Ensuring Titanium Dioxide Quality

With Inline pH Measurement

Securing the desired properties of titanium dioxide requires tight pH control during production; however, process conditions are challenging for in-line pH sensors. METTLER TOLEDO provides a pH system that safeguards measurements at all times.

Background

Titanium dioxide (TiO₂), the naturally occurring oxide of titanium, has a wide range of uses as a whitener and is the preferred pigment for use in paints and coatings where a strong, opaque covering power is required. Due to its low toxicity, it has almost completely replaced applications that historically used lead additives. Currently, the chief sources of titanium dioxide are from ilmenite or rutile ores. Conversion from the mined ores to a useable titanium dioxide product can follow two different routes: the sulfate method and the chloride process.

Sulfate Process

In the sulfate method, ilmenite titanium ore must first be dried to < 0.1% moisture and then ground to a particle size of about 40 μm. The dry grind can be magnetically purified from any metallic iron present, which will eliminate hydrogen evolution downstream. This is followed by a batch digestion step where the ground ore is mixed with 80 – 98% sulfuric acid with an ideal weight ratio between 1.8 – 2.2 of free H₂SO₄. This process is done in steps, adding first water, then dilute sulfuric acid, before adding more concentrated solutions.

The heat of hydration increases the batch temperature. Increased temperature is also due to the exothermicity of the sulfate formation. The reactions can raise the temperature of the medium to 170°C.

Titanium dissolves as titanyl sulfate (TiOSO₄), iron as ferric sulfate Fe₂(SO₄)₃ , and ferrous sulfate FeSO₄. Undissolved species are removed through clarification in a settler and filtration steps. The addition of scrap iron will reduce the ferric sulfate completely to ferrous sulfate, which is removed through crystallization and typically sold as a by-product (copperas).

Caustic soda is added to bring the pH of the strongly acidic titanyl sulfate solution to 1 to 2.5 pH. Between 90-110°C, titanyl sulfate hydrolyzes to titanium dioxide hydrate, which precipitates. pH in the hydrolysis step is essential for producing the desired particle size and degree of flocculation, which in turn affects pigment properties. If the solution is too basic, there will be a considerable effect on the TiO₂ yield as well as the particle size of the growing precipitate. After precipitation, the hydrate can be doped with various alkali (metal compounds or dopants) as per specific pigment grades. The final solid is then calcined at 1000°C and ground to the required consistency for further use.

For maximum yield of valuable product, in-line pH measurement is required to control the hydrolysis and precipitation process. This is typically a challenging measurement as the precipitating solid tends to form a scaled layer on sensor surfaces. This scaling can clog the diaphragm of pH sensors and coat the pH glass, inhibiting measurement ability.

METTLER TOLEDO Solution

METTLER TOLEDO’s InPro™ 4260i sensor is specifically designed for processes with high particle load and scaling solids. The sensor utilizes Xerolyt™ Extra, a solid polymer-type reference electrolyte that allows for an open junction. This means the reference electrolyte is in direct contact with the process medium and eliminates the need for the microporous diaphragms commonly used with common liquid reference electrolyte systems. Such diaphragms are very susceptible to blockage from particles, whereas the InPro 4260i’s open diaphragm resists clogging and is faster and easier to clean.

Intelligent Sensor Management (ISM™) technology in the InPro 4260i provides real-time sensor “health” monitoring, including remaining lifetime and days until the next calibration should be performed. Monitoring the diagnostics means operators can always have complete confidence in the accuracy of their pH measurements.

Learn more about the InPro4260i

The InTrac™ 777e retractable housing enables access to the pH sensor, even during a running process.

In the retracted position, the probe sits in a cleaning chamber where, if desired, a water flush allows rapid cleaning of the sensor. The insertion rod also provides physical protection to the sensor from fast flowing pro cess liquid and solids that could damage the glass membrane. When measurement is not needed, the sensor can remain in the retracted position in a wet environment, ensuring the hydrated layer does not dry out, providing instant measurement availability. The InTrac 777e can be combined with the EasyClean system for fully automated water sensor.

Learn more about the InTrac777e

Now that you have learned how reliable inline pH measurement helps control titanium dioxide precipitation, maximize product yield, and maintain consistent pigment quality, download the full Application Note for a more detailed explanation of the process, key pH measurement challenges, and the benefits of robust sensor technology in demanding production environments.

Explore the complete application note to discover how advanced pH measurement systems ensure accurate process control and stable titanium dioxide production.