Titanium dioxide (TiO2) is a naturally occurring oxide. This inorganic compound is widely used for its bright white pigment, opacity, and UV absorption. It is one of the most produced chemicals globally, found in countless everyday items. This article explores its industrial manufacturing methods.
Raw Materials for Production
TiO2 manufacturing relies on two mineral sources. Ilmenite (FeTiO3), an iron-titanium oxide, is a primary feedstock, abundant in igneous and metamorphic rocks and placer deposits.
Rutile (TiO2), a purer form, is the second primary ore. Rutile deposits are less abundant than ilmenite, but its higher titanium content makes it a desirable, though often more expensive, starting material.
The Sulfate Process
The sulfate process produces titanium dioxide from ilmenite ore. This batch process digests finely ground ilmenite in concentrated sulfuric acid at 150-180 degrees Celsius, dissolving titanium and iron to form titanium sulfate and iron sulfates in an aqueous solution, often called “black liquor.”
Following digestion, the solution undergoes clarification and reduction to eliminate undesirable elements and manage iron. Iron is reduced from ferric (Fe3+) to ferrous (Fe2+) state, facilitating removal through crystallization and filtration. This ensures purity of the final titanium dioxide product.
Hydrolysis involves heating the clarified titanium sulfate solution, causing hydrated titanium dioxide to precipitate. This occurs as a finely divided solid, separating titanium from impurities. Precise control of temperature and acidity influences particle size and characteristics of nascent TiO2.
Finally, calcination, a high-temperature thermal treatment, converts the hydrated titanium dioxide precipitate. This process occurs at 800-1000 degrees Celsius, converting amorphous hydrated TiO2 into crystalline, pigmentary TiO2 (rutile or anatase). Calcination also drives off residual water and sulfuric acid, yielding the bright white pigment.
The Chloride Process
The chloride process is a modern, dominant method for manufacturing titanium dioxide, favored for efficiency and environmental profile. This continuous process utilizes higher-grade titanium feedstocks like natural rutile, synthetic rutile, or titanium slag. Chlorination involves reacting the feedstock with chlorine gas and carbon at 900-1200 degrees Celsius within a fluidized bed reactor, producing gaseous titanium tetrachloride (TiCl4) and chlorides of other metals.
Crude titanium tetrachloride vapor then undergoes purification. Fractional distillation separates TiCl4 from impurities like vanadium oxychloride and iron chlorides by leveraging differences in their boiling points. Purified TiCl4 is a clear, colorless liquid, a refined titanium intermediate.
Purified TiCl4 reacts with oxygen in a reactor at high temperatures, exceeding 1000 degrees Celsius. This oxidation converts gaseous titanium tetrachloride into solid titanium dioxide particles, regenerating chlorine gas. Regenerated chlorine can be recycled back into chlorination, making the chloride process efficient in reagent use.
The chloride process yields a product with higher purity and more consistent particle size distribution than the sulfate process. Recycling chlorine reduces waste and improves the environmental footprint. Its continuous nature allows for greater production volumes.
Comparing the Production Methods
The sulfate and chloride processes are two distinct approaches to producing titanium dioxide. The sulfate process, older, uses lower-grade ilmenite ore as its primary feedstock, offering flexibility in raw material sourcing. In contrast, the chloride process requires higher-grade titanium feedstocks like rutile or titanium slag, which can be more expensive or less available.
Regarding product quality, the chloride process yields a titanium dioxide pigment with higher purity and superior optical properties like brightness and tinting strength. This is due to effective purification of titanium tetrachloride before final oxidation. The sulfate process, while producing a good quality pigment, may contain more residual impurities.
Environmentally, the chloride process is considered more sound. It produces fewer solid waste byproducts, and recycling chlorine reduces its environmental impact compared to the sulfate process, which generates substantial acidic waste. The chloride process is also more energy-efficient due to continuous operation and heat recovery systems.
Primary Applications of Titanium Dioxide
Titanium dioxide is extensively used across industries for its unique optical and protective properties. Its most widespread application is as a white pigment in paints, coatings, and varnishes, providing opacity and brightness. It is also incorporated into plastics, papers, and inks to enhance whiteness and durability. Beyond pigmentary roles, titanium dioxide serves as an active ingredient in sunscreens, blocking harmful ultraviolet radiation. It is also found in cosmetics, food products as a coloring agent, and in some pharmaceutical formulations.