyzed in detail. The results indicated theinterface between β-Sialon grains and/or15R grains is clean in the samplesynthesized at1750℃, and the glassy phase only exist in triple junctions and pockets.These evidences of the mosaic interface structure between β-Sialon and15R, the mostof Mg cations from aluminum dross incorporated into the AlN polytypoid and theclean interface in β-Sialon-15R indicate15R AlN polytypoids in Sialon multiphaseceramics offer an effective path to reduce the glass phase formed from the oxideimpurity in aluminum dross. The analysis results of samples sintered at differenttemperatures indicated the amount of glass phase became less with increasingsintering temperature. At1650℃, the β-15R grain boundary is clean, and the liquidphase penetrated into the β-β grain boundary. The β-β grain boundary became clean at1750℃. The glassy phase only exists in the triple junctions and pockets. The glass phase in aluminum dross-based Sialon ceramics can be effectively decreased bychoosing the right material compositional design and proper sintering parameters.The situation of impurity was also analyzed in detail. The results indicated theimpurities such as NaCl and KCl were evaporated from the samples above1450℃.The main impurity phase is Fe₅Si₃at1750℃, some minor elements impurity such asCr and Mn are contained in Fe₅Si₃phase. Fe₅Si₃has a negative effect on themicrostructure and mechanical properties. Therefore, the impurity Fe should bepriority thought to remove from aluminum dross, and small amount metal impuritiesand salts impurities were no obvious effect on the microstructure and properties ofSialon.