Raw sugar contains between 1 and 3% of non-sugar that must be eliminated.
Filtering is the first step. This consists of eliminating the non-sugar present in the liquid surrounding the crystals. The sugar is mixed with syrup and dewatered. In this manner, we obtain “refined” sugar.
The sugar is then dissolved and purified by:
- - carbonation or phosphatation;
- - discoloration with charcoal or ion exchange.
The syrup obtained, the “fine liquor” is subjected to multilevel crystallization by water vaporization. We then obtain “refined white sugar”.
As a general rule, beetroot sugar purity is more readily obtained than that of cane sugar.
Thermal exchange data
Owing to the high viscosity at 40°C, the cooling process is performed by circulating magma in tubes of diameter 10-15 cm at a velocity above 0.1 m.s-1. The tubes are arranged in a triangle and the global transfer coefficient varies between 20 and 50 W.m-2.K-1. Water circulates on the outside of these tubes and, where appropriate air is used, with finned tubes instead.
On the other hand, the vaporization of water at 70°C occurs in the crystallizers with an internal exchanger, with short tubes of interior diameter 1-1.5 cm and length 1-1.5 m . Circulation requires a draught tube fitted with a marine impeller, with the magma circulating from the top to the bottom.