# Study on Preparation of Aluminum Ash Coating Based on Plasma Spray
## 1 Introduction  
Aluminum ash, also known as aluminum slag, is a by-product of aluminum electrolysis and aluminum smelting. Owing to technical limitations, there is currently no efficacious way to recycle large quantities of aluminum. According to data released by China Aluminum Network, in 2016, the output of primary aluminum was about 32.17 million tons, and the output of alumina was about 60.91 million tons. By preliminary statistics, one ton of aluminum will produce 180-290 kg of aluminum ash [1; 2; 3] during the entire technological process. It is the most common disposal method for recycling the metal aluminum from the ash. After the metal aluminum is extracted, most of the residual ash is directly landfilled, which not only occupies a mass of land, but also generates serious environment pollution [4; 5]. With the strengthening of environmental protection, enterprises gradually began to pay attention to this abandoned resource, and some recycling application technology of aluminum ash came into being. For example, in Italy, Anchitec's related technology in the field of aluminum ash recycling is advanced, and the quality of recycled aluminum is high, thus it can be used in aerospace and automotive fields. A representative enterprise in China is Shandong Zibo Haihui Company, which has developed a comprehensive treatment and utilization method for aluminum ash. The aluminum metals are recovered, and the alumina is returned to the electrolytic cell. This technology properly disposes of harmful gases such as fluoride, chloride, and nitride generated during the treatment of aluminum ash, and theoretically achieves \"0\" emission of aluminum ash [6]. Other scholars havealso achieved certain achievements in the comprehensive utilization of aluminum ash. For example, the aluminum ash is used to prepare refractory materials [7; 8], steelmaking deoxidizer [9; 10], inorganic flocculants [11; 12; 13], chemical raw materials [14; 15; 16], and building materials [17; 18]. However, most of the above applications are in the theoretical and laboratory stages with high production cost and the possibility of causing secondary pollution. These methods cannot be promoted on a large scale, while the accumulation of aluminum ash is still increasing year by year. Therefore, the research for a new application of aluminum ash is urgent and important. Plasma spraying is a method of heating a powdered material such as ceramics, alloys, and metals into a molten or semi-molten state, and spraying it at a high speed on the surface of the pretreated workpiece to form a firm surface layer [19; 20; 21]. At present, in the field of plasma spraying, the price of commonly used spray powder ranges from a few to several hundred dollars per kilogram. Among them, high-purity alumina powder is one of the commonly used spray powders [22], and the key component of aluminum ash is alumina. In some applications, replacing the alumina ceramic coating with an aluminum ash coating not only reduces production costs and turns waste into treasure, but also protects the ecological environment. However, the aluminum ash was intricate in source and contained a lot of impurities, and thus cannot be directly used for plasma spraying. For example, during the aluminum smelting process, the N\({}_{2}\) was passed as a protective gas, which resulted from the fact that aluminum ash comprised a large amount of AlN. The AlN could react with H\({}_{2}\)O at room temperature to produce NH\({}_{3}\), which reduced the quality of the aluminum ash coating. Secondly, the aluminum ash used in the experiment came from disparate factory waste, so it contained a mass of large particle impurities, such as broken glass, stone, and plastic. If sprayed directly, it will generate harm to plasma spray equipment. Besides, as a new type of powder prepared in this study, there are no existing spray process parameters available for reference. These are the difficulties that need to be addressed.