Lime and Carbon Dioxide clarify the Juice
The carbonation process is designed to separate approximately 40 percent of the soluble non-sugar materials produced by the sugarbeets, which were
extracted along with the sugar in the diffuser, from the high sugar content raw juice.
The juice is heated to denature the protein in the juice, and then mixed with slurry of calcium hydroxide, called milk of lime. Then, in a vessel
specially designed for the purpose, the mixture is treated with carbon dioxide gas bubbles. The quantity of gas used is that necessary to
precipitate the majority of the calcium hydroxide added as calcium carbonate and to lower the alkalinity of the resultant mixture to the degree
necessary for the resultant precipitate, called first carbonation sludge, to settle out rapidly.
The first carbonation sludge contains the precipitated calcium carbonate (PCC), a loose amorphous precipitate with a tremendous amount of charged
surface area. Many of the non-sugar materials are either absorbed onto or absorbed into this precipitate. When the mixture from the carbonation
vessel is pumped to another vessel to allow the precipitate to settle out, the settled precipitate contains a great deal of the non-sugars, both
soluble and insoluble, originally in the raw juice.
Filtering to remove sediment
The first carbonation sludge is pumped from the bottom of the settling vessel to the PCC filters, where the sludge is filtered and washed to
remove residual sugar. The dried PCC is then destined for future reuse or sale as soil amendment. The filtrate and washings from the filters,
called sweet water, are returned to the lime calcining operations area for the preparation of milk of lime. Excess sweet water is returned
to first carbonation.
The clear juice from the top of the settling vessel is again heated and then enters another carbonation vessel. There, sufficient carbon dioxide
is bubbled through it to lower the alkalinity to the point where all of the residual calcium hydroxide is neutralized and precipitated as calcium
carbonate. This is done to facilitate concentration of the juice without scaling the heating surfaces with calcium salts. The precipitate formed
is removed by filtration and the precipitate is mixed with the first carbonation sludge prior to its filtration and wash.
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