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Softening

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By Bento, Luis San Miguel
Posted on 2007-07-27    Last edited on 2009-09-25

Sugar solutions contain inorganic compounds generically designed by ashes. These compounds can cause problems during sugar extraction or refining.
-         Some inorganic compounds can decrease the yield of sugar crystallization (named melassigenic compounds as its presence increase the quantity of solids in mother liquors).
-         Other compounds can precipitate during heating, evaporation or crystallization. This will decrease the heat transfer through calandrias or other heating surfaces increasing the evaporation/crystallization time, with increases of energy consumption, color formation and sugar destruction.
-         Other compounds can decrease the filtration rate, as silica.
-         Other compounds can form flocks or sediments in products using sugar, as in soft drinks.
By these reasons it is important to reduce the quantity of inorganic compounds in sugar products. Demineralization will remove all the quantity of inorganic compounds by permuting the cations by H+ and anions by OH-. In softening the objective is to reduce calcium and magnesium as these compounds are related with precipitates formed on heating surfaces. Also these compounds are related with turbidity, flock formation and colour transfer into crystals. For softening, a cationic resin is used, normally in Na form. In his form the sugar solution passing through the resin do not decrease in pH and the solution do not need to be cooled to avoid sucrose inversion.

A decalcification process was studied using a mixed bed resin system. In this system an anionic and a cationic resin are mixed in one column. Colour and calcium are removed this way. Resin regeneration is done by a solution of calcium saccharate, sodium hydroxide and sucrose (Bento, 2001). This mixture is very efficient to remove colourants from anionic resins due the formation of a complex colourant-calcium-sucrose in alkaline solutions (Bento, 1996a). In the same way, the presence of colourants, in alkaline sucrose solution, will enhance the removal of calcium from cationic resin. A final regeneration is done using sodium chloride.
With this regeneration process, in ten cycles, a carbonated liquor decolourization of 90,5% and a decalcification of 90,8% were achieved (Bento, 2001).

Another process to make the liquor decalcification consist in the utilization of strong base anionic resins in carbonate form (Bento, 1990b). In this process anionic colouants from solution are exchanged by the mobil carbonate ion from the resin. Thes ions, in solution will precipitate calcium in solution.  In this way with a single operation with only one resin we achieve a decolourization and a decalcification (Bento, 1990) (European Patent nº EP 0 425 322 A2 and U.S.A. nº 5,096,500). 

Bibliography

Bento L.S.M., 1990, Sugar liquors purification using anionic resins with regenerant recovery, Proc. of
           S.P.R.I. Conf.
, 99-115
Bento L.S.M., 2001, Decolourization and decalcification of sugar solutions, Proc. of S.I.T. Conf. 265;
Zuckerindustrie, 126,  139-140

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