Aluminum processing

1. Is all the aluminum used in human society primary aluminum?

Answer: Not all metallic aluminum widely used in human society is primary aluminum. Current sources consist of primary aluminum production and recycled aluminum production. Recycled aluminum is produced by smelting recovered waste aluminum products. However, primary aluminum production still accounts for the major portion of total aluminum output.

With industrial development and the increase in aluminum production and products, the volume of recycled aluminum production will also grow. In industrially developed countries, recycled aluminum production accounts for approximately 1/3 of total aluminum output.

2. What is the role of aluminum in the national economy and daily life?

Answer: Since the adoption of the molten salt electrolysis method for producing metallic aluminum, aluminum and its alloys have been extremely widely applied. Due to its excellent chemical properties and superior physical performance, aluminum is extensively used in various industrial fields, including aerospace, power and electrical engineering, automotive and transportation, instrumentation, chemical industry, food industry, construction, and even in people’s daily lives.

The output of aluminum ranks first in the non-ferrous metal industry and is second only to steel in the entire metallurgical industry. Since 2001, China has become the world’s largest producer of primary aluminum. In 2001, the production capacity for electrolytic aluminum was about 4.26 million tons, with an actual output of 3.427 million tons. By 2009, the actual aluminum output reached 12.846 million tons.

In practical applications, aluminum alloys are mostly used to improve material performance. Aluminum alloys are divided into two categories: wrought aluminum alloys and casting aluminum alloys. In total aluminum alloy production, wrought aluminum alloys account for approximately 80%, while casting aluminum alloys account for about 20%.

3. What are the major chemical properties of aluminum?

Answer: Aluminum is a relatively active metal that can react with non-metals, acids, and bases.

• Affinity for Oxygen: Aluminum has a great affinity for oxygen. At room temperature, aluminum reacts with oxygen in the air to form a dense and strong alumina ($Al_2O_3$) film, which causes the metal surface to lose its luster. However, this oxide film prevents further oxidation of the metallic aluminum, giving it excellent resistance to atmospheric corrosion (including industrial and marine atmospheres) and water corrosion. During the production and use of aluminum products, scratches to this surface oxide film should be avoided.

• Combustion: When aluminum powder or foil is heated in oxygen, it burns, releasing a large amount of heat and emitting a dazzling white light. The heat of formation for alumina is $1670\text{ kJ/mol}$, so aluminum powder can be used as a solid fuel in rockets or as an aluminothermic reducing agent.

• Reaction with Non-metals: Aluminum reacts with halogens and other non-metals. It reacts with chlorine to form aluminum chloride ($AlCl_3$), releasing $680\text{ kJ/mol}$ of heat. It reacts with fluorine to form aluminum fluoride ($AlF_3$), releasing $1385\text{ kJ/mol}$ of heat. When these are heated with aluminum, volatile sub-valent aluminum chloride ($AlCl$) and sub-valent aluminum fluoride ($AlF$) are formed; upon cooling, they decompose back into aluminum and the corresponding halides.

• Reaction with Acids: Aluminum is soluble in hydrochloric acid and sulfuric acid, reacting with dilute versions to produce hydrogen gas. However, aluminum is insoluble in concentrated nitric acid and organic acids. At room temperature, the surface of aluminum is passivated in concentrated nitric acid, forming a dense and hard oxide film that stops the reaction. Therefore, aluminum containers can be used to transport concentrated nitric acid. Contact with hydrochloric and sulfuric acid should be avoided.

• Reaction with Bases: Aluminum is soluble in alkaline solutions. It reacts with strong alkaline solutions to produce hydrogen gas and aluminates. Contact with alkaline solutions should be avoided.

• Thermite Reaction: Under certain conditions, aluminum can undergo a redox reaction with iron oxide ($Fe_2O_3$) to produce iron and alumina, releasing immense heat and reaching very high temperatures. This is called the thermite reaction. A mixture of aluminum powder and iron oxide is commonly called thermite.

4. What are the major physical properties of solid aluminum?

Answer: Aluminum is a light metal with a relatively low density. The density of solid pure aluminum is approximately $2.7\text{ g/cm}^3$, which is about 1/3 of the density of solid iron or copper. The density of solid aluminum is related to its purity.