Lynne Wong's PhD thesis

acidic to neutral or slightly alkaline by the use of milk of lime, which causes coagulation of
some colloids, and forms a heavy precipitate of complex composition containing insoluble
lime salts, coagulated albumin and some of the fats, waxes and gums.
The phosphate content of the juice is the most important factor in efficient clarification. If
the cane is grossly deficient in natural phosphate, or is otherwise very difficult to clarify,
phosphate may have to be added before liming. Nowadays, liming techniques can
overcome most cane deficiencies, and satisfactorily clarify juice with natural phosphate
levels down to half or less of the normally accepted requirement of 300 ppm P 2 O 5
(Whayman, 1992).
When sugar solution containing soluble phosphate comes into contact with excess calcium
ions, an amorphous calcium phosphate is formed, which crystallizes as octa-calcium
phosphate and hydroxyapatite (Bennett, 1975). The two-stage precipitation first causes
small particles to form, which grow and rearrange into a very intricate floc that entraps and
adsorbs other non-sugars that are precipitated by the reaction change, by the heat, by the
calcium and by the increase in pH. The precipitate entraps most of the fine suspended
solids originally present in the juice. Bennett (1957, 1975) showed that the impurities are
bound by the mechanism of bridging by calcium phosphate precipitation (Fig 1.4).
Figure 1.4. Bridging mechanism formed by calcium and phosphate ions
during cane juice clarification (Bennett, 1975).
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