The seed is not prepared by high supersaturation primary nucleation, but rather by sampling in production, followed by milling, and then maturation.
- 1) Sampling: sugar C, for example, is taken in order to manufacture the seed for sugar B.
- 2) Milling: this operation occurs in a small ball mill of around 1 m3 over 4 h. This takes place as a wet process (isopropanol). The isopropanol leads to the formation of aggregates.
- 3) Maturation: the seed is kept for 4 weeks in a drum (of 1 m3 in volume) that turns slowly, dividing the number of particles by two or three. The mean diameter of the future seeds is of the order of 10 pm, and their size distribution is relatively uniform. The slurry contains from 33 to 35% crystals by weight.
- 4) Addition of seed: the alcohol is vaporized and the crystals remain in suspension in the aqueous sugar solution. The crystallizer’s mother liquor saturation must be in the order of 0.95-1 during the introduction of seed, which allows for the dissolution of microcrystals. Saturation then increases to the following values:
Refined sugar 1.05
White sugar (A) 1.25
Raw sugar (B) 1.20
Sugar after production (C) 1.25
A crystallizer has a draft tube and internal thermal exchange. The tubes are short (1-1.5 m in length) and of low inner diameter (1-1.5 cm). The seed volume must reach 25% of the magma volume. The useful volume is consequently reduced to 75% of the total volume.
In order to eliminate the agglomerates, the seed slurry must be agitated beforehand with a power of 10 kW.m-3.