Titanium Dioxide Manufacturing Process

Titanium dioxide is an inorganic compound found in three crystal forms, anatase, brookite and rutile, known for its opacity and whiteness. These unique properties have made it a highly sought-after substance in many industrial applications as a white pigment.

Raw Materials in the Manufacture of Titanium Dioxide

Titanium dioxide exists naturally in mineral ores usually alongside other elements thus requiring specialized processes to extract its pure form. Titanium dioxide can also by synthesized in controlled environments by reacting titanium compounds such as titanium chloride and titanium isopropoxide.

During titanium dioxide manufacturing process, you will use:

• Ilmenite
• Rutile

Ilmenite

Ilmenite is a naturally occurring mineral ore composed mainly of iron titanium oxide widely available in igneous rocks deposits and sediments. It has a black or dark gray solid appearance with a metallic luster and relatively heavy thanks to its high specific gravity.

When manufacturing TiO2 from ilmenite, there are two main processing techniques – chloride process or sulfate process. Under normal circumstances, you will find between 45% and 60% titanium dioxide. In fact, this forms a bigger percentage of global titanium dioxide production.

Rutile

Rutile makes up the most common and naturally occurring form of titanium dioxide containing over 90%. While less available worldwide compared to ilmenite, it is found in metamorphic and igneous rocks and some beach sands.

There are different appearances for rutile ranging from reddish-brown to black usually with a distinct metallic luster. It is found in nature alongside minerals like ilmenite and zircon and is relatively heavy thanks to its high specific gravity.

Commercial Methods in Titanium Dioxide Manufacturing Process

Chloride Process in Titanium Dioxide Production

The chloride process extracts titanium dioxide from the mineral ores rutile and ilmenite via a continuous chemical process. It is a modern method that currently finds wide usage across the globe thanks to its low energy use and waste production.

The chloride process is conducted in the following stages:

Chlorination

The titania material utilized in the chlorination process determines the necessary quantities of chlorine and coke needed. It also influences the quantity of titanium chloride and waste produced, as well as the rate of build-up.

The chlorination stage involves heating titanium ore and coke in the presence of chlorine gas in a fluidized bed chlorinator at about 1000°C. Crushing, grinding and washing the solid reagents beforehand increase the rate of effectiveness of the fluidization process.

Utilizing high purity reagents reduces the amount of impurities in the process which are removed intermittently to prevent their accumulation. The chlorination stage utilizes a cyclone for solid removal alongside a gas scrubber to eliminate and purify the gas by-products.

The chlorination process converts the metal oxides present in the reagents to chlorides before evaporating them as gases. Hydrogen chloride gas is formed via the combination of chlorine, carbon and moisture content.

Chlorides like those of calcium and magnesium with high boiling points are retained as liquids posing further process difficulties. Oxides of zirconium and silicon do not form chlorides retaining their solid structure.

Carbon reduces the metal oxides attaching to the oxygen molecules and escaping as carbon II oxide and carbon IV oxide. The primary result desired in the reaction of titania, coke and chlorine is the formation of titanium tetrachloride.

Condensation

Titanium tetrachloride has a low boiling point and separating it from the mixture of substances stemming from chlorination process is uncomplicated. Being in vapor form, it’s cooled into a liquid via condensation while the impurities in solid form are separated.

Utilizes titanium tetrachloride that is pumped back for cooling purposes and thus controlling temperature. The condensation unit separates the vaporized titanium chloride and a stream of gas.

Other chlorides formed are also precipitated out as solids after cooling. The solid cyclone is useful in the separation of solids according to grain size with the larger particles sent back to the chlorinator. The gas scrubber cleans the gases produced while the titanium chloride is forwarded for purification.

Purification

The process of making titanium dioxide is dependent on titanium chloride as the primary feed material. Titanium chloride from the chlorination stage is impure with varied impurities like other metal chlorides, vanadium and unused coke and titania.

Such impurities in solid form are eliminated by converting titanium chloride into a gas before reconversion. Vanadium on the other hand is made into a chloride that is insoluble.

This purification step typically involves fractional distillation or other refining techniques to separate and collect pure titanium tetrachloride.

Oxidation

The purified titanium chloride in liquid state is vaporized once again, this time in the presence of hot oxygen. The reaction involve:

2TiCl₄ + O₂ → 2TiO₂ + 2Cl₂

During the oxidation process, the characteristics of the pigment are determined through the use of certain chemical additions. The chlorine gas produced is recycled back into the process for use at the chlorination stage.

Finishing

The fine particles of titanium dioxide that are produced are subjected to further processing depending on the intended application. For instance, surface treatment of particles can involve washing, coating and demoisturization to improve pigment properties.

Finishing can include wet milling to achieve finer particles of titanium dioxide and optimize their distribution. Additionally, inorganic treatment involving the addition of certain compounds for particle improvement can follow.

During filtration process, TiO2 is washed followed by a drying process to remove all moisture.

Steam milling further enhances the refinement process to achieve even finer particles of titanium dioxide.

Packaging

The resulting titanium dioxide in its desired form is finally packaged for distribution to the appropriate area of application.

The chemistry of the chloride process can be summarized as follows:

Sulfate Process in Titanium Dioxide Manufacturing Process

The sulphate process is a batch process utilizing multiple operational units making it more complex than the chloride process. The sulfate process typically employs ilmenite (FeTiO₃) as the titanium dioxide ore and concentrated sulfuric acid as the reducing agent.

It is important to note that in this process, almost every stage is succeeded by a filtration/sedimentation process to eliminate impurities. The sulphate process is conducted in the following stages:

Digestion/Reduction

Highly concentrated sulphuric acid is used to digest the grounded feedstock converting titanium components into titanyl sulphate and iron into sulphate.

When utilizing ilmenite as feedstock a reduction step adds iron to aid conversion of ferric iron into a ferrous for separation later. A sedimentation process follows to remove any undigested material from the liquor.

Crystallization

In this stage, the formed ferrous sulphate heptahydrate known as copperas is removed from the mixture. This iron sulphate can induce a yellowish strain to the titanium dioxide if allowed to remain.

It is nonetheless a useful by-product utilized in the manufacture of chemicals for water treatment, agriculture, cement and food additives.

Hydrolysis

Here, the titanyl sulfate mixture is pre-heated to about 110^oC initiating the hydrolysis process. Addition of water and sodium hydroxide adjusts the pH level resulting in the production of hydrated titanium oxide and sulfuric acid.

Water, temperature, reaction time and pH are the main control parameters in the production of titanium oxide hydrate. The pH is especially useful in determining the titanium dioxide particle growth and impurities precipitation.

Residual iron from the preceding stage is further filtered out as a monohydrate of ferrous sulphate. The sulphuric acid produced is not concentrated enough for reuse in the digestion process and is utilized in two ways.

It is either regenerated into a concentrated acid for use at the digestion stage or neutralized in the production of gypsum. Regeneration involves subjecting the acid to cycles of heating, filtration and concentration whereas neutralization involves addition of a calcium salt.

Calcination

The hydrated titanium dioxide is subjected to high temperatures in a rotating kiln for calcination. Here, the hydrate particles are eliminated forming anhydrous titanium dioxide.

The uncoated particles of anhydrous titanium dioxide are then subjected to a milling process in a ball or sand mill. Agglomerated particles are crushed into a more consistent size which enhances the pigment’s light scattering ability.

Surface Treatment

Exposing titanium dioxide to light results in an electrical charge thanks to its photo activity. This is detrimental as it can result in the degradation of substances titanium dioxide get in contact with.

Surface treatment by coating the titanium dioxide particle with alumina, silica and zirconia mitigates this unwanted effect. Furthermore, you can subject the titanium dioxide particle to other surface treatment to achieve a desired form.

During the surface treatment, you will use water to mix titanium dioxide particles. This is alongside surface treatment in a mixing tank.

During the process, adjusting the pH levels, mix temperature and agitation is critical in determining coating characteristics like thickness and durability.

Final Milling/Packaging

A final milling process succeeds surface treatment to attain the desired size of coated titanium dioxide particles. Thereafter, it is packaged in the defined quantities for shipment and delivery to customers.

The chemistry of the sulfate process can be summarized as follows:

Specialized Methods in Titanium Dioxide Production

Specialized methods of producing titanium dioxide particles are especially utilized to facilitate nanoparticles production. These require special chemical approaches to fulfil finding use in unique applications.

Electrophoretic Deposition

Electrophoretic deposition utilizes an electric field to deposit titanium dioxide particles onto a substrate. In this process, water is mixed with organic compounds of titanium to make a suspension containing particles of titanium dioxide.

The suspension is transferred into a cell with an electrode paring where the cathode is typically the substrate for particle deposition. Application of an electric field between the electrodes causes migration of titanium dioxide particles towards the substrate.

The movement of the particles is inspired by electrophoretic motion thanks to the attraction of the charged particles to the substrate electrode. This deposition type combines electroosmosis (suspension fluid flow), diffusion (particle distribution) and electrophoresis (particle movement).

The titanium dioxide particle produced with electrophoretic deposition offers good adhesion forming coatings that are dense and uniform. Important parameters that influence the coating thickness are voltage applied, pH, suspension concentration and time taken for deposition.

This process also allows you to modify the particles of titanium dioxide by applying dispersants and surface treatment. This improves properties of the particles such as stability and dispersion rate.

Spray Pyrolysis

Spray pyrolysis is a unique technique that thermally decomposes precursor compounds in depositing desired particles in spray form. Titanium compounds like salts are used in the preparation of the precursor before atomization into fine droplets.

The spray formation is targeted to a heated substrate that quickly dries the particles and thermally decomposes them. The titanium precursor consequently leaves solid particles whose decomposition causes reaction with atmospheric air to form titanium dioxide.

Regulating the temperature of spray pyrolysis allows you to achieve the desired particle structure and from of titanium dioxide. Other factors affecting the characteristics of the synthesized titanium dioxide particles are the spray rate, dwell time and type and concentration of titanium precursor.

There are several benefits in utilizing spray pyrolysis for titanium dioxide synthesis and film deposition. It can be used with different materials, is capable of creating uniform coatings over large surfaces and is relatively cost effective.

It is common to use dopants in the spray pyrolysis process to augment the properties of the resulting titanium dioxide. Dopants such as rare earth elements, nitrogen and transition metals help modify the electrical, photocatalytic and optical aspects of the pigment.

Oxidation Methods

Oxidation methods of synthesizing titanium dioxide involve oxidation of titanium metal through anodization or use of oxidants. For instance, anodizing a sheet of titanium in hydrogen fluoride by applying a small voltage, creates nanomaterials of titanium dioxide.

Additionally, heating an anodized plate of titanium to about 500°C in the presence of oxygen for over six hours produces crystals of titanium dioxide. Similarly, using hydrogen peroxide to directly oxidize titanium metal is liable to produce nanomaterials of titanium dioxide.

Pure oxygen, argon-oxygen mix and acetone can be used in the oxidation of titanium as alternative sources of oxygen. Acetone is preferred for high temperature use resulting in highly dense and aligned titanium dioxide nanoparticles.

Utilizing either pure oxygen or a mix containing argon synthesizes titanium dioxide nanoparticles with a crystalline structure.

Microwave-Assisted Methods

Microwave-assisted methods of titanium dioxide synthesis utilize microwave frequencies ranging from 0.3 to 300 gigahertz and wavelengths not exceeding 1 m. There are two primary mechanisms in this process: ionic conduction and dipolar polarization.

Here, molecular behavior such as collision, friction or rotation generates heat that tends to orient with a capricious electric field. This induces movement of ions in a precursor in line with the electric field inducing a friction/collision based temperature rise.

Microwave heating is a process capable of quick heating, enhanced reaction rate, and enhanced yield within a short reaction time. You can undertake microwave heating in two ways: continuous and pulsed microwave heating.

Using microwave radiations in the synthesis of titanium dioxide nanomaterials has the advantage of undertaking rapid transfer of heat that is selective. Consequently, energy is distributed uniformly with enhanced control and reproducibility allowing faster synthesis of titanium dioxide nanoparticles (minutes instead of hours).

Hydrothermal/Solvothermal Methods

Hydrothermal and solvothermal methods are almost similar techniques use in the synthesis of titanium dioxide. These methods subject the precursor materials to a high-pressure and temperature environment, typically in an autoclave, resulting in titanium dioxide nanoparticles formation.

In hydrothermal synthesis, the reaction is undertaken in an aqueous solution containing titanium salts and a mineralizer. The reaction vessel is vacuum-packed and heated beyond the boiling point of water, creating a super/subcritical hydrothermal condition.

The high temperature and pressure conditions promote growth and nucleation of titanium dioxide particles. Adjusting the parameters allows you to regulate the crystallinity, size and shape of the titanium dioxide nanoparticles in the hydrothermal process.

Contrarily, solvothermal synthesis utilizes organic solvents for the reaction medium as opposed to water used in the hydrothermal method. The titanium salts are dissolved in an appropriate solvent, where adding a surfactant can thwart agglomeration and control particle growth.

The mixture is then subjected to a heating process under high pressure and temperature like in hydrothermal synthesis. The primary benefits of using solvothermal method is enhanced crystallinity and better control over particle size and morphology.

Your selection when considering both methods is dependent on factors like the application, desired particle properties and reaction conditions.

Sol-gel Methods

The sol-gel method transforms a precursor solution into a solid gel by employing a series of hydrolysis and condensation reactions. Further processing of the gel can results in the formation of titanium dioxide nanoparticles.

The sol-gel process use in synthesizing titanium dioxide nanomaterials is favored for its low processing temperatures capability and particle homogeneity. This process typically begins with a titanium alkoxide like titanium isopropoxide dissolved in a suitable solvent, usually an alcohol.

It is followed by a hydrolysis reaction initiated by adding some amount of water or acid to the precursor. Hydrolysis leads to the formation of a titanium hydroxide species, which upon undergoing condensation reactions, forms a three-dimensional agglomeration.

Gelation is influenced by parameters such as precursor concentration, solvent composition, pH and temperature. These allow you to tailor the properties of your titanium dioxide particles like size and morphology.

Upon gel formation, a heat treatment stage like drying or calcination follows to eliminate the solvent in the titanium hydroxide species. Calcination subjects the gel to heating at elevated temperatures inducing crystallization converting amorphous titanium hydroxide into crystalline titanium dioxide.

Some of the advantages offered by the sol-gel method in titanium dioxide synthesis includes precise control over particle composition and structure. This method can also be scaled up allowing use of dopants or additives to the titanium dioxide allowing application specific property modifications.

Conclusion

With many options available for titanium dioxide synthesis, the choice of method is dependent on your application and production scale. It is therefore prudent to consider such factors alongside budget and turnaround time when evaluating your options.