How Cane Sugar is Refined - Carbonatation
Carbonatation is achieved by adding milk of lime [calcium hydroxide, Ca(OH)2] to the liquor and bubbling carbon dioxide through the mixture. The gas, which is obtained by cleaning up the flue gas from the boiler, reacts with the lime to form fine crystalline particles of calcium carbonate which occlude the solids. To obtain a stable floc, the pH and temperature of the reaction are carefully controlled.
The filtration is usually undertaken with rotary leaf filters where the liquor is pumped from the outside of the leaf to the middle where the clear liquor is collected. As the layer of floc builds up on the leaf it increases the pressure drop across the system until the filter is effectively choked and taken off line for cleaning. The lime mud that is collected when cleaning the filters is still wet with sugar liquor so it is de-sweetened by slurrying with water - the resultant sweet water is used elsewhere in the process - and re-filtering it to a 50% moisture mud. The mud is then dumped or used as lime on fields.
Phosphatation is a slightly more complex process that is achieved by adding phosphoric acid to the liquor after it has been limed in the same way as above. In the presence of a small amount of lime sucrate a calcium phosphate precipitate is formed which is removed by a flotation process. The clean liquor is usually filtered to remove any remaining fine particles of precipitate. The flotation scum is desweetened by re-slurrying it and floating it again.
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