Sugarcane mill
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 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
- Purification of the juice
- Evaporation of water
- Crystallization
- Centrifugation
- Storage of sugar and molasses
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.
- Mixed juice, in case of milling
- Diffuser juice, in case of diffusion
- Bagasse
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 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
Purification of the juice
The mixed juice has a pH of about 4.0 to 4.5 which is quite acidic. During purification, calcium hydroxide, also known as milk of lime or limewater, is added to the cane juice to adjust its pH to about 7 or 8. This can be done while the juice is still cold or after it has been heated to about 104 °C. It can also be done in phases.The lime helps to prevent sucrose's decay into glucose and fructose. The superheated limed juice is then allowed to flash to its saturation temperature: this process precipitates impurities, which get held up in calcium carbonate crystals. The flashed juice is then transferred to a clarification tank.
In this clarification tank, the suspended solids are sedimented. The supernatant, known as clear juice is drawn off of the clarifier. The clarified juice is then sent to the evaporators. The settled solids can be filtered to produce a juice of poor clarity, which can be recycled for further purification.
Evaporation of water
The evaporation process serves to concentrate the clarified juice. The most widely used evaporator is a multiple-effect evaporator of the Roberts type. The product of this step is syrup of 78 to 86% purity with a soluble solid content of 60-65°Brix and containing 3.5-4.5% invert sugars.The temperature, velocity and retention time in the evaporator are regulated to prevent sucrose inversion, or decomposition of sucrose in glucose and fructose. Another concern is scale formation on the heating surface of the evaporator. The application of a magnetic flow can help to prevent scaling.
Crystallization
Crystallization is done with a single-effect vacuum boiling pan and a crystallizer. In the vacuum pan, the syrup is evaporated until it gets supersaturated with sugar. At this point seed grain is added to serve as nuclei for sugar crystals, and more syrup is added as the water evaporates. The growth of crystals continues till the vacuum pan is full. The crystals and the mother liquor now form a dense mass known as massecuite. The 'strike' is then discharged into a crystallizer.In the crystallizer, the crystallization process of the massecuite continues. The purpose of the crystallizer is to reduce loss of sucrose by it remaining in the mother liquor / molasses, in particular with low-grade massecuites. The crystallizer works by cooling the massecuite. This decreases solubility and again increases saturation, forcing crystallization to continue. Crystallizers are cylindrical or U-shaped vessels equipped with low-speed stirring elements. They are often connected in series for continues operation.
Cooling the massecuite increases viscosity. At the optimum temperature for crystallization, the massecuite is too viscous for the centrifuge to properly separate the crystals from the molasses. However, as the mother liquor of the massecuite is still supersaturated at this point, the viscosity can be reduced without re-solution of the crystals. This can be done by bringing it to a state of saturation by heating or adding water.
Centrifuging
The sugar centrifuge serves to separate the massecuite into sugar crystals and mother liquor / molasses. These centrifuges consist of a cylindrical basket suspended on a spindle. The perforated sides are lined with wire cloth, inside of which are metal sheets containing 400-600 perforations per square inch. The basket revolves at 1000-1800 rpm.While the mother liquor, molasses passes through the holes in the centrifuge, the sugar crystals are retained. After the sugar is purged, it is cut down, making the centrifuge ready for the next badge.
Boiling system
It is quite common for sugar to result from repeatedly undergoing the crystallization and centrifugation steps. This depends on the boiling systemThe most common boiling scheme is the three-boiling system. This method boils the sugar liquors in three crystallization/centrifugation stages, called A-, B- and C-. The sugar resulting from the first stage, A-sugar, is stored. The molasses from the A-centrifugation, A-molasses, are fed to the B vacuum pan. This results in B-sugar and B-molasses. A mix of A-sugar and B-sugar forms the commercial product of the factory.
The B-molasses are of a much lower purity. They are boiled again in the C-pan. While the A and B stage do not always use a crystallizer, it is essential for this low-grade massecuite. The massecuite remains in the crystallizer for more than a day. The C-sugar from the centrifuge is mingled with syrup and used as massecuite seed, and so returns to the start of the process. The molasses resulting from this centrifuge step are called final molasses, or blackstrap. It is a heavy viscous material containing about one-third sucrose, one-fifth reducing sugars, and the remainder ash, organic non-sugars and water. It serves as a base for cattle-feed, industrial alcohol, yeast production and so on.
Boiling in a vacuum pan used to be a batch process, but continuous pan boiling is inherently far more efficient. In the 1970s the first commercially successful continuous vacuum pans were developed. In the 1980s these first pans achieved a better uniform crystal size than that which some factories achieved with their batch process vacuum pans.