Refining of Raw Cane and Beet Sugars

REFINING OF RAW CANE AND BEET SUGARS This consists of three main operations: (I) dissolving the raw sugar in water; (2) decolourizing the resulting solution; and (3) re-crystallization.

Dissolving (or "Melting") the Raw Sugar.

This usually includes two operations. First, washing the raw sugar crystals to remove adhering molasses. Second, dissolving the washed raw sugar in hot water and filtering the resulting liquor to remove in soluble impurities. The raw sugar is mixed with syrup to form a magma which is fed into centrifugal machines (described above). These separate the magma into two parts:—(a) washed raw sugar crystals and (b) affination syrup or wash-syrup. The crystals are sprayed with water in the centrifugal to remove adhering wash-syrup, after which the machine is stopped and the washed raw sugar discharged. From ioo tons of raw sugar (96% sucrose) about go tons of washed crystals (99% sucrose) are obtained by the separation of 10 tons of dissolved sugar and impurities in the wash-syrup. This is called the affincttion process.

The washed raw sugar then passes to large cylindrical vessels (melting tanks) each holding from 5 to 10 tons of sugar and suf ficient hot water to give a liquor containing about 6o% by weight of sugar when all has dissolved. The melting tanks are fitted with mechanical stirrers and perforated steam-coils for heating. The resulting solution is termed washed raw sugar liquor. It is dark brown in colour and contains numerous insoluble impurities in suspension. To remove these, it is filtered through cloth in leaf-filters, filter-presses or bag-filters (see article FILTRATION). The filtered liquor is transparent but strongly coloured.

Decolourizing the Liquor.

This is effected by the action of bone-char (q.v.) in char-filters, large vessels measuring from 20 to 5o ft. in height, and from 5 to Io ft. in diameter. The dark coloured liquor enters at the top, and percolates through the char in from 3 to 6 hours, the rate of descent being regulated by the in let-cock. In passing through the char, the liquor is completely de colourized and flows into a gutter leading to the first liquor tank.

After operating for several hours, the char begins to lose its power, and the treated liquor becomes slightly tinted, this being run into a second gutter leading to the second liquor tank. When the tint increases to a predetermined limit, the liquor-inlet cock is closed, and a cock admitting hot water is opened, this water displacing the tinted liquor remaining in the char, and gradually diluting it. When the outflowing tinted liquor begins to decrease in density it flows into t „o other receivers, one for light liquor until the density falls to 18° Beaume, and another for char water until the density falls to 0.5° Beaume. About 1 ton of water is required per ton of char, and the operation is called sweetening off. After further washing with hot water, which runs to waste, the char is dried and strongly heated in a char-kiln, thereby re storing its activity so that it can be used again. This cycle of operations is repeated several times a week for a few years; at the end of this time the char is of poor decolourizing power and is therefore discarded as "spent char." Crystallization.—The colourless first liquor is boiled and crystallized in the vacuum pan (Plate, fig. 5) in the same manner as cane syrup, but the size to which the crystals grow depends on the class of refined sugar to be made. A very large grain is re quired for crystal or coffee sugar; a medium grain for granulated; and a mixed small grain for cube (or loaf) and caster sugars. In each case, the product discharged from the vacuum pan is a mix ture of crystals and mother-liquor (massecuite) which must un dergo further treatment.

In the manufacture of

granulated, crystal and coarse caster sugars, the massecuite from the vacuum pan is treated as in the manufacture of white granulated sugar direct from cane and beet juices. The final operations of drying, sifting and bagging are shown in Plate, fig. 3. The moist white sugar leaving the centrifu gals is carried by the band-conveyor (Plate, fig. 5 [bottom]) to the raised extremity of the revolving sugar drier (Plate, fig. 3 [top] ).