Role of phosphoric acid in sugar process

Role of phosphoric acid 

Phosphoric acid occurs in the cane in two principal forms:

(a) Soluble phosphates of the juice

(b) In combination as protein in the cell material.

The latter compounds being insoluble, only the soluble phosphates take part in the defecation.

They react with the lime and form a precipitate which constitutes an important part of the floc produced in the juice by the lime.

It has been shown that the greater the amount of phosphoric acid in the juice, the easier

is the clarification. The mean phosphate content of normal juice is approximately 250mg P2O5 per litre.

Unfortunately, certain varieties of cane give a juice poor in phosphoric and which

is difficult to treat.

These juices are improved by adding to them artificially, before defecation, the quantity

of phosphoric acid by which they are deficient.

Action of phosphoric acid on the juice

The phosphoric acid added to the juice precipitates part of the colloids and colouring matter

which it contains.

The precipitate formed with lime is mainly tricalcium phosphate. This unfortunately is a

gelatinous precipitate and difficult to filter.

Advantages

The use of phosphoric acid improves the clarification of refractory juices.

It often permits of eliminating the sulphitation. In other words, it is possible, to a certain

extent, to replace sulphurous by phosphoric acid.

Considered from this angle, phosphoric-acid clarification is much less expensive than

sulphitation, does not require special equipment, avoids corrosion of plates and pipes, and reduces scaling at heaters and multiple effects.

However, phosphoric acid should rather be regarded, not as a replacement, but as an adjunct to the clarification, which should be considered when exceptional difficulties are encountered in subsidation or filtration.

Categories: Clarification