Sugar

In fig. 4 the level of the boiling juice in No. 2 is maintained constant at B by drawing in juice from No. I ; similarly, the level in No. I is maintained constant at A by drawing in juice from an external supply-tank. At the same time, concentrated juice, or syrup, is drawn off continuously from No. 2 by a pump, and at such a rate that the density of the syrup in No. 2, and of that drawn off, remains constant at about 30° Beaume. This syrup contains from 55 to 6o% of dissolved solids (including sucrose). It is, therefore, an unsaturated solution, free from crystals.

Crystallization of the Syrup.

The final stage of evapora tion, accompanied by the crystallization of the syrup, is carried out in the Vacuum Pan. This apparatus is shown in the Plate, fig. 5 (centre). Part of the interior, with steam-heated tubes, is shown on the left. Above, and to the right, is the condenser (shown in section) wherein the vapour generated by the boiling syrup is condensed by sprays of water. A vacuum of about 26" is main tained inside the pan by the combined action of the condenser and air-pump (to right of pan), thereby lowering the boiling point of the syrup to about 13o° F. Cane and beet sugar factories are equipped with three or four pans, each discharging from 10 to 20 tons of crystals after each boiling operation is completed.

Sufficient syrup is first drawn into the pan to occupy only a part of its total capacity, and steam is admitted to the coils which are covered by the syrup. After a period of rapid boiling, the syrup becomes supersaturated and microscopic crystals begin to form. This is called the graining point, and it is important to produce a sufficient quantity of grain as rapidly as possible, in order that the crystals may be of nearly uniform size during all stages of growth. Having formed the grain, the operator reduces the supersaturation by drawing in small charges of unsaturated syrup from the supply-tank, at suitable intervals, thus preventing further formation of grain but causing the previously formed grain to grow until the pan is filled in from 3 to 4 hours. To make a large-grained final product, the operator starts with a smaller volume of syrup, yielding a proportionally smaller quantity of grain, thereby increasing the rate of growth of each grain. Con versely, to make a small-grained final product, he starts with a larger volume of syrup, yielding a larger quantity of grain, thereby reducing the rate of growth of each grain.

The time available for growth can be extended by transferring half the contents of a full pan to a second, empty pan, the two halves being then boiled simultaneously with further additions of syrup to each, causing continued growth of the original grain until both pans are filled. During the process of boiling, the operator is guided by the appearance of small samples of the crystallized syrup which he removes from the pan at short intervals by means of a proofstick. The samples are spread upon a sheet of glass and examined by transmitted light from a lamp. When the pan is full, the mixture of crystals and mother-liquor (called massecuite) is finally concentrated until it contains from 9 to of water. It

is then semi-fluid and contains about 65% by weight of crystals and 35% of mother-liquor. Air is now admitted to the pan, the bottom gate opened, and the massecuite discharged.

Separation of Crystals from Mother-liquor.

This opera tion, called curing the sugar, is effected by centrifugal action in a number of machines operating simultaneously. Each consists of a vertical metal drum, or "basket," which can be rotated on a central vertical spindle at a speed of Soo to 1,200 revs. per min. The cylindrical wall of the drum is made of perforated steel plate and fitted with a lining of woven copper-gauze. After the drum is set in motion, a charge of massecuite enters through a central opening at the top, and the centrifugal action forces the massecuite against the wire-gauze liner which acts as a strainer, allowing the fluid mother-liquor to pass through but retaining the crystals. The separation is complete in about 2 mins. The plate shows the positions of the vacuum pan, massecuite-receiver, centrifugals and band-conveyor.

In the manufacture of raw sugar, the above treatment yields sugar containing from 95 to 98% of sucrose and about 1% of water. The colour, moisture and odour of this sugar are due to residual mother-liquor adhering to the crystals and which cannot be completely separated by centrifugal action alone. The band conveyor (Plate, fig. 5) carries the sugar from the centrifugals to a storage-bin from which the sugar gravitates through chutes into bags placed upon weighing machines.

In the manufacture of white sugar, the separation of the mother liquor is carried much further by spraying water over the layer of sugar in the centrifugal drum whilst this is rotating at full speed. Indeed. two sets of centrifugals may be employed. To conceal any yellowish tint in the sugar, this is sprayed with ultramarine and water (called blueing the sugar) before the machine is stopped. After passing through a rotary drier (described later) the dry white sugar contains from 99.5 to 99.9% of sucrose.

The mother-liquor separated in the centrifugals contains a high percentage of sucrose. It is reboiled in the vacuum pan where it crystallizes, forming second grade massecuite. This is treated in centrifugals where it separates into second grade crystals and second grade mother-liquor. The last is reboiled in the vacuum pan to form third grade massecuite which is discharged into large cylindrical crystallizers, each fitted with a slowly rotating central shaft carrying stirrers (fig. 5)• The mother-liquor, from which no more sucrose can crystallize, is called molasses. It is a highly concentrated solution containing sucrose and various organic and inorganic impurities derived from the original cane juice.

By-products.

These include: (a) the crushed cane (bagasse or megass) which is used as fuel; (b) filter-press cakes, which are used as fertilizer; (c) molasses, which is used for the production of alcohol, or as fertilizer, cattle-food, or fuel.