Sugar mill

A sugar cane mill can refer to a factory that processes sugar cane to produce raw or white sugar. It can also mean the piece of equipment that crushes the sticks of sugar cane to extract the juice.

There are a number of steps in producing raw sugar from cane:

These processing steps will produce a brown or raw sugar. Mill white sugar also known as plantation white sugar can be produced by introducing some form of colour removal process (often sulphitation) between the juice clarification and the juice evaporation stages mentioned above. The raw sugar produced is often refined to produce white sugar. This sugar refining can be done either at a completely separate factory or at a back-end refinery which is attached to the raw sugar factory.

There are two processes for extracting juice from cane:

Juice extraction by milling is the process of squeezing the juice from the cane under high pressure between heavy iron rollers. Imbibition water is used to improve the extraction efficiency of the milling process: Hot water is poured over the cane just before it enters the last mill in the milling train. The juice squeezed from this cane is low in sugar concentration and is pumped to the preceding mill and poured onto the cane just before it enters the rollers, the juice from this mill is the same way pumped back up the milling train. Mixed juice (that is to say cane juice mixed with the water introduced at the last mill) is withdrawn from the first and second mills and is sent for further processing. Milling trains typically have four, five or six mills in the tandem.

Sugarcane diffusion is the process of extracting the sucrose from the cane with the use of imbibtion but without the squeezing by mills. Shredded cane is introduced into the diffuser at the feed end, Hot water is poured over the shredded cane just before the discharge end of the diffuser. The hot water percolates through the bed of cane and removes sucrose from the cane. This dilute juice is then collected in a compartment under the bed of cane and is pumped to a point a little closer to the feed end of the diffuser and this dilute juice is allowed to percolate through the bed of cane. At this point the concentration of sucrose in the cane is higher than the concentration of sucrose in the dilute juice just mentioned and so sucrose diffuses from the cane to the juice, this now slightly richer juice is pumped back up the diffuser and the process is repeated, typically, 12 to 15 times (compared with the four to six times for the milling process)

Sugar cane juice has a pH of about 4.0 to 4.5 which is quite acidic. Calcium hydroxide, also known as Milk of lime or limewater, is added to the cane juice to adjust its pH to 7. The lime helps to prevent sucrose's decay into glucose and fructose. The limed juice is then heated to a temperature above its boiling point. The superheated limed juice is then allowed to flash to its saturation temperature: this process precipitates impurities which get held up in the calcium carbonate crystals. The flashed juice is then transferred to a clarification tank which allows the suspended solids to settle. The supernatant, known as clear juice is drawn off of the clarifier and sent to the evaporators.

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