The use of the clarifier may be described in genoral terms as follows. The juice is raised to a temperature of 80° (176° F.), and sufficient milk of lime is added to neutralize the acid in the juice. The heat is then continued till a scum of impurities has risen to the surface, and com mences to crack. The time occupied in this should be about 10-12 minutes from the commence ment of the operation. The steam is then shut off, and the liquor is allowed to subside for 15-20 minutes, when the scum remains at the top ; some heavy matter will have fallen to the bottom, and between them will be the clarified cane-juic,e, clear and of a pale straw-colour. The clarification being complete, the two-way cock is first turned on to the smaller aperture, until the top scum begins to appear ; the cock is then turned to the large way, and the plug is taken out. The bottom sediment and top scum are conveyed to a cistern, whence they are placed in bags, and any juice remaining in is squeezed out, leaving only a small portion of solid matter behind.
Chemicals.—Of these, the most important and most widely used is slaked lime ; following it come bisulphite of lime, sulphurous acid, lead acetate, and sundry special compounds, as well as antiseptics.
Lime.—The effects of heating are greatly augmented by the simultaneous application of a strong alkaline earth, such as lime, which combines with the liberated acids, and with any carbonates present, and thus forms an insoluble precipitate, which carries down much of the impurities. But any excess of lime beyond what is required to neutralize these acids will re-dissolve the coagulated albumen, and preserve it in a state of solution, until the excess of lime is again neutralized by addition of acid. The operation, which is called " tempering," is thus obviously one of extreme delicacy. The first point to ascertain is the exact amount of lime required by a given quantity of cane-juices. A bottle containing exactly 250 septems (41,-, gal.) is filled with filtered cane-juice of known sp. gr., and the juice is transferred to a beaker over a spirit-lamp, and stirred occasionally with a glass rod till it boils ; after boiling for about a minute, clear saturated lime-water is poured in, a few drops at a time, till the juice shows a neutral reaction. The beaker is then taken off the lamp, and its contents are allowed to settle for a minute. If a coagulum of large flakes floats about in the transparent slightly-coloured liquid, and readily separates and subsides, the points of neutrality and of proper clarification coincide. The exact number of septems of lime-water used is then noted. If the floccules are small, awl do not readily separate, the juice if boiled would throw up scum, and is not properly clarified. More lime-water is added till the indications of proper clarification are attained. The juice will now have a deeper tint than if excess of lime had not been required. The total number of septems of lime-water being noted, the calculation as to bow much quick-lime is required for a given number of gallons of juice is :—If 250 septems of cane-juice required 20 septems of lime-water to render it neutral, and 10 rnore for clarification, then 30 x 40 = 1200, the number of septems of lirne-water that 1 gal. of juice would have taken ;
saturated lime-water at the temperature common in tropical boiling-houses contains 0.00862618 gr. of quick-lime, therefore 1200 x 0.00862618 = 10.351116 gr. of quick-lime required by 1 gal. of juice. As a rough rule, the proportion of 1 septem of lime-water to 250 septems of cane-juice ie nearly equal to 11 dr. of quick-lime to 100 gal. of juice. Hence the number of the test, multiplied by 11 dr. gives the weight in dr. of quick-lime required for 100 gal. of juice. This result is 1 per cont. too little. The test should be frequently repeated.
In the test, saturated lime-water is used, because it is easy to have it of uniform strength ; but on the large scale, to use lime-water would entail great dilution of the juice, and waste of fuel in the subsequent evaporation. Hence " milk " or " cream " of lime is resorted to. The lime used must be thoroughly burned, quickly slaked with clean water (enough water being used to impart a creamy consistence), and carefully filtered through a very fine wire sieve, to remove all fragments of flint and unburnt and unslaked lime. The weight of these impurities removed must be deducted frorn the amount of quick-lime originally taken. Quick-lime can only be kept in perfect condition in closed vessels. The juice being tested as to its density and acidity, and the milk of lime being prepared, the twin process of defecation and clarification commences.
There are several ways of carrying it into operation. One of the most simple is tbat known as " cracking." It necessitates the use of two or more clarifiers, and is conducted as follows. The strained juice is admitted into tho clarifier till sufficient has accumulated to prevent injury by beat. Fire is then made under the clarifier (or steam is admitted into the jacket or coil), and by the time it is full of liquor, the temperature will have risen to about 51° (130° F.). The " temper lime" is then thoroughly incorporated, and the beating continued. A thick greenish-yellow scum appears on the surface, and increases in thickness, changing colour frorn exposure to the air; at about 79°-82° (171°-180° F.), numerous minute air-bubbles form a frothy layer under the thick scum, by and by forcing their way at a few points through the scum, which soon cracks, and shows the bubbles. The beat is then quickly withdrawn, and the contents of the clarifier are allowed to rest for 15-30 minutes or more. Ebullition is avoided, because it would break up the floating scurn and diffuse it through the mass. The time allowed for settling depends OD the nature of the juice and the proper apportioning of the lirne. After settling, there is a layer of coagulum at top, and a precipitate at bottom, while the body of the liquor is bright and transparent, with a more or less deep sherry-tint, and minute flakes floating thick in it. If it is hazy, the beat has not been enough to clarify, or the lime has not been sufficient. After standing, the clear liquor is run off into the filter, and thence to the evaporating apparatus ; the scum aud sediment, with the considerable quantity of juice that invariably accompanies them, are usually run off to the skimmings-cistern, to be used in setting up liquor for rum (see p. 228).