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Decolourization

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By Bento, Luis San Miguel
Posted on 2006-09-18    Last edited on 2010-12-09

In a cane sugar refinery, after Clarification liquor presents a colour of 600 IU ± 100 IU and still contains an appreciable amount of coloured compounds. In order to obtain a white sugar with a colour lower than 45 IU, in a classic three boiling system, it will be necessary to decolourize the liquor until a colour lower than 250 UI. Therefore, a decolourization higher than 60% must be done to the clarified liquor.

In order to make this decolourization, different processes can be used:

- with adsorbents carbons (bonechar, activated carbon granular or in powder);
- by oxidation (ozone; hidrogen peroxide);
- by reduction (sulfitation; sodium hydrosufite);
- by separation (nanofiltration; chromatography);
- by ion-exchange (anionic resins; anionic polymers)

Bone char (animal carbon) was the first decolourizer used in sugar industry. Initially, refineries were supplied with ox bones that, after burning, were mixed with liquors. Nowadays, char is used in cisterns were liquor flows in a down stream way. After the decolourization cycle, char is washed, removed from the cisterns and regenerated by heating at 550 ºC, in presence of a limited air quantity (Riffer, 1993).

After the seventies of last century, char was almost completely replaced by granular activated carbons. These carbons present a specific surface area ten times higher than char, and consequently, their decolourizing capacity is much higher (Field and Benecke, 2000). So, less carbon is needed to make the same colour removal. One disadvantage of these carbons is that they do not remove ash as char does. Char utilization is decreasing in last years, due mainly to the char higher prices in relation to granular carbons.
Regeneration of granular activated carbons demand a higher temperature than char does. However, chemical regeneration of these carbons are being tested recently in order to avoid or reduce kiln regeneration (Bento, 2006).

In some factories, oxidants, as ozone and hydrogen peroxide, are applied as decolourizers. In one sugar mill in Venezuela, sugar obtained from first boiling of clarified syrup, is melted and decolorized with addition of hydrogen peroxide (Mendoza and Dichie, 2002). With this process coloured compounds are oxidized resulting in colourless compounds, that will remain in liquor.

The application of reducing compounds in sugar industry is another way to reduce sugar colour. Normally, in certain countries as India, Brazil and China, sulphur dioxide is used by bubbling it through the juice, liquor or syrup. In this reaction double conjugated bonds, that absorb visible radiation, become saturated. The utilization of sodium hydrosulfite in powder, added directly in vacuum pans, is also used. These processes are being abandoned due to legal restrictions on residual sulphites in food products.

The application of techniques involving tangential filtration, as nanofiltration, has recently been applied in sugar industry. Industrial treatment of resin effluents and experimental tests with cane juices are the known applications in this field. In the effluents treatment, with nanofiltration, more than 90% of colourants are removed from salt solutions. The application of this technology for liquors decolourization is not attractive due high viscosity of sugar solutions at high brix.

Another process to separate colourants in sugar solutions is the application of chromatography. This technique is implemented at industrial level for the treatment of more than 90% of beet molasses in the United States (Kearney, 2002). Although the main objective of this application is to recover sucrose from molasses, there is also an effective separation of non sugars, including colourants. However as the solutions to be treated by chromatography must have a low concentration, around 40º brix, final liquors must be concentrated. This demand a great energy consumption.

Among the decolourizing techniques referred earlier, the utilization of ion exchange resins is the one that have presented the highest increasing of applications in sugar industry, in the last years.

Each decolourize system remove or change crtain colourants. A combination of decolourization system will increase the range of colourants type removed from liquors.

Bibliography

Bento L.B., 2006, "Activated Carbons: adsorption of sugar colourants and chemical regeneration", Proc. of
SIT Conf. ; Zuckerindustrie, December 2
006
Field P.J., Benecke H.P. (2000) “Granular carbon decolorization system” in Handbook of Sugar Refining, Ed.
C.C. Chou, Pub. John Wiley &
Sons Inc., 91-119
Kearney M. (2002) “Cromatographic applications in the cane sugar industry” I.S.J.,104, 194-203
Mendoza J., Espejo D. (2002) “Updates on the use of hydrogen peroxide at Central El Palmar”, S.P.R.I.
Conf
.
Riffer R. (1993) “Decolorization” in Cane Sugar Handbook, Ed. J.P.C. Chen, C.C. Chou, Pub. John Whiley &
Sons, 12ª ed, 460-467

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Decolorization (ion exchange)