pH Control in TiO2 Production: Challenges & Electrode Solutions

Titanium dioxide (TiO₂) is a widely used additive in paints, inks, cosmetics, and plastics. Its high refractive index provides excellent whiteness and opacity, making it ideal for surface coverage. This article reviews the main production processes for TiO₂ and examines the vital role of pH measurement in ensuring final product quality.

Sulfate and Chloride Processes

Two primary methods exist for extracting titanium from ore: the chloride process and the sulfate process. The chloride process uses chlorine gas (Cl₂) and coke (C) at high temperatures to produce titanium tetrachloride (TiCl₄). A secondary oxidation step removes chlorine and yields TiO₂. pH measurement is not typically used in the initial stages of the chloride process, but it becomes important in downstream surface treatment of the TiO₂.

The sulfate process relies on sulfuric acid (H₂SO₄) to leach titanium from ilmenite (FeTiO₃). The reaction produces titanyl sulfate (TiOSO₄). A secondary hydrolysis stage breaks down titanyl sulfate into hydrated TiO₂ and H₂SO₄. Finally, heat removes water to produce pure TiO₂. The reactions are as follows:

Each stage of the sulfate process employs some form of settling or filtration to remove impurities. The crystallizer stage is particularly important for bulk removal of ferrous sulfate (FeSO₄), also known as “copperas.” If not adequately removed, FeSO₄ can cause an undesirable yellowish tint in the final TiO₂ product. The FeSO₄ byproduct is often sold to industries such as water treatment chemicals, cement additives, and even food-related additives.

pH Control in the Hydrolysis Stage

pH control is a critical parameter in the hydrolysis stage of the sulfate process. The incoming mixture of sulfuric acid and titanyl sulfate is often called “black liquor.” This liquid is preheated to about 110°C (230°F) to initiate the hydrolysis reaction. An alkaline mixture of water and caustic soda (NaOH) is used to adjust the pH of the liquid to between 0.5 and 2.5.

Temperature, pH, water addition, and reaction time are the main control parameters. All variables are closely monitored to produce hydrated titanium oxide (TiO₂(OH)₂). pH is a key measurement for controlling particle growth and impurity precipitation. The hydrolysis stage is one of the last steps to control product quality. Downstream filtration and possible additional acid washing prepare the final hydrated titanium oxide for calcination to produce anhydrous titanium dioxide. Waste sulfuric acid is filtered out for purification or gypsum production.

pH Control in Titanogypsum Production

Titanogypsum (CaSO₄) production is a unique byproduct of the sulfate process. The process is similar to flue gas desulfurization used in coal-fired power plants to reduce SO₂. White titanogypsum is ideal for manufacturing gypsum board and cement. If high levels of ferrous sulfate and other impurities are present, red titanogypsum may be produced. Red gypsum is much less valuable but is sometimes sold as a soil additive.

The waste acid from titanyl sulfate hydrolysis is not concentrated enough to be directly reused in the process. The mixture contains 20-23% sulfuric acid along with residual water and impurities such as ferrous sulfate (FeSO₄). Adding quicklime or limestone reacts with sulfuric acid to form calcium sulfite (CaSO₃). The reactions are shown below.

Additional oxidation using air bubbled through the liquid will produce CaSO₄.

pH control in gypsum production helps manage final product quality. As mentioned, white titanogypsum is the more valuable end product. By raising the pH to just above neutral (7.5 pH), ferrous sulfate will convert to elemental iron Fe⁺. As the iron precipitates out of solution, it can be removed, resulting in white titanogypsum.

pH and TiO₂ Surface Treatment

Titanium dioxide is a photoactive chemical. This means it generates an electrical charge when exposed to light. This reaction is undesirable because it can degrade other products in contact with TiO₂. The solution is to encapsulate the TiO₂ particles with a protective layer. Common coatings include silica, zirconia, and alumina. Multiple layers are often applied in a multi-stage batch process to create the desired final product.

After calcination, uncoated TiO₂ undergoes a milling step to eliminate agglomerated particles. Consistent particle size ensures proper light scattering of the TiO₂. After milling, the particles are combined with water and the desired surface treatment agent in a mixing tank. For silica coating, the mixture requires a pH > 8.0. Ammonia or caustic soda is used to raise the pH to the required level. Dilute sulfuric acid (H₂SO₄) is slowly added to initiate the silica deposition process. Silica in the form of sodium silicate (Na₂SiO₃) is slowly added to the mixture. pH, time, temperature, and proper agitation are used to control coating characteristics such as thickness, durability, and hiding power.

Alumina is typically the final coating for TiO₂. After the silica deposition process is complete, aluminum sulfate is added at about pH 3.5. The pH is slowly raised to 7-9, causing hydrated alumina to precipitate and begin slowly bonding to the silica-coated TiO₂ particles. pH, time, temperature, and proper agitation are all closely monitored to produce the desired final product.

After surface treatment, the TiO₂ undergoes final milling to control particle size for the final product requirements.

pH Measurement Challenges in TiO₂ Production

While pH measurement is critical to TiO₂ production, it is quite difficult. The high temperatures in the hydrolysis stage shorten sensor life. Throughout the process, fine particulate matter is known to coat and clog the porous reference junctions used in pH sensors. Once the reference junction is plugged with particulates, the pH sensor’s response to pH changes becomes slow. These applications are typically maintenance-intensive, requiring frequent cleaning and sensor replacement.

Advanced pH Electrode Solutions

Specialized pH electrodes with unique non-porous reference systems bring innovation to industrial pH measurement, especially in harsh conditions. Their solid-state design resists clogging from fine TiO₂ particles, offering longer life and stability in high-viscosity slurries. They also better withstand sulfide poisoning that may occur in sulfate processes, preventing reference potential drift. This significantly reduces maintenance downtime in continuous TiO₂ production, improving overall efficiency.

Summary: Effective pH control is essential throughout TiO₂ manufacturing, from hydrolysis to surface treatment and byproduct gypsum production. Selecting the right pH sensor technology can overcome common challenges like high temperature, particle fouling, and chemical attack, ensuring consistent product quality and reduced operational costs.