How Aluminum Casting Alloys are Refined?

Aluminum casting alloys are the third most abundant element in the world, and the most abundant metal in the earth’s crust. Aluminum contributes to more than 8% of the earth’s core mass. However, it is difficult to refine compared to other metals, such as iron. For this reason, the use of aluminum has lagged behind other metal products while efficient and cost-effective methods were developed to overcome these complexities.

There are many similarities between the aluminum and steel industries. Both rely on the extraction of metals from mineral ores occurring in the earth’s surface. The manufacturing processes of both are energy intensive and involve pouring liquid metal into casts or using continuous casting machines. Aluminum and steel also compete in similar markets for the automotive and aerospace industry. However, there are significant differences in the processing and properties of these metals.

The Processing of Aluminum castingalloys

Bauxite is a sedimentary rock containing high aluminum content; typically, about 46–60%. Bauxite is often covered by several meters of rock and clay, which must first be removed before the bauxite can be recovered. The bauxite then goes through crushing or washing plants before being transported for processing.

In the mid-1880s, two distinct methods were invented and used in series to produce aluminum. The Bayer method uses a chemical process to extract aluminum from bauxite. The Hall-Heroult process uses electrolysis to extract aluminum from the alumina or aluminum oxide produced by the Bayer process.

Bayer process of Aluminum Casting

Bauxite ore is crushed and mixed with caustic soda to produce a slurry containing fine particles of ore. The slurry is held at temperatures between 140˚C–280˚C, depending on the specific ore being processed. During this time, the aluminum dissolves into the caustic soda solution. All the impurities settle out of the solution into a residue called red mud.

The final step in the process is the addition of seed crystals to the caustic soda solution. Dissolved alumina attaches to these seed crystals. The final product from the Bayer process is alumina or aluminum oxide, which has the appearance of a white powder.

Hall-Heroult process of Aluminum CastingAlloys

The reduction unit of an aluminum plant consists of reduction pots or cells which are connected in series. Each pot is made of a steel shell lined with carbon. Molten cryolite (a fluoride mineral) containing aluminum oxide is poured into each pot and carbon electrodes are inserted into the solution from the top. As the current passes through the cryolite solution, aluminum separates from oxygen, forming carbon dioxide gas. Liquid aluminum collects at the bottom of the pot.

Liquid alumina is then sucked from the reduction pots at regular intervals into vacuum buckets. This is transferred into a furnace and cast into ingots in molds, or by a continuous casting machine. Aluminum produced through this process is approximately 99.8% pure. The electrolytic process for aluminum production is very energy intensive, requiring 15MWH per ton of output. Most smelters are therefore located next to a power generator such as hydroelectric plants.