Titanium (Ti) is a highly valued metallic element known for its exceptional performance characteristics. Its most celebrated feature is an impressive strength-to-weight ratio, rivaling many steels while being significantly lighter. The metal also exhibits outstanding resistance to corrosion, even in harsh environments like seawater and various industrial chemicals, and is biologically inert, making it ideal for medical implants. Although titanium is the ninth most abundant element in the Earth’s crust, its natural tendency to bond tightly with oxygen makes extracting it in a pure, usable metallic form a complex and expensive process.
Raw Materials and Initial Preparation
The journey of titanium begins with mining two main mineral sources: ilmenite (\(\text{FeTiO}_3\)) and rutile (\(\text{TiO}_2\)). Ilmenite is the more common ore, often found in beach sands, but requires extensive processing to separate the titanium dioxide from the iron content. Rutile is nearly pure titanium dioxide, making it more desirable but less abundant.
Traditional smelting methods using carbon cannot be used for titanium. Heating titanium dioxide with carbon causes the metal to react readily with both oxygen and carbon, forming brittle compounds like titanium carbide. This contamination makes the resulting metal mechanically unsound and unusable for high-performance applications. Therefore, the initial ore concentrate must undergo rigorous separation and sometimes acid-leaching treatments to yield a highly purified titanium dioxide powder (\(\text{TiO}_2\)), which is then ready for specialized chemical conversion.
The Kroll Process
The inability to use standard high-temperature smelting necessitated the development of the specialized chemical Kroll process. This method bypasses the problems of direct oxygen and carbon contamination by first converting the titanium dioxide into a volatile liquid compound, which is then reduced.
Chlorination
The first stage, known as chlorination, involves mixing the purified \(\text{TiO}_2\) powder with petroleum coke (a reducing agent). This mixture is reacted inside a fluidized bed reactor with chlorine gas (\(\text{Cl}_2\)) at high temperatures, typically around \(1000^\circ\text{C}\). This reaction strips the oxygen from the titanium, producing molten titanium tetrachloride (\(\text{TiCl}_4\)), a colorless, highly corrosive liquid. Since \(\text{TiCl}_4\) is highly volatile, it can be easily distilled and separated from impurities like iron chlorides and vanadium compounds. Meticulous purification of the titanium tetrachloride is essential, as any remaining contaminants will carry through the rest of the process.
Reduction
Once purified, the \(\text{TiCl}_4\) moves to the reduction phase. The liquid titanium tetrachloride is placed into a sealed steel reaction vessel and reduced using a highly reactive metal, most commonly molten magnesium (Mg). This reaction must take place within an inert atmosphere, typically pressurized argon gas, to prevent re-contamination from atmospheric oxygen or nitrogen.
The magnesium reacts with the chlorine in the \(\text{TiCl}_4\) at temperatures between \(800^\circ\text{C}\) and \(900^\circ\text{C}\), producing pure titanium metal and magnesium chloride (\(\text{MgCl}_2\)) as a byproduct. Since the reaction occurs below the melting point of titanium, the resulting pure metal deposits as a porous, solid mass within the reactor, known as “titanium sponge.” The Kroll process remains the dominant industrial method for producing metallic titanium due to the high purity it affords.
Refining and Ingot Production
The titanium sponge produced by the Kroll process is not yet ready for use because it still contains significant amounts of unreacted magnesium and magnesium chloride byproduct. To achieve required purity, the sponge must undergo purification, often involving vacuum distillation at high temperatures to vaporize the residual contaminants. Alternatively, some producers use acid leaching, washing the sponge with dilute acids to dissolve and remove impurities.
Once purified, the sponge is mechanically crushed and pressed into compact blocks or electrodes, preparing it for the critical melting phase. The melting process is almost universally conducted using Vacuum Arc Remelting (VAR). In a VAR furnace, the titanium electrode is melted by an electric arc under an extremely high vacuum. This vacuum environment is strictly maintained to prevent the molten titanium from reacting with atmospheric gases, which would severely compromise the metal’s ductility.
During melting, precise amounts of alloying elements, such as aluminum and vanadium, are introduced to create specific grades, like the widely used \(\text{Ti-6Al-4V}\) alloy. The molten metal solidifies within a water-cooled copper crucible, resulting in a large, homogeneous, cylindrical block known as a titanium ingot.
Shaping the Final Metal
The titanium ingot is the final raw material form and must be mechanically worked to create commercial products. Transforming the ingot into usable shapes requires specialized high-temperature processing because titanium retains its strength and stiffness even when heated.
Hot rolling is a common method, passing the ingot through heavy rollers to flatten it into sheets, plates, and strips. For components requiring high structural integrity, forging applies immense compressive forces to the metal, refining the internal grain structure. Other techniques, such as extrusion, are used to push the heated metal through a die to create long products like bars, rods, and tubing. The stiffness of titanium necessitates robust, specialized tooling and equipment to meet the demanding performance specifications of aerospace and industrial applications.