Brownsugarexporter.com - Sugarcane processing, outlined in
Figure 2, is practiced in many variations, but the essential process consists
of the following steps: extraction of the cane juice by milling or diffusion,
clarification of the juice, concentration of the juice to syrup by evaporation,
crystallization of sugar from the syrup, and separation and drying of the
crystals.
JUICE
EXTRACTION
After
weighing, sugarcane is loaded by hand or crane onto a moving table. The table
carries the cane into one or two sets of revolving knives, which chop the cane
into chips in order to expose the tissue and open the cell structure, thus
readying the material for efficient extraction of the juice. Frequently, knives
are followed by a shredder, which breaks the chips into shreds for finer cane
preparation. The chipped (and shredded) cane then goes through the crusher, a
set of roller mills in which the cane cells are crushed and juice extracted. As
the crushed cane proceeds through a series of up to eight four-roll mills, it
is forced against a countercurrent of water known as water of maceration or
imbibition. Streams of juiceextracted from the cane, mixed with maceration
water from all mills, are combined into a mixed juice called dilute juice.
Juice from the last mill in the series (which does not receive a current of
maceration water) is called residual juice.
The
alternative to extraction by milling is extraction by diffusion. In this
process, cane prepared by rotating knives and a shredder is moved through a
multicell, countercurrent diffuser. Extraction of sugar is higher by diffusion
(an average rate of 93 percent, compared with 85–90 percent by milling), but
extraction of nonsugars is also higher. Diffusion, therefore, is most used
where cane quality is highest—e.g., in South Africa, Australia, and Hawaii.
Occasionally a smaller “bagasse diffuser” is used in order to increase
extraction from partially milled cane after two or three mills. (Residual cane
fibre, after juice is removed, is calledbagasse.)
Disposal
of the large amounts of water used by diffusers is a costly environmental
problem, as cane factories that practice diffusion must operate their own
primary, secondary, and tertiary water-treatment systems.
CLARIFICATION
Mixed
juice from the extraction mills or diffuser is purified by addition of heat,
lime, andflocculation aids. The lime is a suspension of calcium hydroxide,
often in a sucrose solution, which forms a calcium saccharate compound. The
heat and lime kill enzymes in the juice and increase pH from a natural acid
level of 5.0–6.5 to a neutral pH. Control of pH is important throughout sugar manufacture
because sucrose inverts, or hydrolyzes, to its components glucose and fructose
at acid pH (less than 7.0), and all three sugars decompose quickly at high pH
(greater than 11.5).
Heated to
99°–104° C (210°–220° F), the neutralized juice is inoculated, if necessary,
with flocculants such as polyacrylamides and pumped to a continuous
clarification vessel, a large, enclosed, heated tank in which clear juice flows
off the upper part while muds settle below. This settling and separation
process is known as defecation. Muds are pumped to rotary vacuum filters, where
residual sucrose is washed out with a water spray on a rotating filter.
Clarified juice, meanwhile, is pumped to a series of three to five
multiple-effect evaporators. (BD)
