Titanium is a silvery-white transition metal highly valued for its unique properties. It is recognized for its exceptional strength-to-weight ratio, being as strong as some steels but significantly less dense. Titanium also exhibits superior resistance to corrosion, particularly in harsh environments like seawater and chlorine, due to a stable, passive oxide layer that forms on its surface. These attributes make it a preferred material for high-performance applications in aerospace (jet engines and airframes) and in the medical field (biocompatible implants and prosthetics). Although the ninth most abundant element in the Earth’s crust, titanium is never found in its pure metallic form, requiring a complex and energy-intensive process to isolate the usable metal from its chemical compounds.
Where Titanium Minerals Are Found and Mined
The vast majority of the world’s titanium is sourced from two primary mineral ores: ilmenite (an iron-titanium oxide, \(\text{FeTiO}_3\)) and rutile (nearly pure titanium dioxide, \(\text{TiO}_2\)). These minerals are concentrated in two main types of geological deposits, which often dictate the initial mining method. Hard rock deposits, where minerals are locked within igneous or metamorphic rock formations, require conventional, more costly underground or open-pit mining techniques.
The most commercially significant source is placer deposits, also known as heavy mineral sands, which account for roughly half of all titanium mined globally. These deposits are unconsolidated sands found along ancient or modern coastlines and riverbeds, where natural erosion and water action have concentrated the dense titanium-bearing minerals. Extraction from these sand deposits is typically done through large-scale surface mining, such as dredging or dry mining.
In these operations, large volumes of material are processed, but the titanium mineral concentration remains low, often less than five percent of the total mined material. The initial stage yields a raw, low-grade ore mixture of titanium minerals, lighter waste material (gangue), and significant amounts of sand and clay. This mixture must then be prepared for processing to concentrate the valuable titanium components.
Transforming Raw Ore into Usable Concentrates
Immediately following the mining and initial size reduction, the raw ore undergoes beneficiation, a process using physical and mechanical methods to separate heavy titanium minerals from lighter waste. The first major step involves gravity separation, which takes advantage of the difference in density between the titanium minerals and the gangue. Equipment like spiral concentrators use flowing water to allow the heavier minerals to settle and separate from lighter silicates.
The pre-concentrate is then subjected to magnetic separation, which is particularly effective for isolating ilmenite. Since ilmenite contains iron, its slight magnetic susceptibility allows high-gradient magnetic separators to pull the ilmenite particles away from non-magnetic minerals. Rutile, being non-magnetic, passes through this stage and is subsequently separated using electrostatic separation.
Electrostatic separation relies on the difference in electrical conductivity between minerals. Conductive minerals like rutile and ilmenite are separated from non-conductive minerals like quartz by passing them over high-voltage electrical fields. The combination of these physical separation steps progressively increases the titanium dioxide concentration to a highly purified feedstock. This final concentrate, which may be natural rutile, synthetic rutile, or titanium slag, is now ready for the complex chemical isolation process.
The Chemical Isolation of Pure Titanium
The final and most complex step in producing titanium metal is the chemical isolation from its oxide form, a process necessitated by the metal’s strong chemical affinity for oxygen and nitrogen at high temperatures. This is achieved almost exclusively through the Kroll Process, a batch method developed in the 1940s, which is responsible for the metal’s high cost due to its complexity and energy demands.
Chlorination
The process begins with chlorination, where the concentrated \(\text{TiO}_2\) feedstock is mixed with carbon, typically petroleum coke. This mixture is reacted with chlorine gas (\(\text{Cl}_2\)) in a fluidized bed reactor at temperatures around \(1000^\circ\text{C}\). This high-temperature reaction converts the titanium dioxide into liquid titanium tetrachloride (\(\text{TiCl}_4\)), a volatile compound, while removing the oxygen.
The crude \(\text{TiCl}_4\) is then purified through fractional distillation, which separates it from other metal chloride impurities, ensuring the resulting titanium metal will be of high purity. The purified liquid is then transferred to a sealed steel reactor for the reduction stage.
Reduction
In this stage, the \(\text{TiCl}_4\) is reacted with a highly reactive molten metal, most commonly magnesium (\(\text{Mg}\)), in an inert atmosphere of argon gas to prevent contamination by air. The reaction occurs at temperatures between \(800^\circ\text{C}\) and \(1000^\circ\text{C}\), yielding pure titanium metal and a magnesium chloride (\(\text{MgCl}_2\)) salt byproduct. The titanium metal precipitates as a porous, solid mass known as titanium sponge, which is tightly integrated with the liquid \(\text{MgCl}_2\) and unreacted magnesium.
Refining
The final refining involves separating the titanium sponge from the salt and residual reducing agent. This is accomplished either by leaching the mass with an acid or, more commonly, by vacuum distillation. Vacuum distillation vaporizes the \(\text{MgCl}_2\) and magnesium, leaving behind the purified titanium sponge. The sponge is then crushed, pressed, and melted in a vacuum arc remelting (VAR) furnace to produce solid, mill-ready titanium ingots.