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Sulphitation - Clarification

Raw juice received from the Mills is turbid in appearance and contains besides suspended impurities of cane origin like bagacillo, soil and fine particles of Bagasse, a number of impurities either in dissolved State or in colloidal condition. To make good quality pure sugar it is necessary to eliminate maximum quantum of non-sugar impurities from the juice. This is achieved by treating the juice by heat and clarifying agents that results in formation of precipitate which when separated yields transparent clear juice. This process is called clarification and the main objectives here are:

Elimination of suspended impurities and colloids.
Removal of maximum amount of non-sugar component so as to render the subsequent crystallization free from complications for producing white sugar and bring down losses in final Molasses.
To reduce colour precursors and colourants, and imparting clarity and transparency to Juice.

The juice from the Mills is passed through the primary juice heaters, prior to Sulphitation reaction tanks, objectively to reduce its surface tension to facilitate better reaction in Sulphitation and secure solubility of CaSO3 / Calcium Compounds in juice to minimize, aiding the process of precipitation of impurities at the clarifier. The heated limed juice passed through Sulphitation is neutralized by SO2 to maintain its pH. Low pH (below < 7) results in decomposition of sucrose into reducing sugars by hydrolyzing where as high pH (above > 7) results in decomposition of reducing sugar to be avoided since:

Organic acids so formed increase calcium content of juice.
The salts of organic acids increase scaling of evaporator.
Dark coloured compounds so formed increase colour content of juice.
Exhaustibility of Molasses is adversely effected.

Thus as far as possible pH close to 7 should be maintained and the time of contact at high or low pH should be minimum. The mixed juice after Sulphitation is pumped into clarifier tanks where it is allowed to settle and its impurities precipitated and removed to get clear juice. The salient characteristics that need to be checked and maintained during clarification process are:

Rise in sucrose purity from raw juice to clear juice to indicate degree of non-sucrose removed, so as to bring down the formation of Molasses.
pH should remain neutral close to 7.0 so that sucrose remain intact and un-decomposed.
Reducing sugars with their vital role in bringing down sucrose retention in final molasses, should as far as possible not be destroyed and maintained at same level from mixed juice to clear juice.
All chemicals employed like Lime, Phosphate, Sulphur, Settling aids shall be pure so that there is no unwarranted increase in the inorganic salts that results in further undesirable scale formation in evaporator.
CaO content in clear juice is function of :
1. Initial CaO content in raw juice.
2. Organic acids present in the original juice.
3. Acids formed during clarification process.
The increase / addition in CaO content in clear juice from mixed juice should be minimum and is attributable to the neutralization of acids present in the juice with the formation of Organic acids salts of calcium that have not been removed during clarification.
Lime, the principle reagent of clarification, by its addition, elevates the total constituents of total dissolved solids (T.D.S.) and thus viscosity index values, for juice/ syrup down the process. This is contrary to desirable feature of reduction in viscosity of juice/syrup for faster and smoother process. Thus it is necessary that lime consumption (and proportionately Sulphur consumption) is kept at minimum without compromising on the brilliancy (light golden colour), clarity and stability of clear juice.

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