3. DIFFUSION
The actual sugar is inside the beet and has to be extracted. In order to extract the sugar the beet is first cut up into elongated slices.
Sugar is then extracted from the beet by diffusing (see Diffusion) it out with hot water. This is done in a large vessel specially designed for this purpose. Beet slices are fed in continuously at one end and hot water at the other end. A solution of sugar emerges from one end and the exhausted beet slices emerge from the other. The exhausted beet slices, or pulp, are mixed with molasses then dried and sold as an animal feed.
The solution now left to continue for the rest of the process is referred to as the raw juice. This contains about 14% sugar and is black in colour.
4. SATURATION
At the diffusion stage other substances are extracted from the beet as well as the sugar. But before sugar can be produced in a white crystalline form it is necessary to remove as many of these non-sugars as possible. This part of the process is referred to as juice purification.
The main raw materials used in the purification are lime and carbon dioxide gas which are got by burning limestone in a kiln. These substances are added to the juice causing non-sugars to be precipitated out of the solution. The solid material is then filtered off. After juice purification the juice has a light yellow colour.
5. EVAPORATION
The purified juice is a sugar solution containing approximately 14% sugar and 1% non-sugars. It is now necessary to concentrate this solution. This is done by boiling off water from the solution in large vessels known as evaporators. On entering the evaporators, the solution contains approximately 14% sugar. On leaving the evaporators it contains approximately 60%.
6. VACUUM PANS
In order to turn the sugar into a crystalline form it is now necessary to evaporate still more water. This is done at a reduced temperature and pressure in large vessels known as vacuum pans. Syrup is fed to the pans and as the water is evaporated off, the crystals of sugar begin to grow. When the pan is full it contains about 50 tonnes of a mixture of sugar crystals in syrup. The contents are then discharged into large holding vessels known as crystallizers.
7. CENTRIFUGALS
The next step in the operation is to separate the sugar from the syrup. This is done in automatically controlled machines known as centrifugals. In them the syrup is spun off and the sugar crystals remain. The wet sugar is then dried, screened, cooled and sent to large bulk storage silos each of which can contain up to 50,000 tonnes.
The syrup from the centrifugals still contains a lot of dissolved sugar. This syrup is put back through two more boiling stages in order to extract still more sugar.
The final syrup from which it is no longer practical or economical to extract more sugar is known as molasses. It contains some sugar together with non-sugars which were not removed at the juice purification stage. It is mainly used for animal feed.
The total time from beet washing to white sugar is about twelve hours. Depending upon sugar content of the beet, 100 tonnes of beet will give approximately 12-14 tonnes of sugar and 3-4 tonnes of molasses.