Essay On Sugar Precipitation

3. Introduction:

3.1 Basic information:
Refinery carbonation has been used for over 100 years for the refining of raw sugar. It is a cheap process, consists of adding a slurry of calcium hydroxide in water to the raw melt solution. Then CO2 gas is bubbled in to the sugar solution in saturators under controlled conditions of PH and temperature. Generally, the carbon dioxide is added to the saturator in two stages, the gassing carried out in the first saturator, the impurities are absorbed, the particles of CaCO3 precipitated by the reaction of Ca(OH)2 and CO2. The reaction is;
Ca(OH)2 + CO2 CaCO3 + H2O
3.2 Process Description:
Sugar mill carbonation:
After the mill crushes, shreds, or presses the cane or beets to extract juice, it adds calcium oxide (milk of lime). The calcium oxide raises the juice to 11 Ph. or more. The water in the juice react with the calcium oxide to form calcium hydroxide. This chemical addition has three desirable effects;
1. The high PH prevents the sugar from becoming starch or reverting to non-sucrose forms.
2. Organic acids are changed in to salts for later removal by precipitation.
3. Foreign matter is kept in suspension for removal by filtration.

3.3 Injection of carbon dioxide:
The injection of carbon dioxide, called carbonation, usually occurs in multiple stages.

The pH value is continuously measured during both carbonation steps. During the first step the pH value must kept between 10 and 11. To do so, carbon dioxide from the (from the lime kiln process) is introduce at approx. 70 C so that the lime precipitates together with the impurities. If the pH value falls too much due to introduce carbon dioxide the lime decomposes to form hydrogen carbonate. If the value remains too alkaline, the precipitation is incomplete. So, the pH value must be kept between 7.5. pH sensor must be cleaned using an acidic cleaning agent, e.g. amidosulphonic acid