Carbonatation - RefiningTopics:
By Bento, Luis San Miguel
Posted on 2006-09-20 Last edited on 2012-01-24
Raw sugar contains insoluble matter (bagacillo, sand, colloidal matter) that is not separated during Affination. Therefore, affination liquor has a high turbidity, and a clarification process is necessary before decolourization and crystallization.
In Carbonatation process calcium carbonate is formed by reaction of CaO and CO2. The precipitate formed will co-precipitate some high molecular material and suspended solids will be removed jointly with it. The presence of calcium at the precipitate surface can fix anionic colourants that will be removed from liquor.
In this process, affination liquor is mixed with milk of lime (suspension of calcium hydroxide in water or sweet water), in a quantity of circa 0,6% of CaO, on liquor solids. This quantity can vary with raw sugar quality. The presence of polysaccharides can decrease liquor filterability and CaO percentage may be increased up to 0,8%.
The liquor and lime mixture is then contacted with CO2 gas. This gas is obtained from steam boilers, after cleaning, washing and neutralization. This gas, with around 9% of CO2, react with the lime, in two step reactors, in a continual process, with a total reaction time of around 60 minutes.
When lime is added to the liquor, an increase of pH to values higher that 11, occurs. At this high pH hexoses can be degraded forming colour compounds named HADP. In order to avoid this colour formation, the contact time between lime and liquor, before gassing, must be reduced to 2 or 3 minutes. Normally this mixture is done in small tanks with strong agitation.
Reaction of CO2 in carbonataion reactors must be quick enough to decrease liquor alkalinity. By this reason carbonatation is made in two reactors. In the first reactor the final pH must be 9.5 and, in the second reactor, between 8.2 and 8.5. Even in these conditions, an invert sugar destruction of 60% is oberved (Bento, 1999).
Scheme of Carbonatation
The utilization of calcium hydroxide in Carbonatatin has the following functions:
The mud formed after liquor filtration contains a certain amount of sucrose. To recover this sugar, a second filtration of mud slurry is done in plate and frame or in automatic membrane filters. In these filters, mud is compressed and washed with hot water, to extract the majority of sucrose in it.
Carbonatation is a high efficient and robust process used in the majority of cane sugar refineries and in some cane factories. All European cane refineries use this process.
Other advantages of Carbonatation, in comparison with Phosphatation are:
- utilization of less expensive chemicals;
However, Carbonatation presents some drawbacks as:
- boilers gas must be washed and neutralized, originating a great quantity of acidic
Moodley et al., 2002, in lab tests, observed that Carbonataion removes 93% of starch contained in feed liquors.
Bento L.S.M., 1996, Sugar colourants and ion exchange resins: Influence of
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