In South Africa, sugar is processed from sugarcane as raw (brown) sugar and then a portion of the raw sugar is further processed and de-colourised to obtain refined (white) sugar.
Once the sugarcane has been harvested and delivered to a sugar factory, the first step towards obtaining raw sugar crystals is to prepare the cane thoroughly by cutting and shredding the stalks in heavy preparation equipment so as to disintegrate the material and expose the sucrose-containing plant cells in the cane.
The juice, which is rich in sucrose (sugar), is then extracted from the prepared cane by either washing it out (in diffusers) or by repeatedly squeezing and washing (in mills). The remaining fibrous residue is called bagasse and is used mainly as a fuel in the factory’s boilers to make steam. The steam is used for processing requirements and electricity generation. Bagasse may also be further processed to extract valuable down-stream products.
The extracted juice has to be purified and this is usually achieved by a sublimation clarification process that utilises heat and lime. The suspended solids in the juice and the precipitate formed in the clarification process settle to the bottom of the clarifier and are removed as “mud”. The mud is filtered to recover sucrose and this is performed in either dedicated filtration equipment or by using the fibre in the diffuser as the filter medium. The clear juice is removed from the top of the clarifier for further processing to the product sugar crystals.
The clear juice overflow from the clarifiers consists of about 90% water so the next step is to evaporate much of this water in evaporators to produce thick syrup (35% water). This is done in a series of multiple effect evaporators, in which the vapour from one effect is used to evaporate water in the next effect under a lower pressure. In this way, considerable savings in exhaust steam can be achieved. Some vapours are 'bled' off for other heating duties and pan boiling.
The syrup is further boiled (concentrated) in vessels called vacuum pans and then tiny “seed” crystals are added. Sucrose molecules in the syrup deposit (grow) onto these tiny seed crystals to form a massecuite (mixture of sugar crystals and a concentrated liquid). The crystals in the massecuite are further grown in cooling crystallisers. Separation of the raw sugar crystals (also called “A”-sugar) from the surrounding liquid is achieved in centrifuges. The centrifuges make use of screens that when spun at very high speeds, separate the sugar crystals from the liquid portion (now called “A”-molasses and not syrup because it contains less sucrose).
The brown sugar is dried and can be dispatched as a product or kept for further processing into refined sugar.
The “A”-molasses still contains a large amount of sucrose and is therefore boiled again in vacuum pans as described above and will yield a “B”-massecuite, which will be centrifuged again into “B”-sugar and “B”-molasses. The “B”-molasses likewise still contains some sucrose and is boiled one last time to produce “C”-sugar and “C”-molasses (now called final molasses as the recovery of further sugar is not viable).
The “B” and “C” sugars are re-melted and sent back to the syrup so the only sugar product to leave the factory is the very high quality “A”-sugar called “VHP” – very high pol. Some of the “B”-sugar may be used as “seed” crystals in “A”-sugar production.
For every 100 tonnes of cane processed, about 12 tonnes of VHP sugar and 4 tonnes of final molasses is produced.
The purpose of a refinery is to remove the impurities (non-sugars) from the VHP sugar which cause it to have a brownish colour
The raw sugar is dissolved (melted) and the melt colour is then removed by various clarification processes. These processes are used individually or in various combinations, the most common being carbonatation followed by sulphitation. Phosphatation and ion exchange are also used in some refineries.
Water is evaporated from the clarified decolorised melt and up to four crops of refined sugar crystals are boiled from this liquid. After drying the refined sugar is conditioned in silos to remove further moisture. The product is then packaged as per customer requirements.
There are two general categories of refineries viz. back-end refineries and stand-alone or central refineries. Back-end refineries are attached to raw sugar factories and can make use of the factory’s utilities (e.g. steam generated from bagasse). Stand-alone refineries are usually located near to a port or marketplace. The raw material entering a factory with a back-end refinery is sugarcane and that entering a stand-alone refinery is raw sugar.