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at the center, while slightly to the left is the condensing column, which extends through the roof. At the top of each of the condensing columns of the factory in which these photographs were taken is a small covered platform easily seen above the roofs in Fig. 43
The process of manufacture described above yields "raw sugar," which is usually from 95 to 98 per cent pure. The removal of the remaining impurities constitutes the " refining" of the sugar and is usually carried on in places where fuel is more abundant than in the tropical countries where the sugar cane is chiefly cultivated, since about 25 pounds of coal are consumed in refining 100 pounds of sugar. The difference in price between raw and refined sugar is usually 0.7 to 0.9 cent per pound and the cost of refining is estimated at 0.6 to 0.65 cent per pound, leaving a margin of profit so small that it is necessary for the operation to be conducted on a large scale in order to make it remunerative. In the United States the industry is carried on in a relatively small number of large establishments in or near the principal ports on the Atlantic and Pacific coasts.
Nearly all of the three million tons of sugar brought into the United States annually is refined in about 20 establishments. Thus the average output of the refineries now in operation is about 1,000,000 pounds of sugar per day each, some establishments having a much larger output than this.
In principle the refining process consists in washing off as much as is practicable of the molasses which adheres to the crystals of raw sugar, then dissolving the crystals, purifying and decolorizing the solution as thoroughly as possible, and recovering the sugar in a purified state by recrystallization. While the process is simple in principle, the large scale upon
1 Home, School of Mines Quarterly, April, 1911.
which it must be carried out and the extreme precautions necessary to guard against apparently small losses if the industry is to be economically successful require elaborate equipment and constant chemical control.
As the sugar, either in granular form or in solution, is passed through a number of operations in a continuous stream, it is found advantageous to build the refineries several stories high, so that after the first lifting of the material its transportation from place to place for the successive steps of the processes may be effected chiefly by gravity.
The raw sugar is usually carried to the top of the building by means of a bucket elevator and washed by mixing with a small amount of sugar sirup and then separating in centrifugals, the sugar in the centrifugal being sometimes sprinkled with a little water for further purification.
After this washing the sugar usually has a purity of about 99, i.e. of the total solids in the moist sugar about 99 per cent is sucrose.
The sugar is then dissolved in hot water, this step being technically known as melting the sugar. The "melting" process is accomplished by running the sugar into water contained in steam-heated pans, the proportions and the heating being so regulated as to obtain a solution of 280 to 300 Baume and a temperature of 150° to 1700 F. A higher temperature might result in darkening the solution by slight decomposition of some of its constituents.
The hot solution then goes to the blowups for clarification, which is accomplished by adding a very small amount of acid calcium phosphate and then enough milk of lime to make the mixture neutral or very faintly alkaline. The precipitate thus formed carries down such impurities as gums and proteins, as well as suspended particles, and also removes a part of the coloring matter. The precipitate is removed by running the liquid through Taylor filters, which consist of twilled cotton bags about six feet long encased in strong, coarse-meshed hempen sheaths. A single filter box may contain 400 or more of these bags, each attached to the filter head by means of a metal bell and socket.
An alternative method is to mix the liquid with diatomaceous or infusorial earth (" kieselguhr," " celite ") and then filter by means of a filter press, usually of the " shell plate " type.
The filtrate from the bags is clear but not colorless. Most of the color is removed from this filtrate by passing it through boneblack filters. These are large, strong iron cylinders, often 10 feet in diameter and 20 to 30 feet high, filled with boneblack through which the sugar solution flows very slowly, usually at about the rate of one foot per hour. On account of the immense amounts of boneblack required in a modern refinery, this part of the process requires very careful control in order to use the boneblack or " char " as economically as possible. Freshly charred boneblack removes the color from the sugar solution almost completely, but with accumulation of impurities in the pores of the char it naturally becomes less effective until finally the filtrate shows so much color that it must be re-treated and the boneblack must be washed and sent to the " char house" for reburning. Every reburning or "revivifying" leaves the pores of the boneblack somewhat clogged by the added carbon from the absorbed impurities, so that after 10 or 12 reburnings it is no longer economical to use. In the Weinrich oxidizing revivifier the reburning is carried out with a limited supply of air designed to burn out the carbon of the impurities but not that of the original char, and thus to prolong the usefulness of the boneblack.
The sugar solution which has passed the boneblack filter, and is both clear and practically colorless, is evaporated in vacuum pans of 1000 to 2000 cubic feet capacity, wherein the sugar solution is " boiled to grain " and concentrated to a low water content. In order to accomplish this satisfactorily a vacuum is first created in the pan, some sugar solution admitted, and steam then passed through the heating coils and the solution concentrated until supersaturated. The exact point to which the concentration should be carried is determined by an experienced workman, who withdraws samples from the pan by means of a "proof stick," which is a long brass rod sliding through an air-tight fitting in the side of the pan and carrying a cup-like depression by means of which a small sample of the liquid in the pan can be removed without disturbing the vacuum. The test portion thus withdrawn from the pan is examined by drawing between the thumb and finger, and when the exact degree of viscosity necessary to insure the immediate production of " grain " is found, more of the sugar solution is admitted to the pan, thus chilling its contents and starting the crystallization, which is then continued as in the corresponding operation of raw sugar production described above, until the pan is charged with a magma of crystals and mother liquor, which is then dropped into the mixer on the floor below.
In the mixer or crystallizer the mass is thoroughly stirred while cooling and is then allowed to fall into the centrifugals, where the mother liquor, usually known as refinery sirup rather than molasses, is thrown out through the perforated walls of the rotating drum, leaving the mass of crystals, which is sprayed lightly with water for the further removal of the sirup, and usually with a solution of ultramarine or "permitted" blue dyestuff in order to offset the tendency toward a slightly yellowish color due to the very minute trace of mother liquor which still adheres to the crystals.
The washed sugar from the centrifugals is either barreled directly as " confectioner's sugar," pressed into cubical or domino form, or sent to the granulator to be made into the ordinary granulated sugar of commerce.
The granulator is a long inclined revolving cylinder heated by a current of hot air and provided with paddles to keep the