Carbonatation

Encyclopedia : C : CA : CAR : Carbonatation

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Carbonatation is a chemical reaction where calcium hydroxide reacts with carbon dioxide and forms unsoluble calcium carbonate:

[Ca(OH)_2] + [CO_2] [\rightarrow ] [CaCO_3] + [H_2O]

Concrete

It is a process that occurs in concrete where lime (calcium hydroxide) in the cement reacts with carbon dioxide from the air and forms calcium carbonate. Since carbonatation causes a lower pH (acidic) this may lead to corrosion of the steel reinforcing rods and damage to the construction.

Sugar refining

The carbonatation process is used in the production of sugar from sugar beet, It is the introduction of milk of lime (calcium hydroxide suspension) and carbon dioxide enriched gas into the "raw juice", the sugar rich liquid prepared from the diffusion stage of the process, to form calcium carbonate and to precipitate and remove impurities. The whole process takes place in "carbonatation tanks" and processing time varies from 20 minutes to an hour.

Carbonatation involves the following effects:

• The increase in alkalinity coagulates proteins in the juice.
• Calcium carbonate absorbs colourants
• Alkalinity destroys some monosaccharide sugars, mostly glucose and fructose

The target is a large particle that naturally settles rapidly to leave a clear juice. The juice at the end is approximately 15 °Bx and 90% sucrose. The pH of the thin juice produced is a balance between removing as much calcium from the solution and the expected pH drop across later processing. If the juice goes acidic in the crystallisation stages then sucrose rapidly breaks down to glucose and fructose; not only do glucose and fructose affect crystalisation but they are melassagenic taking equivalent amounts of sucrose on to the molasses stage.

The carbon dioxide bubbled through the mixture forms calcium carbonate. The non-sugar solids are incorporated into the calcium carbonate particles and removed by natural (or assisted) sedimentation in tanks.

There are several systems of carbonatation, named from the companies that first developed them. They differ in how the lime is introduced, the temperature and durations of each stage, and the separation of the solids from the liquid.

• Dorr (also Dorr-Oliver) - a continuous process using two tanks with recycling ("1st carbonatation") to build up particle size for natural flocculation. The recycling ratio is about 7:1. The particles are separated under gravity in a thickening stage in a vessel called a clarifier. The clear juice is then gassed further in another tank ("2nd carbonatation") and filtered. The concentrated mud (underflow) is filtered and/or pressed to recover more liquid. The Dorr process is low in maintenance and man-power but susceptible to frost damaged beet. It is favoured in the UK and the USA.
• DDS (Det Danske Sukkerfabrik - "The Danish Sugarfactory")-- multistage process involving pre-liming where the pH of the juice is gradually increased to start precipitation of proteins, followed by addition of further lime and gas. The particles are removed at each stage by filtration.
• RT (Raffinière Tirlemontoise) - another multistage process with a pre-liming stage. Particles also removed by filtration.

Both DDS and RT processes are favoured by European factories.

The clear juice from carbonatation is generally known as "thin juice". it may undergo pH adjustment with soda ash and addition of sulphur ("sulphitation") prior to the next stage which is concentration by multiple effect evaporation.

See also

• Phosphotation - a similar process used in sugarcane processing.

External links

• [Sugar Process at the Crystal Sugar website]
• [How Beet Sugar is Made]
• [Processing technology]

 

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