Aluminum is an essential ingredient in 21st-century life. Countless products we rely on each day would not exist without the lightweight, high-strength, corrosion-resistant and electric-conductive benefits of this versatile, recyclable metal.
Aluminum doesn't exist, however, without two key raw materials that are critical to its production: calcined petroleum coke (CPC) and coal tar pitch (CTP).
Rain Carbon Inc. is the leading, single-point, global supplier of CPC and CTP, raw materials in the anodes that are required in the electrolyic process of producing primary aluminum. Rain Carbon understands the crucial role our raw materials play in the production of this versatile product, and we work closely with smelters around the world to ensure they have a reliable supply of high-quality CPC and CTP. This is vital, because supply interruptions and inconsistent quality can disrupt aluminum smelting operations, and emerging supply and quality constraints can threaten a smelter’s very existence. We have the resources, global diversity of production facilities, expertise and commitment to meet these challenges and help aluminum producers meet current demand and take advantage of new opportunities.
Smelting Process
Aluminum is produced using the electrolytic Hall-Héroult process. Alumina (Al2O3) powder is dissolved in a molten bath of sodium aluminum fluoride known as cryolite. The temperature of operation in modern cells is about 950-960˚C (~1,740-1,760˚F). Electrical current is passed between carbon anodes and a carbon cathode in the cell, reducing alumina to aluminum metal which deposits on the cathode surface. Carbon anodes are consumed in this process, generating CO2 gas. The basic chemical reaction is:
2Al2O3+ 3C → 4Al + 3CO2
A modern electrolysis line, or "potline," is shown above. Cells are connected in series and high-amperage DC current drives the electrolysis reaction. Modern cells operate at currents ranging from 200-500kA, but higher amperage cells are in development. Cells operate at a voltage of 3.8-4.5V; the energy required to produce 1 kg of aluminum is typically 12.5-14 DCkwhr. Aluminum smelting is energy intensive, and access to competitively priced electric power is critical for low-cost production.
The Aluminum Association is an excellent source for additional information on aluminum and its many applications and benefits.
Carbon Anode Production
Carbon anodes are essential to the production of aluminum, as described above.
Anodes used in the Hall-Héroult aluminum process are made from CPC and CTP. Most smelters operate their own anode-producing carbon plant. A handful of stand-alone anode plants supply pre-baked anodes to smelters without plants and to smelters who need anodes because of production shortfalls or maintenance shutdowns.
Pre-baked carbon anodes made from CPC and CTP are used to produce aluminum. In addition, spent anodes (also known as "butts") are used in the anode recipe. A typical breakdown is 67 percent CPC, 20 percent butts and 13 percent CTP. Green anodes are produced first and then baked in large furnaces to a final temperature of approximately 1,150˚C (2,100 ˚F). They are then rodded and used in electrolysis cells. Since anodes are consumed in the smelting process, they must be replaced every 20-30 days, depending on the size and cell design.
CPC and CTP quality directly influences anode quality and performance, so smelters accordingly set critical quality parameters such as sulfur and trace metal impurities (vanadium, nickel, calcium, iron, silicon and sodium). The physical properties of CPC such as bulk density, real density and particle size are also important when making anodes. Because of its lower thermal expansion coefficient, CPC with a sponge coke structure is favored over a shot coke structure.
A modern smelter producing 300,000 MT per year of aluminum requires the production of approximately 500 anodes per day, so consistent CPC and CTP quality from shipment to shipment is very important. Many green petroleum cokes sold to the fuel market are unsuitable for calcination for anode production due to high impurity levels and undesirable structure.