So you want to know about Precipitated Calcium Carbonate?
Precipitated Calcium Carbonate (PCC) is the synthetically formed version of CaCO3. While Ground Calcium Carbonate exists naturally, it lacks the quality used for many of the applications PCC can be used for. With many industrial applications PCC and its price at $8.70/kg according to IndiaMart.com, PCC is shown to be an important chemical for many industries.
How it’s made
PCC is made starting with mined limestone, CaCO3 which is then crushed into a powder and purified to be thermally decomposed at 840 ﹾC and 178 kJ/mole in a calcination reaction resulting in Calcium Oxide (lime) and Carbon Dioxide.
CaCO3 + Heat -> CaO + CO2
Lime is then slaked with water resulting in milk of lime. At the Songadh Plant, a slurry slaker is used for slaking. The minimal of amount of water is used to maintain a stable temperature to produce high quality lime slurry. During the slaking process, the most important measure of efficiency is the specific surface area of the calcium hydroxide particles. The main process control is temperature in the slaker. The optimum temperature for slaking is around 212 ⁰F, but due to the exothermic natural of the reaction, the temperature is reduced to prevent the hydrate particles from crystallizes and agglomeration to reduce the specific surface area of the particles.
CaO + H2O -> Ca(OH)2
Impurities are then separated out again from the slaked lime. After that captured carbon dioxide is mixed with the slaked lime to reform calcium carbonate. Since calcium carbonate is insoluble in water it is precipitated out in a pure form which is where PCC gets its name. Grit and impurities are further removed and the calcium carbonate is now in a water suspension. Morphology, particle size, and other properties of the PCC can be controlled by process variables such as temperature, heating/cooling rate, CO2 pressure, and agitation rate.
Ca(OH)2 + CO2 -> CaCO3 + H2O
Now that high grade calcium carbonate is produced, further drying and filtration can occur to increase solids for use and packaging.
Comparison to GCC
GCC (Ground Calcium Carbonate) is extracted and processed from natural ores compared to PCC which is synthetically produced. Other names for GCC are limestone and chalk. There are plenty of deposits of GCC, but they usually filled with impurities such as feldspar, silicaeous minerals, and heavy metals. GCC may be used similar to PCC, but often lacks control over particle size and shape.
Crystal Structures and Size
There are four basic crystalline forms of PCC crystals. There is aragonite, rhombohedral, scalenohedral, and prismatic. Calcite is the most stable form of calcium carbonate. A summary of the physical properties of other PCC morphologies can be found below.
The particle morphology influences the way light is scattered in the paper which impacts the optical properties of the PCC filler. Particle size is also important and can affect properties such as opacity, bulk, and porosity. For both scalenohedral and prismatic PCC particles, the opacity is highest around 1.0–1.5 µm. For scalenohedral, sheet bulk increases as particle size increases, but at some certain point particles that are too large will disrupt fiber bonding too much to maintain the strength of paper
When choosing the size and shape of PCC fillers, the properties desired need to be considered. All PCC fillers provide the same brightness. To achieve optimum opacity, 1.1–1.5 µm scalenohedral PCC, 0.5–0.9 µm prismatic PCC, and 0.2–0.4 µm rhombohedral PCC should be used. Bulk can be obtained by choosing fillers with particles larger than 1.0 µm. Strength can be maintained by keeping a low surface area. 0.6 µm prismatic PCC and 0.8 µm scalenohedral PCC fillers work best for attaining porosity properties.
Where we get Feed Calcium Carbonate
Limestone mines can be found throughout the world with quarries being very large and long lived. Limestone comes from prehistoric sea animal shells and skeletons where are made of calcium carbonated. Under geologic pressure, they crystalize and become limestone. Any organic matter is removed through oxidation, and high-quality limestone can be found in many places. Different pressures can form limestone at different hardness levels ranging from chalk to marble.
Why we Use PCC
PCC was first used as a coater to improve the brightness, color, and opacity of paper. As technology for PCC production improved in the late-1970s PCC pigments also improved roughness, porosity, printing properties, rheology, and runnability. The main reason to use PCC is its high brightness and sheet gloss. PCC creates a more open sheet that allows for a more ink receptive coated sheet. As a filler, PCC can reduce fiber usage and add optical properties such as opacity and brightness.
Fiber Filler Chemistry
Fillers fill up space between fibers. A good filler needs a range of particle sizes to fill all the different holes left by fibers. Paper is mainly held together by hydrogen bonding between cellulose chains. Refining occurs to open fibrils in cellulose fibers to increase hydrogen bonding sites between fibers. As water is drained, pressed, and dried from the sheet, fibers are brought closer together increasing the number of hydrogen bonding sites. When fillers are added, the surface area of fibers are covered reducing strength qualities in paper. Fillers hold fibers apart increasing air spaces in the paper sheet which will also increase the opacity and brightness of a paper as well. This results in a balance between providing dry-strength additive and enough refining to achieve the desired bulk effect while maintaining strength requirements.
