The Milling Process

IMG

IMG

IMG

CANE PREPARATION

The sugar cane is delivered to the mills by road transport. At the mills the cane is first massed at a weigh bridge before being off-loaded directly onto a large moving table. From there the cane continuously travels through a system of conveyors toth e cane preparation units, namely the cane knives and shredder.

At the cane knives the stick of cane is cut into small pieces of 20mm to 50mm in length before being fed into the shredder which literally shreds all the fibres and opens the inner part of the cane for the extraction process.


JUICE EXTRACTION

At Nakambala two methods are employed to extract the juice from prepared cane, namely the milling or the diffusion process.

DIFFUSION

The prepared cane passes through a huge vessel called a diffuser. Hot water is continuously being sprayed onto the moving bed of finely prepared cane, thereby leaching out the sucrose from the cane.

MILLING

The prepared cane passes through heavy rollers which squeeze out the juice. This process is repeated several times down a tandem of mills. The dry fibre discharged from the last mill is called bagasse. The bagasse so produced is the main source of fuel for boilers, which produce all the energy required by the milling process. Other by-products such as paper, particle board, animal feeds and industrial solvents can also be made from bagasse.


CLARIFICATION

The mixed juice is heated and milk of lime added to it. This neutralises the acids and forms a precipitate that is settled out in the clarifiers. This sediment is then filtered to recover any remaining sucrose before returning it to the fields in the form of filter press as a source of fertilizer. The clear juice from the clarifiers is pumped across the evaporators.


EVAPORATION

The clear juice is pumped to the multiple effect evaporators. This consists of a series of vessels so arranged that each vessel has a higher vacuum than the preceding one, and thereby boiling at a lower temperature.

Steam from the boilers is sent to the first vessel and the resultant vapour from it used to boil the juice in the next vessel and so on. The last vessel boils under a high vacuum. In this process about 85% of the water is evaporated concentrating the juice solids from about 12% to 65%.


CRYSTALLISATION

Crystallisation is accomplished by further evaporation of water under carefully controlled conditions in vacuum pans. As the water is evaporated, the sugar reaches concentrated levels. At this stage seed crystals in the form of a slurry are added and these act as nuclei, growing into larger sugar crystals. Computers are increasingly being used to control this process.

The dense mixture of crystals and mother liquor is known as massecuite. After discharging from the vacuum pans it passes through crystallisers which are large open vessels with slow moving stirrers, where under the action of stirring and cooling, more sugar is exhausted from the mother liqour and is deposited onto the crystal surfaces.


CENTRIFUGING

This stage separates the sugar from the mother liquor. The massecuite is fed to the centrifuge which comprises of a perforated basket lined with a metallic screen which acts as a filter medium. The basket rotates at high speed and the sugar remains behind while the mother liquor now known as molasses passes through the basket perforations.

The resultant molasses is sold either for manufacture of animal feeds, alcohol or as a by-product for the manufacture of other chemicals. The sugar is discharged and either passed through a drier before being packed as raw sugar or transferred to the refinery for further processing.


DISTRIBUTION

Packed raw and refined sugar is loaded either into road or rail trucks and transported to the market.