Fine Points of Calcite in Particle Size Analysis

Calcium Carbonate (Calcite) is used in a wide range of industrial applications in various functions:

• Adhesives and insulation materials: As a filler and viscosity control agent • Animal feed: As a calcium source and digestive aid • Construction: As a filler for concrete, plaster, asphalt and roof covering materials • Fertilizer: As a calcium source and filler • Food: As a supplemental calcium source • Household appliances and hygiene products: As a mild abrasive • Mining: As a bridging component in drilling mud • Paints and Coatings: As a pigment, filler, additive and finishing aid • Paper: As a filler, coating pigment and acidity control agent • Pharmaceuticals: As a filler or anti-caking agent • Plastics: As a filler and rheology control agent • Rubber and Elastomers: As a filler and additive material.

The Importance of Particle Size in Calcite

Calcite, used as both a filler and pigment in the paints and coatings industry, plays an important role in providing both surface smoothness and whiteness and opacity on the coated surface. The particle size distribution of calcite particles used as filler and pigment affects opacity, color, gloss, viscosity, rheology and sedimentation rate. Pigment calcite achieves its optical properties through light scattering efficiency that occurs within a specific narrow particle size distribution band. The effect of particle size on light scattering efficiency and thus on opacity and covering power is the most important factor for pigments. Light scattering efficiency is also dependent on the wavelength of light; smaller particles scatter blue light more, while larger particles scatter red light more. Therefore, particle size distribution also affects undertone. The effectiveness of calcite used as a filler and additive and its effect on rheological properties are again determined by particle sizes. In a fixed volume calcite additive, as particle size decreases, the number of particles and the degree of particle-particle interaction increases. The surface charge surrounding each primary particle and the hydrodynamic layer formed by hydration or adsorption layers amplify the effect of increasing the number of these particles. In this way, a higher effective volume fraction is obtained for a given particle loading, and the suspension viscosity increases. To provide effective functionality in the areas exemplified above, calcite is used in various particle sizes and with different surface coatings. Comprehensive measurements can be performed on all calcite classes from nano size to macro size range within minutes using the Mastersizer 3000 particle size distribution equipment. With the wide measurement range and fast measurement capability provided by the Mastersizer optical measurement equipment, product characteristics can be monitored through process control and finished product quality control tests in different product types and application areas.

Dispersion of Calcite

For primary particle size analysis to be performed with accuracy, the material must be completely dispersed. As particle size decreases, the attractive force between particles increases. Wet and dry dispersion can be achieved with various units available in the Mastersizer 3000 optical measurement unit. The shape of the calcite production process and the desired particle size range determine which dispersion method will be used for measurement of the sample. With the dry dispersion method, larger amounts of samples can be measured, and measurement of coarse particles can also be achieved. However, for measurement of fine particles, wet dispersion is preferred so that distribution control can be easily achieved. With the ability to switch between wet and dry dispersion units in the Mastersizer 3000 optical unit in a very short time, measurements can be performed with both units and result evaluation can be easily accomplished. The dry powder disperser unit disperses the sample with the aid of compressed air. Therefore, the correct pressure must be selected to ensure effective dispersion. With the wet disperser unit, calcite samples are dispersed in water. Brief ultrasound application is needed to disperse agglomerated samples. Sodium pyrophosphate or sodium hexametaphosphate are also used to ensure the dispersion remains stable.

Determining Particle Size of Calcite

Volume-based particle size distribution results are obtained using the laser diffraction method. For example, with Dv10 results, changes in fine particles within the distribution are observed, and with Dv90 results, changes in coarse particles within the distribution are observed. Fine calcite is generally described as below 1 µm and 2 µm, while coarse is described as above 10 µm or 45 µm. Table 1 presents the size distribution parameters of calcite samples in 4 different classes (between 0.1 µm and 1000 µm).

Effect of Theoretical Model Change on Results

In the software used to convert laser diffraction and laser intensity values obtained from the Mastersizer particle size distribution equipment into size results, different calculations can be performed using Mie theory and Fraunhofer approximation. Thus, results can be obtained using both models. The effect of different theoretical model selection on results is shown in Figure 2. The Fraunhofer approximation assumes the optical scattering efficiency coefficient of substances to be 2. In practice, this approach is acceptable for particles larger than 10 microns. On the other hand, Mie theory evaluates light diffraction of fine particles, particularly in the region below 10 microns, more accurately. As can be seen in the graph obtained from calculations made on the measurement result, a difference between Mie theory and Fraunhofer approximation is observed at values smaller than 10 microns.

Why Mie Theory?

The most precise and comprehensive predictions of the laser diffraction behavior of particles are shaped using Mie Theory, and this requires a high amount of computational power. When laser diffraction systems were introduced toward the end of the 1970s, Mie theory could only be used in limited ways because the computational power of systems at that time was insufficient. For this reason, the Fraunhofer approximation, which required less computational power, was more widely used instead of Mie theory. In today's conditions, due to powerful computers being available at accessible prices, there is no longer a need to continue using the Fraunhofer approximation. After the publication of ISO 13320 standard, an international standard was introduced for particle size analysis by laser diffraction. Although the history of laser diffraction method dates back many years, the publication of the standard was an indication that this technique is now fully acceptable. One of the important issues specified in this standard for obtaining reliable measurement results is the correct selection of optical parameters of the particles to be measured. The laser diffraction method performs particle size analysis based on the prediction of how particles scatter the laser light falling on them. The quality of this prediction affects the accuracy of the size analysis result. The most precise and comprehensive predictions of the laser diffraction behavior of particles are shaped using Mie theory. However, when laser diffraction technique emerged toward the end of the 1970s, the era's computational technology was in its infancy and computers had 8K RAM. Due to the lack of technology required for calculations with Mie theory, a Fraunhofer approximation was used to interpret the laser diffraction behavior of particles. Today's technological sufficiency and accessibility allows Mie theory to be used quickly. While the ISO standard states that the Fraunhofer approximation can still be used, it states that this approach gives correct results for particles larger than 50 µm. For measurements with particle distributions smaller than 50 µm, the standard states that Mie theory should be used. Additionally, it states that Mie theory can be used for the entire measurement range. The Mastersizer equipment contains refractive index values in its library for many substances. This facilitates the selection of optical parameters when performing calculations with Mie theory.

Final Conclusion

The laser diffraction method has been used for calcite measurement for a long time due to its wide dynamic measurement range, flexible dispersion options, measurement speed and result repeatability. Reducing particle size brings grinding costs but contributes to the optical, rheological and surface properties of the end product. This increases the value of the fine particle calcite class and particularly requires laser diffraction equipment capable of precise measurements at the nanometer scale for particle size analysis of this class. Mie theory, which can be implemented with technological advancement, can reveal the difference in light diffraction that arises from refractive index differences, the relative transparency of particles and absorption coefficient differences. In the fine particle calcite class where Fraunhofer approximation can lead to errors, Mie theory provides the most accurate result. Abdullah Sandıkçı / Chemical Engineer / Application Engineer / Atomika Teknik Cihazlar Ltd. Kuday Karaaslan / Nuclear Engineer / Application Team Leader / Atomika Teknik Cihazlar Ltd. References • Particle size analysis of calcium carbonates by laser diffraction, AN140211, Malvern Instruments Ltd., 2014. • Particle size analysis of titanium dioxide using the Mastersizer 3000 laser diffraction particle size analyzer, AN131216, Malvern Instruments Ltd., 2013. • Ten ways to control rheology by changing particle properties, WP140820, Malvern Instruments Ltd., 2014. • Laser diffraction for particle size analysis - why use Mie theory?, LabPlus International, Nov., 2000.