Affination & melting The Sugar Refining Process Raw sugar is made up of sugar crystals that have a thin film of molasses on the surface. It is mixed with hot (60°C) syrup which softens the film. This mixture, known as magma, is a dark brown viscous mass. Magma is fed sequentially into a battery of 14 centrifuges, each of which can spin at up to 1,050rpm with a batch cycle time of about two minutes.
This centrifugal force separates the crystals from the raw syrup. The clean raw sugar crystals from these centrifugal machines are then dissolved in water to produce raw melter liquor. The sugar purity of this liquor is about 97-99%. This partially purified sugar liquor now goes to the next refining process, carbonation, which will clarify the liquor and remove some colour.
Carbonatation and Filtration Milk of Lime (slaked lime) is mixed with the raw melter liquor. Carbon dioxide recovered from our boiler flue gas is bubbled through the mixture. The carbon dioxide reacts with the lime to form chalk, which attracts the waxes, gums, resins and other impurities in the liquor at a temperature of 80-85°C.
There are filter presses working in parallel, through which the liquor is now filtered. This removes the chalk and about half of the colour, together with virtually all of the fine debris and insoluble matter. The resulting pressed liquor is then ready for the next stage: decolourisation.
Meanwhile, as the cake from the filtration still contains a fair amount of sugar, it is made into a slurry and passed through a second filtration stage to recover this sugar which is then sent back to the melting process.
Decolourisation The pressed liquor from carbonation is passed through cells containing anionic resins. The decolourised solution is known as fine liquor.
The liquor is further polished by passing it through a column of granular active carbon from a mineral source after which the total colour removal exceeds 90%.
Evaporation and Crystallisation All the previous refining processes have been carried out on liquors that are 60-67% concentration. Evaporation is used at this stage to remove some of the water prior to crystallisation. The resulting syrup, which is known as evaporated fine liquor, is about 74% solids.
The fine liquor is fed into large vacuum pans where the sugar crystals are grown. The syrup is heated indirectly by steam to about 80°C where it boils due to the vacuum applied to the vessel. The use of a vacuum and the resulting reduced temperature helps to minimise the creation of colour during the process.
Crystallisation is controlled by a computer which checks every stage of the process. The computer also determines the timing of the introduction of the seed, the source of the crystals which grow to achieve the maximum yield of the required grade of sugar. We are able to produce sugar crystals of the required size without sieving by varying the volume of seed added. The outputs of the crystallisation process include a range of white sugars including extra fine, caster and granulated.
Separating and Drying The mixture of sugar crystals and liquor (massecuite) is centrifuged to separate the white sugar crystals. The separated liquor which still contains significant amounts of sugar is sent to a second, and then a third crystallisation stage, to ensure that the yield is maximised without compromising quality.
The separated white sugar crystals discharged from the centrifuges still contain up to 1% moisture. This is removed by passing the sugar either into a rotating two-stage dryer or a ‘fluidised’ bed dryer.
The sugar is then conditioned to lower the residual moisture further.
Our advanced process control software and crystal growing routine enables us to provide the size range preferred by customers without damaging the sugar crystals by sieving.