Step 1 - Affination
The raw sugar is mixed with a saturated syrup and then centrifuged to extract the crystals. Surface impurities (molasses) dissolve in this syrup and are removed.
Step 2 - Carbonatation
The sugar is redissoved and calcium hydroxide and carbon dioxide are added to the solution. These react according to the following equation
Ca(OH)2 + CO2 CaCO3 + H2O
Colour, gum and amino acid impurities precipitate out with the calcium carbonate.
Step 3 - Char filtration
Activated charcoal is added to the syrup, removing colour and inorganic ash.
Step 4 - Crystallisation
The solution is boiled under vacuum and the crystal growth monitored to produce particular sizes of crystal.