Carbonatation - Refining

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:

- to form a precipitate of calcium carbonate that removes, after filtration, suspended solids;
- to remove by co-precipitation some high molecular compounds in solution;
- to form a filter aid, the calcium carbonate, that is, in some cases, the only filter aid in Carbonatation filtration;
- to neutralize organic acids in solution;
- to precipitate certain anions as phosphates, sulphates, oxalates and carbonates;
- to remove some anionic colourants by bonding to calcium ions at precipitate
  surface.

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;
- it provokes the precipitation of compounds as sulphates, phosphates and organic
  acids as oxalic and aconitic, that can form incrustations in the evaporation station;
- invert sugars are destroyed with consequent increase of liquor purity;
- the high alkalinity attained will provoke the hydrolysis of phenolic acids esterified
  with polysaccharides. This fact will decrease the affinity of the released phenols, to
  the sugar crystals.

However, Carbonatation presents some drawbacks as:

- boilers gas must be washed and neutralized, originating a great quantity of acidic
  liquid effluents;
- high consumption of electrical power in gas compressors;
- the presence of a great quantity of high molecular weight compounds will difficult
  the filtration;
- colour formation occurs due to invert sugars destruction;
- carbonated liquor presents a significant amount of calcium in solution, about 400
  ppm, that is an inconvenient for decolourization (calcium can form complexes with
  colourants, in alkaline sucrose solutions (Bento, 1996) decreasing ion-exchange
  decolourization efficiency), evaporation (incrustations) and crystallization (the
  presence of high molecular weight compounds in liquor can increase calcium
  presence in sugar crystals, increasing the possibility of sugar solutions turbidity
  (Cosmeur e Mathlouthi, 1999)).
- colourants and ash are returned to Affination, through sweet waters (James et al.,
  1986);
- a tremendous quantity of solid residues (mud) are produced.

Moodley et al., 2002, in lab tests, observed that Carbonataion removes 93% of starch contained in feed liquors.
Carbonatation removes 40- 45% of colour, using 0.9% of CaO. Floc percursors, proteins and polysaccharides, are also removed (Moodley et al. 2002).

Bibliography

Bento L.S.M., 1996, Sugar colourants and ion exchange resins: Influence of
               calcium and sucrose in sugar colourants removal from ion exchange
               resins, Proc. of S.P.R.I. Conf., 121-136
Bento L.S.M., 1999, Study of colour formation during Carbonatation in Cane
               Sugar Refining using GPC with a ELS detector, Proc. of A.V.H. Conf.
James A., M.A. Clarke, R.S. Blanco, Recycling of non-sugars in sugar refineries,
               Proc. of S.I.T. Conf., 225- 252
Moodley M., P.M. Schorn, D. Walthew, 2002, Investigations into Carbonatation,
              Proc. of S.I.T. Conf.

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