How Beet Sugar is Made - Carbonatation
Carbonatation is achieved by adding milk of lime [calcium hydroxide, Ca(OH)2] to the liquor and bubbling carbon dioxide through the mixture. The gas, which is obtained from the manufacture of the lime in the first place, reacts with the lime to form fine crystalline particles of calcium carbonate which occlude the solids. To obtain a stable floc, the pH and temperature of the reaction are carefully controlled. Beet factories use much more lime than cane factories, some 1 to 3% of CaO on beet is used.
The filtration is undertaken with rotary leaf filters where the liquor is pumped from the outside of the leaf to the middle where the clear liquor is collected or in a clarifier where settling occurs. As the layer of floc builds up in a leaf filter it increase the pressure drop across the system until the filter is effectively choked and taken off line for cleaning. The clarifier is run continuously however. The lime mud that is collected from either method is still wet with sugar liquor so it is de-sweetened by slurrying with water - the resultant sweet water is used elsewhere in the process - and re-filtering it to a mud with 50% or less moisture. The mud is then dumped or used as lime on fields.
The efficiency of the factory depends substantially on the use of multiple effect evaporation, as with the raw cane sugar factory. It is even more important for the beet factory because there is no surplus fibre available to porvide fuel for the power house. The greater the number of effects, the less steam is required to drive the first effect. Each subsequent effect is heated by the vapour from the previous effect so has to be operated at a lower temperature and therefore lower pressure. In is not unusual to see 6 and sometimes 7 effects in a beet factory although many cane factories only have 3 or 4 effects.