The first liquor from the filter appears cloudy and is sent back for refiltration, but it soon becomes bright, perfectly colorless and transparent as plate glass. This white liquor is pumped from the liquor gallery into the tanks on the top floor in the pan house, ready for the next process, which will be dealt with presently.
After a filter has been running for from twenty-four to thirty-six hours, depending on the character of the sugar in the liquor, the char becomes “tired” or spent. In other words, it has absorbed so much of the impurities and coloring matter from the liquor passing through it that its capacity to absorb more is gone and the liquor begins to show a slight straw or canary color. The inspector in the liquor gallery immediately notices this and orders the liquor stopped. Immediately afterwards a lower-grade liquor is turned into the filter, which forces the first liquor out before it. In due time the man at the liquor gallery notices the number two liquor coming from the filter and turns it into separate tanks. In time a still lower grade of liquor is turned on and the filter run until the bone-char is absolutely exhausted, when it is ordered “sweetened off.”
Hot water is then turned in at the top of the filter to wash out the remaining sugar liquor which gradually becomes more and more dilute. When its density has been lowered to about thirty-five per cent of solid matter, it is diverted to other tanks, and this is continued until only three-tenths of one per cent of sugar remains in the sweet water, as it is now called. The washing of the char in the filter in this manner, by hot water, is kept up for twelve hours, but as soon as the sugar content falls below three-tenths of one per cent the solution is allowed to run to waste, as the recovery of this small percentage of sugar would cost more than its value.
The sweet water is sent to the evaporators, concentrated to 58.6 per cent of solid matter, and it then begins its refining journey over again.
This long and continued washing of the filters is for the purpose of removing as much as possible of the organic and mineral impurities absorbed by the char.
The washing completed, compressed air is applied to the filter to force out the remaining water. The bottom doors of the filter are then opened and the char, containing about twenty per cent of water, drops to the floor below. Here it passes through mechanical driers and is delivered comparatively free from moisture to the kilns. There it is revivified, that is, the organic matter in the char which could not be removed by washing is converted into carbon by being heated to a cherry red in the absence of air. This is accomplished by allowing the char to pass by gravity through the red-hot retorts of the kilns.
As the wet char leaves the filter, it drops on a moving belt which carries it to large cast-iron hoppers leading to the driers immediately beneath, where the greater part of the moisture is expelled from the char prior to its being treated in the revivifying kilns. The driers are made up of a number of thin, hollow, cast-iron, triangular pipes, enclosed in a large, rectangular, outside casing. The wet bone-char passes over these hollow pipes as it falls slowly through the drier. The hot gases from the furnaces of the kilns below pass through these cast-iron pipes, giving off heat as they ascend, thus driving off the moisture in the char as it falls down over the outer surface of the pipes. At the same time, hot air obtained from cooling the char in the cooling pipes below the retorts is drawn through the drier, coming in direct contact with the char. The moisture given off by the char is absorbed by this hot air and carried out of the drier and building by fans or smokestacks. By this means the water in the char is reduced to ten per cent, and in this comparatively dry, hot state it runs freely by gravity from the bottom of the drier into a second set of hoppers, through which it drops into the retorts of the kiln. The hot gases, after drying the char, pass out at the top of the drier through a flue leading to a stack outside the building.
TOP OF CHAR FILTERS—SHOWING PIPE CONNECTIONS
