Correct Calcium Carbonate Structure

The raw material for PCC is mined limestone, which naturally has a wide range of particle sizes and configurations as well as significant bulk and trace contaminants. The first step in PCC production is the processing of the limestone to calcium hydroxide. Powdered limestone is calcined at 1000 ° C, which breaks down the carbonate to lime (CaO) and carbon dioxide (CO₂). The CO₂ is collected for reuse downstream. Adding water to the lime produces a calcium hydroxide solution. This solution goes through a purification step where unwanted species are removed.

Finally, the calcium hydroxide is reintroduced to the collected CO₂, which results in precipitation of pure calcium carbonate (now referred to as PCC) with particle size and molecular configuration that are dependent on the process conditions.

Different PCC particle shapes result in different bulk densities and surface areas. In comparison to ground calcium carbonate (GCC), PCC has smaller particles sizes as well as a narrower size distribution. Smaller particles are ideal for impact resistance in plastic, and a narrow distribution is desired in oil absorption applications. 

Possible PCC structures include prismatic and scalenohedral forms as well as cubic or cuboidal arrangement. In the past, cubic and cuboidal types were the only forms possible. Later, it was discovered that crystal seeding could produce scalenohedral and prismatic types. It has since been found that different types can be produced through controlling conductivity and without the need for crystal seeding. Within the range of 100 – 1200 μS / cm, scaleonohedral structure will be produced, and be tween 1200 – 6000 μS / cm prismatic or colloidal structures will be favored.

The conductivity is defined by the species that have the ability to transfer electrical charge, namely calcium and hydroxide ions. Another influencing factor is the hardness of the water used (conducting species such as Ca²⁺ and Mg²⁺), so acceptable ranges are dependent on the process conditions at any time. Measuring the conductivity is thus vital to ensuring that the solution has the proper characteristics for the desired products. Using an inline conductivity measurement system becomes of increased importance, as conductivity can vary widely and change quickly, so real - time measurement is essential.