A sugar cane mill is a factory that processes sugar cane to produce raw sugar or plantation white sugar. Some sugar mills are situated next to a back-end refinery, that turns raw sugar into (refined) white sugar. The term is also used to refer to the equipment that crushes the sticks of sugar cane to extract the juice.
Production of raw sugar
There are a number of steps in producing raw sugar from cane:
Harvest and transport to the sugar factory Juice extraction (cane preparation followed by milling or diffusion) Purification of the juice (remove suspended solids from the juice, typically mud, waxes, fibres) Evaporation of water (to concentrate the juice to a thick syrup of about 65°brix) Crystallization Centrifugation (Separation of the sugar crystals from the mother liquor, done by centrifugal machines) Storage of sugar and molasses These processing steps will produce a brown or raw sugar. Raw sugar is generally sent to a sugar refinery 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. A cane sugar mill can also produce sugar that is suitable for direct domestic or industrial consumption. This is called plantation white sugar or mill white sugar, see below.
Harvest and transport to the sugar factory
The overall quality of raw sugar that goes into the factory is dependent on agricultural practices and the cultivar used. Harvesting can be done by machines or by hand. If done by hand, it is normally preceded by burning the field. However, stalks from a burnt field more quickly loose sugar content while waiting to be processed. Cane is transported by truck, narrow-gauge railway, container or cart. On arrival the cane is sold based on weight or sugar content. There are several ways to unload the harvest. Overall, limiting the time between cutting and milling is essential for achieving a high sugar yield and quality.
Juice extraction
Preparation Before the actual extraction of cane juice starts, the cane has to be prepared. This can be done by rotating knives or shredders. There are two modern types of processes for extracting juice from cane:
By tandem mills, and by Diffusion. The products of the extraction phase are:
Mixed juice, in case of milling Diffuser juice, in case of diffusion Bagasse In 2004 and 2005 the Enterprise Sugar mill in Louisiana had a traditional mill and a diffuser, which both processed cane from the same area. Weekly raw juice samples were taken and analyzed. These were found to be very similar, despite the diffuser achieving a higher extraction.
Tandem mills Juice extraction by milling is the process of squeezing the juice from the cane under a set of mills using high pressure between heavy iron rollers. Those mills can have from 3 up to 6 rolls; every set of mills is called a tandem mill or mill train. To improve the milling extraction efficiency, imbibition water is added at each mill. Hot water is poured over the cane just before it enters the last mill in the milling train and is recirculated up to reach the first mill. 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. To improve the milling extraction performance before the cane reaches the first mill, knife and shredder preparation equipment is normally used.
Diffusion Sugarcane diffusion is the process of extracting the sucrose from the cane by osmosis and lixiviation also known as leaching. There are two types of diffusers. One relies on immerging the mat of bagasse in the juice by counterflow. The other relies on percolation of the juice through the mat of bagasse. At a chemical level, the first step is to open the cells. This is usually done by revolving cane-knives and a three roller crusher, which together open most of the thin-walled cells. The juice is then removed from these opened cells by leaching. I.e. the sucrose from these opened cells dissolves in water. The diffusion process proper takes place on the 10-16% of sugar containing cells that have not been opened. First hot water is applied to kill the protoplasm of the cells. This makes that the walls of the cell becomes semipermeable. By osmosis, water or thinner juice can then enter the cell and replace heavier juice until an equilibrium is reached. In this phase sucrose penetrates the walls faster than non-sugar with higher molecular weight. This makes that the purity of the last extracted juice from diffusion higher than that acquired by straight milling, even while diffusing extracts more sugar. In the percolation system process, 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)
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