Crystallization of sugar

Crystallization is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas.

The crystallization process consists of two major events, nucleation and crystal growth

IN SUGAR INDUSTRY…

Crystallisation is the process just after evaporation. During evaporation the clarified sugar cane juice is boiled in evaporators which remove most of the water leaving a thick syrup. Then in the crystallization process the syrup is boiled at low temperatures under partial vacuum and some seedings are added which causes the development and growth of sugar crystals and the outcome is called massecuite (raw sugar crystals mixed with molasses). The sugar crystals and molasses are then separated in centrifugals.

There are normally 3 vacuum pans namely A, B and C. Syrup coming from evaporators enters pan A, where boiling takes place and crystallization begins giving a thicker liquid (massecuite) which comes out and enters the centrifuge.

Schematic representation of sugar crystallization unit. (1) Syrup (liquor A); (2) massecuite A; (3) raw sugar (A); (4) mother liquor A used as feed liquor B; (5) massecuite B; (6) sugar B recycled as magma A; (7) mother liquor B used as feed liquor C; (8) magma C (prepared in batch seeding crystallizer); (9) massecuite C; (10) sugar C recycled as magma B; (11) molasses.

During crystallization, it is necessary to initiate the formation of sugar crystals, in other words, the formation of nuclei through the seeding system. The various seeding techniques include : 1) Traditional (secondary, or shock) seeding2) Full seeding : a. seeding with slurry b. seeding with footing magma

In shock method, the syrup entering vacuum pan A is concentrated and a small charge of powdered sugar is introduced; boiling is continued until a proof slide appears to have sufficient grain. Supposedly, the sugar by its mere presence "shocked" the unstable syrup into spontaneous nucleation to form most of the needed grain. The same procedure is carried out with the other pans.The shock seeding method is successfully carried out by seeding the oversaturated syrup in the pans with a measured amount of standardized very finely ground sugar slurry.

The grainy nature of the crystallized sugar is determined by many factors which include :

• Degree of super-saturation • Rate of cooling • Degree of cooling • Timing, rigor and length of stirring • Temperature at stirring • Seeding • Blend of sugars present • The pH • Presence of crystal growth inhibitors

1. the quality of the crystals is mainly determined by the supersaturation and pan circulation maintained all over the strike,2. while the time needed to reach the required product crystal size is determined by the linear speed of crystal growth