The liming station is one of the most important stations in sugar factory and without correct liming, good clarification cannot be expected. The addition of milk of lime to the raw cane juice is a chemical treatment, and as in all chemical treatments, the correct procedure must be accurately followed.
Composition of Sugar Cane Juice
Raw sugar cane juice is composed of a great number of organic and inorganic compounds, acids, salts etc., in varying amounts. When it comes from the mill tandem, the juice is an opaque liquid varying in color from greenish-gray to dark green, and it carries suspended matter such as fine bagasse (bagacillo), gums, albumin, wax, coloring matter, particles of soil, sand, clay and muck. The normal raw cane juice has pH 5.2—5.5.
Sometimes cane juices are infected with Leuconostoc bacteria, which produce a gummy substance called dextran (C6H10O5 n. This gum is produced by the Leuconostoc fermentation and frequently occurs in sugar cane damaged by frost or insects. Leuconostoc bacteria develop very rapidly in cold alkaline juice and very slowly in a neutral or acid medium and they quickly become lodged in pipes, elbows and valves. Heating juices to the boiling point kills Leuconostoc.
Treatment with Lime
The gums, wax and albumin make the raw sugar cane juice rather viscous and it cannot, therefore, be readily filtered when cold. Liming and heating cause many impurities in the juice to become coagulated and precipitated out. At the same time, the acids are neutralized and any phosphates present are flocculated, adsorbing a large amount of coloring matter, colloids and other impurities. Usually, the lime is added to the raw sugar cane juice in the form of milk of lime, for better dispersion and quicker reaction.
Quality of Lime
In preparing the milk of lime it is more advantageous to use already prepared hydrated lime, rather than to burn limestone and slake it at the factory. The lime must be carefully selected. It should contain over 95% of Ca(OH)2 and not more than 1% MgO, and should be almost free of iron, aluminum oxides and sand. The lime should be finely ground and pass through a 400 mesh. Lime which meets these specifications will actually be more economical to use than a cheaper grade of lime, because in preparation of milk of lime of about 5°Baumé all the lime will be in suspension, and when dispersed in the raw cane juice it will react much faster with acids and compounds in the sugar cane juice.
Incorrect Use of Lime
Incorrect liming of juice tends to increase sucrose losses and reduce the recovery and sugar yield.
All the lime used in excess of the amount required to neutralize the acids and precipitate impurities has a destructive action upon reducing sugars, which are transformed into soluble lime salts which increase the color and viscosity of cane juices. In other chemical reactions, excess lime upon heating attacks reducing substances to produce acids which may further invert sucrose. . It also cause the formation of soluble calcium salts that will increase scaling in evaporators. These soluble salts will increase the viscosity of syrup, massecuite and molasses, affecting sugar recovery in the pans and increase the quantity of final molasses and sugar losses.
Commonly Poly acrylamide is used as a polyelectrolyte solution. Its use is 3-4 ppm on cane. It improve the flocculation, rapid settling of impurities and produce mud in smaller volume. It has no effect on pH. It should be noted that water used for making this solution should be of high quality i.e. condensate water.
Liming of Sugarcane Juice
The raw juice is heat up to 70-75 °C temperature in primary heaters for the purpose of destroying the Leuconostoc bacteria before liming, otherwise it creates an alkaline medium in which the bacteria develop rapidly. After liming, the juice is further heat slightly above its boiling point say 103-105°C before sending it into the clarifier.
Lime is used as lime sucrate. Lime sucrate is the 50% mixture of both heated juice and syrup. Lime sucrate has a better results for clarification than only lime.
However, heating the raw sugar cane juices to a high temperature prior to liming is dangerous, since raw juice has a low pH, and substantial inversion of sucrose can occur. Proper sanitation of mill tandem and periodical steaming will prevent infection of juice with Leuconostoc bacteria.