A basic cane mill consists of three grooved rollers. Prepared cane is squeezed between the rollers, thus forcing the juice out of the fibre. The bagasse left in the last set of roller will be used for fuel to power the boilers to produce steam and generate electricity for use in subsequent sugar production process.
Juice from a milling tandem contains a large amount of cane fibre, dirt and unwarranted particles that falls out with the juice between the rollers of the mills. To make it lucid and clear juice the juice is poured over a wire-mesh screen. And then the juice is heated and lime is added to neutralise the natural acidity. It is then placed in a Clarifier which produces a clear juice that is light in colour and free of suspended matter.
The clarified juice is then put through multiple evaporators to extract up to 60–65% of water from it, leaving the juice becoming concentrated or syrup with 60–65 brix. Multiple effect evaporation is the system where juice is boiled in series in several vessels, with steam fed to vessel 1 only. Vapour from vessel 1 boils the juice in vessel 2, vapour from 2 boils the juice in 3, and so on until vapour from the final vessel goes to waste.
Sugar boiling is conducted under “vacuum pan” process in order to lower operating temperatures, but some small scale manufacturer of sugar “open pan” processes. The syrup produced by the evaporators is concentrated further in pans. As the concentration rises the dissolved sugar crystallises and the work of the pans is to grow sugar crystals in several steps to maximise the amount of sucrose recovered in raw sugar.
When the massecuite is discharged from the pans it is retained in crystallisers, where the sugar crystals continue to grow through cooling rather than boiling. The crystallisers are U-shaped vessels where massecuite is slowly rotated and allowed to cool. This technique is often referred to as “crystallisation in motion”. Rotation promotes even cooling of the massecuit, which helps to achieve uniform crystal growth. Uniform crystal growth can also be encouraged by placing granulated massecuit from a previous batch into the crystalliser with the fresh massecuit. This process is known as “seeding”.
A centrifugal is a machine that separates crystals from molasses. Centrifugation involves spinning massecuite in a perforated basket; centrifugal force acts on the molasses, forcing it through the perforations while the sugar remains on the basket wall. Water and steam may then be sprayed onto the crystals to wash off the remaining molasses.
In a drier, the moisture is driven off from the surface of the liquor layer covering the crystal by passing heated air around the sugar crystals. The product from the process described so far is a raw sugar (Brown sugar) that can be used as is, or sent to a refinery to be converted to a white (refined) sugar.
Bagasse is the final residual of the cane after it has underwent crushing/milling process. Nowadays, bagasse is used as fuel in generating electricity and steam the sugar production process. Furthermore, bagasse can be used as raw material for producing paper, insulating board, and hardboard, as well as furfural, a chemical intermediate for the synthesis of furan and tetrahydrofuran. The end product derived from sugar refining is blackstrap molasses.
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