Calcium Carbonate – production
Processing and production of industrial fillers based on calcium carbonate
UK calcium carbonates are a nationally significant source of raw materials. Commercial calcium carbonate grades, in the form of powders, granules and slurries, are produced in 2 ways: through the extraction and processing of natural ores or synthetically through chemical precipitation.
Ground calcium carbonate, commonly referred to as GCC, is primarily based on limestone and chalk in the UK, though marble stone is imported and processed at a few locations.
Precipitated Calcium Carbonate (PCC) is produced through a recarbonisation process or as a by-product of some bulk chemical processes.
Dolomite fillers are also produced from indigenous stone, similar to GCC, but as the UK has no viable source of high whiteness dolomite rock, again this material is also imported but processed locally by a few BCCF member companies.
Though there are extensive reserves of chalk, limestone and dolomite in the UK, in reality only a few deposits are of sufficiently high quality to be able to provide raw materials for industrial and agricultural uses. Only if the purity, colour, thickness and homogeneity are acceptable is commercial extraction worthwhile.
SEM of ground calcium carbonate (GCC)
After quarrying, further treatment is required to process natural calcium carbonates (and dolomite, which is very similar) of the highest quality, known generically as Ground Calcium Carbonate (GCC).
The production process maintains the carbonate very close to its original state, ending up in a finely ground product delivered either in dry or slurry form (water based suspension).
Bulk tanker delivery of fine limestone powder
Generally, the processing includes washing, sorting of undesirable contaminants, grinding, size classification of particles and possibly drying. Depending on the circumstances and intended uses, the order and necessity of those different steps vary. At the outlet of the process, the material is delivered in bags or in bulk when dry, or as bulk tankers for slurries by road or rail.
Surface treatment of GCC is another aspect of adding value to the basic material. Such coatings aim to match the surface tension of calcium carbonate fillers with that of the compounds (e.g. thermoplastics) in which they are incorporated.
Most commercial producers use the recarbonisation method of PCC production where limestone is converted into calcium oxide (lime) and carbon dioxide by calcination at high temperatures. After calcination the lime is slaked with water and the resulting milk of lime is purified and re-carbonised with the carbon dioxide obtained directly from the calcination process.
Silos used for storing calcium carbonate prior to delivery by bulk tanker to the customer
This produces a water-based suspension of CaCO3. A cake comprising upto 60% solid matter (depending on particle diameter) is then obtained by filtration. This filter cake is then dried and subsequently disagglomerated in grinders.
Depending on the chemical composition of the milk of lime used and on the purifying stages during production, food and pharmaceutical grades as well as technical grades can be produced.
Coated grades of PCC are also produced by introducing fatty acids or other additives at the suspension stage prior to filtration.
SEM of scalenohedral precipiated calcium carbonate (PCC)
The fineness of the grain, as well as the morphology of the PCC crystals can be modified during the process by controlling temperature, concentration and time. There are three main crystal morphologies: Calcite, Aragonite, and Vaterite. Within each morphology, several crystal forms are possible. This opens up interesting possibilities of tailoring PCC products to specific use applications.