Harvesting Cane Sugar Plants
Sugar cane plants are grown in tropical climates until maturity when the stalks achieve a three to five centimeter diameter and stand two to three meters tall. They are then cut either manually or with a mechanical harvester and trucked to a mill. The chopped leaves and tops are left in the field. At the mill, the stalks are removed from the trucks with cranes and dropped into mill feed shoots. Whole trucks can also be lifted to dump the load of cane stalks into a feed hopper. As cane shoots feed the stalks toward the mill, a mechanical leveler rakes the stack of cane stalks to a more consistent height. The cutting knives then shred the stalks into small fiber strips.
The strips are fed to a series of four to six cane crushing mills which squeeze the sugary juice out of the cane stalks and further reduce the size of the fibers. Water at 70 °C is added countercurrent to the fiber movement during the milling operation to assist the extraction of the sugar from the cane stalk fibers. The resulting mill juice has a sugar concentration of approximately 17 °Brix. The remaining fiber, called bagasse, is sent to the boiler where it is burned to produce steam and electricity for the mill. The bagasse can produce most, all, or even excess energy for the mill's needs.
Heating and Clarifying Brix Mill Juice
The 17 °Brix mill juice is heated to 105 °C and mixed with lime, flocculants, and CO2, or SO2 and H3PO4, then sent to a clarifier to start the clarification process. In the clarifier, the lime destroys non-crystallizable invert sugars and amino acids. The lime and CO2 or SO2 and H3PO4 combine to form a carbonate or phosphate precipitate which attracts color and other impurities.
The color-laden precipitated solids sink and are discharged from the bottom of the clarifier, while the clarified juice overflows from the top. Alternatively, the clarifier may use air to float the floc to the surface where it is skimmed off while the clarified juice flows out the bottom.
The clarified juice passes through pressure or vacuum filters to remove any residual colored precipitate. The filtered juice, now at 14 °Brix due to some dilution during the lime defecation process, passes through a multiple-effect evaporator to raise the concentration to approximately 70° Brix. The concentrated syrup is fed to the vacuum pans where further evaporation and supersaturation of the sucrose solution occurs. Crystallization of the sucrose occurs in the first vacuum pan and the resulting mixture of sugar crystals and syrup, called massecuite, is dropped into a centrifuge.
Cane Sugar Crystallization
The centrifuge spins the mixture at high speed to separate the lighter colored sucrose crystals from the darker colored syrup. The sucrose crystals are retained by centrifugal screens. After the dark syrup is spun off, the raw sugar is dropped out of the bottom of the centrifuge.
The tan, raw sugar crystals from the first (A pan) crystallization are either sent to an attached cane refinery for further purification or they are dried and shipped to non-associated refineries. The machine syrup, still containing a large amount of sugar, but at lower purity and higher color, is sent to the B vacuum pan for further crystallization of sucrose. The B pan sugar recovered is mixed with the concentrated syrup fed to the A pan. The B pan machine syrup is fed to the C pan. Then, the C pan sugar is added to B pan feed. C pan machine syrup is fed to a crystallizer. Crystallization of sucrose becomes more difficult as the impurity level in the syrup rises. In the crystallizer, the sucrose is further reduced until the remaining syrup contains only about 50% sucrose. This syrup, at 80 °Brix, is called blackstrap molasses and is sold as an animal feed additive.