In the double carbonation process, the purification is effected in two ways. The carbonic acid, in uniting with the lime in the midst of the juice, forms carbonate of lime, which, on precipitating, carries with it a large quantity of organic matters. The scums of the first carbonation are thus very dark. Thc supply of carbonic acid is stopped when its further action would redissolve the colouring matters. In the second carbonation, the lime-boiling destroys the matters which resist the first carbonation. The excess of lime is finally removed by carbonic acid.
The apparatus and its manipulation may be described as follows. At the exit from the presses and the diffusors, the juice re,ceived either directly into the carbOnation-boilers, or into a tank communicating with a pump or monte-jus, for filling the carbonators placed at a higher level. Tbe monte-jus, pumps, and defecating- and clarifying-boilers will be found described under the section on Cane-sugar. The carbonating-boilers are of various forms, composed essentially of large rectangular tanks (Figs. 1310,1311), generally of greater depth than width. Around their circum ference, passes a steam-worm c of large diameter, to rapidly heat the /nags of liquor. At the bottom of the tank, rune a pipe a, which separates into two branches, or takes the form of the tank. This pipe is pierced beneath with small holes, whose total area is less than the section of the pipe ; the end, it rises in front of the boiler, and bears a tap u within the operator's reach. It then con ducts to the carbonic acid source, and serves for introducing this gas into the liquor. The bottom of the boiler is inclined towards the front, and has in the lowest part a large plug d, or a tap for rapidly drawing off the liquid. Thermometers are attached for ascertaining the temperature. The boilers are the same for the first and second car bonation, except that the flrst produces a tena cious scum which must be beaten down. This is effected in two ways : either by furnishing the boilers with ledges, and a cover provided with a long chimney, when the scum stops at a small height in this pipe ; or by placing at the top of the boiler, throughont its whole length, perpen dicularly to the side where the workman stands, and on each side, two pipes of small diameter pierced laterally with little holes, through which steam is passed at high pressure. The escaping
steam blows the ecum back into the boiler. This latter apparatus, termed " Evrard's skimmer," works well, but requires much steam.
Below each carbonating-tank, is placed a decantation-vessel, generally of the same form and dimensions, into which the liquid flows when let ont of the carbonators by the plug. These decantation-vessels, whose floor is also inclined and furnished with a plug, have in front an external tap, connected inwardly with a flexible tube provided with a float which maintains the mouth of the tube at the clear surface of the liquor. When the turbid carbonated juic,e has been run into these vessels, it is allowed to settle and clarify itself, and is then decanted. The clear juice is received in a conduit which conveys it to the second carbonation, or to the filtration. When the float reaches the deposit, the workman closes the tap, opens the ping, and lets out the semi-solid mass into a trough connected with the filter-presses (p. 1848). In some works, the decantation-vessels are dispensed with, the operations being conducted in the carbonator.
The lime and carbonic acid employed in the operations are usually made at the factory. With this object, a large coutinuous lime-kiln, Fig. 1312, is bnilt near. The gases escaping from the calcination of the limestone contain 25-30 per cent. of carbonic acid gas; they are drawn away from the exit of the kihi by the suction of a larg,e pump, whose speed is regulated according to the state of the kiln. This pump forces the gas into the general pipe serving all the carbonators, which pipe is furnished with a safety-valve. Between the kiln and the pump, the gas traverses a " washer," a vertical cylinder with perforated trays, entering at the bottom by a perforated pipe, and escaping at the top by the pipe f, while a stream of water, conveyed by the pipe Ay, falls in showers over the trays and comes into contact with the ascending gas. According to the richness of the gas, tbe kiln is regulated : it is thus necessary to make frequent tests of the gas. One of the most convenient instruments for this pelves° is that of Wigner and Harland. The lime to be used in defecating is first slaked in special tanks furnished with agitators. It is then diluted with sufficient water, carefully strained, and constitutes a milk of lirne having a density of 20°-25' B.