Titanium dioxide (\(\text{TiO}_2\)) is an inorganic compound recognized for its intense whiteness and high opacity. It is a naturally occurring mineral that is chemically inert, meaning it resists reaction with other substances, which makes it a highly stable commercial product. This versatility allows it to function as a pigment, known as titanium white, which provides brightness and hiding power in a vast array of materials. The compound is widely used in paints, coatings, plastics, paper, and textiles, and specialized grades are incorporated into sunscreens to block ultraviolet light and even as a food additive (E171).
Primary Geological Sources
The journey of titanium dioxide begins with the mining of two primary mineral ores containing titanium: ilmenite and rutile. Ilmenite is the most common and economically significant source, which is a titanium-iron oxide mineral with the chemical formula \(\text{FeTiO}_3\). This ore is less pure, typically containing between 30% and 40% titanium dioxide, with the rest being iron oxide that must be chemically removed during processing.
Rutile, the other main source, is a naturally purer form of titanium dioxide, often containing 90% to 98% \(\text{TiO}_2\). This mineral is less abundant and consequently more expensive than ilmenite. Both ilmenite and rutile are primarily sourced from two geological deposit types: hard rock formations and, more commonly, mineral sands. The valuable mineral sands are typically found along coastlines, where natural processes have concentrated the heavy titanium-bearing minerals. These deposits are mined to extract the raw material necessary for conversion into the final \(\text{TiO}_2\) product. Because of ilmenite’s greater abundance and lower cost, it accounts for the majority of the world’s titanium concentrate production.
Transforming Ore: The Sulfate Process
The sulfate process is one of the two commercial methods used to convert raw titanium ore into refined titanium dioxide pigment. This older technology is particularly well-suited for processing the more abundant, lower-grade ilmenite ore and titanium slag. The process starts with the digestion of finely ground ore using concentrated sulfuric acid (\(\text{H}_2\text{SO}_4\)) at high temperatures. This reaction dissolves the titanium and iron components, forming a porous solid mass called the digestion cake, which is then dissolved in water to create a titanium sulfate solution, or “black liquor”.
The next step focuses on removing the iron impurities, which would otherwise discolor the final white product. This is achieved by reducing the iron from its ferric (\(\text{Fe}^{3+}\)) state to its ferrous (\(\text{Fe}^{2+}\)) state using scrap iron. The solution is then cooled to crystallize the ferrous sulfate heptahydrate, which is filtered out.
The purified titanyl sulfate solution then undergoes hydrolysis, where water is used to precipitate the titanium. Steam is blown into the clarified solution, which causes the soluble titanyl sulfate to convert into insoluble, hydrated titanium oxide. This precipitate is washed and filtered to separate it from the remaining sulfuric acid.
The final stage is calcination, where the hydrated titanium oxide is heated in a rotary kiln to high temperatures. This heating drives off the remaining water and converts the material into anhydrous titanium dioxide (\(\text{TiO}_2\)). Controlling the temperature during calcination determines the final crystal structure, allowing the production of either the anatase or rutile form of the pigment.
Transforming Ore: The Chloride Process
The chloride process represents the more modern and generally more efficient method for producing titanium dioxide, favored for its ability to yield a high-purity rutile pigment. This method typically requires a higher-grade feedstock, such as natural rutile or synthetic rutile. The initial step is chlorination, where the titanium-bearing material is reacted with chlorine gas (\(\text{Cl}_2\)) and carbon (coke) at temperatures around \(1000^\circ\text{C}\) within a fluid-bed reactor.
This high-temperature reaction converts the titanium into a volatile intermediate compound, titanium tetrachloride (\(\text{TiCl}_4\)). Simultaneously, the impurities present in the ore are also converted into various metal chlorides. The resulting gas stream containing the \(\text{TiCl}_4\) is then purified through fractional distillation to remove these unwanted metal chloride byproducts.
The purified, liquid \(\text{TiCl}_4\) is then ready for oxidation. This involves reacting the titanium tetrachloride with pure oxygen at temperatures exceeding \(1000^\circ\text{C}\). This step yields the final, pure \(\text{TiO}_2\) pigment and regenerates the chlorine gas.
The chlorine gas produced during the oxidation step is captured and recycled back to the beginning of the process for reuse in the chlorination stage. This closed-loop system contributes to its greater efficiency and reduced generation of waste compared to the sulfate process. The resulting titanium dioxide is collected as a fine solid from the gas stream, possessing a controlled particle size that results in a superior pigment quality.