Cane Sugar Refining - Purolite

Cane Sugar Refining with Ion Exchange Resins
Cane Sugar Refining with Ion Exchange Resins
Sucrose is extracted from the sugar cane stalk in a cane sugar Mill, purified and
crystallized into a tan colored raw sugar. The sugar we consume must be further refined
so as to not impart color, flavor or odor to the foods and beverages it sweetens. In a cane
sugar Refinery the raw sugar is remelted and the sucrose is further decolorized with
ion exchange resins, carbon or bone char and crystallized into white sugar for industry,
consumers and confectioners. Although the ion exchange resins are used in the refinery,
it is useful to understand the mill operations also.
Cane Sugar Mills
Sugar cane plants are grown in tropical climates until maturity when the stalks achieve
a 3-5 cm diameter thickness and stand 2-3 meters tall. In some locations, the fields
are burned prior to harvest to remove the leaves from the stalks, eliminate green matter
from the fields and drive rodents out to make manual cutting easier. The stalks are
then cut and trucked to the mill. In other locations the fields are not burned and the
leaves remain on the cane sugar stalk during harvesting. Cutting can be done manually
or with a mechanical harvester. The leaves and tops are chopped and left in the field.
At the mill, the stalks are removed from the trucks with cranes and dropped into the
mill feed shoots. Whole trucks can also be lifted to dump the load of cane stalks into the
feed hopper. As the cane shoots feed the stalks toward the mill, a mechanical leveler rakes
the stack of cane stalks to a more consistent height. The cutting knives then shred the
stalks into small fiber strips.
The strips are fed to a series of 4-6 cane crushing mills which squeeze the sugary
juice out of the cane stalks and further reduce the size of the fibers. Water at 70° C
is added countercurrent to the fiber movement during the milling operation to assist the
extraction of the sugar from the cane stalk fibers. The resulting mill juice has a sugar
concentration of approximately 17° Brix. The remaining fiber, called bagasse, is sent to
the boiler where it is burned to produce steam and electricity for the mill. The bagasse
can produce most, all or even excess energy for the entire mill’s needs.
The 17° Brix mill juice is heated to 105° C and mixed with lime, flocculants and
CO 2 and sent to a clarifier. In the clarifier the lime destroys non-crystallizable invert
sugars and amino acids. The lime and CO 2 combine to form a calcium carbonate
precipitate which attracts color and other impurities which adsorb onto the solids.
The color-laden precipitated solids sink and are discharged from the bottom of the
clarifier while the clarified juice overflows from the top. Alternatively, the clarifier
may use air to float the floc to the surface where it is skimmed off and the clarified
juice flows out the bottom.
The clarified juice passes through pressure or vacuum filters to remove any residual
colored precipitate. The filtered juice, now at 14° Brix due to some dilution during
the lime defecation process, passes through a multiple effect evaporator to raise the
concentration to 60° to 70° Brix.
The concentrated syrup is fed to the vacuum pans where further evaporation and
supersaturation of the sucrose solution occurs. Crystallization of the sucrose occurs in the
first vacuum pan and the resulting mixture of sugar crystals and syrup, called massecuite,
is dropped into a centrifuge.
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