DIFFUSION BATTERY—SHOWING DIFFUSION CELLS IN STRAIGHT LINES
After being thoroughly agitated, the mixture is treated with carbonic acid gas obtained from the lime-kilns, as the result of the decomposition of limestone and the combustion of the fuel used for burning it.
By this process some of the impurities in the juice are removed and the color reduced to a brilliant amber. As is the case in the cane mills and refineries, it is essential to keep the juice hot throughout the process. The carbonation is continued until the juice is only slightly alkaline, when it is passed through filter presses for the removal of the precipitated lime carbonate and other solid matter. The solid matter in suspension is retained in the frame of the press, and, as soon as the frame is full, the cake is washed by passing water through it. When the sugar content of the cake has been sufficiently reduced, the press is opened and the cake discharged and sent to the fields to be used as a fertilizer.
As a rule, the filtration is repeated for the elimination of any solids that may have passed through the first filtration. This double filtration is usually practiced in all the filtrations in the course of the juice through the factory.
The juice after being filtered a second time is again treated with carbonic acid gas for the further reduction of the caustic lime and then undergoes a third filtration, following which it is sent to the evaporators for concentration.
CONCENTRATION OF JUICE
When the juice reaches the evaporators it contains about eighty-two per cent of water, which, by concentration, must be reduced to about forty per cent. As already explained, the removal of water is generally accomplished in multiple-effects. The apparatus consists of a number of boiling bodies connected in such a manner as to secure a progressive decrease in atmospheric pressure.
The thin juice enters the first body where evaporation takes place under a slight pressure. The steam for this evaporation is usually the waste or exhaust from the engines and pumps. The vapors generated by the evaporation of water from the juice in the first body enter the heating tubes of the second body and are used in further concentrating the somewhat concentrated juice from the first body. The evaporation in this second body is conducted at a higher vacuum and corresponding lower temperature than in the first body. This proceeding is continued until five or even six bodies are used in the series. The last body is usually under a vacuum of about twenty-six inches of mercury.