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Sugar from sugar cane

posted Jun 6, 2011, 9:33 AM by Moges Ashagrie   [ updated Jun 6, 2011, 9:38 AM ]

The bulk of sugar cane is cut by hand (manually) with a cane knife but sometimes mechanical cutting is also being practiced in some other countries. The cut sugar cane is then loaded to vehicles and transported to the mill. At the first end of the factories the cane is usually weighted, washed and chopped in to smaller pieces before the cane is feed to mills (Tandems) for juice extraction. 
Juice extraction is mostly done by passing the chopped cane through a series of rollers or horizontal mills. The rollers are laid in a triangle which are supported by a mill housing made of cast iron or cast steel.

The prime objective in sugar cane milling is to extract the greatest possible amount of sucrose from sugar cane, and to make the final bagasse as dry as possible so that it will burn readily in the boilers.

The tandems are a train of mills preceded by various combinations of cane preparation devices. The power required by the mills is obtained from steam turbines followed by gear boxes for speed reduction.

During the last few years diffusers have been installed in various sugar factories instead of mills. This new method of extracting sugar from sugar cane has proved advantageous from the technical point of view. Diffusion has been accepted as an efficient way of achieving high extraction. The capital investment and maintenance costs of diffusers are lower than those of mills but the moisture of the bagasse obtained here is high hence reducing boiler efficiency.

Next to this important process, juice extraction, the raw cane juice is weighted and carried to liming process. The fibrous part called bagasse is transported to furnaces for burning.

The liming station of the cane juice is one of the most important stations in a raw-cane sugar factory. 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 cane juice has  PH 5.2 – 5.4.

The gums, wax and albumin make the raw sugar juice rather viscous and it cannot, therefore, be readily filtered when cold. Liming and heating causes 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, solids 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.

The next process after liming of sugar cane juice is clarification. Without good clarification of sugar cane juice, the production of good quality raw sugar is impossible. The purpose of clarification is the precipitation and removal of all possible non sugars, (organic & inorganic) and the preservation of the maximum sucrose and reducing sugars possible in the clarified juice.

The greatest part of sugar cane consists of soluble inorganic compounds or ashes. A certain amount of fiber, mainly cellulose, also remains in sugar cane juice after crushing, which passes through the cush-cush screen in the form of bagacillo. The raw cane juice is generally limed to PH 8 in order to obtain clarified juice of about PH 6.8-7.2 clarified juice is concentrated to a syrupy consistency before it is sent to the vacuum pans to be crystallized in to raw sugar. The concentrate is made in several evaporators connected in series called a multiple effect. The juice travels from one vessel to another because of the gradual increase of vacuum. The vapors obtained in each body of the multiple effects serve to heat the calanderia tubes and to evaporate additional water in the following vessel.

And after being evaporated in a multiple effect evaporator to be a syrupy consistency, clarified juice must be evaporated further for the sugar to crystallize. This is accomplished in a vacuum to form a heavy mixture of crystals and mother liquor, called massecuite.

The raw sugar massecuite is then crystallized by cooling. On this process residual syrup incapable of crystallizing called black strap molasses is separated. Finally Batch & continuous centrifugals are used to separate the liquid and hard phases of raw sugar.