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Crystallization


Crystallization Pan
The most important step in the separation process is that of crystallization. It is more than 99 percent effective as a purification step and without its use further separations would have to be made in order to purify the sugar acceptably for commerce today.

Crystallization takes place in three major steps. These steps are known as white crystallization, high raw or high re-melt crystallization, and low raw or low re-melt crystallization.

Crystallization takes place in specially designed vessels called “pans”. Each pan of sugar boiled is called a “strike”. In white strikes, standard liquor is introduced into the pan to a level known as the charge level. This level is generally just above the pans heating surfaces. The syrup is then boiled under vacuum, while continuously adding standard liquor feed to maintain the charge level, until it reaches a concentration where no further sugar would dissolve in the syrup if it were added. This point is known as the “saturation point”.

The pan is then carefully raised in concentration above the saturation point to a preset “supersaturation point”. Then a pre-measured quantity of very fine milled sugar crystals is added as a seed on which the sugar will deposit and become macroscopic crystals. The standard liquor feed is then increased to maintain crystal growth. When the pan is nearly full, the feed is stopped, and the percent solids of the mixture of sugar crystals and syrup in the pan raised to above 92 percent by evaporation. A great deal of crystal growth occurs during this “brixing-up” process.


Centrifuges separate massecuite from syrup
At this point the mixture of crystals and syrup from which the crystals have been grown is called “white massecuite” or white “fill-mass”. The pan is now “dropped”, which means that vacuum on the pan is released, and the massecuite allowed to flow by gravity to a mixer above the white centrifugals. The mixer keeps the massecuite in suspension until the syrup can be separated from the crystals by centrifugation. Specially designed cooling/heating mixers, called “crystallizers”, are sometimes used to promote further crystallization prior to centrifugation.

The process of boiling high raw and low raw strikes is virtually identical to boiling white strikes. The differences are: high raw strikes are boiled using a feed stock consisting of the syrup separated from the white massecuite, called “high green” instead of standard liquor; and the low raw strike is boiled from a feed stock consisting of the syrup separated from the high raw messecuite called “low green”, “intermediate green” or “machine syrup”. Low raw strikes always go to crystallizers before one step, and sometimes two-step (affination), separation of syrup from crystals by centrifugation. The syrup separated from the low raw massecuite is called molasses.

Molasses is one of the three final end products of the factory. It contains about 60 percent of the soluble non-sugars originally extracted with the beet and its sucrose content approaches 50 percent. The non-sugars concentration, however, prevents further crystallization within reasonable time limits. The quantity of molasses produced varies with the amount of non-sugars in the beets as processed and the type of equipment available for crystallization, ranging from about 3-6 percent on beet.

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