Always draw enough and only enough lime to last the factory until about the time of the next draw. In this way the lime will be drawn cold, which is much to be preferred to the drawing of the hot lime, as it will then be in the form of cold ashes and be burned thoroughly. It is only in extra ordinary cases that more than 10 per cent coke to rock is necessary, and more than that amount is dangerous, as it often culminates in fuzing the rock too much, and a consequent bridging of the kiln.
About half an hour before charging the kiln, the kiln-boss should look in at the peek holes above the top of the rock, and see whether the fire is coming through. If so, half an hour afterward would be the time for charging the kiln ; but, should the fire not have broken through, the time for charging should be delayed until the fire shows. On the con trary, should the top of the rock be red hot, the kiln should be charged at once. If, in looking into the kiln, one side be found hot and the other dark, the lower charging doors on the dark side should be opened, while the others should be closed, thus forcing the draft up through the dark side and producing an even fire.
The kiln should always be charged and run in conjunction with the gas analysis. Should the gas by analysis show a large percentage of oxygen, the pumps should be allowed to slow down ; should the analysis show a large quantity of carbonic oxid, the pumps should be speeded up. In drawing in this way,—that is, drawing simply enough lime to keep the house running,—one will find that the amount of gas will always take care of itself.
A rock having over one per cent of magnesium should be carefully watched, as it causes great trouble in the juices ; and a rock containing a large percentage of silica should be avoided, as it fluxes and helps to bridge in the kiln. A rock of 96 per cent carbonates, as a rule, is considered excellent.
At all times it is very desirable to load the kiln with nothing but uniform sizes of rock. No rock smaller than one-half the size of a man's fist should be put into the kiln, as the small pieces will tend to stop the draft ; and no pieces much larger than the size of the two fists should be used, as the very large rocks will not be burned through entirely. The top of the rock should alwa-s be four to six feet below the gas pipe in the kiln. The coke should
be of open grain, and at the same time be fairly compact, for then it will hold its heat during the required time to give the best results. The coke should be free from sulfur, as there is a possibility of sulfuretted hydrogen being generated, which, when carried into the carbonatation tanks, might do considerable harm.
The importance of care in addition of lime to the juices cannot be overestimated, and, if the liming is not done according to the various rules that have been applied only after years of experience, the succeeding operations will prove failures and the final product will be of little value as a marketable commodity. The changes which take place during the carbonatation are both mechanical and chemical. In the chemical nature, the lime forms compounds with the sugar and the impurities present in the juice. Some of these compounds are of complex combination, while others are very simple in their composition.
The gas.—The gas which is produced in the kiln and pumped into the tanks of this first carbonatation station has a great affinity for the lime, breaking up the compound it forms with the sugar and form ing an insoluble lime carbonate, thus setting the sugar free and leaving it in solution in the juice. The gas is pumped into the tanks through pipes which extend to the bottom of the tanks and are there divided into three sections. From each of these sections it passes through the perforations in the pipes and bubbles upward through the juice. The moment the gas comes in contact with the juice it causes a change to take place ; there is a thicken ing of the juice in proportion to the degree of con centration and the amount of sugar present. Dur ing this period there is no precipitation, but rather a gelatinous consistency, which decreases with the length of time that the juice is acted on by the gas. At the beginning of the carbonatation there is ex treme frothing, which gradually diminishes and finally ceases altogether. At this point the precipi tate forms, settling rapidly and easily, and is read ily filtered. The juice is then ready for filtration, and is pumped through the filter presses, leaving behind the heavy deposit, while the translucent yel lowish liquid passes on into the second carbonatation tanks.