Manufacture of Cane Sugar.
—In the manufacture of sugar from the sugar cane the first process, naturally, after the harvest, is the expression of the juice from the cane. At the time of harvesting the canes are topped in such a way as to cut off the green portion of the upper part of the stalk and the leaves also are removed.
Fig. 76.—cane Field Partly Harvested.—(Photographed by H. W. Wiley.)
There are two methods of extracting the juice from the cane, one similar to that described for the sugar beet but used very little. Only one or two factories in the United States use this method of extraction. The most common method of extraction is by passing the canes through heavy mills. These mills are made of great strength so as to bear an immense pressure without breaking. The largest mills have a capacity of grinding from 500 to 1000 tons of cane a day. Many of them grind only from 200 to 500 tons per day. The mills are nearly always placed in series, that is, the cane is subjected to a double pressure. The first mill is uniformly composed of three rollers of the same size and set so that the first and second are not quite so close together as the second and third. The second mill also often consists of three rollers the same as the first mill but sometimes only two. Occasionally a third is used. It is quite customary to sprinkle the crushed cane as it comes from the first mill with water before it enters the second mill, thus securing a greater degree of extraction. The residue from the mill is called bagasse and is commonly carried directly to the furnace and used as fuel, furnishing steam, to evaporate the juice and drive the mill. The mills extract from 75 to 80 percent of the weight of cane in juice. The sugar cane contains about 88 percent of its weight of sugar juice. It is seen, therefore, that a considerable portion of the sugar remains in the bagasse. By the process of diffusion a larger proportion of the sugar is extracted than by milling, but the resulting juices are very much diluted and require a greater combustion of fuel for evaporation.
Clarifying the Juice.
—The juice as expressed from the cane is a dirty-looking mass and requires to be clarified before it is concentrated. It is a very common practice to subject the fresh juice to the fumes of burning sulfur. In all cases the first step in the clarifying is the addition of lime to neutralize the natural acidity of the juice and facilitate the coagulation of the dissolved matter. The limed juice is next subjected to heating and as the boiling point approaches a separation of the suspended and coagulated matter takes place, the light coming to the top and the heavy falling to the bottom. The common method of separating these bodies is by skimming the top coagulum and settling the bottom portion and drawing off the clear juice therefrom. In addition to this to get a more complete separation the heated juice may be run through a filter press.
The clarification of sugar cane juice, as is seen, is much more simple than that of beet juice. The method employed for the clarification of beet juice is sometimes used for cane juice but not very frequently.
Evaporation of Clarified Juice.
—After the clarification is completed the further treatment of the juice is exactly the same as that for the sugar beet.